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Understanding the difference between diesel and gasoline engines begins with understanding the combustion process the two engines use. Both are internal combustion engines that use pistons to create mechanical energy. However, what happens inside to move those pistons is key to the differences in operation and efficiency between diesel and gasoline engines.
In gasoline engines, fuel is mixed with air, then compressed by a piston, where it is then ignited with a spark from a spark plug. The burning fuel releases energy used to push the piston down, which turns the crankshaft of the engine.
Diesel engines use a slightly different process to produce mechanical energy. As the piston rises in the cylinder, air in the cylinder is compressed. As the pressure builds up, the temperature of the air in the cylinder rises dramatically. Combustion occurs when, near the top of the compression stroke, diesel fuel is sprayed through an injector nozzle into the charge of hot air in the cylinder. The fuel is instantly ignited by the hot air and the combustion gases force the piston back down the cylinder, causing the crankshaft of the engine to turn. No spark is needed because diesel fuel autocombusts in the superheated air of the cylinder under compression. The lack of a spark represents the major difference between diesel and gasoline engines.
For those interested in fuel mileage, there can be a big difference between diesel and gasoline engines. When trying to make a decision between diesel and gasoline engines, buyers should consider the fuel efficiency that can be realized, but there are other factors to look at as well. Diesel engines use a fuel that is not as clean as gasoline. Therefore, it is necessary to provide more maintenance to the engine. However, if taken care of, diesel engines generally last much longer, in part because diesel fuel is not as corrosive as gasoline. It is not uncommon for diesel engines to last anywhere from 200,000 to 500,000 miles (322,000 to 800,000 km) in non-commercial vehicles. Commercial vehicles, many times, get at least 500,000 miles (800,000 km), but it is also common for diesel engines to keep working well beyond that point. This durability can be a huge factor when choosing between these engines.