Compression ignition engines use highly compressed air to ignite fuel, without relying on a spark plug. They use dense, oily petroleum-based fuels and have higher compression ratios, producing better fuel efficiency and performance in wet environments. Diesel is the most common fuel, but biodiesel is also being developed. These engines are more efficient at low revs and have higher constant power output, making them longer-lasting than other types.
Compression ignition is an internal combustion process that relies on the heat generated by 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 of a spark plug to ignite the combustible fuel-air mixture in its cylinders. This type of ignition system uses the extreme heat generated by compressing air to very high pressures to provide the ignition needed 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 features, such as excellent fuel efficiency, better continuous high-power output, and improved performance in wet 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 typically feature preignition pressures of approximately 8 to 14 bar (200 psi). Compression ignition engines feature much higher compression ratios producing preignition pressures up to 40 bar (580 psi). The air gets hot when the 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 that provide the ignition necessary 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 is only drawn into the cylinder at the beginning of the compression stroke, and fuel is drawn in only at the top of the stroke. By this stage, the air in the cylinder has been heated by compression to a point where the atomized fuel vaporizes and ignites, pushing the piston down and driving the crankshaft in the process. The fuel is introduced into the cylinder by an injection system that sprays it evenly into hot pressurized air. This aerosol is designed to produce droplets of a size that promotes uniform 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—distillates from crude oil or even alcohol and natural gas. Considerable attention has also been paid in recent years to the production of compression fuels from vegetable oil products such as soybean and coconut extracts. Also known as biodiesel, these fuels generally require some modification to existing engines, although some recently developed types can be used as a direct replacement in regular diesel engines.
Engines based on compression ignition systems present several distinct advantages over their spark ignition brethren. Fuel efficiency is undoubtedly one of the most important; Diesel engines produce excellent fuel consumption figures. Diesel engines are also much more efficient at low revs, especially at idle speeds. Compression ignition engines are also much less prone to failure in wet environments due to the lack of the high voltage electrical system necessary in gasoline engines. Diesel engines also generally feature higher constant power output figures and generally last longer than other types.
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