10.17 Combustion Chambers for SI Engines – Thermal Engineering

10.17 ❐ COMBUSTION CHAMBERS FOR SI ENGINES

The design of combustion chamber involves the shape of the combustion chamber, location of the spark plug, and disposition of inlet and exhaust valves.

10.17.1 Basic Requirements of a Good Combustion Chamber

The three main requirements of a SI engine combustion chamber are high power output with minimum octane requirement, high thermal efficiency, and smooth engine operation.

  1. Higher power output requires the following:
    1. High compression ratio
    2. Small or no excess air
    3. No dead pockets
    4. An optimum degree of turbulence
    5. High volumetric efficiency
  2. High thermal efficiency requires the following:
    1. High compression ratio
    2. Small heat loss during combustion
    3. Good scavenging of exhaust gases
  3. Smooth engine operation requires the following:
    1. Moderate rate of pressure rise during combustion.
    2. Absence of detonation: compact combustion chamber, proper location of spark plug and exhaust valve, and satisfactory cooling of spark plug points.

10.17.2 Combustion Chamber Design Principles

  1. To achieve high volumetric efficiency, the largest possible inlet valve should be accommodated with ample clearance around the valve heads.
  2. To prevent detonation, the length of the flame travel from the spark plug to the farthest point in the combustion space should be as short as possible.
  3. To reduce the possibility of detonation, the exhaust valve should be near the spark plug.
  4. The exhaust valve should be kept small due to its hot surface, but to compensate for this, a high lift should be employed.
  5. The correct amount of turbulence should be created in the combustion chamber by properly positioning the inlet valve. Turbulence should be preferably created by squish.
  6. The shape of the combustion chamber should be such that the largest mass of the charge burns as soon as possible after ignition with progressive reduction in the mass of charge burnt towards the end of combustion.
  7. To ensure high thermal efficiency and less air pollution, the surface–volume ratio of the chamber should be minimum in the beginning.
  8. In the end gas region, surface–volume ratio should be large to achieved good cooling in the ‘detonation zone’.
  9. The spark plug should be so positioned so that it will be scoured of any residual exhaust products by the incoming charge.
  10. The exhaust valve head should be well-cooled as it is the hottest region of the combustion chamber.
  11. The sparking plug points should be sufficiently cooled to avoid pre-ignition effects.
  12. There should be good scavenging of exhaust gases.
  13. Thickness of wells must be uniform for uniform expansion.
  14. It is desirable to employ a plain flat-topped piston on manufacturing grounds.
  15. To achieve maximum thermal efficiency, the highest possible compression ratio should be used.

10.17.3 Combustion Chamber Designs

The various types of combustion chambers are shown in Fig. 10.62.

  1. T-head combustion chamber: In the earliest engine, the T-head design shown in Fig. 10.62(a) was used. Its disadvantages are that it has two camshafts and it is prone to detonation.
  2. L-head or side valve combustion chamber: This is shown in Fig 10.62(b). Its advantages are as follows:
    1. A side valve engine is easy to manufacture.
    2. It is easy to enclose and lubricate the valve mechanism.
    3. Its head can be removed without disturbing the valve-gear and pipe work for decarbonising.
    4. It provides a compact layout.
    5. Cooling of exhaust by inlet charge is more effective as both are situated by the side.

    Its shortcomings are as follows:

    1. There is lack of turbulence in the incoming charge.
    2. It is extremely prone to knocking as the flame has to travel long distance.
    3. It is extremely sensitive to ignition timing due to slow combustion.

    Figure 10.62 Examples of typical combustion chambers: (a) T-head type, (b) L-head type, (c) L-head side valve type, (d) I-head type, (e) F-head type

  3. Ricardo turbulent L-head side valve design: This arrangement is shown in Fig. 10.62(c). This gives higher flame speed and reduces the knocking tendency. The entire combustion space is concentrated over the inlet and exhaust valves. This provides sufficient turbulence during the compression stroke and increased flame velocity. The ratio of area to volume of the space above the piston is large, which provides sufficient quench area essential to avoid knocking.
  4. Overhead valve or I-head combustion chamber: In this arrangement, both the valves are located in the cylindrical head as shown in Fig. 10.62(d). This arrangement is superior to side valve or L-head arrangement when high compression ratios are used for the following reasons:
    1. Lower pumping losses and higher volumetric efficiency from better breathing of the engine from larger valves or valve lifts and more direct passage ways.
    2. Less distance for the flame to travel and therefore greater freedom from knock.
    3. Less force on the head bolts and therefore less possibility of leakage.
    4. Absence of exhaust valve from block results in more uniform cooling of cylinder and piston.
    5. Lower surface-volume ratio and, therefore, less heat loss and less air pollution.
    6. Easier to cast and hence lower casting cost.
  5. F-head combustion chambers: A combustion chamber in which one valve is located in the head and other in the block is known as F-head combustion chamber, as shown in Fig. 10.62(e). This arrangement is a compromise between L-head and I-head combustion chambers. Figure 10.62(e) shows exhaust valve in head and inlet valve in block. This gives a rather poor performance. The modern F-head engines have inlet valve in the head and exhaust valve in the block. The valves are inclined with plug located in the flat roof. This provides the shortest flame travel and ensures minimum knocking tendency.
10.18 ❐ COMBUSTION IN CI ENGINES

In the CI engines, only air is compressed through a large compression ratio during the compression stroke, raising highly its temperature and pressure. At this stage, one or more jets of fuel are injected in the liquid state, compressed to a high pressure by means of a fuel pump. Each minute droplet, as it enters the hot air, is quickly surrounded by an envelope of its own vapour and after an appreciable interval, is inflamed at the surface of the envelope. As soon as this vapour and the air in contact with it reach a certain temperature and the local air-fuel ratio is within combustion range, ignition takes place.

10.18.1 Stages of Combustion

The stages of combustion in the CI engine are shown in Fig. 10.63.

  1. First stage—ignition delay period: During the first stage, some fuel has been admitted but has not yet been ignited. The ignition delay is counted from the start of injection to the point where the pressure-time (crankshaft rotation) curve separates from the pure air compression curve. The delay period is a sort of preparatory phase.
  2. Second stage—rapid or uncontrolled combustion (premixed flame): In the second stage, the pressure rise is rapid because during the delay period the fuel droplets have had time to speed over a wide area and they have fresh air all around them. This period is counted from the end of the delay period to the point of maximum pressure on the indicator diagram. About one-third of the heat is evolved during this period.

    Figure 10.63 Stages of combustion in a CI engine

  3. Third stage—controlled combustion (diffusion flame): At the end of the second stage, the temperature and the pressure are so high that the fuel droplets injected during the last stage burn almost as they enter and any further pressure rise can be controlled by the injection rate. The period of controlled combustion is assumed to end at maximum cycle temperature. About 70%–80% of total heat of the fuel is supplied during this period.
  4. Fourth stage—after burning: Due to poor distribution of fuel particles, combustion continues during the part of the remainder of the expansion stroke. This after-burning can be called the fourth stage of combustion. The after-burning period is about 70°–80° of crank angle from the TDC. The total heat evolved during the end of entire combustion process is 95–97% and 3–5% of heat goes to unburnt fuel.

10.18.2 Delay Period or Ignition Delay

In Fig. 10.64, the delay period is shown on pressure-crank angle (or time) diagram between points a and b. Point a represents the point of injection and point b represents the time at which the pressure curve (caused by combustion) first separates from the compression curve (non-firing or motoring curve). This ignition delay period can be roughly divided into two parts.

  1. Physical delay: The period of physical delay is the time between the beginning of injection and the attainment of chemical reaction conditions. In the physical delay period, the fuel is atomised, vapourised, mixed with air, and raised in temperature.
  2. Chemical delay: In this period, the pre-flame reactions start slowly and then accelerate until local ignition takes place. Generally, chemical delay is longer than the physical delay. The delay period refers to the sum of physical and chemical delay. The ignition lag in SI engine is basically equivalent to the chemical delay in CI engine. There is no component like physical delay in SI engine.

Figure 10.64 Pressure-time diagram illustrating ignition delay

10.18.3 Variables Affecting Delay Period

  1. Fuel: A lower pre-ignition temperature means a wider margin between it and the temperature of the compressed air and hence delay period. A higher cetane number means a lower delay period and smoother engine operation. Other properties of fuel which affect delay period are: volatility, latent heat, viscosity, and surface tension.
  2. Injection pressure or size of droplet: The smaller the size and greater the number of droplets, the larger will be aggregate area of inflammation and therefore the greater the uncontrolled pressure rise. The size of the droplets depends on the injection pressure. Therefore, lower the injection pressure the lower the rate of pressure rise during the uncontrolled phase and smoother the running.
  3. Injection advance angle: The delay period increases with increase in injection advance angle because the pressures and temperatures are lower when the injection begins. When the injection advance angles are small, the delay period reduces and operation of the engine is smoother but the power is reduced because larger amount of fuel burns during after-burning. The optimum angle of injection advance is 12°–20° before TDC.
  4. Compression ratio: Increase in compression ratio reduces the delay period as it raises both temperature and density. However, higher compression ratio decreases the volumetric efficiency and power. It also decreases mechanical efficiency. In practice, the engine designer uses the lowest compression ratio which would satisfy the needs of easy cold starting and light load running at high speeds.
  5. Intake temperature: Increasing the intake temperature would result in increase in the compressed air temperature, which would reduce the delay period. However, it would reduce the density of air and hence volumetric efficiency and power output.
  6. Jacket water temperature: Delay period is reduced with increase in jacket water temperature or compressed air temperature increases.
  7. Fuel temperature: Increase in fuel temperature would reduce both physical and chemical delay periods.
  8. Intake pressure or supercharging: Supercharging reduces the auto-ignition temperature and hence reduces delay period.
  9. Speed: As the engine speed increases, the loss of heat during compression decreases with the result that both the temperature and pressure of the compressed air tend to rise, thus reducing the delay period.

 

Table 10.9 Effect of variables on delay period in CI engine

  1. 10. Air-fuel ratio (load): With increase in air-fuel ratio the combustion temperatures are lowered and hence the delay period increases. With increase in load, air-fuel ratio decreases operating temperature increase and hence, delay period decreases.
  2. 11. Engine size: The engine size has little effect on the delay period in milliseconds but crank angle decreases.
  3. 12. Type of combustion chamber: A pre-combustion chamber gives shorter delay period as compared to an open type of combustion chamber.

The summary of the effect of delay period in a CI engine is given in Table 10.9.

10.19 ❐ KNOCKING IN CI ENGINES

The fuel injection takes place over a definite period. Consequently, the first droplets injected are passing through the ignition delay, additional droplets are being injected into the chamber. If the ignition delay of the injected fuel is small the first droplets start burning in a relatively short time after injection and relatively small amount of fuel will be accumulated in the chamber. Therefore, the rate of pressure rise will be smooth as shown in Fig. 10.65(a). However, if the delay period is longer, the accumulation of fuel will be larger as the actual burning of the first droplets is delayed. When actual burning starts, the accumulated fuel and additional fuel injected starts burning and can cause too rapid rate of pressure rise. This rapid rate of pressure rise causes pulsating combustion and creates heavy noise. This pulsating variation of pressure due to sudden burning of accumulated fuel is shown in Fig. 10.65(b). This type of abnormal combustion is known as knocking. This knocking occurs during the initial period over the delay period when the burning of the first portion of the fuel is uncontrolled, thus giving rise to an excessive rate of pressure rise.

Figure 10.65 Combustion in CI engine: (a) Without knocking, (b) With knocking

10.19.1 Factors Affecting Knocking in CI Engines

The factors which influence the delay period also influence the knocking tendency of CI engines are as follows.

  1. If the injection of fuel is too far advanced, the rate of pressure rise during auto-ignition is very high and cause knocking.
  2. Inferior fuels (having lower cetane number) promote diesel knock but this can be avoided by using better types of fuel (higher cetane number).
  3. Fuel injection parameters (better penetration and distribution) and better combustion chamber design (split type) also influence knocking tendency of the engine.
  4. Initial condition of the air (higher temperature or higher pressure in case of supercharged engine or both) influences knocking tendency. Higher pressure and temperature reduces knocking tendency.
  5. The fuel having longer delay period and higher self-ignition temperature leads to knocking.

With proper design of combustion chamber and injecting the fuel uniformly over a hot surface of the combustion chamber, the detonation can be avoided.

10.19.2 Controlling the Knocking

If the delay period is long, a large amount of fuel will be injected and accumulated in the chamber. The auto-ignition of this large amount of fuel may cause high rate of pressure rise and high maximum pressure which may cause knocking in diesel engines. The methods for controlling diesel knock are as follows:

  1. Cetane number of the fuel used: Cetane number is a scale for comparing the ignition delay period of various diesel fuels. A higher Cetane number means a lower delay period, and smoother engine operation, because it reduces the self-ignition temperature. Some compounds called additives or dopes are used to improve combustion performance of fuels. Various additives used for diesel fuels are: Isopropyl nitrate, Amyl nitrate, Heptyl peroxide and Butyl peroxide. These dopes delay the auto-ignition but they increase NOx drastically.

