How about I just post a Diesel 101 write-up? Though most of what I will say is on here already (and accessible from the search feature), it's not all in one place.
Diesel engines differ from gas engines in a number of ways that mostly relate to two key differences: the fuel used, and the way it is ignited.
Burning diesel instead of gasoline allows a much higher compression ratio (it also requires a much higher compression ratio, as we'll see). This makes for a more efficient engine. While a gas engine can only run as high a compression ratio as the fuel octane will allow, there is no theoretical limit to the compression ratio a diesel can have.
Diesel fuel is also a more "efficient" fuel in the sense that a gallon of diesel contains more BTUs (potential heat energy) than does a gallon of gasoline. It's the difference between a pound of wood and a pound of coal-- which will give more total heat when burned?
Diesel engines ignite the fuel with no spark. Thus, no spark plugs or distributor and associated wiring.
Thermodynamics tells us that when we compress a gas, it gets hotter. You've seen this effect when you feel a spray can get cold as the compressed gas expands-- same principle, opposite case.
So the diesel compressed the air so much that it's hot enough to burn the fuel when it's injected. Since there's no fuel before injection, there's no risk of detonation ("spark knock"). But this requires a certain amount of compression to get the air hot enough-- thus, a diesel both allows and requires a higher compression ratio.
A gas engine is regulated by controlling the amount of air it's allowed to ingest. It takes in air and then mixes fuel with it at a constant ratio. So gas engines have a "throttle" that limits air intake and increases pumping losses.
Diesels have no "throttle"-- they always take in as much air as they can. They are regulated by HOW MUCH fuel is injected into the cylinder. Fuel and air are mixed in a varying ratio, so a diesel at light loads can run VERY lean. At idle, they can run as lean as 100:1 (air:fuel ratio), while a gas engine MUST operate around 13-14:1. So a diesel has a lot more flexibility as far as fuel mixing.
Diesels are stronger engines because they HAVE to be. They operate under conditions that would break or melt a lot of gas engines. Ever seen a gas engine with 24:1 compression? The ignition of diesel under pressure is much more violent than gas, because not only is it under more pressure, but it's releasing more energy.
Any more questions, please fee free to post or PM me.
Justin
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