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.
- - - - -
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.)
