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.
Keywords: HCCI combustion, combustion characterization, hydrocarbons, CO,
emissions, HCCI implementation, gasoline HCCI, external EGR, exhaust gas
retention, exhaust gas rebreathing, diesel HCCI, port injection, early injection,
multiple injection, auto-ignition.
