Study On Spray & Combustion Characteristics Of LPG Compression-Ignition Engine

The Liquefied-Petroleum Gas(LPG), which is of the better property of low exhaust emissions and rapid evaporation, is used extensively as an important substitute for the diesel and gasoline in the engines. However, the LPG is combusted in the chamber by premixing with the air in outside of the engine cylinder today, which leads to the low charge- efficiency and high exhaust emissions and can't better use of the LPG's capability of low exhaust emissions. In order to avoid these shortcomings, fueling the engine with the LPG injected directly into the cylinder will be the future aim of the LPG engine development. After studying most of the articles on the use of the LPG in the engine this paper researches the property on the spray and the combustion of the LPG injected directly into the cylinder in theory and experiments.Through spraying into the constant volume chamber with LPG or diesel or some kinds of mixtures of LPG and diesel, the effects of spray parameters and mixture ratio for LPG and diesel on the property of the spray were investigated and the characteristics of the sprays of all kinds of fuels were compared each other. The experiment shows that the penetration distance of the LPG is the shortest among the fuels and the velocity of the evaporation is the fast at the same spray parameters. It also finds that the higher the ratio of LPG in the mixture fuel, the better the spray property of the fuel. In addition of the experiment, this paper also performs the numeric simulation of the spray process for the LPG and diesel in the constant volume chamber with KIVA-3V program and analyzes the spray field in detail. Through comparison with the experiment, it finds that they are consistent with each other.Based on the detailed comparisons and analysis to many kinds of spray impingement models, two representative spray impingement models, wall spread and splashing, are introduced in this thesis. In the model, the exchange of heat and momentum between the wall, droplets and the gas above liquid film are modeled. The spray impingement of LPG is simulated and compared with that of diesel. Due to LPG quickly evaporating, its film thickness is much smaller than the one of diesel, which shows that the accumulation of LPG on the wall is not serious to be of advantage to combustion process.In order to perform the combustion experiment with the various mixture ratio of LPG and diesel injected directly into the cylinder, this paper designs a fuel supplement system, which can premix the LPG and diesel in the mixture corrector randomly. By use of the AT89C52 SCM, the system can control the open times of the electromagnetic valve through controlling the width of the electrical current pulse to determine the flux of the LPG and diesel and mix the LPG and diesel with predetermined ratio, which can be changed randomly.In this thesis, we study the combustion of the mixture of LPG and diesel in the ZS195 diesel engine by injecting it directly into the cylinder. It shows that with the suitable ratio of LPG, the maximum combustion pressure of the mixture and the exhaust emission of CO are higher but the HC and NOX are lower than that of the diesel. Through the experiment, it can be found that the mixture fuel consumption and the exhaust temperature are lower but the combustion noise is higher than that of the diesel and also can find that when the LPG ratio is too high, the combustion can't occur.As a new manner of combustion, Homogeneous Charge-Compression Ignition(HCCI), is regarded as an important technology advancement of engine because of its great potentials of reducing the fuel consumption and the exhaust emissions. In this thesis, the homogeneous charge compression ignition of LPG in ZS195 by inducing the LPG into the cylinder through the intake pipes and injecting the DME into the cylinder as an ignition additive was conducted. Through the experiment, it can be found that the engine can run over a wide range of load and when the LPG is too high in the cylinder, the combustion can't occur steadily and the exhaust emissions and noise would increase. In the experiment, it can also be found that the emissions of the HC, CO, NOx, HCHO and CH3OCHO would increase when the LPG concentration became high in the cylinder and all the emissions would decrease as the load became heavy.