    Figure 10.66 Required rate of fuel supply to avoid detonation

  2. Compression ratio: Increase in compression ratio reduces the delay period as it raises both temperature and density. At the same time the minimum auto-ignition temperature decreases which reduces the time of reaction when fuel is injected. Thus the delay period decreases. With increase of compression ratio the unused air would be much more decreasing the volumetric efficiency and power. It further decreases the mechanical efficiency due to increase in weight of reciprocating parts.
  3. Air-fuel ratio: With decrease in air-fuel ratio (richer mixture) the combustion temperatures are increased and cylinder wall temperatures are increased and hence the delay period decreases. With increase of load air-fuel ratio decreases, operating temperature increases and hence delay period decreases.
  4. Injection pressure: Lower the injection pressure, the lower the rate of pressure rise during the uncontrolled phase and smoother the running of engine. Lower injection pressure gives rise to fuel droplets of smaller size to obtain largest surface-volume ratio resulting in less physical delay. Large droplets of fuel results in too low subsequent burning.
  5. Controlling the rate of fuel supply: It can be reduced by controlling the initial rate of fuel delivery without affecting efficient combustion condition. A small amount of fuel should be supplied until combustion starts and more fuel should be supplied as the inside combustion chamber condition is suitable (because of high temperature inside) to burn more fuel. In this case, the cam shape is so designed that it gives a low initial injection followed by a main injection at a higher rate as shown in Fig 10.66.
  6. Knock reducing fuel injector: This type of injector is developed so that it can avoid the sudden increase in pressure inside the combustion chamber due to the accumulated fuel. In this case, the injection pressure of 100 bar is used with a semi-flexible needle valve.

10.19.3 Comparison of Knocking in SI and CI Engines

It is interesting to compare the knocking phenomenon in SI and CI engines. This phenomenon in both engines is fundamentally similar as it involves auto-ignition, subject to the ignition-lag characteristics of A:F mixture supplied to the engine.

  1. In SI engines, the knocking occurs near the end of combustion, whereas in CI engine, this occurs at the beginning of combustion as shown in Fig. 10.67.

    Figure 10.67 Knocking in SI and CI engines

  2. The knocking in SI engine takes place in a homogeneous mixture; therefore, the rate of pressure rise and maximum pressure is considerably high. In case of a CI engine, the mixture is not homogeneous and the rate of pressure is lower than in an SI engine.
  3. The question of pre-ignition does not arise in CI engines as the fuel is supplied only near the end of compression stroke.
  4. It is easier to distinguish between knocking and non-knocking operations in SI engines as the human ear easily finds the difference. However, in a CI engine, the normal ignition is by auto-ignition and the rate of pressure-rise under normal operation is considerably higher (10 bar against 2.5 bar for SI engines) and causes high noise. The noise level becomes excessive under knocking condition. Therefore, there is no definite distinction between normal and knocking combustion.

The knocking in SI engines is due to the auto-ignition of the last part of the charge. To avoid this, the fuel must have a long-delay period and high self-ignition temperature.

To avoid knock in the CI engine, the delay period should be as small as possible and the fuel self-ignition temperature should be as low as possible.

The factors which reduce knock in SI and CI engines are given in Table 10.10.

Due to this dissimilarity in the time of starting of the knock in SI and CI engines, the conditions which reduce the knock tendency in the SI engine increase the knock tendency in CI engines. The fuels which are better from the point of view of avoiding detonation in SI engines may promote detonation in CI engines. Therefore, good SI fuels are poor CI fuels. In terms of fuel rating, diesel oil has high cetane number (40–60) and low octane number (30) and petrol has high octane number (80–90) and low cetane number (20).

 

Table 10.10 Factors which reduce knock in SI and CI engines

10.20 ❐ COMBUSTION CHAMBERS FOR CI ENGINES

The most important function of the CI engine combustion chamber is to provide proper mixing of fuel and air in a short time. For this purpose, an organised air movement, called air swirl, is provided to produce high relative velocity between the fuel droplets and air.

There are three basic methods of generating swirl in a CI engine combustion chamber as follows.

  1. By directing the flow of the air during its entry to the cylinder, known as induction swirl. This method is used in open combustion chambers shown in Figs 10.68 (a) to (d).
  2. By enforcing air through a tangential passage into a separate swirl chamber during the compression stroke, known as compression swirl. This method is used in swirl chambers. This type of chamber is shown in Fig. 10.69.

Figure 10.68 Open combustion chambers: (a) Shallow depth chamber, (b) Hemispherical chamber, (c) Cylinderical chamber, (d) Toroidal chamber

Figure 10.69 Ricardo swirl chamber comet, mark II

Figure 10.70 Combustion-induced swirl chambers: (a) Precombustion chamber, (b) Lanova air-cell combustion chamber

  1. 3. By use of the initial pressure rise due to partial combustion to create swirl turbulence, known as combustion induced swirl. This method is used in pre-combustion chambers and air-cell chambers. These type of chambers are shown in Figs 10.70(a) and (b).

The comparison of induction and compression swirl is given in Table 10.11 and comparison of combustion characteristics in Table 10.12.

 

Table 10.11 Comparison of induction and compression swirl

 

 

Table 10.12 Comparison of combustion chamber characteristics (Mainly for four-stroke non-supercharged high-speed engines)

10.21 ❐ LUBRICATION SYSTEMS

All moving parts of an internal combustion engine have relative motion and rub against each other. The lubrication is required to reduce this rubbing action and increases the life of engine. The purpose of lubrication is to reduce the rubbing action between different machine parts having relative motion and to remove the heat generated inside the cylinder. Due to friction between rubbing parts and combustion of fuel in the cylinder, the temperature of various parts, cylinder, and piston may rise enormously if the engine is not cooled. The lubrication system will also be seriously affected, leading to damage to the cylinder and piston. Therefore, it is essential to provide lubrication and cooling to various engine parts.

10.21.1 Functions of a Lubricating System

The important functions of a lubricating system are as follows:

  1. Lubrication: The main function of the lubricating system is to keep the moving parts sliding freely past each other and thus, reduce the engine friction and wear.
  2. Cooling: To keep the surfaces cool by taking away a part of their heat through the oil passing over them. This requires oil to possess good oxidation stability.
  3. Cleaning: To keep the bearings and piston rings clean of the products of wear and combustion by washing them away.
  4. Sealing: The lubricating oil must form a good seal between piston rings and cylinder walls. Therefore, it must possess adequate viscosity stability.
  5. Reduction of noise: Lubrication reduces the noise of the engine and other sliding parts.

10.21.2 Desirable Properties of a Lubricating Oil

The desirable properties of a lubricant are as follows:

  1. It should be available in a wide range of viscosities.
  2. There should be little change in the viscosity of the oil with a change in temperature.
  3. The oil should be chemically stable at all temperatures encountered in its application.
  4. The oil should have sufficient specific heat to carry away frictional heat without abnormal rise in temperature.
  5. It should be commercially available at a reasonable cost.

10.21.3 Lubricating Systems Types

There are two types of lubricating systems:

  1. Splash lubricating system: The arrangement of a splash lubricating system is shown in Fig. 10.71. This method is generally used for a vertical engine with a closed crankcase. The sump is located at the bottom of the crankcase. When the engine crankshaft rotates, the big end of the connecting rod splashes oil by centrifugal action. The connecting rod big end has a hollow pipe called a scoop which is fitted to the bearing cap and pointed towards the direction of rotation of the crankshaft. The lubricating oil passing through the scoop lubricate the big end bearing and gudgeon pin bearing. All other parts are lubricated by the splash. Excess oil is collected in the troughs, is provided with overflows, and collected in the main sump. The level of the oil in the trough is maintained constant. The dripping from the cylinders is also collected in the sump. The oil from the sump is recirculated with the help of a pump.

    The limitations of this system are as follows:

    1. Inability to regulate the quantity of oil splashed against the cylinder wall.
    2. Inability to keep the oil from getting past the piston head into the combustion chamber, burning with the fuel and passing out with exhaust gases.
  2. Pressure feed lubricating system: This system is shown in Fig. 10.72. Such a system supplies oil under pressure directly to the connecting rod bearing, crankshaft bearings, valve gear, and the camshaft drive. Indirect supplies reach the cylinder walls, gudgeon pin, the distributor, and pump drives.

    Oil is carried in the sump and circulated by the gear pump which sucks from the sump through a strainer. The pump delivery pressure is controlled by a relief valve and the oil passes through a very fine filter before it reaches the main distributor gallery from the various bearings, surfaces, and gears.

    After lubricating the big end bearings, the oil is fed to the gudgeon pins through the oilway in the connecting rod and further squirted into the cylinder wall.

  3. Charge lubricating system: It does not require oil-filter and oil sump. In this system, the lubricating oil is pre-mixed with the fuel which carries the lubricating oil in the cylinder to lubricate the piston and cylinder. Most of the oil burns with the fuel and is carried away with the exhaust gases. The lubricating oil cannot be recovered in this system. The system is generally used for two stroke spark ignition engines of scooters and motorcycles.

    Figure 10.71 Splash lubrication system

    Figure 10.72 Wet sump high pressure lubrication system

    The main disadvantages of this system are as follows:

    1. Carbon deposits due to burning of oil on the spark plug
    2. Non-recovery of the oil used

    The advantages of this system are as follows:

    1. It does not require a separate lubricating system and is the most economical one.
    2. There is no risk of failure of lubrication system.
    3. The lubricating oil supplied is regulated at various loads and speeds by the increased fuel flow.

10.21.4 Lubricating System for IC Engines

Various lubricating systems used for internal combustion engines may be classified as follows:

  1. Mist lubrication system: This system is used for two-stroke cycle engines. Most of these engines are crank-charged, that is, they employ crankcase compression and thus, are not suitable for crankcase lubrication.

    Such engines are lubricated by adding 2–3% lubricating oil in the fuel tank. The oil and the fuel mixture is inducted through the carburettor. The gasoline is vapourised; and the oil, in its form of mist, goes through a crankcase into the cylinder. The oil which impinges on the crankcase walls lubricates the main and connecting and bearing, the rest of the oil which passes on to the cylinder during charging and scavenging periods, lubricates the piston, piston rings, and the cylinder.

    The two-stroke engine is very sensitive to a particular oil and fuel combination. The composition of fuels and lubricants used influence the exhaust smoke, internal corrosion, bearing life, ring and cylinder bore wear, ring sticking, exhaust and combustion chamber deposits, and one of the most irritating and difficult problem of spark plug fouling and whishering. Therefore, specially formulated ashless oils are used for two-stroke engines.

    The fuel/oil ratio used is also important for good performance. A fuel/oil ratio of 40 to 50:1 is optimum. Higher ratios increase the rate of wear and lower ratios result in spark plug fouling.

    The main advantage of this system is simplicity and low cost because no oil pump, filter, etc., are required. However, this simplicity is at the cost of many troubles some of which are enumerated as follows:

    1. Some of the lubricating oil remains invariably in combustion chamber. This heavy oil when burned, and still worse, when partially burned in the combustion chamber, leads to heavy exhaust emissions and formation of heavy deposits on the piston crown, ring grooves, and the exhaust port which interferes with efficient engine operation.
    2. One of the main functions of the lubricating oil is protection of anti-friction bearings, etc., against corrosion. Since the oil comes in close contact with acidic vapours produced during the combustion process, it rapidly loses its anti-corrosion properties resulting in corrosion damage of bearings.
    3. For effective lubrication, oil and the fuel must thoroughly mixed. This requires either separate mixing prior to use or use of some additive to give the oil good mixing characteristics.
    4. One important limitation of this system is oil starvation of the working parts, especially when the throttle is closed on a descent on a long hill. A closed throttle means no fuel, and, hence, no oil. The prolonged absence of oil so produced may result in overheating and piston seizure. This oil starvation can be controlled if the driver periodically releases the throttle to replenish for the complete absence of oil while descending a hill.
    5. Due to high exhaust temperature and less efficient scavenging, the crankcase oil is diluted. In addition, some lubricating oil also burns in the combustion chamber. This results in about 5–15% high lubrication consumption for two-stroke engines as compared to four-stroke engines of similar size.
    6. Since there is no control over the lubricating oil, once introduced with fuel, most of the two-stroke engines are over-oiled most of the time.

    Some manufacturers use a separate oil injection pump to inject the oil directly into the carburettor and the amount of the oil is regulated according to engine load and speed. This system completely avoids the oils starvation problem discussed above. The oil consumption is also reduced. This results in less deposits and less spark plug fouling problems. Since in this system the main bearings are excluded from the crankcase and instead receive lubricating oil from a separate pump, the corrosion damage of bearings is also eliminated.

  2. Wet sump lubricating system: The wet sump lubricating system is shown in Fig. 10.73. In this system, the sump is always full of oil. Oil is drawn from the sump by an oil pump through an oil strainer. A pressure relief valve is provided which automatically maintains the delivery pressure constant. If the pressure exceeds the predetermined pressure, the valve opens and allows some of the oil to return to the sump and relieves the oil pressure in the system. The oil from the pump goes to the bearings and part of it passes through the filter which removes solid particles from the oil. As all the oil is not passed through the filter, the system is known as by-pass filtering system. The advantage of this system is that a clogged filter will not restrict the flow of oil to the engine.
  3. Dry sump lubricating system: In the dry sump lubricating system as shown in Fig. 10.74, the sump does not contain oil and all the oil required for lubrication remains in circulation. High speed racing cars and military jeeps use this type of system.

    An auxiliary tank is used to supply the oil to the main bearings with the help of the pump. The oil returns to the tray and is then returned to the auxiliary tank by the scavenging pump. If the filter gets clogged, the pressure relief valve opens, permitting oil to flow by passing the filter and reaches the supply tank. The oil is then recirculated to the bearings from the supply tank. A separate oil cooler is used to cool the oil.

