Characteristics of homogeneous charge compression ignition (HCCI) combustion and emissions of n-heptane

This paper reports the outcome from a systematic investigation carried out on HCCI ( Homogeneous Charge Compression Ignition) combustion of a diesel type fuel. The n-heptane was chosen in this study to study the HCCI combustion characteristics of diesel engines with premixed charge by port fuel injection. Measurements were carried out in a single-cylinder, 4-stroke and variable compression ratio engine. Premixed n-heptane/air/EGR mixture was introduced into the cylinder by a port fuel injector and an external EGR system. The operating regions with regard to Air/Fuel ratio ( A/F) and EGR rate were established for different compression ratios and intake temperatures. The effects of compression ratios, intake temperatures, A/F and EGR rates on knock limit, autoignition timing, combustion rate, IMEP and engine-out emissions, such as NO(x), CO, and unburned HC, were analysed. The results have shown HCCI combustion of n-heptane could be implemented without intake charge heating with a typical diesel engine compression ratio. The attainable HCCI operating region was mainly limited by the knock limit, misfire, and low IMEP respectively. Higher intake temperature or compression ratio could extend the misfire limit of the HCCI operation at low load area but they would reduce the maximum IMEP limit at higher load conditions. Compared with conventional diesel combustion, HCCI combustion with diesel type fuels would lead to extremely low NOx emissions ( less than 5 ppm) and smoke free exhaust, but would produce higher HC and CO emissions. An increase in intake temperature or compression ratio helped to reduce HC and CO emissions.