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Compression ignition is an internal combustion process which relies on the heat generated from highly compressed air to ignite a fuel/air mixture. Unlike spark ignition systems, a compression ignition internal combustion engine does not rely on the arc from a spark plug to ignite the combustible mixture of air and fuel in its cylinders. This type of ignition system utilizes the extreme heat generated by compressing air to very high pressures to provide the ignition necessary to complete its combustion cycle. The fuels used in these systems are typically dense, oily petroleum-based products. Engines with this ignition system have several beneficial characteristics such as excellent fuel efficiency, better continuous high power output, and improved performance in damp environments.
Conventional gasoline internal combustion engines draw a mixture of fuel and air into their cylinders where it is compressed and ignited by an electric arc from a spark plug. These engines generally feature pre-ignition pressures of approximately 8 to 14 bar (200 psi). Compression ignition engines feature far higher compression ratios which produce pre-ignition pressures of as high as 40 bar (580 psi). Air becomes hot when temperatures in these engines at the point of ignition are typically in the region of 1,022° Fahrenheit (550° Celsius). It is these high air temperatures which provide the ignition required to complete the combustion cycle.
The fuel used in the compression ignition process is not pre-mixed with air prior to introduction into the cylinder as is the case with spark ignition systems. Air alone is drawn into the cylinder at the start of the compression stroke, and the fuel introduced only at the top of the stroke. At this stage, the air in the cylinder has been heated by compression to a point where the atomized fuel becomes vaporized and ignites, pushing the piston down and driving the crankshaft in the process. The fuel is introduced into the cylinder by an injector system which sprays it evenly into the hot air under pressure. This spray is designed to produce droplets of a size promoting even vaporization and efficient ignition.
Compression ignition fuels are generally oily petroleum products with a higher density than gasoline. The most common of these is diesel although this type of engine can run on a variety of fuels distilled from crude oil or even alcohol and natural gas. Considerable attention has also been given in recent years to the production of compression fuels from vegetable oil products such as soya bean and coconut extracts. Also known as biodiesels, these fuels generally require some modification to existing engines although some recently developed types may be used as a direct substitute in regular diesel engines.
Engines based on compressive ignition systems feature several distinct advantages over their spark ignition siblings. Fuel efficiency is certainly one of the most important; diesel engines produce excellent fuel consumption figures. Diesels are also far more efficient at lower revolutions especially at idle speeds. Compression ignition engines are also far less prone to fail in damp environments from the lack of the high tension electrical system needed on gasoline engines. Diesel engines also generally feature superior constant power output figures and typically last longer than other types.