Homogeneous Charge Compression Ignition (HCCI) Combustion

Abstract

Homogeneous charge compression ignition (HCCI) combustion is a distinct combustion concept, which can be implemented in internal combustion engines. Its development began thirty years ago and is still the focus of many researchers worldwide. The main features which attract attention to HCCI engines are of both environmental and energysaving character. Due to the premixed nature of HCCI combustion and the relatively lean mixtures used, NOx and soot emissions are but a fraction of the ones emitted by conventional spark ignition (SI) or compression ignition (CI) engines. Moreover, the relatively rapid combustion process and the unthrottled operation provide the potential for high thermal efficiency. Apart from these favorable attributes of HCCI combustion, significant issues have to be resolved. These issues are related to the high unburned hydrocarbons and carbon monoxide emissions, which are emitted during HCCI operation. Moreover, technical issues have arisen regarding the implementation of the HCCI combustion concept to actual engines. The latter is related to difficulties in controlling the ignition timing and the combustion rate over a wide load-engine speed range. The ignition timing must be adequately controlled if the thermal efficiency is to be kept high; the combustion rate control is of importance, since the high combustion rates encountered in HCCI combustion increase the peak combustion pressures and the pressure rise rates, thereby limiting the maximum attainable load. The present chapter presents the main features of HCCI combustion, namely its characterization based on experimental data, the pollutant emissions formation processes, the effect of major operating parameters on HCCI combustion and the various strategies used for the realization of the HCCI combustion concept to gasoline or diesel HCCI engines.