10.21.5 Lubrication of Different Engine Parts

  1. Lubrication of main bearings: The main bearings are lubricated satisfactorily with the help of a ring or a chain-type feeder. The arrangement of the system is shown in Fig. 10.75.

    The oil ring rests on the main shaft where a small portion of the main bearing shell has been cut away as shown in the figure. The lower end of the oil ring is allowed to submerge in the oil bath as shown in Fig. 10.73. The oil ring rotates with the main shaft and carries the oil from the oil bath to the bearing and is distributed to the bearing through the oil groove. The surplus oil flows to the ends of the bearing and drops back into the oil bath. Chains or more rings are provided instead of single ring to carry more oil.

    Figure 10.73 Wet sump lubricating system

    Figure 10.74 Dry sump lubricating system

    Figure 10.75 Ring lubrication

    This type of lubrication is more useful for medium speed engines because at high speeds the oil will be thrown off due to centrifugal force but at low speeds, the amount of oil carried is not sufficient.

  2. Lubrication of cylinder and small end bearing of connecting rod: The cylinder and a small end bearing (gudgeon pin) are lubricated with the help of side feed lubricator as shown in Fig. 10.76.

    The oil is supplied to the surface of piston through the hole provided in the cylinder wall. The oil falling on the surface of the piston is spread over the surface of the cylinder walls.

  3. Lubrication of crank and gudgeon pin: The scoops are provided to the caps of big-end-bearing of the connecting rods as shown in Fig. 10.76. These scoops dip into the splash toughs when the pistons are at BDC and splashes the oil in all directions with the rotation of cranks. A thin mist of splashed oil settles on the surface of the cylinder and on the other parts which are to be lubricated. The main bearings and cam shaft bearings are provided with small pockets and the oil splashed by scoops is collected in those pockets and is used for lubrication. The surplus oil falls back into the oil sump.

    Figure 10.76 Side feed lubricator

    Many times, a through hole is provided in the connecting rod and it is connected with the scoop; therefore, some oil is supplied to the crank pin and through the hole to the gudgeon pin. The oil level in the splash tough is maintained with the help of oil pump. The arrangement of scoop with connecting rod is shown in Fig. 10.77.

  4. Lubrication of big end (crank pin) and small end (Gudgeon pin) of connecting rod: The crank pin and gudgeon pin are lubricated by ring as shown in Fig. 10.78.

The arrangement of the system used is shown in Fig. 10.78. In this system, the oil is supplied to the ring from the oil feed pump which is fitted concentric on the crank shaft. The centrifugal force is given to the oil as the oil ring rotates with the crank shaft and the lubricating oil is forced to the crank pin and to the gudgeon pin through the holes provided in the crank pin and connecting rod as shown in Fig. 10.77. This system of lubrication is known as ring lubrication.

Figure 10.77 Scoope used in splash lubrication

Figure 10.78 Lubrication system for big end of connecting rod

10.22 ❐ NECESSITY OF IC ENGINE COOLING

The cooling is provided to avoid the effects of overheating as follows:

  1. The high temperature reduces the strength of the piston and piston rings and uneven expansion of cylinder and piston may cause the seizure of the piston.
  2. The high temperature may cause the decomposition of the lubricating oil and lubrication between the cylinder wall and piston may break down, resulting in a scuffing of the piston.
  3. If the temperature around the valve exceeds 250°C, the overheating of the valves may cause the scuff of the valve guides, due to lubrication break down. Further increase in temperature may cause the burning of valves and valve seats.
  4. The tendency of the detonation increases with an increase in temperature of the cylinder body.
  5. The pre-ignition of the charge is possible in spark ignition engines if the ignition parts initially are at high temperature.

10.22.1 Types of Cooling Systems

There are two types of cooling system as per the coolant used—air cooling and water cooling system.

Air Cooling System

In this system, air is used as a cooling medium and it is used for small capacity engines. Earlier, it was used for big capacity air-craft engines as water cooling was not practically possible as the weight of water cooling system is very high compared with air-cooling system.

As mentioned earlier, the heat transfer coefficient for air-cooling is very low. This heat transfer coefficient can be increased further by using the forced flow of air over the engine surface as done in aero-engines. The heat transfer coefficient with air-cooling with forced circulation is also considerably lower (50 W/m2-K) when compared with water cooling system. The other method of increasing the heat transfer rate from the cylinder’s surface is to increase the surface area by providing the fins. The use of fins increases the heat transfer surface area by 5 to 10 times of its original value. The air-cooled systems, the forced circulation of air with increased surface area by using the fins, is commonly used in practice for aero-engines and motor cycle engines.

The sectional view of an engine-cylinder with fins is shown in Fig. 10.79. More fins are used near the exhaust valve and cylinder head where the possibility of occurrence of maximum temperature exists.

The cooling fins are either cast integral with the cylinder and cylinder head or they are fixed to the cylinder block separately. The number of the fins used are two to three in case of cast fins and four to five in case of machined fins per centimetre. The height of the fin depends on the type of material and the manufacturing process used. Generally, the height of the fin used lies between 2 cm and 5 cm and the fin spacing is limited to 2.5 mm.

Advantages of Air Cooling

  1. Water jacket, radiator and water pump are not required. This reduces the size of the engine and its weight.
  2. Simpler engine design.
  3. Less sensitive to climatic conditions engine.
  4. Due to small thermal losses, the specific fuel consumption is low.
  5. Better warm-up performance results in low wear of cylinder.
  6. Sustained engine performance due to reduced carbon deposit on combustion chamber.
  7. Easier control of cooling systems compared to water-cooled engines.
  8. An air-cooled engine can take up some degree of damage.

Disadvantages of Air Cooling

  1. Air fan creates noise.
  2. The volumetric efficiency is lower due to higher cylinder head temperature.
  3. High specified output engines cannot be air-cooled due to the complex nature of the fins that are required.
  4. Air cooling results in higher engine temperature. This necessitates the provision of bigger clearances between the various parts of the engine.

Figure 10.79 Air cooling of engine cylinder

Water Cooling System

The water cooling system is further subdivided into two groups:

  1. Thermo-siphon system: The arrangement of the system is shown in Fig. 10.80. The force (pressure head) required to circulate the water through the system is the difference in pressure head due to hot and cold water. This force is given by

     

    F = h(ωcωh) = h × Δω

     

    where h = Height of the radiator tubes through which the water is circulated.

    ωc = Weight density of cold water.

    ωh = Weight density of hot water.

    The difference in density is limited as the rise in temperature of the water passing through the engine is limited. The rate of circulation is less as the force causing the flow of water is limited. This cannot be used for heavy duty engines as the heat carrying capacity of this system is limited. The water as passed through the radiator is cooled by the flow of air passed over the radiator tubes by the cooling fans. The cooled water rises to the cylinder jacket; takes the heat from the cylinder wall and then it enters into the radiator from the top header and comes down. It is cooled as it is passed through the radiator tubes.

    The limitations of this system are as follows:

    1. The engine should be placed as low as possible in relation to the radiator as the force causing the flow is limited by the temperature difference of hot and cold water.
    2. The water level in the system should not fall below the level of the delivery pipe; otherwise, the circulation of water in the system will stop. This causes the boiling of water and formation of steam, resulting in further loss of water which may damage the engine in a short period.
    3. The use of this system is recommended for small capacity engines.
    4. The main drawback of this system is that cooling depends only on the temperature and is independent of engine speed. The rate of circulation is slow.

    Due to these limitations, this system is rarely used at present.

    Figure 10.80 Thermo iphon system

    Figure 10.81 Forced pump system

  2. Forced pump system: In a thermo-siphon system, the height of the radiator should be kept above the engine as the flow of water takes place by natural circulation.

    To avoid this limitation, a pump is incorporated in the system for water circulation. The arrangement of the system is shown in Fig. 10.81. The pump is driven from the engine shaft with the help of the belt.

    The major advantage of this system is effective and positive cooling of all the parts of the engine. It can easily take the overload as the engine speed increases, water circulation is also increased and same effective cooling can be maintained by this system.

Advantages of Forced Circulation System

  1. The increased rate of coolant flow improves the heat transfer between the gases and water. This reduces the quantity of water to be circulated and thereby reduces both the bulk and weight of the engine because smaller coolant passages can be used.
  2. The high temperature areas such as spark plug; exhaust valve seats and exhaust ports can be effectively cooled and reduces local overheating.
  3. The formation of steam pockets in critical areas that restrict the coolant circulation and hence the heat transfer, can be avoided by maintaining the pressure in the system.
  4. Increased coolant velocities allow a higher mean temperature difference between the water and the air passing through radiator matrix. This reduces the size of the radiator.
  5. When the vehicle is operated in mountainous regions, loss of water through boiling may occur in thermo-siphon system is used because the boiling temperature of water decreases with the decrease in atmospheric pressure at high altitudes.

Disadvantages of Forced Circulation System

  1. There is a possibility of overcooling as the cooling is independent of temperature.
  2. There is a possibility of overheating the engine when the automobile is climbing a hill where the fuel burning rate is increased and water flow rate is reduced as part of the head is lost to overcome the inclination of the road.
  3. As soon as engine stops, the cooling also stops. This is undesirable because the cooling should continue for some time till the temperatures are brought to normal values.

Cooling With Thermostatic Regulator

An important feature of the cooling system of present-day cars is the inclusion of a thermostat. This is generally fitted in the upper radiator hose and its purpose is to prevent the circulation of water through radiator until the cylinder block and head have reached a temperature of about 75°C. If the water circulation starts immediately as the engine has been started (as in the absence of thermostat), the engine cannot run efficiently when all the water in the cooling system has been raised to a temperature near the boiling point (90°C). By preventing circulation before the engine is warm, the engine gets warm much more quickly. In consequence, there is less danger of the oil being washed away from the cylinder walls by wet petrol drawn into the cylinders while the engine is cold. Less petrol is wasted while the engine warms up and the car can be driven more quickly after the engine has been started. The arrangement of this system is shown in Fig. 10.82.

The thermostat consists of a valve attached to a bellows containing volatile liquid such as ether. Heating of bellow by the water around them causes vapourisation of the liquid, expansion of the bellows, and therefore, opening of the valve.

When the forced circulation is used, it is necessary to use a bypass pipe from the engine side of the thermostat (as shown in Fig. 10.80) to the lower water hose so that the circulation pressure of the water cannot force to open the valve before the correct temperature has been reached.

Figure 10.82 Thermo-syphon cooling system for IC engines

Evaporative Cooling System

This is not preferred until there is acute shortage of cooling water. The arrangement is shown in Fig. 10.83. In this system, the water coming out of engine jacket is allowed to be heated to 100°C. This method of cooling utilises the latent heat of water to obtain cooling with minimum water. The cooling circuit is such that the coolant is always maintained in liquid form but the steam formed is flashed off in a separate vessel as shown in Fig. 10.83. This system is generally used for big-capacity stationary engines used for power generation (2 MW).

Advantages of Water Cooled Engines

  1. The engine design is compact due to high heat transfer coefficient of water.
  2. Due to high latent heat of water, overheating troubles are eliminated.
  3. The engine can be installed anywhere in the vehicle.
  4. The volumetric efficiency is higher than air-cooled engine.

Disadvantages of Water-cooled Engines

  1. The need for a radiator and a pump increases the weight and dimensions of the engine.
  2. It requires more maintenance.
  3. The engine performance is more sensitive to climatic conditions.
  4. The warm-up performance is poor resulting in greater cylinder wear.
  5. Pump absorbs slightly higher power than fan.

Open and Closed System

In open this system, the hot water coming out from the engine jacket is not circulated through the pump directly but it is cooled either in cooling tower or spray pond as shown in Fig. 10.84.

Figure 10.83 Evaporative cooling system

Figure 10.84 Open cooling system

With the help of a spray, the water is cooled by partly evaporating the water (1%) and absorbing the latent heat required from the remaining water. This evaporated water is carried by the air. The effective cooling of water depends upon the relative humidity of the surrounding area. This system is generally used where more than adequate water is available.

In closed system, the hot water coming out from the engine is cooled in the radiator, heat exchanger, or cooling tower. In this system, the same water is used repeatedly without exposing to the air directly as in the case of an open system.

Impurities in water and the consequent danger of scale formation are major problems in an open system. Due to water evaporation, which does not carry impurities, the concentration of salt in the water after every pass increases and danger of scaling also increases. In closed system, the hot water cooling methods are shown in Figs 10.85(a) to (c).

In a closed system, the same water remains in the system and recooled repeatedly. If it is pure at the start, it remains pure throughout the period of use and there is no danger of scaling.

Figure 10.85 Closed cooling systems: (a) Closed system (radiator type), (b) Closed system (heat exchanger) using raw water, (c) Closed system using cooling tower

Advanced Cooling System

There are two main disadvantages with the conventional cooling systems. First, large volume of coolant in the primary circuit can lead to a slow engine warm up and second, the cooling system tends to overcool the engine parts when the engine is running at part load conditions. To overcome these difficulties, advanced cooling systems are used.

