In this article, we discuss compression ignition engines (CI Engines) or four stroke diesel engines and a comparison between compression ignition engines and spark ignition engines.
Compression Ignition Engines (CI Engines) or Four Stroke Diesel Engines
The working of a compression ignition engine is similar to spark ignition engine, except for the following two major points:
In compression ignition engines the air is only drawn during the suction stroke instead of the air-fuel mixture drawn in SI engines.
In CI engines the fuel is injected by using an injector into the cylinder and the high-pressure and temperature mixture is burnt. Hence there is no need for the spark plug.
The compression ignition engine cycle consists of the following four strokes
- Suction stroke
- Compression stroke
- Expansion/Power stroke
- Exhaust stroke
1. Suction stroke
During this stroke air is drawn into the cylinder and the piston moves from TDC to BDC.
The exhaust valve remains closed during this stroke.
2. Compression stroke
During this stroke, the piston moves from BDC to TDC and both valves remain closed.
The air sucked during the suction stroke is compressed hence there is a rise in the pressure and temperature of the air.
Before the end of this stroke, a fine spray of fuel is injected into the compressed air which is at a high temperature and hence combustion takes place.
3. Expansion or power stroke
During this stroke both the valves remain closed and the piston moves from TDC to BDC.
The heat energy released by the combustion of fuel results in a rise in the pressure of the gases which drives the piston downwards. Hence, power is developed.
4. Exhaust stroke
In this stroke exhaust valve is opened and inlet valve remains closed and the piston moves from BDC to TDC.
The upward movement of the piston forces the burnt gases out of the cylinder through the exhaust valve.
Comparison between Compression Ignition Engines and Spark Ignition Engines
The comparison between compression ignition engines and spark ignition engines is as follows:
Compression Ignition Engines
1. The diesel engine works on the diesel cycle.
2. In these engines, only air is sucked during the suction stroke.
3. Fuel supply is controlled by the fuel pump.
4. Spark plug is not required.
5. Compression ratio is high (about 14 to 22).
6. Due to the high compression ratio, the efficiency of these engines is high.
7. Due to the high compression ratio, starting of these engines is difficult.
8. Diesel engine is heavier due to high pressure.
9. Due to their heavy weight these engines cannot rotate at high speed.
10. The operation of these engines is noisy.
11. Running cost is low since it uses cheaper fuel.
12. Initial cost is high.
13. These engines are used in heavy-duty vehicles like buses, trucks, etc.
Spark Ignition Engines
1. The petrol engine works on the Otto cycle.
2. In these engines, the air-fuel mixture is sucked during the suction stroke.
3. Fuel supply is controlled by the throttle valve in the carburetor.
4. For ignition, a spark plug is required.
5. Compression ratio is low (about 6 to 12).
6. Due to the low compression ratio, the efficiency of these engines is low.
7. Due to the low compression ratio, starting of these engines is easy.
8. Petrol engine is light in weight.
9. Due to their lightweight these engines can rotate at high speed.
10. The operation of these engines is silent.
11. Running cost is more since it uses costlier fuel.
12. Initial cost is low.
13. These engines are used in light-duty vehicles like motorcycles, scooters, cars, etc.