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, auto-ignition 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 NO_x 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.

A multi-pulse injection strategy for premixed charge compression ignition (PCCI) combustion was investigated in a four-valve, direct-injection diesel engine by a computational fluid dynamics (CFD) simulation using KIVA-3V code coupled with detailed chemistry. The effects of fuel splitting proportion, injection timing, spray angles, and injection velocity were examined. The mixing process and formation of soot and nitrogen oxide (NO_x) emissions were investigated as the focus of the research. The results show that the fuel splitting proportion and the injection timing impacted the combustion and emissions significantly due to the considerable changes of the mixing process and fuel distribution in the cylinder. While the spray, inclusion angle and injection velocity at the injector exit, can be adjusted to improve mixing, combustion and emissions, appropriate injection timing and fuel splitting proportion must be jointly considered for optimum combustion performance.

While variable valve actuation or variable valve lift (VVL) is used increasingly in spark ignition (SI) engines to improve the volumetric efficiency or to reduce the pumping losses, it is necessary to understand the impact of variable valve lift and timing on the in-cylinder gas motions and mixing processes. In this paper, the in-cylinder flow characteristics for reduced maximum valve lifts (MVLs) were examined in a modified four-valve optical SI test engine in which the combustion system is suitable for gasoline direct-injection (GDI) operations. Three different MVLs of 6.8 mm, 4.0 mm, and 1.7 mm were tested and particle image velocimetry was employed for the measurements. It is expected that the investigation will be helpful in understanding and improving GDI combustion when a VVL system is used. The results showed that a reduced MVL could significantly enhance the in-cylinder swirl motion in all three measured horizontal planes. By comparing the swirl ratios for different MVLs, it can be found that a reduced MLV can considerably increase the swirl ratio during the intake and compression processes. In general, the swirl centre induced by a lower MVL is more stable and can stay closer to the cylinder centre. In addition, the reduction in the MVL can also increase both the high-frequency fluctuating kinetic energy and the low-frequency fluctuating kinetic energy. These may contribute to an improvement in the air-fuel mixing but also to an increase in the cycle-to-cycle variation.