The main purpose of these systems is to maintain more uniform temperature and less sensitive to the load and speed of the engine. The friction loss between the piston and the cylinder is reduced if the liner temperature is raised. The significance of reduction in friction loss increases as load is reduced because this leads to reduction in fuel consumption. A rise in liner temperature may increase the emission of NOx. This can be reduced either by retarding the ignition timing or by using exhaust gas recirculation (EGR). Both these remedies impose a fuel consumption penalty and the fuel saving gained by reducing the frictional loss. The NOx emission is problematic in diesel engine at full load conditions.

With SI engines, another problem that can occur at full load is the reduction in the knock margin caused by higher cylinder temperatures. This also can be remedied by EGR or retarding the ignition timing.

A few methods used for this purpose are described in brief.

10.22.2 Precision Cooling

It is developed for cooling diesel engines and to provide cooling only where it is needed; at a rate proportional to the local heat flux.

A precision-cooled system has small local passages which are used to cool critical regions such as injector nozzle, exhaust valve region, and valve guides, with much space in the head filled with air.

With precision cooling, many regions are not directly cooled and their temperatures rise, but the system is designed to keep the temperature within safe limits. The corresponding benefit is a more even temperature distribution, producing less thermal strain and possible elimination of hot spots to allow higher compression ratios in SI engine.

10.22.3 Dual Circuit Cooling

In this process of cooling, there are two separate cooling circuits—one for the head and the other for the cylinder block. The higher block temperature reduces friction losses and reduces fuel consumption, particularly at part loads. The lower coolant temperature in the cylinder head tends to reduce the indicated thermal efficiency but the risk of knock is reduced in SI engines and higher compression ratio can be used. The overall effect is an improved efficiency.

Increasing the coolant temperature passing through cylinder block reduces the fuel consumption and hydrocarbon emissions but increases NOx emissions.

10.22.4 Disadvantages of Overcooling

Should we cool the engine as much as we can? No. The engine must never be overcooled. The engine must always be kept sufficiently hot to ensure smooth and efficient operation. Extremely low engine temperatures may be difficult to start and above all, the low temperature corrosion assumes such a significant magnitude (see Fig. 10.86) that the engine life is greatly reduced. At low temperatures, the sulphurous and sulphuric acids resulting from combustion of fuel (fuel always contains some sulphur) attack the cylinder barrel. The dew points of these acids vary with pressure, and hence, the critical temperature, at which corrosion assumes significant properties, varies along the cylinder barrel. To avoid condensation of acids the coolant temperature should be greater than 70°C. Thus, the cooling system should not only cool but must also keep the cylinder liner temperature above a minimum level to avoid corrosion and ensure good warm up performance of the engine.

Figure 10.86 Effect of temperature on corrosion

10.23 ❐ ENGINE RADIATORS

The function of the radiator is to cool hot water coming out of the engine with the help of air. In the radiator, the heat transfer from hot water to the air takes place by conduction and convection.

The cooling effect in the radiator is enhanced by providing fins over the surface of tubes carrying the hot water to be cooled. The fins are also so arranged that some turbulence is generated in air passages which further help to increase heat transfer coefficient to the air side.

A commonly used cross-flow type radiator is shown in Fig. 10.87. The fins are not shown.

10.23.1 Radiator Matrix

Figure 10.87 shows four different types of matrices (fins) commonly used in practice.

In the tube and fin type shown in Fig. 10.87(a), a series of long tubes extending from top to bottom of the radiator are surrounded by simple straight metallic fins. The matrix has greater structural strength.

The improvement over the tube and fin type is the tube and corrugated fin type matrix as shown in Fig. 10.87(b). The water tubes are made of an oval-shape section and zig-zag copper ribbons are used to provide secondary heat transfer areas and air turbulence. This combines the good heat transfer characteristics of film type matrix and structural strength of the tube and fin type matrix.

Figure 10.87(c) shows the film-type matrix which is also known as the ribbon-cellular matrix. It consists of a pair of thin metal ribbons soldered together along their edges so as to form a water-way running from header tank to the bottom tank of the radiator. The zig-zig copper ribbons forms an air passage. In this case, the metal surface area is significantly increased and higher turbulence is also created.

Figure 10.87 Different types of radiator matrices: (a) Plate type matrix, (b) Ribbon cellular type matrix, (c) Corrugated fin matrix, (d) Honey comb matrix

Earlier, radiators used honey-comb matrix as shown in Fig. 10.87(d). The hexagons were packed in contact and bound by solder. This provided a continuous water passage between the circular parts of the tube. The cooling air was passed through the circular tubes. This type of matrix is rarely used in present types of radiators.

In all the aforementioned matrices, the air flow was in cross-flow (perpendicular to the paper).

The materials used for matrices must have the following properties:

  1. It must have good conductivity (Cast iron = 60 W/m-K, Aluminium = 210 W/m-K and Copper = 400 W/m-K).
  2. The material must have good corrosion resistance.
  3. It must possess the required strength.
  4. It must be easily formable.
  5. It must be easily available at a reasonable cost.

Yellow brass and copper meet all these requirements and are widely used.

10.23.2 Water Requirements of Radiator

The quantity of cooling water (Q) required to be circulated is given by

Figure 10.88 Variation of amount of air required with power for air-cooled radiator

where (∆T) is permissible rise in temperature and the value of K depends upon specific fuel consumption and compression ratio.

The cooling water carries 4000 kJ/kWh for large engines and 550 kJ/kWh for small engines. If the rise in temperature is limited to 10–15°C, the outlet cooling water temperature is limited to 60°C for small or medium engine and 80°C for automobile engines.

Figure 10.88 shows the amount of air required for air-cooled radiator. It is clear from the graph, petrol engine requires much more air than diesel engine.

The power required for cooling the engine is given by

where Af and ∆Tf are the fin area and temperature difference between the fin average surface temperature and air temperature, ρa is air density, and Ae is effective area.

The equation is developed with following assumptions.

  1. A:F ratio supplied to engine is constant.
  2. The temperature change in fin is assumed small.
  3. The density and temperature of the cooling media are assumed to be unaffected by its flow through the radiator fin.

10.23.3 Fans

As mentioned earlier, an engine driven fan is mounted behind the radiator to increase the air flow over the radiator tubes. This increases the air-side heat transfer coefficient and ultimately overall heat transfer coefficient.

A radiator fan is usually made of pressed steel blades, four or six in number depending upon the size of the radiator (capacity of engine). The power consumed by the fan depends upon the amount of air handled, diameter of the fan, its shape, and speed of the engine.

Figure 10.89 shows the effect of speed and diameter of fan on the quantity of air flow. Figure 10.90 shows the effect of fan diameter and air flow on the power consumed by the fan. For the given flow rate of air, the power required decreases slightly as the fan diameter increases, whereas for the same engine speed, the power required increases with the fan diameter.

Figure 10.89 Variation of diameter of fan on the quantity of air flow

Figure 10.90 Effect of fan diameter and air flow on the power consumed

10.24 ❐ COOLING OF EXHAUST VALVE

In the liquid cooled engines, the spaces around the exhaust valve seating should be sufficiently large for the coolant to flow. Sometimes, the water circulation provides jets of water to be directed on the valve seating metal.

An efficient valve cooling requires methods of conducting the heat from the valve head to the cylinder, improved guides and metal, also to the lower part of the valve stem operating under cooler air conditions.

In order to enhance the conductivity of the exhaust valve, the practice of making the valve stem and even the heads hollow is becoming popular. The hollow stem is filled with sodium which melts at 97°C and boils at 880°C. Thus, at valve operating temperatures, the valve is filled with conducting material. Such a valve is shown in Fig. 10.91 with a tapered plug seal driven into the hollow stem. The end of the stem is provided with a Stellite button, fused on the seat to prolong the wearing life of stem and end.

Figure 10.91 Cooling of exhaust valve

10.25 ❐ GOVERNING OF IC ENGINES

Governing is the process of varying the fuel supply to the engine in accordance with the load demand so that the engine runs at practically constant speed. There are four methods of governing of IC engines.

  1. Hit and miss governing: This method is used for small engines and is illustrated in Fig. 10.92. The rotational motion of the cam C actuates the rocker R through the roller D. The roller carries a pecker G which strikes against the pecker block H and lifts the valve V against the spring S. When the speed of the engine becomes excessive, the pecker block is lifted by the rod E as the sleeve of the governor rises up. The pecker is now unable to strike against and lift the valve off its seat and the engine performs an idle cycle because no fuel is now being supplied. This method of governing is quite simple but owing to the violent explosions which usually occur as a result of extra scavenging which take place immediately after a mixed explosion, produces uneven turning moment necessitating the use of heavy flywheel which increases the friction at the bearings and lowers the mechanical efficiency of the engine.

    Figure 10.92 Hit and miss governing

  2. Qualitative governing: In this method, a centrifugal governor is used to control the supply of fuel, whereas the supply of air remains constant. Thus the quality of air-fuel mixture is altered. This method is widely used in all heavy oil engines.
  3. Quantitative governing: In this method, the quantity of air-fuel mixture flowing into the cylinder is varied. This may be done by decreasing the lift of the inlet valve or by throttling the mixture before it is made to enter the cylinder. This air-fuel ratio of the mixture is kept constant. This gives a more even turning moment and closer limits of speed variation. This method is widely used for governing petrol and large gas engines.
  4. Combination method: This method uses qualitative and quantitative methods simultaneously.
10.26 ❐ RATING OF SI ENGINE FUELS-OCTANE NUMBER

The tendency to knock depends on the composition of fuel as mentioned earlier. Fuels differ widely in their ability to resist knock. The rating of a particular fuel is done by comparing its performance with that of a standard reference fuel which is usually a combination of iso-octane and n-heptane.

Iso-octane offers great resistance to knock and is arbitrarily assigned a rating of 100 octane number, n-heptane, a straight chain paraffin, on the other hand, detonates very rapidly therefore it is assigned a rating of zero-octane number. It is possible to match the knocking tendency of all commonly used fuels by blending these two reference fuels in different proportions.

The percentage of iso-octane by volume in a mixture of iso-octane and n-heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine under given standard operating conditions, is termed as ‘octane number’ of the fuel. An octane number of 80 means that the fuel has the same knocking tendency as a mixture of 80% iso-octane and 20% n-heptane by volume.

It has already been mentioned that the knocking tendency of an engine increases with increasing compression ratio. The higher the octane number of the fuel, the greater will be its resistance to knock and higher will be the compression ratio which may be used without knocking. The power output and specific fuel consumption are functions of the compression ratio and therefore, these can be considered as a function of the octane number of the fuel. This fact indicates an importance of the octane number rating of fuels for SI engines. Higher octane number fuels increase the efficiency of the engine as high compression ratio can be used with higher octane number fuels.

The knocking tendency of the fuels used in aero-engines is more as they are supercharged engines; therefore, it is necessary to use the fuels having the highest octane number (largest resistance to knock). The fuels superior in anti-knock qualities to iso-octane (having octane number greater than 100) are required. The addition of tetra-ethyl-lead (TEL) to gasoline produces marked an effect in reducing the knock tendency of the fuel. The addition of various amounts of one of these compounds (TEL is commonly used) to iso-octane produces fuels of greater anti-knock quality than iso-octane alone. The anti-knock quality of fuels above 100 octane number in the USA is measured in terms of cc (cu-cm) of TEL per US gallon of iso-octane. A fuel having an octane number of 110 means that that fuel has the same tendency to knock as a mixture of 10 cc of TEL in 1 US gallon of iso-octane. The importance of such fuels (commonly used in aero-engine) has diminished with an introduction of turbojet turbines in aircraft.

10.26.1 Anti-knock Agents

The knock resistance tendency of a fuel can be increased by adding anti-knock agents. The anti-knock agents are substances which decrease the rate of preflame reaction by delaying the auto-ignition of the end mixture in the engine until the flame generated by spark plug passes through the end mixture.

Among all, TEL [Pb(C2H5)4] is the most powerful antiknock agent. It has helped to improve the efficiency of the engine and to increase the specific output of SI engines. Its use will not improve the performance of the engine which is not knocking unless the spark is advanced, the compression ratio is increased, or a higher inlet pressure is used to take the advantage of an increase in octane number. Advancing the spark will improve the performance of the engine only if optimum spark for a non-knocking fuel is not already used.

Some other anti-knock agents with their relative effectiveness are listed in Table 10.13.

 

Table 10.13 Anti-knock agents

Knocking essentially results in an incomplete and non-uniform combustion of the fuel. It is caused by the sudden and spontaneous ignition of the unburned mixture, ahead of the flame front advancing through the combustion chamber. Partial oxidation reactions of the hydrocarbons in this mixture produce reactive intermediates that lead to such spontaneous ignition and overheating of the engine. This overheating can result in the wearing of engine parts. Antiknock agents react with these intermediates and reduce their ability to propagate the oxidation that causes knocking. Octane number of the gasoline is a measure of the effectiveness of the antiknock agent it contains. Higher this number lesser is the knock. Table 10.14 shows some of these agents and their relative effectiveness.

 

Table 10.14 Relative effectiveness of antiknocks

Gasoline composition, methods of test, and antiknock concentrations affect both the absolute and relative effectiveness of different compounds, even the same element. Moreover, some compounds can function as an antiknock under some conditions but promote knock under other. Therefore, the comparison of active elements in various compounds is representative but subject to considerable variation.

The effectiveness of an element can vary greatly, depending on the specific compound considered. Some aromatic amines are not antiknocks while others are twice as effective as N-methylamine (nitrogen in other forms is not effective). Most metallic compounds possessing the antiknock property have metal to carbon bonds but there are exceptions. In case of organometallics, it is believed that the products released by the thermal decomposition of the compound at an appropriate stage of the precombustion process are the active antiknock species.

