Compilation of exam questions on IC engine for thermal engineering students.
Exam Question # 1. Differentiate between two stroke cycle and four stroke cycle engines.
Ans. Certain engines work on four stroke cycle principles while others work on two stroke cycle principle. Each cycle has its own advantages.
(1) In two stroke cycle engine, every revolution has an active stroke; hence a more even torque is produced which is not the case with four stroke cycle engine as it gives one active stroke in two revolutions.
(2) Two stroke cycle engine gives one working stroke per revolution and thus for the same speed and a given size, increase of power is obtained over the four stroke cycle engine. Four stroke cycle engine generally gives higher efficiency due to the positive scavenging action during the exhaust. While in case of two stroke cycle engines burnt gases are always left in the dead pockets behind from the previous stroke and these dilute the next charge as a result mean effective pressure is reduced.
(3) The quantity of burnt gases left in the two stroke cycle engine is greater than that in the four stroke cycle.
(4) One reason for lower efficiency of two stroke cycle engine is that with the two stroke cycle engine both inlet and exhaust valves are open at the same time, hence a portion of the fresh charge often escapes unused through the exhaust ports.
(5) The thermodynamic efficiency of an engine is only dependent on the ratio of compression. For the same stroke and same clearance volume, the effective compression ratio is lower in the case of two stroke cycle engine.
(6) If crankcase compression is used, the two stroke cycle engine has a higher power for a given weight.
(7) The two stroke cycle engine will run in either direction if it is of the valve-less type.
It may be said that two stroke cycle compression ignition engines are better for slow and moderate speeds. In marine installations where the engine room is short two strokes opposed piston engines are used.
Exam Question # 2. Explain the advantages and disadvantages of C.I. engine over S.I. engine?
Ans. The following are the advantages and disadvantages of C.I. engine over the S.I. engine.
(1) The C.I. engine operates at high compression ratio as compared to S.I. engine, hence the thermal efficiency is high. The compression ratio of S.I. engine is limited to 7 because of the problem of detonation and knocking, hence the thermal efficiency is low.
(2) The specific fuel consumption is low for diesel engines because of the high thermal efficiency and less dilution of exhaust gases.
(3) Power produced per unit volume of the cylinder is higher in case of diesel engine
(5) Fuel price is much cheaper as compared to the S.I. engine fuels
(6) The supercharging is possible, which increases the efficiency of the engine
(7) Higher volumetric efficiency
(9) The quantity governing is possible
(10) In case of two stroke engine the operation is efficient.
(1) It has low operating speeds
(2) The weight of the engine is more
(3) Power to weight ratio is low
(4) High strength mechanical components are required
(5) Injection system replacement and maintenance cost is high.
Exam Question # 3. What are the two types of gas calorimeters?
(i) Boy’s Gas Calorimeter:
It consists of:
(a) Burner of two jets
(b) Funnel or chimney
(c) Radiator coils.
The inlet water first passes through the outer coils downwards then returns upward through the inner coils which are heat insulated by a partition from the outer coils, finally it flows around suitable channels on the exterior of the metal casting immediately above the chimney, and passes into a mixer with a labyrinth formed of coiled brass strips. At the top of this chamber the outlet thermometer is fixed. Two luminous flames from two jets are employed.
The central chimney is always too hot for condensation of water to take place in it. At the beginning of experiment water is poured into the bottom of the vessel until it overflows at the spout provided. Condensation water flows from this spout during a run and is collected and measured for the net calorific value determination.
In this calorimeter, the gas burns at a burner in a central flue, the hot products of combustion pass to the top of the central flue and then descend through small metal tubes arranged in a double circle around the central flue. The gases leave the instrument at the bottom where its temperature is measured.
The water is supplied from a Constant level tank and flows through the calorimeter in a reverse direction to gas flow, as a result flue gases leaving the apparatus are cooled to atmospheric temperature.
The proper admixture of the various streams of warm water is accomplished by numerous baffles constituting a labyrinth below the thermometer which reads the temperature of the exit water. The water vapour which condenses in the bottom of the vessel is drained and measured for the net calorific value determination.
The gas calorimeter can also be used for the determination of the calorific value of some fuel oils. For this, a suitable burner is placed in the calorimeter while a pipe feeds the oil from a container outside the calorimeter.
The oil is usually fed under pressure in a similar manner to that used in a primus stove. From knowledge of the quantity of fuel burnt and the readings from the calorimeter the calorific value of the fuel can be determined.
The gas calorimeter the fuel is burnt at constant pressure whereas in the bomb calorimeter, it is burnt at constant volume. Some fuels can be tested in both calorimeters.
Exam Question # 4. Discuss the functions of various parts of I.C. engines.
Ans. Parts common to petrol and diesel engine are:
The cylinder is hollow vessel which contains gas under pressure and guides the piston. The cylinder is filled with the fresh charge of mixture and the combustion of the fuel is carried out in the engine cylinder. The upper end of the cylinder has clearance volume. The upper end has Top Dead Center (TDC) and Bottom Dead Center (BDC).
The piston reciprocates between the TDC and BDC which is called stroke. The stroke volume or swept volume is the volume swept by the piston. The cylinder has a cylinder head on the top and the space between the cylinder head and TDC is known as clearance volume. Sometimes, the cylinder is fitted with the liner. The cylinder is made of carbon steel or cast iron. The liners are made up of steel with chromium plating.
