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What's the difference between a diesel and petrol engine?

At first sight, diesel and petrol engines are actually quite similar. They are internal combustion engines, which turn the fuel’s chemical energy into mechanical energy. The main difference between the two engines is their combustion sequence:

Petrol engines

  • For the intake stroke, the fuel is amalgamated with air.
  • The compression stroke sees the piston move upwards, where a fusion of fuel and air is compacted.
  • The fuel and air is then ignited via a spark plug for the ignition stroke.
  • For the exhaust stroke, the piston moves up and drives the exhaust through the exhaust valve.

Diesel engines

  • For the intake stroke, the intake valve widens, allowing the air to enter, which then makes the piston move down.
  • The piston then moves up with the compressed air, which can reach 540°C in the compression stroke.
  • For the combustion stroke, the fuel is infused, goes through the ignition, and the piston moves down.
  • Next, the piston rises again and thrusts the exhaust through the exhaust valve.

What’s better?


Back in the day, diesels used to be extremely loud and eject dark plumes of smoke, but since low sulphur diesel was introduced, they drive very economically and quietly. Diesel is the fuel of choice of HGVs as they give more torque, meaning they have more power.

Diesels are also said to retain 63% of their value after three years. Although diesel costs more each time you refuel, you get more miles to the gallon thanks to diesel’s rich energy consistency – it’s up to 30% richer than petrol. Although diesel has fewer CO2 emissions, it’s been known to produce particles that are linked to asthma.


Petrol cars are on the whole cheaper to buy than diesel cars. Although these are less economical (30% less economical in comparison to diesel cars), these tends to be £900 cheaper to own across a 3 year period.

If you’re after a quieter engine, petrols are often less noisy in comparison to diesel engines. Petrol is also slightly less expensive (10% less to be exact) each time you go to fill up your tank.

Consider your driving needs

If you know you’ll cover a lot of miles and regularly use motorways, then a diesel engine is ideal for you. If you know you will only be driving locally and won’t be covering that many miles, a petrol car with a small engine is more convenient.

If you’re still undecided about what engine to choose, why not pop into one of our showrooms to talk about your needs with one of our specialists?