TEL is the most widely used (also known longest) of all antiknocks. A hydrogen atom in ethane is substituted by the heavy metal atom of lead in this organometallic compound. The average TEL usage, at present, in motor gasoline (2.0 gms lead per gallon in regular and 2.7 gms in premium) usually takes up the octane number by 6 to 10. It is a mixture of tetra methyl and ethyl leads, and the intermediate lead alkyls produced by these two are also commercially used as antiknocks. These higher volatile alkyls offer advantages of better vapourisation and are the most effective in some multicylinder engines. Concentrated and alkyls are hazardous to handle. Therefore, these antiknock agents are blended with gasoline in refineries under same conditions; the resulting leaded gasoline can be handled as a motor fuel safely. The use of leaded gasoline, however, is not a perfect solution to the problem. It leads to the emission of lead into the atmosphere which is known to be hazardous.

10.26.2 Performance Number

The detonation tendency is also measured in terms of performance number. The performance number (PN) is defined as

where KLIMEP represents conception of knock limited indicated mean effective pressure.

The performance number is obtained for a specified engine, under a specified set of conditions by varying the inlet pressure.

The PN method of rating enables to develop a scale beyond 100 octane number. There is a relationship between performance number and Octane number as shown in Fig. 10.93.

It has already been mentioned that by adding TEL to iso-octane increases knock resistance tendency. Figure 10.96 also shows the quantity of TEL by volume in millilitre per litre of gasoline on X-axis for octane numbers exceeding 100 and performance number on y-axis.

10.27 ❐ HIGHEST USEFUL COMPRESSION RATIO

In the development of SI engines, it was of great interest to use higher compression ratio as the efficiency and power output of the engine increase with an increase in compression ratio. The maximum compression ratio of any SI engine is limited by its tendency to knock. Therefore, previously, the rating of SI engine fuels was based on the highest useful compression ratio (HUCR) that could be employed in a given engine under a given set of conditions. To find the highest compression ratio of a fuel, the compression ratio of the engine is increased (using a variable compression ratio engine) till knocking became audible with certain temperature conditions and with mixture strength and ignition, both adjusted to give highest efficiency. The HUCR of a few fuels determined by using Richardo E6 engine are listed in Table 10.15.

Figure 10.93 Relationship between performance number and octane number

Table 10.15 HUCR for various fuels

Through the measurement of HUCR of various fuels, a measure of their anti-knock value can be obtained.

The effect of anti-knock agents on HUCR is shown in Fig. 10.94.

Figure 10.94 Effect of anti-knock agents on HUCR

Figure 10.95 Variation of properties of fuel with cetane number

10.28 ❐ RATING OF CI ENGINE FUELS

Knocking is also encountered in CI engines, with an effect similar to that in SI engines, although it is due to a different phenomenon. Knock in the CI engine is due to sudden ignition and abnormally rapid combustion of accumulated fuel in the combustion chamber. Such a situation occurs because of ignition lag in the combustion of fuel between the time of injection and actual burning. As the ignition lag increases, the amount of fuel accumulated in the combustion chamber, before starting of combustion, also increases. When combustion actually takes place, abnormal amount of energy is suddenly released, causing an excessive rate of pressure rise which results in an audible knock.

A CI engine knock can be controlled by decreasing ignition lag. The shorter an ignition lag, there is less tendency to knock. A good CI engine fuel, therefore, will ignite more readily, that is, it will have short ignition delay. Furthermore, ignition lag affects the starting, warm-up, and production of exhaust smoke in CI engines.

The property of ignition lag is generally measured in terms of cetane number. Cetane, a straight chain paraffin with good ignition quality, is arbitrarily assigned a rating of 100 cetane number (CN). It is mixed with alpha-methyl naphthalene, a hydrocarbon with poor ignition quality, which is assigned zero CN. The mixture is matched with a fuel under test in standard (CFR) engine running under prescribed conditions. The CN of the fuel is then defined as the percent by volume of cetane in a mixture of cetane and alpha-methyl-naphthalene that produces the same ignition lag as the fuel being tested, in the same engine and under the same operating conditions.

If a test fuel has a CN of 40, it means a mixture containing 40% cetane and 60% a-methyl naphthalene by volume gives the same ignition delay as tested fuel.

The straight chain paraffins are the most suitable fuels for CI engines.

The graph shown in Fig. 10.95 shows the relationship of the other properties of the fuel with CN which are also responsible for the knocking of the engine.

10.29 ❐ IC ENGINE FUELS

The desirable properties of IC engine fuels are as follows:

  1. High energy density
  2. Good combustion qualities
  3. High thermal stability
  4. Low deposit forming tendencies
  5. Compatibility with the engine hardware
  6. Good fire safety
  7. Low toxicity
  8. Low pollution
  9. Easy transferability on board vehicle storage

10.29.1 Fuels for SI Engines

Gasoline

It is a mixture of various hydrocarbons such as paraffins, olefins, napthalenes, and aromatics. The requirements of an ideal gasoline are as follows:

  1. It should mix readily with air and afford uniform manifold distribution, i.e., it should easily vapourise.
  2. It must be knock-resistant.
  3. It should not pre-ignite easily.
  4. It should not tend to decrease the volumetric efficiency of engine
  5. It should be easy to handle.
  6. It should be cheap and easily available.
  7. It should not corrode engine parts.
  8. Its calorific value should be high.
  9. It should not form gum and varnish.

Knock Rating of SI Engine Fuels

Highest Useful Compression Ratio: The HUCR is the highest compression ratio at which a fuel can be used without detonation in a specified test engine under specified operating conditions and the ignition and mixture strength being adjusted to give best efficiency. The HUCR of different fuels is given in Table 10.16.

Rating of SI Engine Fuels

The rating of fuel is done by comparing its performance with that of a standard reference fuel which is usually a combination of iso-octane and n-heptane. Iso-octane offers great resistance to deformations and is arbitrarily assigned a rating of 100-octane number. On the other hand, n-heptane detonates very rapidly, and is assigned a rating of zero-octane number.

Octane Number

It is percentage of iso-octane by volume in a mixture is iso-octane and n-heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine under given standard operating conditions.

The octane scale is extended beyond 100 by adding TEL to iso-octane.

 

Table 10.16 HUCR for different fuels

10.29.2 Fuels for CI Engines

Diesel fuels are used for CI engines. These are petroleum fractions that lie between kerosene and lubricating oils.

The main desirable characteristics of diesel fuels are as follows:

  1. Cleanliness—carbon residue, contamination, sulphur, etc.
  2. Ignition quality—cetane number
  3. Fluidity—viscosity, pour point, etc.
  4. Volatility—flash point and carbon residue

Rating of CI Engine Fuels

The rating of a diesel fuel is measured in terms of its cetane number. Cetane, a straight chain paraffin with good ignition quality, is arbitrarily assigned a rating of 100-cetane number. It is mixed with a-methyl naphthalene, a hydrocarbon with poor ignition quality, which is assigned zero-cetane number.

Cetane Number

This is defined as the percentage of volume of cetane in a mixture of cetane and α-methyl naphthalene that produces the same ignition lag as the fuel being tested, in the same engine and under the same operating conditions.

10.30 ❐ ALTERNATIVE FUELS FOR IC ENGINES

The various alternative fuels for IC engines are as follows:

  1. Alcohols: ethanol and methanol
  2. Hydrogen
  3. Biogas
  4. Producer (or water) gas
  5. Biomass generated gas
  6. LPG and LNG
  7. CNG
  8. Coal gasification and coal liquefication
  9. Non-edible vegetable oils
  10. Non-edible wild oils
  11. Ammonia

10.30.1 Alcohols

Alcohols are of two types—ethanol (C2H5OH) and methanol (CH3OH) which can be produced from sugarcane waste and other agricultural products. Ethanol can be produced by the termination of vegetables and plant materials such as sugarcane, molasses, starchy matter, cellulose material (wood, sulphite, waste liquor from paper manufacture), hydrocarbon gases, and so on.

The use of ethanol in IC engines gives better performance than on a gasoline engine. The power output, BSHC, maximum thermal efficiency, and engine torque are higher as compared to petrol engines. Alcohols are not suitable fuels for CI engines.

Methanol behaves much like petroleum. It can be used directly or mixed with gasoline. Some of the important features of methanol as fuel are as follows:

  1. The specific fuel consumption with methanol as fuel is 50% less than petrol engine.
  2. Exhaust CO and HC are decreased continuously with blends.
  3. Methanol can be used as supplementary fuel in heavy vehicles.

Methanol can be produced from coal, natural gas, oil shell, farm waste, etc. Methanol emits less amount of CO2 and other polluting gases as compared to gasoline-fuelled vehicles. However, methanol has high latent heat and gives cold starting problems, is more corrosive, and more toxic as compared to petrol, gives high levels of formaldehyde emission. The important objectionable emissions are CO, HC, NOx, and aldehydes.

10.30.2 Use of Hydrogen in CI Engines

There are two methods by which hydrogen can be used in CI engines as follows.

  1. By introducing hydrogen with air and using a spray of diesel oil to ignite the mixture by the dual fuel mode. The limiting conditions are when the diesel quantity is too small to produce effective ignition, that is failure of ignition and when the hydrogen air mixture is so rich that the combustion becomes violent. In between these limits, a wide range of diesel-to-hydrogen proportions can be tolerated.
  2. By introducing hydrogen directly into the cylinder at the end of compression. Since the self-ignition temperature of hydrogen is very high, the gas spray is made to impinge on a hot glow plug in the combustion chamber by surface ignition. It is also possible to feed a very lean hydrogen air mixture during the intake into an engine and then inject the bulk of the hydrogen towards the end of the compression stroke.

Advantages of Hydrogen as IC Engine Fuel

  1. Low emission: Essentially, no CO or HC in the exhaust as there is no carbon in the fuel. Most exhaust would be H2O and N2.
  2. Fuel availability: There are a number of different ways of making hydrogen, including electrolysis of water.
  3. Fuel leakage to environment is not a pollutant.
  4. High energy content per volume when stored as a liquid. This would give a large range vehicle range for a given fuel tank capacity.

Disadvantages of Hydrogen as IC Engine Fuel

  1. Requirement of heavy, bulky fuel storage both in vehicle and at the service station. Hydrogen can be stored as a cryogenic liquid or as a compressed gas. If stored as a liquid, it would have to be kept under pressure at a very low temperature. This would require a thermally super-insulated fuel tank. Storing in a gas phase would require a high pressure vessel with limited capacity.
  2. Difficult to refuel.
  3. Poor engine volumetric efficiency. When a gaseous fuel is used in an engine, the fuel will displace some of the inlet air and less volumetric efficiency will result.
  4. Fuel cost would be high in the context of present–day technology and availability.
  5. High NOx emissions because of high flame temperature.
  6. Can detonate.

10.30.3 Biogas

Biogas is produced from organic waste (cow dung) which is easily available. The primary advantages of biogas is its ability to operate the engine on lean mixture, which reduces exhaust HC and CO2 concentration in the engine exhaust. It gives less deposits and shows clean burning characteristics as compared to petrol and diesel oil.

SI engines can be operated on biogas after starting the engine by using petrol. Biogas can be used in SI engine in two forms as follows:

  1. To run the engine entirely on biogas
  2. Dual fuel engine operation on biogas and petrol

The advantages of using biogas as fuel in a CI engine are as follows:

  1. The gas-air mixture provides uniform mixture in a multi-cylinder engine at all times.
  2. There is virtually no CO emission in exhaust due to lean operation of the engine.
  3. NOx emissions are reduced by about 60%.
  4. Soot is virtually eliminated and exhaust is found to have less pungent odour than diesel oil.

10.30.4 Producer (or Water) Gas

Producer gas is a mixture of CO, H2, and N2. It is obtained from the partial oxidation of coal, or peat when burnt with insufficient quantity of air. In using the producer gas in an SI engine, the problem remains as to how and with which fuel it has to be produced economically. The producer gas can be used with an SI engine but with a lesser output.

10.30.5 Biomass-generated Gas

Biomass comprises all kinds of agricultural waste such as stalks, husks of different crops, residues like bagasse, and so on. Biomass gasification gives producer gas.

10.30.6 LPG as SI Engine Fuel

LPG consists of hydrocarbons of such volatility that they exist as gases under atmospheric conditions, but can be readily liquefied under pressure. LPG contains mainly members of paraffin hydrocarbons up to C4. It consists of propane and butane, to some extent.

Advantages of LPG in SI Engine

  1. LPG contains less carbon than petrol. Therefore, emission is much reduced by its use.
  2. LPG mixes with air at all temperatures.
  3. In multi-cylinder engines, a uniform mixture can be supplied to all cylinders.
  4. There is no crankcase dilution because the fuel is in the form of vapour.
  5. Automobile engines can use propane if they have high compression ratio.
  6. LPG has high antiknock characteristics.
  7. Its heat energy is about 80% of gasoline, but its high octane value compensates the thermal efficiency of the engine.
  8. Running on LPG translates into a cost saving of about 50%.
  9. The engine may have a 50% longer life.