(2) Cylinder Head:
The cylinder head is fitted on the top of the cylinder. The end of the cylinder is closed by means of a removable cylinder head. The cylinder head has port such as inlet and exhaust port. In these ports, the inlet valve and exhaust valve is fitted. The passages are being cut in the cylinder head for flow of the inlet charge and hot gases. The inlet valve and port are connected to the passages in the cylinder head.
The piston is reciprocating in the cylinder and slides over the cylinder walls. It is made up of hollow shape with the material like cast iron, steel and aluminum alloy. The top surface of the piston is called crown which receives the force of the gas or gas pressure.
The force of gas compels the piston to move and the gas pressure is converted to mechanical force. The wall of the piston is grooved in which the piston rings are fitted. The bottom surface of the piston is connected to piston pin or gudgeon pin.
The piston rings are the hollow circular disc with the ends open. The piston rings are fitted in the piston wall grooves and lie between the piston and cylinder wall. The piston rings allow the piston to move over the cylinder wall and prevent leakage of gases from the cylinder.
There are two types of rings:
(ii) Oil control ring
The function of compression ring is to prevent flow of gases in from the cylinder wall and piston reciprocating over the walls; it maintains a gas tight pressure on the cylinder.
The oil control ring controls the flow of oil over the cylinder wall and scraps the lubricating oil on the wall of the cylinder.
(5) Piston Pin or Gudgeon Pin:
The piston pin is hollow or solid bar which is fitted in the piston wall at ends and in the middle it is fitted with the small end of the connecting rod. The piston pin is fitted with a bearing or bush between the piston pin and small end of connecting rod.
(6) Connecting Rod:
The connecting rod converts the reciprocating motion of the piston into rotary motion at the crank shaft. The connecting rod one end is small end and other end of the connecting rod is big end. The small end of the connecting rod is fitted with the piston pin and the big end of the connecting rod is fitted with crank pin bearing. The middle portion of the connecting rod is made of I-section in order to reduce the weight of connecting rod.
The crankshaft consists of main pin bearing and the crank pin bearing. The crank pin bearing is fitted with the big end of connecting rod and main pin bearing is fitted with in the casing. The big end of the connecting rod is fitted with the main pin bearing. The crank shaft main pin bearing are supported on the casing and allows the main pin to rotate freely. There are grooves are cut in the crank pin and main pin bearings.
These are main pin bearings fitted in the crank shaft. The main pin bearings are made up of bi-metallic material or ball bearings which are made of two halves.
(9) Crank Case:
This is the bottom portion of the engine and it is casing for closing of the engine. The crank case contains the oil and the oil can be used for lubrication purpose.
The flywheel is heavy disc either hollow or solid fitted with the crank shaft. The basic objective of the flywheel is to store the energy.
The engine develops power during the power stroke and in rest of the stroke, the power is absorbed for smooth operation of the engine. This results into fluctuation of speed and energy. The fluctuation of speed, can be controlled by the flywheel. The flywheel absorb the energy when it is surplus and releases the energy when it is deficient. The flywheel is solid wheel and the weight of the flywheel is utilized for absorption of energy.
The governor is used for controlling the speed of the engine. The governor helps in maintaining the speed of the engine constant when the load varies. This is done by the governor by controlling the supply of fuel.
(12) Valves and Valve Gear System:
This the system used for operation of inlet and exhaust valves in case of four stroke engine. The inlet valves and exhaust valves are used to connect to the rocker arm and the rocker arm oscillates about a valve rod. One end of the rocker arm is connected to the valves which are opened against the spring force.
The other end of the rocker arm is connected to the push rod and the push rod moves up and down with the help of the cam. The cam is fitted on the cam shaft and the cam shaft is connected to the crankshaft with the timing gears. The cam lifts the follower at the proper moment and the follower is lifted upward because of the movement of the cam.
Exam Question # 5. What are the components of gas producer in IC engine?
Ans. Gas engines employ gaseous fuels. The plant which generates gas is known as the gas producer plant. The gas producers may either of the suction type or pressure type.
The essential components of the suction type of gas producer are:
(i) The generator
(ii) The scrubber and
(iii) The vaporizer
When the engine starts, it draws its supply through the gas producer plant. Air enters the vaporizer and picks up steam in its passage through the vaporizer. The mixture of air and steam passes down in the pipe and is then led to the fire grate. Going through the generator it comes in contact with red hot fuel where the gas is formed.
From the upper part of the generator, the gas passes to the scrubber. In the scrubber the gas is cooled and the impurities are removed and then the gas passes to the engine.
There are four zones in the gas generator:
(i) Ash zone
(ii) Combustion zone, where the reaction 1100˚ C and where the reaction C + O2 = CO2 takes place.
(iii) Reduction zone, where the temperature is near about 1000°C and where the reactions –
CO2 + C = 2CO and
H2O + C = CO + H2 take place.
(iv) Distillation zone.
The gas resulting from the producer plant is a mixture of hydrogen, carbon monoxide, carbon dioxide and nitrogen.
The efficiency of the gas producer plant may be as high as 90%.
Exam Question # 6. Explain the structure of gas engine.
Ans. Gas engines use either natural gas or manufactured gas. This engine requires an ignition system which usually consists of a sparking plug and coil or magneto ignition circuits. The engines usually run on the four stroke cycle principle. Generally the admission valve controls both the admission of gas and air as shown in fig. 13-92. There are two valves on the same side.
The bottom valve, which is attached to the steam closes off the cylinder and immediately above it is the air admission port. Part way up the valve seat. The gas supply is fed in, above this valve. When the whole valve seat. The gas supply is fed in, above this valve.
