Numerical Simulation Of Effects Of Combustion Chamber Geometry And Exhaust Gas Recirculation On Combustion And Emissions Of Diesel Engines

The soot and NOxemissions from the diesel engines have adverse impact onhuman health and the environment. In the traditional combustion mode, there exists atrade-off relationship between the soot and NOxemissions, therefore simultaneouslyreducing the soot and NOxemissions is the research hotspot at home and abroad. Thecombustion chamber geometry of a diesel engine is believed the key factor to themixture formation, and the good mixture is not only beneficial to improve thecombustion efficiency, but also can reduce the pollutant emissions of diesel engines.The effects of combustion chamber geometry on combustion and emissions of dieselengines are researched in this paper by the numerical simulation method.The low temperature combustion (LTC) has the potential of reducing the dieselengine emissions. With the increase of EGR rate, the soot emission increases firstlyand then decreases which forms the soot-bump. It is important to reduce the peakvalue of soot-bump and the corresponding EGR rate of the peak value of soot-bumpin order to improve the thermal efficiency and reduce the emissions of diesel engines.The effects of injection timing and swirl ratio as well as injection pressure on the peakvalue and the corresponding EGR rate of soot-bump under the condition of EGR arestudied by the numerical simulation method. The effects of injection timing and swirlratio as well as injection pressure on the emissions of different combustion modes arealso analyzed.Through the above research, the major results are listed as follows.The peak pressure of the new designed Bowl-4is the highest and the peakpressure of Bowl-4is the closest to the top dead center. When the EGR rates are20%and35%, the average combustion temperature of the prototype combustion chambergeometry is the highest and the NOxemissions are the largest. The effect ofcombustion chamber geometry on the in-cylinder maximum CO content is negligible,but the effect of combustion chamber geometry on the CO oxidation rate is obvious.The effect of combustion chamber geometry on NOxand soot changes with theincrease of EGR rate. From the comprehensive consideration of combustion andemissions, adopting the Bowl-4combustion chamber could improve the performanceand emissions of the diesel engine.With the advance of injection timing, the peak value of soot-bump decreases and the corresponding EGR rate of the peak value increases. The soot emission decreaseswith the advance of injection timing in the traditional combustion mode, while thesoot emission increases with the advance of injection timing in the low temperaturecombustion mode when the EGR rate exceeds60%. With the increase of swirl ratioand injection pressure, the soot curve becomes flat, the peak value of soot-bumpdecreases and the corresponding EGR rate of the peak value increases. It is necessaryto introduce more exhaust gas to obtain the low temperature combustion at high swirlratios and high injection pressures. In the low temperature combustion mode, theeffect of injection timing, swirl ratio and injection pressure on NOxis negligible andthe NOxemission almost approaches zero. As for CO emission, there exists an optimalswirl ratio to reduce the CO emission in maximum. At the same swirl ratio andinjection pressure, the corresponding EGR rate of soot-bump peak value is almost thesame as the EGR rate when the CO emission increases drastically.