ORIGINAL: earlwb
The diesel engines seem to run more quietly than a glow or gasoline engine does. I think it is the way the fuel is burned more completely inside the engine before it is let out. Thus the mufflers can be smaller with less volume to them.
Iagree and also have viewed it like this, because the fuel burns completely and instantly the rest of the downstroke provides an additional expansion chamber for the gases.
This is why diesels can get away with such small external expansion chambers for mufflers, their internal thick walled 'expansion' chamber compliments it well.