When the whole valve assembly is pushed down, as a result of inlet cam action, then both valves open and the engine aspirates a mixture of air and gas. On the valve steam there is a gas valve stop. This stop ensures that the gas valve is pushed off its seat in the valve open position. When the valve is closed, there is a slight clearance between the stop and the gas valve to ensure that the valve is spring loaded onto its seat.
Sometimes there are separate ah- valve and gas valve. Both these valves are operated by separate cams mounted on the cam shaft. Fig. 13-93 shows such a gas valve.
Gas engines on the whole are of fairly heavy construction and are usually of single cylinder with the cylinder horizontally arranged. They run at fairly low speed and have a comparatively low output. They were widely used for electric power generation and for driving factory machinery. Due to availability of gas in our country they are now being used.
Exam Question # 7. What are dual fuel engines? Discuss its advantages.
Ans. Many engines are being developed to work on either gas or oil or to a combination of oil and gas. Such an engine manufactured by Nordberg Company develops 1585 kW at 400 r.p.m. This engine is 12 cylinder radial engine. As a gas engine, it employs a spark ignition and as an oil-Diesel engine compression ignition is used with the Oil serving as the pilot fuel.
During the compression stroke, there is a lean mixture, which will not spontaneously ignite at the compression pressure. The small amount of oil injection is necessary to start combustion.
The oil is injected so that burning occurs at cover end dead centre.
The following advantages are claimed for the gas-diesel engines:
(1) It has the high efficiency of the Diesel cycle.
(2) Smooth combustion with little tendency to knock
(3) Thorough mixing of the air and gas before combustion
(4) Clean burning with utilization of all the air when needed.
(5) Lowest cost fuel.
Typical performance of dual-fuel engines shows that less than 5% of the energy is supplied as pilot oil and the rest as the natural gas. The thermal efficiencies of more than 31% have been obtained.
Exam Question # 8. Explain the occurrence of abnormal combustion in C.I. engine with the help of a diagram.
Ans. The abnormal combustion in C.I. engine occurs if the fuel is not burned in the engine cylinder at proper moment or fuel ignites in the engine cylinder abnormally. This is known as detonation or knocking in C.I. engine.
The knocking or detonation is an abnormal combustion in C.I. engine. This is combustion of the fuel in the engine cylinder suddenly. The normal combustion in C.I. engine is gradual combustion of fuel in the engine cylinder resulting into the smooth generation of power.
The detonation or knocking is the spontaneous combustion of fuel due self-ignition of fuel. If the fuel has higher ignition delay period mean the period required for the vaporization, mixing and combustion, then the fuel may remain accumulated in the engine cylinder and self-ignite suddenly rather than combustion of fuel as soon as it is injected in the engine cylinder. This results into sudden rise of very high pressure and operation of the engine becomes erratic and uncontrollable. Fig. 13-89 shows the knocking or detonation in the engine cylinder.
Fig. 13-89 shows the P-θ diagram for the knocking in the C.I. engine. The P-θ diagram shows that the knocking in the engine cylinder resulting into very high rise in the erratic manner in the engine cylinder and uncontrolled combustion of fuel.
The detonation and knocking in the engine cylinder can be detected by the humming or hammer sound from the engine cylinder. The knock can be heard from the engine cylinder with the continuous metallic sound.
Exam Question # 9. What are the factors affecting detonation or knocking in the S.I. engine?
Ans. There are various factors affecting the detonation or knocking in the S.I. engine:
(i) Compression Ratio:
The compression ratio of the engine is the main factor affecting the detonation or knocking in the S.I. engine. Since the combustion in the C.I. engine is mainly because of self-ignition of fuel. The minimum compression ratio is required in the C.I. is 12 for diesel fuel. The compression ratio is mainly dependent on the clearance volume and total volume of the engine cylinder.
The combustion in S.I. engine depends upon the compression ratio of the engine. The combustion in S.I. engine is dependent on the compression ratio. The combustion process of the C.I. engine is mainly because of the combustion of the fuel due to self-ignition as it is injected in the cylinder. The high compression ratio is desirable in the engine because the possibility of self-ignition of fuel increases with increase in compression ratio. But due to mechanical constraint CR is limited to 22.
(ii) Speed of the Engine:
The speed of the engine affects the possibility of detonation or knocking in C.I. engine. Increase in the speed of the engine reduces the time for transfer heat to cooling system and gas temperature is higher which reduces the possibility of detonation in C.I. engine.
(iii) Quality of Fuel:
The quality of fuel is a very important factor. If the fuel quality is poor then the combustion of fuel occurs by delayed self-ignition of fuel which will be responsible for detonation or knocking in C.I. engine.
Exam Question # 10. What is scavenging of C.I. engine? Name the methods of scavenging.
Ans. The removal of burnt products of combustion from the engine cylinder is known as scavenging. This poses a problem for two stroke cycle engines as the exhaust stroke is short. Therefore, some type of blower or compressor is used to remove the exhaust gases from the cylinder of two stroke cycle engines. The two stroke cycle Diesel engine is adaptable to the scavenging process since the air does not carry fuel with it as is the case with petrol engines.
The scavenging pressure does not normally exceed 30 kN/m2.
Some of the methods of scavenging are:
(1) Cross scavenging
(2) Through scavenging
(3) Loop scavenging
(4) Opposed piston arrangement.
Exam Question # 11. Explain the types of abnormal combustion in S.I. engine. How can it be prevented?
Ans. The abnormal combustion in S.I. engine occurs if the fuel is not burned in the engine cylinder at proper moment or fuel ignites in the engine cylinder abnormally.
