How are petrol and diesel engines different?

In theory petrol and diesel engines are almost the same; they are both internal combustion engines that convert fuel into kinetic energy by burning it. In practice though the two engines are very different in many areas including construction and the way in which they burn fuel. Below we have outlined the main ways in which petrol and diesel cars differ;

What they are made of


Diesel engines tend to be made of cast iron as they require a higher compression ratio than that of a petrol engine and materials such as aluminium do not have as long a life as cast iron. This higher propensity for withholding pressure works hand in hand with the fact that almost all diesel engines are turbocharged.


Petrol engines do not have quite such a high compression rate and can be made of lighter materials such as aluminium. Even when petrol engines are turbocharged they tend to have reinforced cylinder liners whilst still being made up of aluminium.

How they burn fuel


Petrol is the most familiar system for internal combustion and is known as a four stroke cycle these four steps are;

  • Intake – The air and fuel are sucked into the cylinder through the intake valve.
  • Compression – The air and fuel mixture are compressed by the piston.
  • Ignition – The spark plug ignites the mixture causing them to explode and force the piston down.
  • Exhaust – The spent gases are expelled through the exhaust valve.


Diesel engines combustion system is slightly different;

  • Intake (air only) – Air alone is sucked into the cylinder.
  • Compression – The air is compressed, the friction created by this heats the air quickly.
  • Injection – The diesel is injected into the cylinder, the hot air causes the diesel to combust and forces the piston down.
  • Exhaust – The spent gases are expelled and generally rerouted to a turbocharger via the exhaust manifold.

The differences between the engines may seem incredibly small but if for example you were to put petrol in a diesel car the entire combustion would cease to work and you would be risking permanent damage to your car. Thankfully we are always on hand to help and can drain the unwanted fuel from your tank and ensure that no damage is caused, our number is 0800 634 9827 and is always great to have in your contacts, just incase.