Because diesel and gasoline have significantly different properties, the engines designed to burn the fuels have to be different as well. Gasoline is extremely volatile. It releases fumes easily, and those fumes are easily ignited by a spark. The addition of the spark makes gasoline engines operate at a higher speed and can start much more easily than a comparable diesel engine. Diesel, on the other hand, is heavier and more stable. Because of this stability, it does not take to a spark too easily. The engines don't use a spark, just compression. This makes the engine slower, and harder to start manually. (If it's too cold, for example, a diesel won't start at all without an additional input of heat - that's what glow plugs are for.) Also, diesels have a phenomenon named (appropriately) dieseling, which is a condition where the engine does not stop when you want it to, only when it runs out of fuel or oxygen, which is potentially dangerous (and quite annoying). While most automobiles, for example, have ways of reducing the risk of dieseling, it still is possible for it to happen. Each engine has its uses. Gas engines are faster and more responsive, diesels are more efficient and provide higher torques.