There are two types of abnormal combustion:
The moment of ignition of fuel in the engine cylinder has been controlled by the spark plug and the moment of feeding the spark in the engine cylinder by ignition system. The spark is fed to the engine cylinder normally 20-30° before TDC. The pre-ignition is abnormal combustion where the ignition of the fuel occurs even before the spark is fed in the engine cylinder.
The ignition of fuel occurs during the compression stroke because some external aid or hot spots. Fig. 13-85 shows hot spark plug electrodes or hot spot in the cylinder. The fuel is ignited due hot spot of the spark plug points or carbon deposits on the surface piston or cylinder head or hot inlet/exhaust valve.
These hot spot pre-ignite the fuel during compression stroke before the spark is fed in the engine cylinder, then the pressure rises in the cylinder during compression stroke and this results into very high power required during the compression stroke.
The power required for compressing the gas is more than power available during the expansion stroke. This results into very low power or negative power from the engine even when the fuel is burned in the engine cylinder.
Fig. 13-84 shows the P-θ diagram for the pre-ignition of fuel where low power is developed or negative power is developed from the engine even though the fuel is burned in the engine cylinder.
The knocking or detonation is abnormal combustion in S.I. engine. This is combustion of the fuel in the engine cylinder suddenly. The normal combustion in S.I. engine is gradual combustion of fuel in the engine cylinder resulting into the smooth generation of power.
The moment of combustion of fuel is determined by the moment of feed the spark in the engine cylinder. The flame generated in the engine cylinder because of the feeding of spark burns the rest of the fuel in the engine cylinder in the gradual manner.
The detonation or knocking is the spontaneous combustion of fuel due self-ignition of fuel. If the fuel has less ignition delay period mean the period required for the vaporization, mixing and combustion, then the fuel may self-ignite on its own rather than combustion of fuel in the flame.
This happen when the flame is travelling in the cylinder burning the fuel gradually the last part of the fuel in the cylinder reaches to the may reach to self-ignition temperature and explode suddenly resulting into spontaneous combustion of the fuel. Fig. 13-87 shows the sudden self-ignition of fuel. This results into sudden rise of pressure and operation of the engine becomes erratic and uncontrollable. Fig. 13-86 shows the knocking or detonation in the engine cylinder.
The P-θ diagram shows that the knocking in the engine cylinder results into very high rise in the erratic manner in the engine cylinder resulting into uncontrolled combustion of fuel.
The detonation and knocking in the engine cylinder can be detected by the humming or hammering sound from the engine cylinder. The knock can be heard from the engine cylinder with the continuous metallic sound also.
The knocking in S.I. engine has various bad effects on the operation of the engine:
(1) Reduction in the efficiency of the engine
(2) Increase of fuel consumption of the engine
(3) Wear and tear of parts and components
(4) Breakage of the gaskets and studs
(5) Carbon deposits
(6) Noise and roughness of engine operation.
The knocking can be prevented by following methods:
(1) Preventive maintenance of part
(2) Better quality of fuel
(3) Reduction in compression ratio
(4) Increasing engine RPM
(5) Retarding spark
(6) Reducing pressure in the inlet manifold
(7) Water injection increases the delay period.
Exam Question # 12. Define cetane & octane number.
It is percentage of n-cetane in the mixture of n-cetane and a-methyl naphthalene which provides similar combustion characteristic as similar to the test fuel in a similar working environment. The n-cetane is considered to be the best fuel for C.I. engine, and α-methyl naphthalene is treated as the worst fuel for C.I. engine.
Therefore the mixture of n-cetane and α-methyl naphthalene is prepared in the different proportions and tested in the given environment of the engine. Then the combustion characteristics of the test fuel are matched with the mixture of n-cetane and α-methyl naphthalene.
The percentage of n-cetane in a mixture of n-cetane and α-methyl naphthalene, which matches with the combustion characteristic of the test fuel is octane number of the fuel. The higher octane number of fuel is considered to be the best for C.I. engine. The cetane number of fuel in generally varies between 40-45.
It is percentage of iso-octane in the mixture of iso-octane and n-heptane which provides similar combustion characteristic as similar to the test fuel in a similar working environment. The iso-octane is considered to be the best fuel for S.I. engine and n-heptane is treated as the worst fuel for S.I. engine.
Therefore the mixture of iso-octane and n-heptane is prepared in the different proportion and tested in the given environment of the engine. Then the combustion characteristic of the test fuel is matched with the different mixture of iso-octane and n-heptane.
The percentage of iso-octane in a mixture of iso-octane and n-heptane which matches with the combustion characteristic of the test fuel is octane number of the fuel. The higher octane number of fuel is considered to be the best fuel for S.I. engine.
The octane number of fuel generally varies between 80-85. The octane number of fuel can be improved by use of the additive like tetra-ethyl lead or tetra methyl lead. The maximum octane number is 100.
Exam Question # 13. Discuss the meaning of Highest Useful Compression Ratio (HUCR)?
Ans. It is the highest compression ratio at which a fuel can be used in a specified engine under specific set of the operating conditions upto which the detonation or knocking does not occur in the engine.
A variable compression ratio engine is used for determining the HUCR. The compression ratio of the engine is varied and the compression at which the detonation starts is the HUCR of the engine under the given fuel and specified conditions.
Exam Question # 14. What do you mean by performance number?
Ans. The performance number is another measure of the detonation. It is the ratio of knock limited mean effective pressure of the test fuel to the knock limited mean effective pressure of the iso-octane. The knock limited mean effective pressure is mean effective pressure at which the knocking starts in the engine with given fuel under the given operating conditions.
