Diesel and petrol engines are quite similar in their function of converting chemical energy available with the fuel to mechanical energy. Both are internal combustion engine, burning the fuel and air mixture inside a cylinder but the difference lies in the way the fuel air mixture is ignited.

To understand the difference better, let us see how an engine works. Both petrol and diesel engine has four strokes, the difference lies in how these 4 strokes are executed.

1. Intake or suction stroke:

In a diesel engine the intake valve opens with piston moving down allowing air to enter the combustion chamber whereas in a petrol engine air fuel mixture is allowed. The air and fuel is mixed separately in a place a called carburetor in the inlet manifold and diesel engine does not require one.

2. Compression stroke:

The piston now moves up compressing the air in diesel engine and air-fuel mixture in the petrol engine. This stroke considerably increases the temperature inside the combustion chamber.

4 stroke diesel and petrol engines

3. Combustion or power stroke:

In a diesel engine, just before the piston reaches the top of the cylinder, fuel is injected directly into hot compressed air and ignited. The ignition causes the temperature to rise and pushes the piston down. Petrol engine is similar, but the air and fuel is already mixed in carburetor and a spark plug is used to ignite this mixture. The timing of fuel injection and sparking is crucial for the performance of an engine.

4. Exhaust stroke:    

The piston moves back to the top pushing the remains from the combustion through the outlet or exhaust valve.

This basic difference in the execution of the 4 strokes demands different type of construction between the two. A diesel engine is heavy and bulky due to the high compression ratio (1: 25 maximum) it needs to handle and has a direct fuel injection system, delivering fuel at high pressure into the combustion chamber. A petrol engine is comparatively light and uses a spark plug for ignition of the fuel.

Usage:

Petrol engine is normally used in cars and bikes, where the initial torque required is generally less and they work at high speeds.

Diesel engines are used for heavy loads like trucks and buses which require higher initial torque and works at lower speeds.

The cost of a diesel engine is higher than that of petrol. Even though the initial cost of a diesel engine is high, it overcomes this disadvantage through better fuel economy (miles per gallon or miles per liter). Normally 20% to 30% more than petrol engine.

Difference between diesel and petrol engines

• Diesel allows air inside the combustion chamber whereas it is air and fuel mixture for petrol.
• Diesel engine has direct injection system whereas petrol has spark plug for ignition and carburetor for air and fuel mixing.
• Diesel engine is heavier than a petrol engine.
• Diesel engine is costlier than a petrol engine.
• Diesel has better fuel economy.
• Diesel engine operates at low speed with higher torque whereas petrol engine operates at high speed with lesser torque.
• Fuel: Diesel is used in diesel engine and petrol for petrol engine.

Above images obtained from http://commons.wikipedia.org