The diesel engine is a lot like the gasoline engine. Both use the four stroke system of intake, compression, ignition and exhaust. Both are piston type engines. The difference lies in the method of igniting the fuel mixture. Instead of using spark plugs, ignition of the fuel is accomplished by the heat of highly compressed air. In a diesel engine, only air enters through the intake valve. Compressing the air causes it to heat up and at the point of highest compression, fuel is sprayed into the cylinder. The two coming in contact with each other causes the ignition without the use of a spark plug. The high air temperature is achieved by a much higher compression ratio. The diesel engine compresses it's air to one twentieth it's original volume compared to a gasoline engine's one sixth compression ratio. Instead of a spark plug to ignite the fuel mixture, a diesel engine uses glow plugs which heat the cylinder when the engine is cold.
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