Performance number = KLIMEP of test fuel / KLIMEP of iso-octane
Exam Question # 15. What are the various parameters influencing thermal efficiency of an engine?
Ans. The thermal efficiency of an engine is dependent upon a number of different influencing factors which these include the following:
(1) Mixture Strength:
The thermal efficiency depends upon the mixture strength. As the mixture is rich the efficiency is less because large amount of fuel remains unburnt. When the mixture is lean then the most of the fuel is burned and hence the thermal efficiency is high.
(2) Compression Ratio:
The thermal efficiency increase with increase in the compression ratio. The air standard efficiency increases with increase in the compression ratio of the engine. Therefore, it is desirable to have high compression ratio for high efficiency.
But the compression ratio in the petrol engine is limited to 8-9 because of the possibility of detonation or knocking as abnormal combustion. In case of diesel engine, the compression ratio is minimum 12 and it is limited to 22 due to material strength problem. Fig. 14-27 shows the variation of CR versus thermal efficiency.
(3) Engine Speed:
The speed of the engine has effect on the thermal efficiency of the engine. The thermal efficiency varies with the speed and becomes maximum at particular speed. When the speed is lower then the efficiency is low because of the heat transfer to the cooling medium or radiation. But when the speed becomes higher the frictional losses increases at higher rate which reduces the thermal efficiency. Fig. 14-28 shows the variation of speed versus thermal efficiency.
(4) Throttle Opening:
The thermal efficiency depends upon the opening of throttle or load on the engine. It increases with increase in throttle opening or load. This occurs because at a particular speed the frictional losses remains same and therefore the thermal efficiency is low at lower loads. Fig. 14-29 shows the variation of load versus thermal efficiency.
(5) Valve Timing:
The value timing effect the mass flow rate of air. The inlet valve opens earlier to TDC and closes after BDC. The suction of the charge is more because of the increase in timings. If the inlet valve opening and closing is changed, this reduces the power output and volumetric efficiency of the engine. The exhaust valve opens earlier to BDC and closes after TDC.
This result into the discharge of exhaust gases from the cylinder. This increases power output and volumetric efficiency of the engine. Fig. 14-30 shows the variation of valve timing versus thermal efficiency.
(6) Ignition Timing:
The ignition timing in the spark ignition engine is very important. If the ignition of fuel occurs at proper time, then the high pressure rise is possible just after the compression stroke. The ignition of fuel occurs before the TDC at the end of the compression stroke. The proper timing result into increase in the higher thermal efficiency due to complete combustion of fuel. Fig. 14-31 shows variation of thermal efficiency versus ignition advance angle.
(7) Nature of Fuel Employed:
The type and quality of fuel effect the thermal efficiency of the engine. Good quality of fuel results into high thermal efficiency of the engine. The gaseous fuel provides higher thermal efficiency due proper mixing of fuel with air. But the power output of the gaseous fuel is less. Fig. 14-32 shows the variation of thermal efficiency with size.
(8) Dimension of Engine:
The dimensions of the engine as cylinder bore and stroke length effect the thermal efficiency of the engine. The large bore and stroke length are undesirable because it reduces the thermal efficiency of the engine.
(9) Combustion Chamber Design:
The combustion chamber is the main area of the engine where the combustion of fuel occurs. The design of the combustion chamber is more important. If the combustion chamber is properly designed then the thermal efficiency increases and fuel consumption reduces. Fig. 14-33 shows surface to volume ratio versus thermal efficiency.
(10) Inlet Charge Temperature:
The temperature of the inlet air is very important. If the temperature of air is low, then the density of air is high and large quantity of air is sucked in the engine cylinder. But due to low temperature of air, less combustion of fuel occurs. If the temperature of air is high, then less mass of air is admitted in the cylinder and more fuel burn. This increases the thermal efficiency.
(11) Cylinder Temperature:
The cylinder temperature effect the thermal efficiency of the engine. If the cylinder temperature is more, then the fuel is vaporized earlier and combustion of fuel is more. However if the wall temperature is more than more heat losses occurs from the engine.
Exam Question # 16. What are the different cooling systems used in IC engine?
Ans. The basic purpose of cooling system is to maintain engine temperature constant to a low level to keep material properties and to extract heat continuously from engine cylinder.
Petrol engines are either water cooled or air cooled. When sufficient water is available, the circulating water is allowed to run to waste, otherwise the same water is used over and over again after re-cooling it in the radiator. In air cooled engines the fins are cast or shrunk onto the cylinder and the cylinder head to increase the heat radiating surface. The air cooling is more effective if air is passed over the fins.
In the air cooling system, the engine cylinder is cooled by the flowing air. The flow of air is created by the fan circulating the air in the engine cylinder. The cylinder head and cylinder surface is provided with fins to improve the heat transfer from the cylinder.
A fan is installed before the cylinder and the air flow is created around the cylinder and head surface over the fins. The flowing air sucks the air from atmosphere and creates a forced circulation of air. Due to convection process the air cool the cylinder and cylinder head surface.
The air cooling system provides number of advantages:
(1) It cools the cylinder at faster rate
(2) It requires less components
(3) Less maintenance of parts
(4) The design of engine is simpler
(5) There are no cooling pipe and radiator
(6) No water leakages in the system
(7) Weight per unit power produced is less
(8) Installation of engine is easier.
(1) The system can be used only for single cylinder
(2) Need proper flow of air
(3) The system has more noise
(4) Engine surface runs at higher temperature
(5) Non-uniform cooling
(6) Less output.
