Why they make this noise?

[Nima]

Hi:
Just because of my curiosity, Why diesel engine make the loud engine noise? you know, the one that they make at idle. Is there anyway to reduce that? Thanks

[pjmoreland]

It's my understanding that diesel engines have way higher compression than gas engines, and that is why they are so much louder. Also they have no spark plugs. Combustion occurs because the diesel and air mixture gets compressed until it ignites spontaneously. In a gas engine, the ignition timing is controlled precisely by the distributor, whereas in a diesel engine I doubt it is nearly as precise since combustion begins somewhere near when the piston is at the top of the compression stroke. I'm guessing that when the engine is idling, combustion happens slightly earlier in the combustion stroke because the pistons are moving slower, so this might cause some valve knock and a loud idle.

[Nima]

hum. That makes sense some how but now I am thinking: do they use any variable valve timing? if not, wouldn't that help? second, if this theory about noises is right, why don't they use some kind of auxilary ignition system which could control the ignition timimng at idle? It is interesting.

[chevymad]

Newer diesels basically do what you're thinking. They have injectors that are operated by the ecm. By setting when the fuel is injected into the cylinder the "timing" can be varied. They've also found that higher injection pressures and releasing small amounts earlier then the rest in a 2nd injector event can make the engine run quieter.

[71Chevy4-speed]

Chevymad is right about the injectors controlling timing. Personally I like the noise of diesel's but I grew up around semi's and john deeres so I like the sound of a working motor. Higher compression is part of the reason they are louder, a normal chevy 350 has 8:1 or 8.5:1 from the factory, while many diesels run upwards of 18:1 to 22.5:1 is not uncommon for compression ratio's. Actually the reason the 24 Valve Cummins in the dodge trucks are so loud is because of the injector pump. If you've noticed the brand new Cummins are quieter than the older 24 valve ones, the reason for this is that they use 3 short injector pulses compared to one longer one. Most of the noise on the new Cummins is from the injector pump. One of the guys I work for has over 500 hp at the rear wheels of his 99 Dodge with over 1000ft-lbs, He's running 50 psi of boost, had to have the head specially ringed to handle it, bigger injectors, twin turbo'd, 4 inch exhaust, exhaust break, chip, and all kinds of goodies. His truck actually got back to normal sound level when he twin turbo'd it, it was a lot louder.