Disadvantages of LPG in SI Engine

  1. Engines are normally designed to take in a fixed volume of the mixture and air. Therefore, LPG will produce 10% less power for a given engine at full throttle.
  2. The ignition temperature of LPG is somewhat higher than petrol. Therefore, running on LPG could lead to a 5% reduction in valve life.
  3. A good cooling system is quite necessary, because LPG vapouriser uses engine coolant to provide the heat to convert the liquid LPG to gas.
  4. The vehicle weight is increased due to the use of heavy pressure cylinders for storing LPG.
  5. A special fuel feed system is required for LPG.

Liquefied natural gas (LNG) comes from dry natural reservoirs, mainly CH4 (methane) with a very small percentage of ethane and propane. The major difficulty in using this gas is its very low boiling point (–161.5°C)

10.30.7 Compressed Natural Gas

Compressed natural gas (CNG) is composed of methane, ethane, and propane with traces of other gases. Methane is the main constituent. It has very good anti-knock qualities and burns at much higher temperature as compared with petrol. It can be safely used in engines with a compression ratio up to 12:1. It gives higher thermal efficiency as compared to gasoline engine. The pollutants emitted by CNG are also reduced. It is nontoxic and has been widely used in three-wheelers and buses.

Advantages of CNG

  1. It is cheaper than petrol.
  2. It is engine friendly–gives lower maintenance cost and longer life.
  3. It is safe–it is lighter than air and safely dissipates to atmosphere.
  4. It mixes easily with air completely and gives proper combustion.
  5. It is odourless.

10.30.8 Coal Gasification and Coal Liquefaction

These fuels are mainly used for power generation than IC engines.

10.30.9 Non-edible Vegetable Oils

The vegetable oils including soya bean oil, cotton seed oil, sunflower oil, rapeseed oil, palm oil, coconut oil, and linseed oil have cetane numbers and calorific values comparable with those of diesel oil and can be used as alternative fuels for diesel engine. The main disadvantage is their high viscosity which gives difficulties with fuel injection and cold flow pumping.

10.30.10 Non-edible Wild Oils

Of all the non-edible wild oils, mahua and neem oils have the highest production. Their use as IC engine fuels has not become popular yet.

10.30.11 Ammonia

Ammonia has been demonstrated as a practical fuel for engine applications. Ammonia does not contain carbon and hence, there is an absence of two major pollutants, HC and CO, in the exhaust. These are its added advantages. The performance characteristics of engine using NH3 as fuel are similar to the performance characteristics obtained with petroleum fuels.

The pros and cons of alternative fuels are given in Table 10.17.

 

Table 10.17 Pros and cons of alternative fuels

 

Summary for Quick Revision

  1. An internal combustion engine is a device in which the fuel is mixed with air and burned inside the engine to generate power.
  2. The IC engines can be either of spark ignition or compression ignition type.
  3. The scavenging process refers to clearing the cylinder after the expansion stroke with fresh-air charge.
  4. The process of mixing air and fuel, outside the engine cylinder, in proper proportions, is called carburetion.
  5. For a simple carburettor:
    1. Approximate analysis:
      1. Mass of air per second,
      2. Mass of fuel per second,

        Af = area of fuel nozzle, Cd = coefficient of discharge.

      3. For z = 0,
    2. Exact analysis:
  6. There are two mechanically operated petrol injection systems—combustion chamber injection and continuous port injection. The first method is not being used nowadays.
  7. Fuel injection systems for CI engine are air injection system and solid injection.
  8. Fuel nozzle design:
    1. Fuel consumed per cycle,
    2. Duration of injection in seconds,
    3. Mass of fuel supplied per second,
    4. Volume of fuel injected per second,

    where d = orifice diameter, n = number of orifices, θ = crank angle duration, Ni = number of injections per minute, N = rpm, Cdf = coefficient of discharge of fuel nozzle.

  9. Fuel ignition systems are battery ignition system, magneto ignition system, and electronic ignition system.
  10. Ignition lag (or delay) is the phase during which some fuel has already been admitted but has not been ignited.
  11. The phenomenon of detonation occurs in SI engines when the temperature of unburned mixture exceeds the self-ignition temperature of the fuel during the ignition lag. This gives rise to spontaneous or auto-ignition of fuel at various pin-point locations.
  12. The phenomenon of pulsating variation of pressure due to sudden burning of accumulated fuel during the delay period in CI engines is called knocking.
  13. There are two types of lubrication systems—splash lubrication systems and pressure feed lubrication system.
  14. There are mainly two types of cooling systems—air cooling system and water cooling system.
  15. There are four methods used for governing of IC engines—hit and miss governing, qualitative governing, quantitative governing, and combination methods.

Multiple-choice Questions

  1. An Otto cycle on internal energy (U) and entropy (s) diagram is shown in:
  2. Which of the following statements is “true”?
    1. The term “Knock” is used for on identical phenomenon in a spark ignition and compression ignition engine.
    2. “Knock” is a term associated with a phenomenon taking place in the early part of combustion in a spark ignition engine and the later part of combustion in a spark ignition engine.
    3. “Knock” is a term associated with a phenomenon taking place in the early part of combustion in a spark ignition engine and the later part of combustion in a compression ignition engine.
    4. None of the above
  3. The essential function of the carburettor in a spark ignition engine is to
    1. meter the fuel into air stream and amount dictated by the load and speed
    2. bring about mixing of air and fuel to get a homogeneous mixture
    3. vapourise the fuel
    4. distribute fuel uniformly to all cylinders in a multi-cylinder engine and also vapourise it.
  4. Match List I with List II and select the correct answer using the codes given below the lists:
    List I List-II
    A. Pre-combustion chamber 1. Compression swirl
    B. Turbulent chamber 2. Masked inlet valve
    C. Open combustion chamber 3. Spark ignition
    D. F-head combustion chamber 4. Combustion induced
    5. M-Chamber

    Codes:

           A  B C D

    1. 4  5  3  2
    2. 1  3  5  2
    3. 2  3  1  5
    4. 4  1  2  3
  5. Which of the following factors increase detonation in the SI engine?
    1. IncrEase spark advance
    2. Increased speed
    3. Increased air-fuel ratio beyond stoichiometric strength
    4. Increased compression ratio

    Select the correct answer using the codes given below:

    Codes:

    1. I and III
    2. II and IV
    3. I, II, and IV
    4. I and IV
  6. Which one of the following curves is proper representation of pressure differential (y-axis) vs velocity of air (x-axis) at the throat of a carburettor?
  7. Match List I with List II and select the correct answer using the codes given below the lists:
    List I
    (Elements of a complete carburettor)
    List-II
    (Rich-mixture requirement)
    A. Idling system 1. To compensate for dilution of charge
    B. Economiser 2. For cold starting
    C. Acceleration pump 3. For meeting maximum power range of operation
    D. Choke 4. For meeting rapid opening of throttle

    Codes:

           A  B C D

    1. 1  2  3  4
    2. 1  3  4  2
    3. 2  3  4  1
    4. 4  1  2  3
  8. Match List I with List II and select the correct answer using the codes given below the lists:
    List I
    (SI engine problem)
    List II
    (Characteristic of fuel responsible for the problem)
    A. Cold starting 1. Front end volatility
    B. Carburettor icing 2. Mid-range volatility
    C. Crankcase dilution 3. Tail end volatility

    Codes:

           A  B C

    1. 1  2  3
    2. 1  3  2
    3. 2  3  1
    4. 3  1  2
  9. The air-fuel ratio for idling speed of automobile petrol engine is closer to
    1. 10:1
    2. 15:1
    3. 17:1
    4. 21:1
  10. Consider the following statements:
    1. The performance of an SI engine can be improved by increasing the compression ratio.
    2. Fuels of higher octane number can be employed at higher compression ratio.

    Of these statements

    1. both I and II are true
    2. both I and II are false
    3. I is true but II is false
    4. I is false but II is true
  11. The object of providing masked inlet valve in the air passage of compression-ignition engines is to
    1. enhance flow rate
    2. control in flow
    3. induce primary swirl
    4. induce secondary turbulence
  12. Which one of the following events would reduce the volumetric efficiency of a vertical compression ignition engine?
    1. Inlet valve closing after bottom head centre
    2. Inlet valve closing before bottom dead centre
    3. Inlet valve opening before top dead centre
    4. Exhaust valve closing after top dead centre
  13. As compared to air standard cycle, in actual working, the effect of variation in specific heats is to
    1. increase maximum pressure and maximum temperature
    2. reduce maximum pressure and maximum temperature
    3. increase maximum pressure and decrease maximum temperature
    4. decrease maximum pressure and increase maximum temperature
  14. Reference fuels for knock rating of SI engine fuels would include
    1. iso-octane and alpha-methyl naphthalene
    2. normal octane and aniline
    3. iso-octane and n-hexane
    4. n-heptane and so-octane
  15. Consider the following measures:
    1. Increasing the compression ratio
    2. Increasing the intake air temperature
    3. Increasing the length of dimension of the cylinder
    4. Increasing the engine speed

    The measures necessary to reduce the tendency to knock in CI engines would include

    1. I, II, and III
    2. I, II, and IV
    3. I, III, and IV
    4. II, III, and IV
  16. Match List I with List II and select the correct answer using the codes given below the lists:
    List I
    (Operating condition)
    List II
    (Approximate air fuel ratio)
    A. Idling 1. 16
    B. Part load operation 2. 10
    C. Full load 3. 12.5
    D. Cold start 4. 3

    Codes:

           A  B C D

    1. 2  1  3  4
    2. 1  2  4  3
    3. 2  1  4  3
    4. 1  2  3  4
  17. Generally, in Bosch type fuel injection pumps, the quantity of fuel is increased or decreased with change in load, due to change in
    1. timing of start of fuel injection
    2. timing of end fuel injection
    3. injection pressure of fuel
    4. velocity of flow of fuel
  18. Match List I with List II and select the correct answer using the codes given below the lists:
    List I
    (Operating mode of SI engine)
    List II
    (Appropriate air-fuel ratio)
    A. Idling 1. 125
    B. Cold starting 2. 9.0
    C. Cruising 3. 16.0
    D. Maximum power 4. 22.0
    5. 3.0

    Codes:

           A  B C D

    1. 2  4  5  1
    2. 1  3  4  2
    3. 5  1  1  2
    4. 2  5  3  1
  19. Knocking in the SI engine decreases in which one of the following orders of combustion chamber designs?
    1. F head, L head, I head
    2. T head, L head, F head
    3. I head, T head, F head
    4. F head, I head, T head
  20. The two reference fuels used for cetane rating are
    1. cetane and isooctane
    2. cetane and tetraethyl lead
    3. cetane and n-heptane
    4. cetane and a-methyl naphthalene
  21. Match List I with II, in respect of SI engines, and select the correct answer using the codes given below the lists:
    List I List II
    A. Highest useful compression ratio 1. Ignitable mixture
    B. Cold starting 2. Knock rating of fuels
    C. Limiting mixture strength 3. Detonation
    D. Delay period 4. Chain of chemical reactions in combustion chamber

    Codes:

           A  B C D

    1. 2  3  1  4
    2. 3  2  1  4
    3. 2  3  4  1
    4. 3  4  2  1
  22. By higher octane number of SI fuel, it is mean that the fuel has
    1. higher heating value
    2. higher flash point
    3. lower volatility
    4. longer ignition delay
  23. Which of the following factors would increase the probability of knock in the CI engines?
    1. Long ignition delay of fuel
    2. Low self-ignition temperature of fuel
    3. Low volatility of fuel

    Select the correct answer using the codes given below:

    Codes:

    1. I, II, and III
    2. I and II
    3. I and III
    4. II and III
  24. List I gives the different terms related to combustion while List II gives the outcome of the events that follow. Match List I with List II and select the correct answer.
    List I List II
    A. Association 1. Pseudo shock
    B. Dissociation 2. Knock
    C. Flame front 3. Endothermic
    D. Abnormal combustion 4. Exothermic

    Codes:

           A  B C D

    1. 3  4  1  2
    2. 4  3  1  2
    3. 3  4  2  1
    4. 4  3  2  1
  25. Which one of the following engines will have heavier flywheel than the remaining ones?
    1. 40 kW four-stroke petrol engine running at 1500 rpm
    2. 40 kW two-stroke petrol engine running at 1500 rpm
    3. 40 kW two-stroke diesel engine running at 750 rpm
    4. 40 kW four-stroke diesel engine running at 750 rpm
  26. Consider the following statements:

    Knock in the SI engine can be reduced by

    1. supercharging
    2. retarding the spark
    3. using a fuel of long straight chain structure
    4. increasing the engine speed

    Of these statement

    1. I and II are correct
    2. II and III are correct
    3. I, III, and IV are correct
    4. II and IV are correct
  27. Consider the following statements:

    The injector nozzle of a CI engine required to inject fuel at a sufficiently high pressure in order to

    1. be able to inject fuel in a chamber of high pressure at the end of the compression stroke.
    2. inject fuel at high velocity to facilitate atomisation.
    3. ensure that penetration is not high.