It is indirect cooling system. The cooling of the engine cylinder by use of water as a medium for cooling. Then further the water is cooled by forced circulation of air in the radiator. The engine cylinder has got passages cut in the cylinder head and surrounds the engine cylinder.
The cooling medium flow through the passages and cools the cylinder head and cylinder. It extracts the heat from the cylinder surface and the cylinder head. The passages are connected to the radiator by the hose pipes. The radiator is a heat exchanger having fins.
The water is stored in the radiator and flows through the hose pipe to the engine cylinder block. The water returns to the radiator after cooling the engine from the return pipe. The radiator cools the water using the forced air send through the cooling fans.
The water cooling system provides number of advantages:
(1) The system can be used for single or multi-cylinder engines
(2) Flow of air can be less
(3) The system has less noise
(4) Engine surface runs at lower temperature
(5) Uniform cooling
(6) More output.
(1) It cools the cylinder at slower rate
(2) It requires more components
(3) More maintenance of parts
(4) The design of engine is complex
(5) There are cooling pipe and radiator
(6) Coolant leakages in the system
(7) Weight per unit power produced is more
(8) Installation of engine is complicated.
Exam Question # 17. What are the types of dynamometers used for measuring the power of engines?
Ans. There are four types of dynamometer:
(1) Rope Brake Dynamometer:
This is the simplest dynamometer used for measurement of brake power. It consists of drum on which the rope is wound. One end of rope is connected to the spring balance and the other end of the rope is loaded with the weight.
The resulting tension due to spring balance and weight creates a resistive load on the drum. The drum shaft is connected to the engine crank shaft and the frictional load of the rope and drum applies load on the engine.
The surplus torque of the engine is absorbed by the resistive torque of the rope and drum keeping the speed of the engine is constant. If the radius of the drum is R, spring balance reading, S, and load placed on the rope W.
Torque absorbed by rope brake dynamometer
Torque on the engine = (W – S) R g.
If N be the speed of the engine crankshaft in r.p.m.,
Brake power = [(W – S) R x 2πN g] / 60 watts.
(2) Band Brake Dynamometer:
The band brake dynamometer is used for measurement of brake power. The resistive force is applied on the engine by the band brake and surplus power of the engine is absorbed by the dynamometer. It consists of a band wound around the drum and two ends of the band brake are under the spring balance as shown in fig.14-11.
The tension is exerted by the band due to spring balance at two end of the band and a resistive force is exerted between the band and drum due to the friction between the band and drum surface. The load is applied on the engine. The reading of spring balance on the end of the band is S1 and S2, then the torque applied by drum can be calculated.
Torque is applied on the drum by the tensions of the drum and engine power is absorbed.
The torque is given by-
= (S1 – S2) R g
Where R radius of the drum
Brake power = [(S1 – S2) R x 2πN g] / 60 watts.
(3) Pony Brake Dynamometer:
The pony brake dynamometer uses brake shoe to apply load on the engine. It consists of brake shoes made of wood and are clamped on to the rim of the brake wheel by means of bolts. The pressure on the rim can be varied with the help of nuts and springs. It is used for testing of slow speed engine of capacity 70 KW.
(4) Hydraulic Dynamometer:
For high speed engines, the type of brake widely used is the Heenan and Froude hydraulic dynamometers. It works on what is known as “torque reaction principle”. It consists of a rotor running in a casing through which water flows steadily via the inlet and outlet pipes.
The rotor is coupled to the engine output shaft, and the casing is freely mounted on bearings fitted to the trunnion brackets. At the periphery of the rotor, there is a series of semi-elliptical pockets and at the inside of the casing, there is an identical set of pockets, so that when the rotor is driven by the engine, the water is flung out of the pocket by centrifugal action and transferred to the pockets in the casing.
This results in a tendency to turn the casing with the rotor. The casing is prevented from rotating by the resistance of the spring and the static load applied to the torque arm which projects from the casing.
A hand wheel is provided on the top of the balance frame to adjust the torque arm to a horizontal position; this is facilitated by a small pointer. The amount of load and hence the torque absorbed by the dynamometers can be varied by controlling the flow of water. This is usually done by operating another hand wheel, which slides thin metal plates between the rotor and the casing pockets, thus blanking-off a number of effective pockets.
The torque on the casing can be determined from the length of the torque arm, R, spring balance reading, S, and static load W. By the principle of torque reaction, this is equal to the torque transmitted by the engine.
The value of the constant K is usually stamped on the name plate attached to the dynamometers. Thus the brake power formula for the Heenan and Froude dynamometers may be reduced to the simple form
Brake power = [(W – S) Ng] / K watts.
(5) Electric Dynamometer:
In this case, the electric dynamometer is used for measurement of power. The engine is connected to the electrical generator and the generator develops electric power. The electric power is consumed by the electrical load either the electric heater or bulb load. The power generated is measured by using the voltage and current or energy meter.
Exam Question # 18. Explain the working of spark ignition engines.
Ans. The spark ignition engines are used for petrol fuels and therefore they are known as petrol engines. The petrol engine works on the principle of spark ignition in which fuel is burned by the ignition of fuel due to a spark by an ignition mechanism.
In this engine, the petrol fuel is used as a fuel and it is mixed with air in a device known as carburetor. The carburetor mixes the air and fuel in the desired proportion according to the requirement. The mixture of air and fuel is supplied to engine cylinder during the suction stroke.
