Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I guess it's related to their basically running off knocking, but if I were to explain it to someone like this, it wouldn't convince me neither.

[Edit] From

The Loud knock is caused by the introduction of high pressure cold fuel into very high cylinder pressures.

Now, who's right?

share|improve this question
Forgot to upvote your question. +1 Good question. – DucatiKiller Feb 26 at 0:02
Only indirectly but I suppose from a certain perspective that is true. I think it's more because of the actual combustion that generates the sound not the injection itself. The valve that opens may have a sort of ticking noise with the timing. Also I was to understand the question to be more about diesel as opposed to gasoline, and the intrinsic differential factors are of course the compression ignition and not in how the fuel is injected although it is done so somewhat differently in diesel engines. – JaredW82 Feb 26 at 0:08
up vote 5 down vote accepted

The engine operates on high compression to ignite the fuel at very low RPMs which gives it it's distinctive sound. The lower RPM makes the ignition of each cylinder easier to hear and the compression ignition system gives it it's rattle like sound. This is as opposed to gasoline engines that operate by a spark/compression ignition system. The key to understanding this lies in understanding the diesel engine. See: the Wiki page for this.

In short, the distinctiveness of your typical diesel sound comes from a combination of it's properties of low RPMs and the high compression ignition system. The actual sounds are generated at combustion and the opening and closing of valves. You really cannot her the injection of the fluids into the chambers. Also, as another poster fist pointer out (DucatiKiller), there is sometimes a piston slap that make a audible sound but not always in newer engines.

Piston SlapPiston Slap2

The high pitch hiss noise you hear from some diesels as they accelerate comes from the turbo charger turbine spinning up and sucking in air at a high rate. Don't mistake the hiss I refer to with the air-brake system, as this is altogether something else.

See:How a Turbo Charger works.

share|improve this answer
atypical should probably be replaced by typical unless you meant that for some reason – cat Feb 25 at 0:41

Two types of sounds

There are two sound types that emerge from a diesel engine.

  • Combustion Noise

  • Mechanical Noise

Combustion noise is created by the compression ignition process which compresses the air fuel mixture of the gas which creates higher temperatures upon increased compression until combustion occurs.

Mechanical noise is created primarily by piston slap where the piston rocks back and forth upon the wrist pin in the bore of the cylinder making a tapping noise that transfers through the solid case and emanates from the surface.


Diesel Engine System Design

Combustion Noise from a Running Diesel Engine Based on Transient Combustion Noise Generation Model

share|improve this answer
I would rephrase the Combustion noise statement. With the compression ignition first the air is compressed until it is very hot, there is no fuel in the cylinder at that time. Once the compression is near complete an injector sprays the fuel directly into the cylinder. The interaction of the fuel with the hot air as it ignites is what creates that unique noise. – vini_i Feb 25 at 9:50
I've given the accepted answer to JaredW82 but this one is good and interesting too, thanks. – Arch Stanton Mar 2 at 8:49

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.