    Of these statements

    1. I and II are correct
    2. I and III are correct
    3. II and III are correct
    4. II and IV are correct
  28. Match List I with List II select the correct answer:
    List I
    (SI engine operating mode)
    List II
    (Desired air-fuel ratio)
    A. Idling 1. 13.0
    B. Cold starting 2. 4.0
    C. Cruising 3. 16.0
    D. Full throttle 4. 9.0

    Codes:

           A  B C D

    1. 4  3  2  1
    2. 2  4  1  3
    3. 4  2  1  3
    4. 2  4  3  1
  29. Compensating jet in carburettor supplies almost constant amount of petrol at all speeds because
    1. the jet area is automatically varied depending on the suction.
    2. the flow from the main jet is diverted to the compensating jet with increase in speed.
    3. the diameter of the jet is constant and the discharge coefficient is invariant.
    4. the flow is produced due to the static head in the float Chamber.
  30. If methane undergoes with the stoichiometric quantity of air, then the air-fuel ratio on molar basis would be
    1. 15.22:1
    2. 12.30:1
    3. 14.56:1
    4. 9.52:1
  31. For maximum specific output of a constant volume cycle (Otto cycle)
    1. the working fluid should be air
    2. the speed should be high
    3. suction temperature should be high
    4. temperature of the working fluid at the end of compression and expansion should be equal
  32. In a SI engine, which one of the following is the correct order of the fuels with increasing detonation tendency?
    1. Paraffins, olefins, naphthenes, and aromatics
    2. Aromatics, naphthenes, paraffins, and olefins
    3. Naphthenes, olefins, aromatics, and paraffins
    4. Aromatics, naphthenes, olefins, and paraffins
  33. Consider the following statements:

    Detonation in the SI engine can be suppressed by

    1. retarding the spark timing
    2. increasing the engine speed
    3. using 10% rich mixture

    Of these statements

    1. I and III are correct
    2. II and III are correct
    3. I, II, and III are correct
    4. I and II are correct
  34. For the same maximum pressure and temperature
    1. Otto cycle is more efficient than diesel cycle
    2. diesel cycle is more efficient than Otto cycle
    3. dual cycle is more efficient than Otto and diesel cycles
    4. dual cycle is less efficient than Otto and diesel cycles
  35. Velocity of flame propagation in the SI engine is maximum for a fuel-air mixture which is
    1. 10% richer than stoichiometric
    2. equal to stoichiometric
    3. more than 10% richer than stoichiometric
    4. 10% leaner than stoichiometric
  36. Divided chamber diesel engines use lower injection pressure compared to open chamber engines because
    1. pintle nozzles cannot withstand high injection pressures
    2. high air swirl does not require high injection pressures for atomisation
    3. high injection pressures may cause overpenetration
    4. high injection pressure causes leakage of the fuel at the pintle
  37. For the same maximum pressure and heat input, the most efficient cycle is
    1. (a) Otto cycle
    2. (b) Diesel cycle
    3. (c) Brayton cycle
    4. (d) Dual combustion cycle
  38. The order of values of thermal efficiency of Otto, diesel, and dual cycles, when they have equal compression ratio and heat rejection, is given by
    1. ηotto > ηdiesel > ηdual
    2. ηdiesel > ηdual > ηotto
    3. ηdual > ηdiesel > ηotto
    4. ηotto > ηdual > ηdiesel
  39. In an air-standard Diesel cycle, r is the compression ratio, ρ is the fuel cut-off ratio and g is the adiabatic γ, its air standard efficiency is given by
  40. Consider the following statements:
    1. Octane rating of gasoline is based on isooctane and iso-heptane fuels which are paraffins.
    2. Tetraethyl lead is added to gasoline to increase octane number.
    3. Ethylene dibromide is added as scavenging agent to remove lead deposits on spark plugs.
    4. Surface ignition need not necessarily cause knocking.

    Which of these statements are corrects?

    1. I, II, III, and IV
    2. II, III, and IV
    3. I and IV
    4. I, II, and III
  41. Consider the following statements:
    1. Recycling exhaust gases with intake increases emission of oxides of nitrogen from the engine.
    2. When the carburettor throttle is suddenly opened, the fuel air mixture leans our temporarily causing engine stall.
    3. The effect of increase in altitude on carburettor is to enrich the entire part throttle operation.
    4. Use of multiple venturi system makes it possible to obtain a high velocity air stream when the fuel is introduced main venturi throat.

    Which of these statements are correct?

    1. I and III
    2. I and II
    3. II and III
    4. II and IV
  42. Match List I (Air-fuel ratio by mass) with List II (Engine operation mode) and select the correct answer using the codes given below the lists:
    List I List II
    A. 10:1 1. CI engine part load
    B. 16:1 2. SI engine part load
    C. 35:1 3. SI engine idling
    D. 12.5:1 4. CI engine full load
    5. SI engine full load

    Codes:

           A  B C D

    1. 3  2  1  5
    2. 4  2  1  5
    3. 3  1  2  4
    4. 4  1  2  3
  43. Consider the following statements:

    In down draft carburettor, a hot spot is formed at the bottom wall which is common for intake and exhaust manifolds. This helps to

    1. improve evaporation of liquid fuel
    2. provide higher thermal efficiency
    3. reduce fuel consumption
    4. lower the exhaust gas temperature

    Which of these statements are correct?

    1. I, II, and IV
    2. I, II, and III
    3. I, III, and IV
    4. II, III, and IV
  44. In a petrol engine car, which one of the following performance characteristics is affected by the front-end volatility of the gasoline used?
    1. Hot starting and vapour lock
    2. Engine warm-up and spark plug fouling
    3. Spark plug fouling and hot starting
    4. Vapour lock, engine warm-up, and spark plug fouling
  45. In the operation of four-stroke diesel engines, the term ‘squish’ refers to the
    1. injection of fuel in the pre-combustion chamber
    2. discharge of gases from the pre-combustion chamber
    3. entry of air into the combustion chamber
    4. stripping of fuel from the core
  46. Consider the following statements regarding the advantages of fuel injection over carburetion in SI engines:
    1. Higher power output and increased volumetric efficiency.
    2. Simple and inexpensive injection equipment.
    3. Longer Life of injection equipment.
    4. Less knocking and reduced tendency for back-fire.

    Select the correct answer using the codes given below:

    1. I, II, and III
    2. I, II, and III
    3. II and III
    4. I and IV
  47. Stoichiometric air-fuel ratio by volume for combustion of methane in air is
    1. 15:1
    2. 17.16:1
    3. 9.52:1
    4. 10.58:1
  48. Auto-ignition time for petrol-air mixture is minimum when the ratio of actual fuel-air ratio and chemically correct fuel-air ratio is
    1. 0.8
    2. 1.0
    3. 1.2
    4. 1.5
  49. Consider the following statements regarding knock rating of SI engine fuels:
    1. Iso-octane is assigned a rating of zero octane number.
    2. Normal heptane is assigned a rating of hundred octane number.
    3. Iso-octane is assigned a rating of hundred octane number.
    4. Normal heptane is assigned a rating of zero octane number.

    Which of the above statements are correct?

    1. I and II
    2. II and III
    3. III and IV
    4. IV and I
  50. In spark ignition engines knocking can be reduced by
    1. increasing the compression ratio
    2. increasing the cooling water temperature
    3. retarding the spark advance
    4. increasing the inlet air temperature
  51. The tendency of knocking in CI engine reduces by
    1. high self-ignition temperature of fuel
    2. decrease in jacket water temperature
    3. injection of fuel just before TDC
    4. decrease in injection pressure
  52. Consider the following statements relevant to the ignition system of SI engine:
    1. Too small a dwell angle will lead to the burning of condenser and contact points.
    2. Too small a dwell angle will result in misfiring.
    3. Too large a dwell angle will result in burning of condenser and contact points.
    4. Too large a dwell angle will result in misfiring.

    Which of the above statements are correct?

    1. I and II
    2. II and III
    3. III and IV
    4. IV and I
  53. Consider the following statements in respect of automobile engine with thermosyphon cooling:
    1. Heat transfer from gases to cylinder walls takes place by convection and radiation.
    2. Most of the heat transfer from radiator to atmosphere takes place by radiation.
    3. Most amount of heat transfer from radiator to atmosphere takes place by convection.
    4. Heat transfer from cylinder walls take place by conduction and convection.

    Which of the above statements are correct?

    1. I, II and IV
    2. I, III, and IV
    3. II, III, and IV
    4. I and II
  54. Which of the following action(s) increase(s) the knocking tendency in the SI engine?
    1. Increasing mixture strength beyond equivalence ratio (ϕ) = 1.4
    2. Retarding the spark and increasing the compression ratio.
    3. Increasing the compression ratio and reducing engine speed.
    4. Increasing both mixture strength beyond equivalence ratio (ϕ) = 1.4 and the compression ratio
  55. Which of the following features(s) is/are used in the combustion chamber design to reduce SI engine knock?
    1. Spark plug located away from exhaust valve, wedge-shaped combustion chamber, and short flame travel distance
    2. Wedge-shaped combustion chamber
    3. Wedge-shaped combustion chamber and short flame travel distance
    4. Spark plug located away from exhaust valve, short flame travel distance, and side valve design
  56. Which of the following factor(s) increase(s) the tendency for knocking in the CI engine?
    1. Increasing both the compression ratio and the coolant temperature
    2. Increasing both the speed and the injection advance
    3. Increasing the speed, injection advance and coolant temperature
    4. Increasing the compression ratio
  57. Match List I with List II and select the correct answer using the codes given below the lists:
    List I
    (SI engine operational mode)
    List II
    (Air-fuel ratio by mass)
    A. Idling 1. 4:1
    B. Cruising 2. 10:1
    C. Maximum power 3. 12.5:1
    D. Cold starting 4. 16:1
    5. 14.8:1

    Codes:

           A  B C D

    1. 2  4  3  1
    2. 5  4  1  3
    3. 2  3  5  1
    4. 5  3  1  4
  58. Consider the following statements for a multi-jet carburettor:
    1. Acceleration jet is located just behind the throttle valve.
    2. Idle jet is located close to the choke.
    3. Main jet alone supplies petrol at normal engine speeds.

    Which of the statements given above are correct?

    1. I, II, and III
    2. I and II
    3. II and III
    4. I and III
  59. The stoichiometric air-fuel ratio for petrol is 15:1. What is the air-fuel ratio required for maximum power?
    1. 16:1 – 18:1
    2. 15:1
    3. 12:1 – 14:1
    4. 9:1 – 11:1
  60. For which of the following reasons, do the indirect injection diesel engines have higher specific output compared to direct injection diesel engines?
    1. They have lower surface to volume ratio.
    2. They run at higher speeds.
    3. They have higher air utilization factor.
    4. They have lower relative heat loss.

    Select the correct answer using the code given below:

    1. I and II
    2. II only
    3. II and III
    4. III and IV
  61. Consider the following statements:
    1. In a carburettor, the throttle valve is used to control the fuel supply.
    2. The fuel level in the float chambers is to be about 4 to 5 mm below the orifice level of main jet.
    3. An idle jet provides extra fuel during sudden acceleration.
    4. A choke valve restricts the air supply to make the gas richer with fuel.

    Which of the statements given above are correct?

    1. II and IV
    2. I and III
    3. I, II, and III
    4. II, III, and IV
  62. The knocking tendency in compression ignition engines increases with
    1. increase of coolant water temperature
    2. increase of temperature of inlet air
    3. decrease of compression ratio
    4. increase of compression ratio
  63. Which of the following cannot be caused by a hot spark plug?
    1. Pre-ignition
    2. Post-ignition
    3. Detonation
    4. Run-on-ignition

    Select the correct answer using the code given below:

    1. I and IV
    2. II only
    3. II and III
    4. III only
  64. Which of the following combustion chamber design features reduce(s) knocking in SI engines?
    1. Spark plug located near the inlet valve
    2. T-Head
    3. Wedge-shaped combustion chamber
    4. Short flame travel distance

    Select the correct answer using the code given below:

    1. I and III
    2. III only
    3. III and IV
    4. I and II
  65. A 4-stroke diesel engine, when running at 2000 rpm has an injection duration of 1.5 ms. What is the corresponding duration of the crank angle in degrees?
    1. 18°
    2. 36°
    3. 15°
  66. Consider the following statements:
    1. In the SI engines, detonation occurs near the end of combustion, whereas in CI engines, knocking occurs near the beginning of combustion.
    2. In SI engines, no problems are encountered on account of preignition.
    3. Low inlet pressure and temperature reduce knocking tendency in SI engines but increase the knocking tendency in CI engines.

    Which of the statements given above are correct?

    1. I, II, and III
    2. Only I and II
    3. Only II and III
    4. Only I and III
  67. The tendency of petrol to detonate in terms of octane number is determined by comparison of fuel with which of the following?
    1. Iso-octane
    2. Mixture of normal heptane and iso-octane
    3. Alpha methyl naphthalene
    4. Mixture of methane and ethane
  68. For the same indicated work per cycle, mean speed and permissible fluctuation of speed, what is the size of flywheel required for a multi-cylinder engine in comparison to a single-cylinder engine?
    1. Bigger
    2. Smaller
    3. Same
    4. Depends on thermal efficiency of the engine
  69. Consider the following statements:
    1. For a diesel cycle, the thermal efficiency decreases as the cut-off ratio increases.
    2. In a petrol engine, the high voltage for spark is in the order of 1000 V.
    3. The material for centre electrode in spark plug is carbon.

    Which of the statements given above is/are correct?

    1. Only I
    2. Only I and II
    3. Only II and III
    4. I, II, and III
  70. Consider the following statements:

    In order to prevent detonation in a spark ignition engine, the charge away from the spark plug should have:

    1. low temperature
    2. low density
    3. long ignition delay

    Which of the statements given above is/are correct?