The air fuel mixture is compressed in the cylinder during the compression stroke. Therefore the fuel which in the form of fine droplets which are vaporized because of the high temperature of air. The hot air creates a turbulence in the engine cylinder which is responsible for proper mixing of air and fuel.
The spark is ignited in the engine cylinder at the end of compression which result in generation of a flame near to the spark plug. The spark occurs at the spark plug at the proper instance and the ignition mechanism supplies a high voltage current which creates spark at spark plug electrodes.
The flame at the spark plug rises to very high velocity inside the engine cylinder and the fuel is gradually burned by the high velocity flame in the engine cylinder. The combustion of fuel result in a generation of high force on the piston, which performs a work and the piston is forced during the expansion stroke.
The exhaust gases are exhausted from the engine cylinder. The spark ignition engine is dependent on the ignition quality of fuel. If the fuel used in the engine is of vary high quality then efficiency of engine is very high. These engines use petrol as a fuel and it and be made either as a two stroke or a four stroke cycle engine. The petrol must be correctly metered into the intake air during the suction stroke and the mixture of air petrol must be ignited at the correct instant.
The former function is performed by the carburetor and the latter by ignition system. The ignition system can be either coil or magneto type and is generally called the high tension ignition system. The two engines in which the mixture of air and fuel is supplied to engine cylinder from crankcase and fuel air mixture is ignited at the end of compression stroke by ignition mechanism.
Exam Question # 19. Explain the functions of carburetor.
Ans. It is a device use for preparing air fuel mixture of the required quality according to the requirement of air fuel mixture. It is generally accepted that a chemically correct air fuel ratio is required for proper combustion of fuel.
The engine is subjected to varying speed and load conditions like acceleration, starting, high power, cold start etc. The requirement of air and fuel varies according to the load conditions. If the economy of fuel is desired then fine air/fuel mixture is most desirable.
In case of petrol, fuel air ratio of 14:1 is considered to be the chemically correct air fuel ratio. The ratio of 20:1 is considered to be the best air fuel ratio for fuel economy conditions where as a ratio of 12:1 is considered to be suitable for high power and acceleration conditions. The ratio of 8:1 is desired during the starting conditions. Therefore, the carburetor has to supply fuel having ratio from 8:1 to 20:1.
A simple carburetor consists of a float chamber, float and main jet. The purpose of the float chamber is store minimum quantity of fuel. The float chamber is supplied with fuel from the fuel tank. The fuel is automatically controlled by a needle valve which is supported by a float floating on a fuel in the float chamber.
If the level of fuel is rising in the float chamber. The float automatically closes the needle valve and supply of fuel is stopped. The float chamber is connected to a main jet which is placed at the throat of venturi. The suction pipe of the engine cylinder is connected to a venturi.
When the air flows through the suction pipe, the velocity of air increases in the venturi and the pressure drops at the throat of venturi. The float chamber is vented to atmosphere and a pressure difference occur between the throat of venturi and the float chamber.
This result in the flow of fuel through the main jet. The fuel is spread in the flowing air and mixes with air to form a homogenous mixture. The metered quantity of fuel is supplied to the engine cylinder.
The simple carburetor is designed with the air fuel ratio supplied at a constant rate. But the ratio of air and fuel mixture varies with respect to speed of the engine. If the engine runs at very high speed then the quantity of fuel sucked is more and at low speed it is less. Therefore the simple carburetor is unsuitable for the varying speed engine operating conditions.
Exam Question # 20. What is the function of fuel pump in petrol engine?
Ans. The fuel pump is used for the petrol engine because the fuel from the fuel tank is brought to the carburetor by use of the fuel pump from the fuel tank. It consists of a diaphragm pump which sucks the fuel from the fuel tank and supplies it to the float chamber of the carburetor.
It has cam which is connected to the crank shaft and the cam revolve along with the crank shaft. The follower is lifter by cam and it is connected to the diaphragm by bell crank lever. The diaphragm is pulled against the spring force resulting into increase in volume on the upper part of the diaphragm.
The suction is created on the top part, this suction result into sucking of fuel in the pump by opening of the valve. The fuel is filled in the upper portion of the diaphragm and then the valve gets closed. The diaphragm is forced into upward direction which forces the outlet valve to open and allows the fuel to flow towards the float chamber. Therefore the fuel pump maintain constant supply of fuel to the float chamber when the engine starts running.
Before we study the ignition system, we shall describe the spark plug, which is the essential component of the ignition system.
Exam Question # 21. Discuss the functions of spark plug.
Ans. A spark plug consists of a central porcelain insulator, through which passes the central electrode. This central electrode extends for a short length through the bottom of the insulator. The central electrode has an external contact at the top.
A metal screw surrounds the bottom part of the insulator and makes a gas tight seal with the insulator. Onto the bottom of the metal screw is welded a metal tongue, which bends over to lie across the end of the projecting central electrode but with a small gap between the tongue and the electrode. This gap is called the spark gap.
Across spark gap high tension electric spark jumps to ignite the charge in the engine cylinder. The spark plug is screwed into the cylinder head such that the spark gap projects into the combustion space. Copper-asbestos washer is provided between the plug and the cylinder head to make a gas tight seal.
The primary circuit consists of the battery, ammeter, ignition switch, primary winding, contact breaker points and condenser. This circuit is completed through the engine block and frame back to the battery.
When the breaker points are closed, current flows in the primary circuit building up from zero to a maximum value and at the same time a magnetic field is established in the coil. When we want a spark in the engine, the breaker point is opened by the distributor cam.
This opens the primary circuit, but the current still tries to flow in the same direction and builds up in the condenser, which is in parallel with the breaker point. The flow of current is reversed when the battery voltage is less than the voltage across the condenser plates.
