A petrol engine can fire the spark plugs slightly earlier or later to account for factors like revolution speed and engine temperature.

However a diesel only fires when the mixture is compressed. There are no spark plugs to say "burn... NOW"

I am aware that glow plugs are used to pre-heat the top of each diesel cylinder, but they're always warm and can't give an initial ignition to the mix.

So, How does a diesel engine not suffer from late ignition when cold, or preignition/pinking when hot?

This is a follow-on question to Why do Diesel engines have a higher compression ratio than gasoline engines?

2 Answers 2


Diesel engine timing is accomplished through the fuel injectors (they inject fuel at the proper times). Proper timing prevents pre-ignition because there's no fuel in the cylinder yet to ignite. Glow plugs and/or well-designed high pressure injectors ensure the diesel is in a suitable state for combustion and prevent delayed ignition. If the engine is warm enough to start then it's warm enough to run: assuming the injector timing is correct, if the air isn't hot enough to ignite the fuel when it's injected near peak compression, it's not going to become hot enough while it's expanding again; it just wouldn't ignite at all.

Note that diesel engines don't compress fuel the whole way through the compression stroke. They compress fresh air and heat it up, then fuel is injected into the hot air at the proper time, where it ignites (animation).

It's pretty easy to find all this out though if you Google for how diesel engines work. There's a lot of good animations and stuff out there, too.


It's true that electronic injectors inject fuel at proper timing after the engine ECM checks for coolant and oil temperatures. And remember that there is a thermostat that maintain the engine's overall temperature so even in classic mechanical engines they still operate in a certain range of temperature. Also it takes time to get to the peak compression and the homogeneous mixture of air/fuel. So everything is quite slower in diesel engines than petrol engines

  • What do you mean "it takes time to get to the peak compression"? - that's based on the compression ratio...
    – Solar Mike
    Commented Dec 9, 2018 at 7:16
  • @SolarMike - I think he's saying it takes time for the events to occur. Commented Dec 9, 2018 at 14:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .