Experiment and analysis on path optimization of low-temperature premixed combustion for light-duty vehicle diesel engine

Abstract: The combustion process of diesel engines has a significant effect on the power, economy, and emission performance of a vehicle. As internal combustion engines tend to be highly effective, energy-saving, and environmentally friendly, all the scholars of the world in this area concentrate on creatively developing the combustion theories and techniques for the oncoming generation of engines in order to break through the emission limits of conventional diesel engines. Aiming at the difficulties of forming the homogeneous charge and organizing the combustion progress for the compact structure of light-duty vehicle diesel engines, the research in this paper was based upon advanced injection combined with high proportion cooling EGR to achieve the low-temperature premixed combustion. Taking advantage of a pressure information acquisition and performance bench test, the effects of fuel injection strategies and intake system parameters on the low-temperature premixed combustion process and pollutants emissions were studied systematically, which explores the effective ways for forming the homogeneous charge and controlling the low-temperature combustion progress so that the theoretical principle of low-temperature premixed combustion techniques for engineering application was built. Based on an in-cylinder pressure information acquisition and performance bench test, the effects of combustion control parameters such as injection timing, injection pressure, intake oxygen concentration, intake temperature, and pilot-injection on a low-temperature premixed combustion process and emission performance were studied. This revealed that taking the advanced injection strategy can prolong the ignition delay period, and improving the fuel-air mixture is the key to reducing the soot emission. Diminishing the intake oxygen concentration can control the temperature of premixed combustion effectively, restrain the production of the NOx emission, and put the ignition phase of a premixed combustion which is too early off, which is beneficial to improving the combustion efficiency and enhancing the fuel economy. Increasing the fuel injection pressure makes the NOx and soot emissions reduced in the low-temperature combustion mode when the load ratios are 10% and 25%, while it brings about the increment of the NOx emission and the decrement of the fuel efficiency in a moderate duty. In the low-temperature combustion mode, the combustion efficiency can be dramatically improved by decreasing the intake temperature and putting off the ignition phase, the in-cylinder temperature can be decreased, and the production of NOx is restrained at the same time. The soot emission can be reduced by raising the intake temperature appropriately when the load ratio is 10%, while intake cooling plays an important role in cutting the soot emission down at the load ratios of 25% and 50%.Multi-injection strategy is an effective method to extend the operating scope of low-temperature combustion and ameliorate the trade-off relationship between the NOx and soot emissions. Pilot injection can reduce the main injection amount, and shorten the ignition delay period of the main injection fuel. As a result, the exothermic phase advances and the peak value of the heat release rate descend so that the soot is successfully restrained. What's more, different injection parameters have different effects on the soot emission. Compared with the original engine, the NOx emissions decreased by 97.8%, 80.7% and 62.1% respectively when the load rates were 10%, 25%, and 50%, and the soot emissions decreased by 76%, 93.9%, and 47.1%, while the brake specific fuel consumptions increased slightly.