Why do diesels sound different than gasoline engines.
 

I know all the differences between diesel and gasoline engines and can tell the difference in sound a block away. However, I cannot explain why they sound different. What exactly are we hearing when we hear that distinctive clatter? Is it the sound of higher compression? That doesn't seem likely since higher compression gasoline engines don't change their sound very much. Could it be the sound of compression ignition? Probably. When a gas car 'diesels', it does sound like a diesel.
Why is this sound so distinctive and different than spark ignition?

I used to work at Cummins and can tell you that prototype fuel systems almost completely alleviate the "diesel" sound. Mainly these work through pilot injection (a small quantity well before TDC) then the main injection.

Perhaps the difference is the stratification, where the diesel fuel plume may not completely fill the cylinder, while a gas engine with its homogeneous charge does completely fill the engine. The pilot injection mimics that and reduces noise significantly. Maybe someone has heard a GDI (gasoline direct injection) engine and can comment if they make any type of diesel noise.

The clatter is a result of ignition of the fuel lagging behind injection -- this is most evident in direct injection low compression engines. Quite a bit of fuel is injected before it starts burning and it goes "snap" when it does light. Diesels inject fuel over a rather long period under power, but only a tiny spritz of short duration at idle, so the entire charge can explosively burn at once. Gasoline engines are set up so that the fuel/air mix is over rich for explosion, so it just burns fast rather that making a bang.

On top of that, the air charge in a diesel is just about as high as it gets -- the ability of the air to completely fill the combustion chamber goes down as the speed of the engine goes up. Hence a very lean condtion, also very hot. Gasoline engines are throttled -- remember, the intake has substantial vacuum at idle, and it is this thin air that is compressed. The result is a very small amount of air and low pressure at idle (the cause of most air pollution from gas cars). Nothing to make a bang with.

Diesels move large amounts of air all the time -- max flow is about 1000 rpm on a non-turbo -- and turbo models move tremendous amounts under load. This means there is a lot more air moving through the exhaust, too. Most diesels are very quiet even with open pipes, much to the dismay of teenagers who want noisy exhaust. Only when you get the engine under full load do you get much noise -- some list members can attest to this, after finding out their downpipes were broken and not knowing it!

Gas engines have higher peak pressures as the fuel burns, and waste more heat out the exhaust -- more noise, but different than a diesel.

As far as it goes, it is hard to tell my 300D is a diesel when it is fully warm -- just a bit more rumble and vibration than a gas engine. The Volvo, on the other hand, rattles something fierce.

Peter

I have a Case tractor that seems to work different than other Diesels.

It is quiet at idle but knocks at full load.

Engine oil doesn't get a black as the MB Diesels with same fuel and engine oil.

P E H