Both diesel and gasoline engines convert chemical energy from fuel into mechanical energy through a series of explosions. The way these explosions happen is the major difference between these two engines.
In a gasoline engine, the explosion process is:
- Intake stroke – fuel is mixed with air
- Compression stroke – piston goes up, mixture of fuel and air is compressed
- Ignition stroke – fuel/air is ignited through the use of a spark plug
- Exhaust stroke – piston goes up, pushes exhaust through the exhaust valve
In a diesel engine, the explosion process is:
- Intake stroke – intake valve opens, air in, piston goes down
- Compression stroke – piston goes up, air compressed (heated in excess of 540°C)
- Combustion stroke – fuel is injected (right time), ignition, piston goes down
- Exhaust – piston goes up, pushes exhaust through the exhaust valve
Diesel engines have no spark plug. They need high compression ratios to generate the high temperatures required for fuel auto ignition (the higher the cetane number, the better the ignition).
Compression is much higher with a diesel engine (14:1 to 25:1) than a gasoline engine (8:1 to 12:1). Gasoline engines use lower compression ratios to avoid fuel auto ignition (engine knock). Higher compression ratios lead to higher thermal efficiencies and better fuel economies.
Read about Common Problems with diesel fuel