What is the difference between a gasoline engine and a diesel engine?
In a diesel engine there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine gets better mileage. The excellent web site on How Stuff Works goes into the details of both types of engines with super animated drawings that are much better than my trying to describe the engines operation in words.
