The homogeneous charge compression ignition-HCCI (also to be known as controlled auto ignition-CAI) engine concept has the potential to be highly efficient and to produce low NOx, carbon dioxide and particulate matter emissions. It experiences, however, problems with cold start, running at idle and high loads that together with controlling the combustion over the entire speed/load range limits its practical application. A solution to overcome these problems is to operate the engine in ‘hybrid mode’, where the engine operates in HCCI mode at low, medium and cruising loads and switch to spark ignition (SI) mode (or diesel mode-CI) at a cold start, idle and higher loads.
To operate such ‘hybrid mode’ engine, a transition between SI and HCCI and SI modes, as a result of changes in engine speed and load must be seamless in operation, whilst keeping all relevant engine and combustion parameters in an acceptable range.
A successful transition strategy has to compensate changes in the fuelling rate (and hence in the charge composition) that occurs with different loads, and changes in the time available for the gas exchange process (as a consequence of different) at different engine speeds. The problem is even more exacerbated with the fact that HCCI combustion is controlled by the charge temperature, composition and pressure and therefore the use of a direct control mechanism such as in the SI combustion is not applicable.
The exhaust gas recirculation (EGR), obtained by trapping residual gases inside the cylinder appears to have the potential to enable fast and smooth transition from SI to HCCI to SI, and to provide, in the certain range, control for the HCCI combustion.
The aim of this paper is to present the development and investigation of using TRG, obtained by the Fully Variable Valve Timing (FVVT) system, for a transition from SI to HCCI to SI mode at different engine speeds and loads. The experiment is performed on a single cylinder research camless engine (equipped with the FVVT system) fuelled with commercially available gasoline fuel (95 ON). The results obtained show that different parameters such as the valve profile, throttle position, fuelling rate and spark timing have the major influence on the quality of transition and subsequent operations in HCCI and SI mode. The results also indicate that at different engine operating points (speed and load) different controlling parameters have to be adjusted accordingly.