    1. Only I
    2. Only II
    3. Only III
    4. I, II, and III
  71. Where does mixing of fuel and air take place in case of diesel engine?
    1. Injection pump
    2. Injector
    3. Engine cylinder
    4. Inlet manifold
  72. Consider the following statements regarding ce­tane:
    1. It is standard fuel used for knock rating of diesel engines.
    2. Its chemical name is n-hexadecane.
    3. It is a saturated hydrocarbon of paraffin series.
    4. It has long carbon chain structure.

    Of the above statements

    1. I, III, and IV are correct
    2. I, II, and III are correct
    3. I, II, and IV are correct
    4. II, III, and IV are correct

Review Questions

  1. Define an engine.
  2. Define a heat engine.
  3. How IC engines are classified?
  4. Define an internal combustion engine.
  5. What is an external combustion engine?
  6. List the applications of IC engines.
  7. List four advantages of internal combustion engine over external combustion engine.
  8. Describe the salient features of SI engine.
  9. Differentiate between SI and CI engines.
  10. What are the main differences between 4-stroke and 2-stroke engines?
  11. Explain the working of:
    1. Four-stroke SI engine
    2. Four-stroke CI engine
  12. Explain the working of 2-stroke SI engine.
  13. Compare 4-stroke and 2-stroke engines.
  14. Compare SI and CI engines.
  15. List the merits of two-stroke engines over 4-stroke engines.
  16. Draw the p-v diagram for (a) 4-stroke petrol engine (b) 2-stroke petrol engine.
  17. Why the actual p-v diagram for a 4-stroke diesel engine differ from the theoretical diagram?
  18. Explain the working of 2-stroke diesel engine.
  19. What is the function of flywheel in an internal combustion engine?
  20. What is the necessity of valve-timing diagram?
  21. What is scavenging process in IC engines?
  22. List four applications of Wankel engine.
  23. Why do actual p-v diagrams differ from the theoretical diagrams of IC engines?
  24. Define diagram factor.
  25. Write four assumptions generally made while drawing the theoretical p-v diagram for 2-stroke petrol engine.
  26. Suggest the IC engines used for the following:
    1. Bus
    2. Motor cycle
    3. Scooter
    4. Moped
  27. What are the essential requirements of an ideal carburettor?
  28. What do you understand by compensation? Explain.
  29. Explain with a neat diagram how the power and efficiency of an SI engine vary with A:F ratio.
  30. Why are rich mixtures required for starting and during idling of an engine? Explain with the help of a neat sketch.
  31. Draw a simple type of carburettor and explain its working.
  32. What do you understand by stoichiometric A:F ratio?
  33. What are the requirements of a good injection system?
  34. What is solid injection? What are its advantages over air-injection system?
  35. What are the basic requirements of an ideal ignition system?
  36. Explain the working of a battery ignition system with the help of a neat sketch.
  37. What are the main drawbacks of a battery ignition system? How can these be overcome?
  38. Explain the working of a magneto ignition system with the help of a neat sketch.
  39. Compare battery and magneto ignition systems.
  40. Describe the working of an electronic ignition system.
  41. What are the lubricating systems for IC engines?
  42. Differentiate between splash and pressure lubrication system and describe their operations with the help of neat diagrams.
  43. What is meant by dry and wet sump lubrication? Where is dry sump preferred? Why?
  44. Explain the lubrication of the following parts:
    1. Crank pin and gudgeon pin
    2. Main bearing
    3. Cylinder and piston
    4. Exhaust valve
  45. What is the necessity of cooling system for an IC engine?
  46. Name the cooling systems generally used for I.C. engines.
  47. What is the necessity of I.C. engine cooling?
  48. Discuss the types of cooling systems for an IC engine.
  49. What are the effects of under cooling and over cooling of an engine?
  50. Draw a neat diagram of thermostatic-controlled water cooling system and explain its working.
  51. Discuss the merits and demerits of water cooling with air cooling system.
  52. Define the function of a radiator. Discuss different types of matrices used with these radiators.
  53. What is the function of fan in radiator system? Discuss its performance with change of speed and load on the engine.
  54. What do you understand by evaporative cooling? When is it preferred?
  55. What is the difference between open system and close system cooling used for IC engines? Discuss their relative merits and demerits.
  56. Discuss the relative merits and demerits of air cooling system over water cooling system.
  57. What is overcooling? What are its disadvantages?
  58. Define combustion phenomenon in I.C. engines.
  59. What are the stages of combustion in S.I. engines?
  60. What is ignition lag?
  61. List four factors affecting delay period.
  62. List four factors which affect the velocity of flame propagation.
  63. What is the phenomenon of knocking in S.I. engines?
  64. List four factors which reduce knocking in S.I. engines.
  65. What are the effects of knocking?
  66. Write four design principles for combustion chambers.
  67. What are the stages of combustion in C.I. engines?
  68. Write four factors which reduce delay period in C.I. engines.
  69. What is diesel knock?
  70. What is knocking in C.I. engines?
  71. List four factors which affect knocking in C.I. engines.
  72. Give the names of few anti-knock agents.

Exercises

10.1 The venturi of a single jet carburettor has throat diameter of 75 mm and fuel nozzle diameter of 5 mm. Find the A:F ratio of the mixture supplied to the engine for the following data: Cda = 0.93, Cdf = 0.68, ∆p across venturi = 0.15 bar, ρa = 1.29 kg/m3, ρf = 720 kg/m3. Neglect nozzle lip and air compressibility effect.

[Ans. 13.3:1]

10.2 A four-cylinder, four-stroke petrol engine of 82.5 mm diameter and 115 mm stroke has 80% volumetric efficiency, when running at 3000 rpm. The pressure head causing the flow is 11.03 cm of Hg. Determine (a) the size of the venturi, and (b) the fuel nozzle diameter if the A:F ratio of mixture supplied is 14:1. Take Cda = 0.84, Cdf = 0.7, ρa = 1.29 kg/m3, ρf = 700 kg/m3.

[Ans. 22.3 mm, 4.15 mm]

10.3 The diameter of a simple venturi is 75 mm and fuel nozzle diameter is 5 mm. Nozzle lip = 5 mm, and Cda = 0.85, Cdf = 0.7, ρa = 1.29 kg/m3, ρf = 720 kg/m3

Determine (a) the air-fuel ratio supplied by the carburettor if the pressure drop causing air flow is 0.14 bar neglecting nozzle lip and (b) the air-fuel ratio when nozzle lip is considered.

10.4 A simple carburettor fitted on a SI engine consumes 6.4 kg/h. The nozzle lip = 3 mm, df = 1.27 mm, Cda = 0.8 and Cdf = 0.6. The atmospheric air pressure and temperature are 1.013 bar and 15.5°C and ρf = 760 kg/m3. The A:F ratio supplied by the carburettor = 15:1. Determine (a) the venturi diameter, and (b) the drop in venturi pressure.

[Ans. 21.5 mm, 37 cm of H2O]

10.5 A single jet carburettor is to be designed to give an A:F ratio of 15:1 and fuel supply of 24.5 kg/h. The atmospheric air pressure and temperature are 1.013 bar and 288 K. Calculate (a) the venturi diameter if the air velocity is limited to 97.5 m/s, and (b) the fuel nozzle diameter.

Take Cda = 0.84 and Cdf = 0.66.

[Ans. 33.8 mm, 2 mm]

10.6 A four-stroke, eight-cylinder CI engine develops 500 kW at 1200 rpm and consumes 0.175 kg/kWh of fuel whose specific gravity is 0.89. Determine the diameter of a single orifice injector if the injection pressure is 160 bar and combustion chamber pressure is 40 bar. The period of injection is 30° crank rotation. Take Cdf = 0.9.

10.7 A four-stroke, six-cylinder oil engine, 115 mm diameter and 140 mm stroke operates with A:F = 16:1. The suction condition in the engine are 1 bar and 300 K. The volumetric efficiency of the engine is 76%. Determine the maximum amount of fuel supplied per hour to the engine running at 1500 rpm. The injection pressure is 125 bar and combustion chamber pressure is 40 bar. The fuel is supplied for 20° rotation of the crank. Calculate the diameter of the orifice assuming each nozzle has single orifice. Take ρf = 760 kg/m3 and Cdf = 0.64.

[Ans. 21.7 kg/h, 1.76 mm]

10.8 A six-cylinder, four-stroke engine, 8 cm bore and 12 cm stroke runs at 3600 rpm. The volumetric efficiency of the engine is 0.8. If the maximum head causing the flow is limited to 11.765 cm of mercury, find the throat diameter of the venturi required. Find the diameter of the nozzle orifice if the desired A:F ratio is 15:1. Assume Cda = 0.9, Cdf = 0.7, ρa = 1.3 kg/m3, ρf = 720 kg/m3.

[Ans. 28 mm, 1.69 mm]

10.9 The throat diameter of a carburettor is 8 cm and nozzle diameter is 5.5 mm. The nozzle lip is 6 mm. The pressure difference causing the flow is 0.1 bar. Find (a) A:F ratio supplied by the carburettor neglecting nozzle lip. (b) A: F ratio considering nozzle lip, and (c) and minimum velocity of air required to start the fuel flow. Neglect air compressibility. Take Cda = 0.85, Cdf = 0.7, ρa = 1.2 kg/m3, ρf = 750 kg/m3.

[Ans. 10.2:1, 10.28:1, 8.57 m/s]

10.10 The A:F ratio of a mixture supplied to an engine by a carburettor is 15:1. The fuel consumption of the engine is 7.5 kg/h. The diameter of the venturi is 2.2 cm. Find the diameter of fuel nozzle if the lip of nozzle is 4 mm. Assume ρf = 750 kg/m3, Cda = 0.82, Cdf = 0.7 atmospheric pressure 1.013 bar and temperature = 25°C. Neglect compressibility effect of air.

[Ans. 1.245 mm]

10.11 A simple carburettor is designed to supply 6 kg of air and 0.45 kg of fuel per minute to a four stroke single-cylinder petrol engine. The ambient air is at 1.013 bar and 27°C. (a) Calculate the throat diameter of the choke (venturi) when the velocity of air is limited to 92 m/s. Take ρf = 740 kg/m3 and velocity coefficient = 0.8 (b) If the pressure drop near the fuel nozzle is 75% of that at the venturi, calculate fuel nozzle diameter. Take Cdf = 0.6.

[Ans. 35.2 mm, 23.4 mm]

10.12 Determine the size of fuel orifice to give air-fuel ratio of 12:1. The diameter of venturi throat is 3.5 cm and vacuum at the venturi is 6.9 cm of Hg. The pressure and temperature of atmospheric air are 1.013 bar and 25°C. The nozzle lip is 5 mm. Assume Cda = 0.9, Cdf = 0.7, ρf = 760 kg/m3. Consider the compressibility of air.

[Ans. 0.63 mm]

10.13 An engine having a simple single jet carburettor consumes 6 kg/h of fuel. The level of fuel in the float chamber is 3 mm below the top of the jet when engine is not running. Ambient conditions are 76 cm of Hg pressure and 20°C temperature. The jet diameter is 1.3 mm and its coefficient of discharge is 0.60. The discharge coefficient of air is 0.84 and A/F ratio is 15. Determine the critical air velocity and the throat diameter. Express the pressure depression in cm of water. Take density of fuel = 700 kg/m3.

[Ans. 5.847 m/s, 19 mm, 32 cm of water]

10.14 A six-cylinder, four-stroke oil engine develops 200 kW at 1200 rpm and consumes 0.3 kg/kWh. Determine the diameter of a single orifice injector if the injection pressure is 200 bar and combustion chamber pressure is 40 bar. The injection is carried out for 30° rotation of the crank. Take ρf = 900 kg/m3 and Cdf = 0.7. Each nozzle on a cylinder is provided with a single orifice.

[Ans. 1.005 mm]

10.15 A four-stroke, six-cylinder oil engine operates on A:F = 20:1. The diameter and stroke of the cylinder are 10 cm and 14 cm, respectively. The volumetric efficiency is 80%. The condition of air at the beginning of compression are 1 bar and 300 K. Determine (a) the maximum amount of fuel that can be injected in each cylinder per cycle. (b) If the speed of the engine is 1500 rpm, injection pressure is 150 bar, air pressure during fuel injection is 40 bar and fuel injection is carried out for 20° of crank rotation, determine the diameter of the fuel orifice assuming only one orifice being used. Take ρf = 760 kg/m3 and Cdf = 0.67.

[Ans. 0.023 kg/s, 0.575 mm]

ANSWERS TO MULTIPLE-CHOICE QUESTIONS
  1. c
  2. c
  3. a
  4. a
  5. d
  6. c
  7. b
  8. a
  9. a
  10. d
  11. a
  12. b
  13. b
  14. d
  15. b
  16. a
  17. b
  18. d
  19. a
  20. d
  21. b
  22. d
  23. b
  24. d
  25. d
  26. d
  27. a
  28. a
  29. b
  30. d
  31. a
  32. a
  33. d
  34. b
  35. a
  36. b
  37. a
  38. d
  39. c
  40. a
  41. a
  42. c
  43. b
  44. c
  45. c
  46. b
  47. d
  48. b
  49. c
  50. c
  51. c
  52. c
  53. b
  54. b
  55. a
  56. a
  57. d
  58. a
  59. b
  60. c
  61. a
  62. c
  63. b
  64. c
  65. b
  66. d
  67. b
  68. b
  69. a
  70. d
  71. c
  72. a