Burning techniques are totally different when comparing Diesel and Gasoline. In Gasoline engines, air and fuel mixture is injected into the cylinder (in a non direct fuel injection system) this mixture is then compressed and thus temperature of the mixture increases, but not to the ignition temperature, this is why we need a spark plug in gasoline engines to start this small fire. After reaching the top dead level of the cylinder (or sometimes even before that ) spark plug releases its charge and start a very small explosion inside the cylinder head. This small fire ball propagates as far as there is enough fuel and oxygen. While this explosion propagates, the piston is being pushed down and turning the flywheel through the crankshaft. As you can see, you don't need to compress this air and fuel mixture till ignition temperature and you don't want to do it!!! This is why it is possible to design gasoline engines with short strokes. Short strokes means short reciprocating movements, which means faster rotational speed.

In diesel engines, there is no spark plug, because ( and I'm not 100% sure about that ) Diesel is not a good fuel in propagating a controlled fireball. It just doesn't work very well in diesel. Because of that, Only air is being compressed inside a diesel engine during the compression cycle. Somewhere during compression stage, diesel is being injected inside the cylinder and starts to spread homogenously inside the cylinder, compression continues and the mixture is mixed and heated up till it reaches ignition temperature. Once ignition temperature is reached, the whole mixture explodes at the same moment. As you can see, there is no flame propagation in diesel engines, the whole mixture just explodes in a single moment. And in order to reach ignition temperature you need a long enough stroke to be able to compress air till it reaches ignition temperature of the fuel. Longer stroke means slower cycle, but higher torque. This is why Diesel engines have more torque but less speed than their gasoline counterparts.

Hope this answers your question.

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Okay...

The Diesel engine (named after Rudolf Diesel, who invented it) was designed to burn any flammable liquid you put in it--Herr Diesel originally intended to run it on vegetable oil. OTOH, a spark ignition engine requires fuel with a minimum amount of volatility, so gasoline has to be a very highly refined product.

The Diesel cycle compresses air inside the cylinder until it's hot enough to ignite the fuel. At that point, the injection system squirts a premeasured dose of fuel into a combustion chamber in the cylinder head. It ignites instantly. Because the flame front is confined to the little chamber the fuel's being injected into, there has to be a LOT of excess air in there. To get it, the engine is "oversquare"--the stroke is longer than the bore. (A Detroit Diesel DD15 is 5.47" bore, 6.43" stroke. It's an inline 6 engine that displaces 14.7 liters.

Now here's where it gets fun: The DD15 comes in eight different horsepower ratings, and the only difference between the eight engines is the fuel injector system--the more power your engine has, the more fuel the injector holds. (There's also a different program in the computer, but that's another issue.) Before the EPA got all antsy about diesel emissions you could take your truck to Pittsburgh Power, have them install higher-flow injectors and have a much more powerful truck for relatively little money. (You can't do that now; the current electronic fuel injection systems on big rigs keep you from doing that.)

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