Why do Diesel engines have that distinctive sound to them when compared to gasoline engines?
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The distinctive diesel rattle has to do with the difference in the way diesel engines ignite their fuel versus gasoline engines.
With a gas engine, fuel is sprayed into the cylinders along with air. The air and fuel is allowed to mix before it is squished by the piston as it moves upwards in the cylinder. When the piston reaches the top, a spark ignites the compressed air-fuel mixture.
This spark sets off something called a flame front. If you’ve ever lit a gas stove or grill, you might have noticed that the flame kind of “runs” down or around the burner as it lights. The gas is coming out all around the element, but it takes time for the flame to make its way around. It happens so slowly, you can actually watch it happen.
The same thing happens inside a gasoline engine’s combustion chamber. The spark sets off something called a “flame front”, that propagates smoothly throughout the air-fuel mixture. This (relatively) slow, even process creates a smooth expansion of the gasses inside the cylinder.
With a diesel engine, there is no spark plug, and the fuel is not added to the cylinder until the air has already been squished by the piston. The fuel is forced into the cylinder just before the piston reaches the top, and it all ignites simultaneously in an explosion. This happens much more quickly than the flame front propagation in a gasoline engine.
Because of the way sound saves work, this gives a diesel engine combustion event a much harsher clacking noise. In a gasoline engine, the flame front spreads the explosion out over a much longer period of time. We’re talking milliseconds here, but when it comes to sound frequencies, the difference between 300 ms and 3 ms makes a big difference in how something sounds.
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