Investigation of the combustion and emission characteristics of partially premixed compression ignition in a heavy-duty diesel engine

Partially premixed compression ignition in a diesel engine is a combustion mode between diffusion combustion and homogeneous charge compression ignition combustion, the combustion controllability and the emission performances of which are close to those of a homogeneous charge compression ignition combustion engine; the mixing period plays a key role in realizing partially premixed compression ignition, and a long premixed process before combustion is necessary to make the fuel and air mix well. Partially premixed compression ignition combustion in a boosted six-cylinder heavy-duty diesel engine is realized by adjusting the injection timing and the rate of exhaust gas recirculation based on a single injection. The effects of the injection timing, the exhaust gas recirculation rate and the load rate on the combustion and the heat release pattern are studied. The factors which influence the mixing period and the ignition delay and its regularity are researched. Endoscope technology and the two-colour method are also used to gain an insight into partially premixed compression ignition combustion. The results show that both early injection and late injection have a long mixing period, which helps to form a more homogeneous mixture, and no diffusion combustion is found in the heat release rate curves. Premixed combustion and low-temperature combustion are the key factors in reducing the particulate matter emissions and nitrogen oxide emissions simultaneously. However, the low-temperature combustion and the dilute mixture may lead to incomplete combustion; consequently, for relatively late-injection conditions, the hydrocarbon and carbon monoxide emissions increase dramatically and the fuel consumption becomes worse. In these partially premixed combustion patterns, the effect of the injection pressure on the particulate matter and nitrogen oxide emissions is not clearly observable. At lower load rates, partially premixed compression ignition combustion displays a low-temperature and a high-temperature two-stage heat release. When the engine load rate is increased to 50%, diffusion combustion appears in the early-injection modes, which leads to higher nitrogen oxide and particulate matter emissions.