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What is a Compression Ignition?

Compression ignition is the process where fuel ignites due to the high temperature created by compressing air in an engine's cylinder. This efficient method powers diesel engines, offering robust performance and fuel economy. Intrigued by how this technology shapes our world? Discover the mechanics behind compression ignition and its impact on modern transportation. What could this mean for the future of engines?
Paul Scott
Paul Scott

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.

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Woman posing

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.

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Discussion Comments


Many thanks for using wiseGEEK and for your query. Again a difficult question to give a simple answer to. Most engines are designed with peak performance criteria in mind. These generally balance power plant output with economy and engine life.

As I understand it, increasing ignition pressures in turbo blown mills can dramatically increase performance but can present problems with decreased engine life and may lead to engine or turbo damage. Apologies if this does not answer your question fully - hopefully one of the CI gurus could give you a better description. --Paul S.


What is the relationship between the pressure ration of a turbocharged engine and power out of that engine?


Many thanks for your query and sorry for not responding earlier.

If I understand your question correctly, you'd like to know what the cylinder pressure is directly after ignition. That's a tough one as it depends on a number of widely differing factors.

First, the engine's compression ratio will play a role. As this may be anything from 14:1 to 23:1 it would be impossible to generalize.

Fuel grades and engine types (low, medium or high speed types)also play a role in determining combustion pressure ratios. Under correction, I'd say a safe average is to work on double the injector pressure point, but I may be wrong.

Perhaps someone with a little more in depth CI knowledge could shed a little light on this one.

Again, thanks for the interest.


What is the PSI at ignition?

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