Four-Stroke SI and Diesel Engines | Mechanical Engineering

In a four-stroke engine, the entire cycle operation is completed in four strokes of the piston or in two revolutions of the crankshaft. In case of SI engines, petrol is used as a fuel which is highly volatile and a lighter fluid, whereas in the case of CI engines, diesel is used as a fuel.

Four-Stroke SI Engine (4S-SI):

A four-stroke petrol engine consists of a cylinder covered with a cylinder head while the other end is connected with crank case. The cover end has the provision of inlet and exhaust valves. These valves operate mechanically by cam mechanism with the help of rocker arms.

A spark plug is fitted at the top of the cover which initiates the ignition of the petrol. A trunk piston reciprocates inside the cylinder. The connecting rod and the crank transmit forces to the crankshaft which converts the reciprocating motion to rotary motion.

The SI engine works on the principle of theoretical Otto cycle. The Otto cycle is also known as constant volume cycle because the addition of heat takes place during constant volume.

The complete cycle of operation of a four-stroke SI engine consists of the following four strokes:

(a) Suction or intake stroke

(b) Compression stroke

(c) Expansion stroke

(d) Exhaust stroke

(a) Suction Stroke:

During the suction stroke, the inlet valve is kept opened and the exhaust valve remains closed. The piston travels from TDC to BDC in case of vertical cylinder engine. The suction pressure is found little less than the atmospheric pressure. During this stroke, a mixture of petrol vapor and air (prepared by carburetor) in correct proportion is drawn into the cylinder [Fig. 4.33(a)].

(b) Compression Stroke:

During the compression stroke, both the inlet and exhaust valves are closed. The mixture of petrol vapor and air already drawn in suction stroke is compressed when the piston returns from BDC towards TDC.

The compression ratio in the case of SI engines is 6-9. At the end of compression stroke, since the temperature achieved is less than the ignition temperature of petrol, sparking is done instantaneously by spark plug (Fig. 4.34), and thus, the compressed mixture is ignited with the help of electric spark.

In this case, there is an ignition system which does this work at proper timing. The combustion starts leading to high temperature and pressure. The fuel starts burning. During this combustion phenomenon, the chemical energy of fuel is converted into heat energy producing nearly 2000°C [Fig. 4.33(b)].

(c) Expansion Stroke or Power Stroke:

During the power stroke, inlet and exhaust valves stay closed. The piston moves from TDC to BDC and the crankshaft completes half revolution. High-pressure burnt gases push the piston towards BDC.

The reciprocating motion of the piston causes to produce mechanical work which is transferred to the crankshaft through connecting rod. As the piston moves towards the BDC, the pressure of burnt gases decreases gradually. When the piston reaches BDC, the exhaust valve is almost open [Fig. 4.33(c)].

(d) Exhaust Stroke:

During this stroke, the inlet valve is closed and the exhaust valve is opened. The piston moves from BDC to TDC and the crankshaft revolves half revolution. As the piston moves up, the burnt gases are expelled out from the cylinder.

This movement of the piston is done by the inertia effect of the flywheel [Fig. 4.33(d)], Fly wheel is a heavy weight wheel mounted on shaft which has high moment of inertia. During operation, the fly wheel absorbs energy which is supplied during this period.

Four-Stroke Diesel Engine (4S-CI):

The operation and construction of a four-stroke diesel engine (CI) is similar to that of a four-stroke petrol engine (SI), but CI engines operate at higher compression ratio (11-22) as compared to SI engines.

In case of a diesel engine, a fuel injector or fuel atomizer is used instead of a spark plug. An injector is connected with a high-capacity fuel pump which supplies the fuel (diesel) to the injector at high pressure. The fuel is injected to the combustion chamber.

A CI engine works on the principle of theoretical diesel cycle developed by Rudolph diesel in 1897. The cycle is a constant pressure cycle because the supply of heat takes place at constant pressure.

The complete cycle of operation of a four- stroke CI engine consists of the following four strokes of processes:

(a) Suction or intake stroke

(b) Compression stroke

(c) Expansion stroke

(d) Exhaust stroke

(a) Suction Stroke:

During this stroke, the inlet valve is kept opened and the exhaust valve remains closed. The piston moves from TDC to BDC in vertical cylinder engine. The suction pressure is found little less than the atmospheric pressure. During this stroke, a fixed amount of air is sucked in the cylinder through the opening of an inlet valve. At the end of this stroke, the entire cylinder is filled with air.

(b) Compression Stroke:

During this stroke both the valves are kept closed. The piston moves from BDC to TDC. The amount of air which was sucked during the suction stroke is compressed during the compression stroke. The piston returns back from BDC to TDC. The compression ratio in this case varies from 11 to 22. The compression ratio in CI engine is high as compared to that in SI engine.

At the end of compression process, pressure and temperature are very high. The temperature achieved is more than the ignition temperature of diesel which is more than the ignition temperature of petrol in SI engine. When the compression ends, the diesel fuel is injected in the form of spray by the injector. As soon as the diesel particles come in contact with the high-temperature compressed air, they catch fire instantaneously.

The auto-ignition of the diesel fuel initiates the combustion. As a result, hot gases are released. The release of heat is continuous by the continuous supply of diesel. The high-pressure gas forces the piston to move further. The rate of injection maintains the combustion process at constant pressure even if the piston moves.

(c) Expansion Stroke:

The downward movement of piston towards BDC causes to develop mechanical work which is transferred to the crankshaft through connecting rod. As the piston moves down, the gas pressure gradually decreases. This is an expansion stroke. Now the entire cylinder is filled with hot gases, only to be expelled out for fresh cycle to continue.

(d) Exhaust Stroke:

During this stroke, both inlet and exhaust valves are closed. Piston starts moving from BDC to TDC which pushes out the product of combustion. Due to high pressure achieved in the cycle, the CI engine is considered stronger than SI engine.

The length of cylinder is also greater in case of CI engine. Due to this, CI engines are heavier than SI engines. Since four-stroke diesel engines produce more power than SI engines, they are used in heavy locomotives such as trucks, tractors, jeep, diesel locomotive, etc.