Spark-ignition engine

The spark-ignition (SI) engine is one of the leading members of the large family of reciprocating internal combustion engines, i.e. engines that produce work by the direct action on a piston of the pressure arising from the ignition of a combustible mixture, with the reciprocating motion transformed into rotation via a connecting rod-crank system. The term "spark ignition" derives from the fact that ignition is initiated by a spark, generally of electrical origin, at a specific point in the cycle. A homogeneous mixture of air and fuel vapour is essential for good ignition, and this calls for fuels with good volatility, such as petroleum spirits, hence the more familiar name of "gasoline engine". Its ubiquitous use in road traction, and particularly in automobiles, is the reason for its great popularity; few other propulsion systems are currently able to compete with it in this application.