system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do Diesel engines have that distinctive sound to them when compared to gasoline engines?
gpt
Yet another question for an engineer

Diesel engines don't use a spark plug, instead igniting their air-fuel mixture via pressure and heat.

With a gasoline engine, the mixture is well mixed before the ignition happens. If it gets too hot, you get pre-ignition, or knock. (The technical term for that mixture is that it's homogenous, meaning it's the same throughout)

Diesel engines are NOT well mixed (meaning their air-fuel is heterogeneous, or not the same throughout). In the spots that are mixing early combust (explode) before the actual power stroke. The clicking and clacking is the SAME pre-ignition/knock you hear from a gas engine, but in small pockets as the pressure is raised to combust the main body of the mix.

Those early, smaller combustions travel back through the engine mounts and drivetrain and we hear them.

On older diesels, you could also hear the mechanical fuel injectors clacking away.