The reversal of the direction of the current in primary circuit reverses the magnetic field in the coil, causing a high voltage to be induced in the secondary winding, when the contact breaker in the primary circuit is opened. The distributor rotor connects with a contact to a spark plug and hence a high voltage will be set up across the gap of the spark plug.
The high intensity voltage occurs across the gap, thus igniting the cylinder charge. The condenser across the contact breaker points prevents excessive arcing across the contact breaker.
Exam Question # 22. Describe ignition advance mechanism.
Ans. For a variable speed petrol engine, it is absolutely necessary to change the time at which ignition spark occurs if greatest efficiency and power are to be obtained. It has been found practicable to control the spark advance only as a function of speed and intake manifold pressure.
The centrifugal masses in the bottom of the distributor head moves out as the speed increases, causing the head to rotate with respect to .the breaker point, thereby advancing the spark. A spring loaded diaphragm connected to the manifold is used to advance the distributor head and to override the centrifugal masses.
Exam Question # 23. What are the types of ignition system?
Ans. The petrol engines use an electric spark for the ignition of the compressed charge at the end of compression stroke. Hence these engines are known as Spark Ignition (S.I.) engines. This high tension ignition system uses the fixed gap spark plug as the means by which ignition is accomplished in the engine cylinder.
In magneto ignition system, battery is not required. The magneto may consist either of rotating magnets in fixed coils or rotating coils in fixed magnets.
In case of rotating magnet type of magneto, as the poles of the magnet pass between the shoes of the coil a rapid change of magnetic flux in the coil takes place. As a result a current is induced in the primary winding of the coil.
When the current is at its maximum value the contact breaker points are opened by means of a cam which is mounted on the magneto shaft and then the circuit operates in the same way as the coil ignition system.
For a two pole magneto, there will be two current reversals per revolution of the rotating magnet and hence, two sparking plugs can be fed per revolution of the magneto. The direction of the current through the sparking plug is not important.
In a magneto system, the speed of the rotor must be governed to cover the number of cylinders and the number of poles. For a four cylinder four stroke cycle petrol engine, two pole magneto rotor must rotate at engine speed giving four current reversals in two revolutions of the crank shaft. If the same magneto is used for an eight cylinder engine the rotor should run at twice the engine speed.
There is a minimum speed at which a magneto should be run to supply the necessary current. This speed is called the coming in speed which is near about 100 r.p.m.
This is known as breakerless ignition system. In this system, there are no contact breaker point, cam and sliding blocks etc. The wear of these parts results into improper performance of engine and starting problems. The contact breaker system has wearing of components and it results into increase in the spark gap. This reduces the voltage and timing of spark.
In this system, the distributor cam, breaker plate, CB points and condenser is replaced by an armature and electronic control module known as amplifier. This creates magnetic pulses which trigger an electronic control unit to create high voltage in the special coil.
It consists of ignition coil and magnetic pick up. The revolving armature has tooth. As the tooth of the revolving armature approaches the magnetic pick up induces a voltage which signals the electronic control unit, to turn of the ignition coil.
Then a timing device in the electronic control unit turns the circuit on again after collapsing magnetic field which induces high voltage in the secondary winding of the ignition coil. This generation of high voltage results into feeding of spark at the spark plug electrode points.
The conventional contact breaker point ignition system is known as inductive storage system because energy stored in the magnetic pick up field of the ignition coil which is used to create spark. The similar concept is used in the capacitive discharge system.
This system uses a transistor for switching the primary circuit current. This is known as transistor inductive storage system. The ignition coil is used as high voltage transformer. A large size capacitor is used for the storage of energy.
The capacitor is charged to about 300 volts and when it is triggered, it discharges its energy to the primary winding of the ignition coil. This steps up voltage upto 30000 volts in the secondary windings. This voltage is supplied across the spark plug.
Exam Question # 24. What are the advantages of breakerless electronic ignition system?
Ans. The various advantages of breakerless electronic ignition system are:
(1) Period checking of the system and cleaning is not required
(2) There are no changes in the ignition timings, it is perfectly maintained
(3) It produces high tension sparking voltage regularly in very short time
(4) There is no starting problem
(5) Longer life
(6) Better efficiency
(7) Fuel economy
(8) Less maintenance cost.
Exam Question # 25. Discuss the functions of electronic fuel injection system with advantages and disadvantages.
Ans. The electronic fuel injection is the injection in the petrol engine cylinder during the suction stroke in the metered quantity. The supply petrol fuel from the carburetor is not possible as the requirement of quality of mixture during varying load conditions. Therefore the electronic fuel injection system is used for injection of the fuel in the individual cylinder.
This system consists of the injector connected to the fuel pump and the fuel pump is connected to the fuel tank. The engine parameters like air flow rate, fuel flow rate, speed, temperature, exhaust gas temperature and constituents are detected by the sensors and the signal is send to Electronic Control Unit (ECU).
The signals from the different sensors are analysed by ECU and then the fuel flow and injection of fuel to individual cylinder is controlled by ECU. This results into supply fuel to individual cylinders in the metered quantity. The fuel economy can be achieved by the use of electronic fuel injection system.
(1) Better starting and acceleration
(2) Higher fuel economy
(3) High power is available
(4) Thermal efficiency is very high
(5) High compression ratio
(6) The fuel supply is not affected by position of vehicle
(7) Less carbon deposit.
(1) High initial cost
(2) Replacement and maintenance cost is high
(3) More bulky and heavy.