Why do Diesel engines have that distinctive sound to them when compared to gasoline engines?
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We use a spark plug in gasoline to ignite a mix of gas and air, so we know exactly when we can control the start of combustion. Diesel engines rely on a mix of diesel and air self combusting, so despite all the advances we’ve made, there is still a bit of variance between the cylinders when that self ignition point happens.
If you look at it on a graph (of pressure rise rate within the cylinder) Gasoline will be the same every cycle (assuming we don’t advance/retard the ignition) and diesel will show that rise moving forward and backwards slightly. This is that slightly uneven noise you get especially from older diesels that makes them stand out as a noise
The ELI:5 (this was way harder than I thought)
To make the car go, we set fire to some gasoline and air which we’ve mixed together and we use it to push a piston. In a gas engine we use a spark plug to set fire to this mixture, but a Diesel engine squeezes the mix a lot lot more until the mix begins to burn on its on, usually with a Big Bang! Because we turn on the spark plugs on a gas engine, we get to say “I want to start burning the mixture Now!” But in a Diesel engine there is a tiny tiny bit of difference everytime we get to the combustion (burning time) which means that it changes ever so slightly every time. The noise of the engine comes from this burning, so the petrol is smooth because it’s the same every time, but the diesel changes a little bit and the sound it makes is therefore a bit faster and slower sometimes.
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