Spark-ignition engine - Components and control strategy

Engine control in the broadest sense of the term can be achieved by mechanical or hydraulic means, as was the case, for example, in the 1950s, with the widespread use of a device acting, by means of an expandable element, on the flow of coolant to regulate engine temperature so as to make it independent of operating conditions. Temperature control has improved engine behavior by better controlling heat exchange, improving combustion and reducing the duration of mixture enrichment required when the engine is cold. It has also extended engine life by better stabilizing mechanical clearances.

In this article, however, we'll be focusing on so-called digital control, as achieved by means of dedicated actuators, driven electrically by an electronic unit, according to information from sensors sensitive to engine operating parameters. Indeed, the great innovation of recent decades has been the emergence of digital processors that are compatible with the automotive environment in terms of robustness and cost, and fast enough to perform what we call "real-time" control. These processors are capable of calculating and refreshing the control of the various actuators in a sufficiently short time, given the speed of change in operating conditions and engine requirements.

For example, the value of ignition timing used to be determined mechanically, and its variation as a function of engine speed and load was obtained by means of centrifugal and pneumatic corrections. It is now the result of an algorithm based on a map search and a number of corrections. The performance of today's processors means that the algorithm in question can be executed before each ignition command.

This article will focus solely on the functional aspect of engine control, and more specifically on the spark-ignition engine.

The complete study of the subject includes the articles :

• BM 2 970 – Spark-ignition engine. Components and control strategy (this article) ;

• BM 2 971 – Spark-ignition engine. Control features.