Research On Combustion Characteristics In A High Compression Ratio Diesel Engine Under Late Injection Conditions

The new combustion concept of diesel engine can be a good way to deal with the trade-off relations between Nitrogen Oxides(NOx) and Soot emissions in traditional combustion. It is clean and efficient, which plays a great help on alleviating the serious situation of the current energy crisis and environmental pollution. New concept combustion with late injection has attracted a widespread attention because of its advantages in load expanding and combustion phase controlling. Compression ratio(CR) is one of the most important parameters of diesel engine, which has great influence on the performance of diesel engine. Research has shown that it is good for the power performance, fuel economy, cold start of diesel engine to increase the CR appropriately. The combustion and emission characteristics of a high CR diesel engine was studied in this thesis with the simulation method.In this thesis, a three-dimensional numerical simulation model was established with KIVA-3V code based on a high compression ratio six-cylinder common-rail diesel engine. The model was verified by the experimental results from different aspects, and they agreed well with each other. A set of effective methods were introduced and adopted to calculate the initial boundary conditions. The effects of injection timing, EGR levels and injection strategies on diesel combustion characteristics were studied under high pressure and late injection conditions. The results showed that:Relative to the TDC, advancing or delaying the injection timing increased the ignition delay, which resulted in more uniform distribution of fuel, IMEP decreased. High CR diesel engine was relative insensitive to the variation of injection timing compared to low CR diesel engine. For 40% load condition, advancing or delaying the injection timing were both help to realize the premixed combustion, cylinder temperature rose, equivalence ratio decreased, so Soot emission declined, but NOx emission went up. For 40% load condition, cylinder temperature and NOx emission both went down as the injection timing delayed. But Soot emission increased. In addition, the engine would miss fire when injection timing delayed to 8oCA ATDC.As EGR rates increased, the cylinder temperature at injection timing declined, the ignition delay increased, which was helpful to form a more homogeneous mixture, the equivalence ratio at injection timing decreased. The ignition delay period of high CR engine almost increased linearly compared to low CR engine. The cylinder pressure, IMEP, highest cylinder temperature and total heat release all decreased as EGR rates increased, NOx emission went down, but Soot emission rose.As times of injection increased, ignition delay period decreased, the equivalence ratio rose and distributed more concentrated. The changes of cylinder pressure, cylinder temperature, heat release rate flatten out with the increase of injection times, resulted in the improvement of engine's steadiness and reliability. The NOx emission curves show that it can reduce NOx emission to advance injection timing, but it's harmful to advance too much; Soot emission increased first and then slightly drop as injection times increased.