| With the intensified energy crisis and increasingly stringent emission regulations, natural gas engines are increasingly important. Diesel engine fueled with pilot-ignited directly-injected natural gas adopts high-pressure direct injection technology, under the premise of ensuring engine dynamics and economy, to keep NO emissions by 45%, CO2 emissions by 25%, PM significantly decreased, and HC emissions far lower than diesel engines.In diesel engine fueled with pilot-ignited directly-injected natural gas, fuel injection and mixture formation, etc. play an important role in the combustion process of the cylinder. Therefore, the study of the impact of injection timing and injection pressure of nature gas on the combustion process is necessary to reveal the theory of combustion of diesel engine fueled with pilot-ignited directly-injected natural gas.In this paper, STAR-CD software is used to do the simulation calculation on cylinder injection and combustion process of diesel engine fueled with pilot-ignited directly-injected natural gas. Main items are as follow:1 Geometric modeling for diesel engine fueled with pilot-ignited directly-injected natural gas by using three-dimensional CAD software Pro/E.2 Mesh of geometric model by using CFD pre-processing software ICEM-CFD.3 Physical modeling and doing simulation calculation by using three-dimensional CFD simulation software STAR-CD.4 Verify the validity of the model by comparing the experimental data with foreign countries.5 Study the influencing law of injection timing and injection pressure of nature gas on the combustion process in order to provide a theoretical basis for engineering application and development. The conclusions of this simulation study are as follows:1 If the injection timing for pilot diesel is 10BTDC, the injection duration is 1°CA, and the fuel delivery per cycle is 2mg, the injection timing for natural gas is 5BTDC, as the natural gas starts to inject, the pilot diesel has already burnt about 94.5%and the combustion procedure has finished.2 The HPDI system makes the maximum value of the velocity achieve 500m/s, and the maximum value of TKE 1.5e+O4m2/s2. The increase of TKE can greatly promote the flame speed, which is greatly significant for natural gas engine.3 As the delay of injection timing of natural gas, the maximum of the mean pressure and mean temperature decrease gradually and the timings for the maximum value increase. With every delay of 5°CA of the injection, the mean pressure reduces by about 16%.As the delay of injection timing, the impact of high pressure of injection on the flow weakens, and the concentration and the range of the mixture gas reduce, which leads to the delay of the timing of the high temperature and the range decrease for the high temperature region, and this is beneficial for the emission of NO, but not conducive to the emission of HC, CO and PM.5 As the increase of injection pressure, the injection speed increases and the injection duration decreases. When the injection pressure are 17MP,21MP and 25MP, the injection duration are10.6°CA,9.5°CA and 8.5°CA respectively. With the increase of injection pressure, the maximum of the mean pressure increases, and the timing of the maximum value is 367°CA, but the mean pressure increases less than 4%. It can be believed that the increase of the injection pressure of natural gas has little impact on the mean pressure. However, the maximum of the mean temperature increases with the increase of the injection pressure, and the timing of the maximum is advanced. As the increase of injection pressure, the injection penetration and the utilization of air increase, which can greatly improve the mixture of natural gas and air and the range of the high temperature region expands. This causes the increase of NO emission and the decrease of HC emission. |
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