Physics expert witness Louis A. Bloomfield, Professor of Physics at the University of Virginia, answers the question “What is the difference between spark ignition engines and diesel engines?”
Just before burning their fuels, both engines compress air inside a sealed cylinder. This compression process adds energy to the air and causes its temperature to skyrocket. In a spark ignition engine, the air that’s being compressed already contains fuel so this rising temperature is a potential problem. If the fuel and air ignite spontaneously, the engine will “knock” and won’t operate at maximum efficiency. The fuel and air mixture is expected to wait until it’s ignited at the proper instant by the spark plug. That’s why gasoline is formulated to resist ignition below a certain temperature. The higher the “octane” of the gasoline, the higher its certified ignition temperature. Virtually all modern cars operate properly with regular gasoline. Nonetheless, people frequently put high-octane (high-test or premium) gasoline in their cars under the mistaken impression that their cars will be better for it. If your car doesn’t knock significantly with regular gasoline, use regular gasoline.
A diesel engine doesn’t have spark ignition. Instead, it uses the high temperature caused by extreme compression to ignite its fuel. It compresses pure air to high temperature and pressure, and then injects fuel into this air. Timed to arrive at the proper instant, the fuel bursts into flames and burns quickly in the superheated compressed air. In contrast to gasoline, diesel fuel is formulated to ignite easily as soon as it enters hot air.
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