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Exhaust Gas Recirculation (EGR) is an important parameter for control of diesel engine combustion, especially to achieve ultra low NOx emissions. In this paper, the effects of EGR on engine emissions and engine efficiency have been investigated in a heavy-duty diesel engine while changing combustion control parameters, such as injection pressure, injection timing, boost, compression ratio, oxygenated fuel, etc. The engine was operated at 1400 rpm for a cycle fuel rate of 50mg. The results show that NOx emissions strongly depend on the EGR rate. The effects of conventional combustion parameters, such as injection pressure, injection timing, and boost, on NOx emissions become small as the EGR rate is increased. Soot emissions depend strongly on the ignition delay and EGR rate (oxygen concentration). Soot emissions can be reduced by decreasing the compression ratio, increasing the injection pressure, or burning oxygenated fuel.

The effects of cooled EGR on combustion and emission characteristics in HCCI operation region was investigated on a single-cylinder diesel engine, which is fitted with port injection of DME and methanol. The results indicate that EGR rate can enlarge controlled HCCI operating region, but it has little effect on the maximum load of HCCI engine fuelled with DME/methanol dual-fuels. With the increase of EGR rate, the main combustion ignition timing (MCIT) delays, the main combustion duration (MCD) prolongs, and the peak cylinder pressure and the peak rate of heat release decreases. Compared with EGR, DME percentage has an opposite effect on HCCI combustion characteristics. The increase of indicated thermal efficiency is a combined effect of EGR rate and DME percentage. Both HC and CO emissions ascend with EGR rate increasing, and decrease with DME percentage increasing. In normal combustion, NOX emissions are near zero.

By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a PRF fuel with octane rating between 0 and 100. The influence of PRF fuel’s octane number on the combustion characteristics, performance and emissions character of homogeneous charge compression ignition (HCCI) engine was investigated. The experiments were carried out in a single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delayed, the combustion rate decreased, and the cylinder pressure decreased. The HCCI combustion can be controlled and then extending the HCCI operating range by burning different octane number fuel at different engine mode, which engine burns low octane number fuel at low load mode and large octane number fuel at large load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load.