Automobile Engine - Petrol Vs Diesel

Both diesel and petrol are taken out of mineral oil but diesel has better fuel efficiency and lower Carbon dioxide emission. Thus, with respect to the engine concept and combustion process, a diesel engine is more efficient for the same power output. This means diesel consumes lesser fuel. Though diesel is considered to be heavier, it is more efficient than petrol and the engine efficiency is higher by about 20%.

Unlike petrol engines, there is no spark plug-in diesel engine. Ignition of the mixture is achieved by the pressure of air.

Functioning of the petrol engine

1. Suction stroke: When the piston comes down a low pressure is created in the upper part of the cylinder. This causes the inlet valve to open through which the mixture of petrol and air from the carburetor enter the cylinder.

2. Compression stroke: During the upward movement of the piston, the mixture gets compressed causing the inlet valve to close. The volume of the mixture drops to about one eighth of the original volume. Pressure and temperature of the mixture rise enormously.

3. Power stroke: As the piston gets to almost the last part of the upward stroke, the mixture is so highly compressed that its temperature and pressure reaches the maximum point. At the same time an electric spark at the top of the cylinder causes the mixture to ignite and explode. This piston is forced down within the cylinder. This movement is transmitted to the crankshaft through the piston rod. The crankshaft is attached to the car wheel. Hence the movement of the crankshaft is transmitted to the wheel of the car.

4. Exhaust stroke: As the piston comes down to the lower part of the cylinder the pressure inside the cylinder falls allowing the exhaust port to open. As the wheel makes the piston go upwards the waste mixture is discharged.

Functioning of the diesel engine

1. Suction stroke: As the piston comes down air is drawn into the cylinder.

2. Compression stroke:As the piston goes up the drawn air is compressed to high pressure and temperature.

3. Power stroke: At the end of the compression stroke, the air is compressed to the maximum increasing in its temperature and pressure. Now the fuel is injected into the air. Ignition takes place immediately causing the piston to go down. Its movement is transmitted to the crankshaft and to the flywheel and then to the car wheel.

4. Exhaust stroke:As the piston goes up to complete the cycle, the exhaust vapours are discharged out through the exhaust valve.

Advantages of Diesel Engine

• It has higher torque
• It gives higher power output
• It is economical to use diesel as only 70% needs to be spent for fuel for getting the same mileage as a petrol car
• Relaxed driving is ensured especially for long distances

Advantages of Petrol Engine
• It is cheaper
• Maintenance is easier
• Engine noise is lesser

The choice between the two is dependent on individual requirements.