In this article, we discuss four stroke spark ignition engines (SI Engines) or four stroke petrol engines and a comparison between four stroke spark ignition engines and compression ignition engines.
Four Stroke Spark Ignition Engines (SI Engines) or Four Stroke Petrol Engines
In a four stroke spark ignition engine, the cycle is completed in two revolutions of the crankshaft and four strokes are performed.
1. Suction stroke
2. Compression stroke
3. Expansion/Power stroke
4. Exhaust stroke
Each stroke consists of 180 degrees of crankshaft rotation, hence the cycle consists of 720 degrees of crankshaft rotation for one cycle.
The four stroke spark ignition engine consists of the following strokes.
1. Suction stroke
This stroke starts when the piston is at TDC and about to move downwards.
During this stroke inlet valve remains open and the exhaust valve remains closed.
Due to the low pressure created by the downward-moving piston, the charge (air-fuel mixture) is drawn into the cylinder.
At the end of this stroke, the inlet valve closes.
2. Compression stroke
During this stroke, the compression of fresh sucked charge takes place by the return stroke (BDC to TDC) of the piston.
During this stroke, both the inlet and exhaust valve remains closed.
Just before the completion of the compression stroke, a charge is ignited by a spark plug and combustion takes place.
3. Expansion or power stroke
The products of combustion exert pressure on the piston hence forcing it to move downwards i.e. TDC to BDC.
During this stroke, both the inlet and exhaust valve remains closed.
Power is developed during the expansion of gases hence this stroke is also called a power stroke.
During a power stroke, both temperature and pressure decrease.
4. Exhaust stroke
At the end of the power stroke, the exhaust valve opens but the inlet valve remains closed.
During this stroke, the piston moves from BDC to TDC, and the burnt gases inside the cylinder are exhausted.
At the end of this stroke exhaust valve closes but some residual gases remain in the cylinder.
In this way, again the inlet valve opens and the new cycle starts.
Comparison between Four Stroke Spark Ignition Engines and Compression Ignition Engines
The comparison between spark ignition engines and compression ignition engines is as follows:
Four Stroke 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 these engines is easy.
8. Petrol engine is light in weight.
9. Due to their light weight 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.
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 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.