Study On The Regularity Of In-cylinder Combustion And Heat-work Conversion Process For Natural Gas Engine

Energy crisis and environmental pollution are the two major c hallenges for the world today, and developing clean alternat ive fuels is one of the effective ways to solve these two problems. Natural gas has become an ideal subst itute for gasoline and diesel due to its rich resources, convenient transportat ion and low emission pollution. The study of natural gas engine in China starts relat ively late, and the research on the in-cylinder combustion process is also not very sufficient, at the same time, the advantages and disadvantages of the engine in-cylinder combustion process and the improvement of the heat-work conversion process direct ly determine the comprehensive performance of the engine. Therefore, it is very important to study the law of the in-cylinder combust ion and the heat-work conversion process to improve the performance of natural gas engine.An experimental study was carried on a heavy-dut y natural gas engine used on trucks, and explored how the operat ing parameters and design parameters affected the in-cylinder combust ion heat release law and the heat-work conversion process based on the comparison and analysis of the performance data and combust ion process parameters. Based on the analyses, the general trend of the heat-work conversion process of this t ype natural gas engine was summarized. The thesis research results show that:(1) The combust ion qualit y index locates between 1.5 and 3.0 at full load. The combustion efficiency of lean-burn natural gas engine is greater than 98%. 50% combustion position is mainly affected by ignit ion advance angle, and 50% combustion posit ion advances with the increase of ignit ion advance angle. The combustion duration increases with the increase of the load, and the trend is more obvious at low load. In addit ion, the influence of excess air coefficient is obvious on combustion durat ion. 50% combustion position lags behind combust ion start ing posit ion by 12~14oCA, and the peak pressure po sit ion lags behind 50% combustion posit ion by 4~5oCA.(2) Engine speed and load have an obvious influence on the indicated heat efficiency of high pressure cycle, and the influence of the speed is greater than the load. The combust ion characterist ic parameters which make the best heat-work conversion efficiency are tabulated as follows: the combustion start ing po sition locates at 2~8oCA before top dead center; 50% combust ion posit ion locates at 6~10oCA after top dead center; the maximum explosion pressure posit ion is posit ioned at 10~15oCA after top dead center; effect ive expansion ratio is about 10. The combustion duration has double effects on the heat-work conversion efficiency, and there is an opt imal combustion duration, which makes the heat-work conversion efficiency reach the best value.(3) The closer 50% combustion position is to top dead center, the lower the high pressure cycle indicator specific gas consumption is; the higher effect ive expansion rat io is, the higher the indicated heat efficiency of high pressure cycle is, but the indicated heat efficiency of high pressure cycle is also affected by engine speed.The above rules can provide data to support the value of the boundary condit ions, the choice of the key design parameters and the o ptimizat ion of the operating parameters in the numerical model stage of the conceptual design process of natural gas engine, on the other hand, it can also provide theoretical support to improve the heat-work conversion process of natural gas engine.