【作者】

Joel Hiltner

【摘要】

论文已在中国上海举行的2013年CIMAC大会上发表，论文的版权归CIMAC所有。The onset of end gas knock remains one of the primary factors limiting the thermal efficiency, fuel flexibility, specific power output, and transient capability of lean burn natural gas engines. In the past two decades, huge strides have been taken to improve the performance of these engines, based largely on a fundamental understanding of the impact of thermodynamic design and fuel properties on the resistance of a given combustion platform to engine knock. Simulation tools based on this knowledge have led to optimized designs as regards expansion ratio, Miller Cycle utilization, and other design approaches aimed at limiting end gas temperatures. Two basic engine phenomena whose impact on knock is less well understood are in-cylinder heat transfer and overall combustion rate, phasing, and stability. In-cylinder heat transfer has a profound impact on charge temperatures and can lead either to increased or decreased knock tolerance for a given design depending on a number of factors. Combustion chamber surface temperatures are a function of detail design, engine load, and engine operating condition and have a direct impact on heat transfer rates during compression and the early part of combustion. In-cylinder bulk flow and turbulence level induced, for example, by swirl and squish also impact the heat transfer rate and thus the unburned gas temperature. The influence of these heat transfer effects on knock tendency is not well documented. Combustion rate and stability affect knock behavior as well for a given engine geometry. The faster heat release rates that are desirable from a thermal efficiency standpoint can either increase or decrease detonation margin as they tend to drive up peak temperatures while at the same time reducing the time avail-able for end gas chemistry and pushing the engine operating point to leaner conditions in order to maintain constant NOx emissions levels. Poor combustion stability which often results from very lean operating conditions can also impact knock margin as some very fast burning cycles give end gas conditions far away from the mean operating point typically analyzed with simulation tools. A combination of test results, simulation data and statistical tools can be utilized to under-stand these combustion stability effects. This paper seeks to provide insight into the relationship between these secondary effects (heat transfer and combustion behavior) and combustion system development as regards knock avoidance. Simulation results and engine test data are provided that highlight the role of heat transfer and heat release on knock tendency. De-tailed single cylinder engine measurements including surface temperatures, heat release rates, and inferred heat transfer rates are provided to clarify the performance trends. Multiple zone cycle simulation results are used to provide further clarification of the knock impact of heat transfer and heat release for a given engine geometry. Combustion system development that includes an understanding of these processes inevitably yields production engines with superior market potential. The availability of a fundamental framework for these physical processes, supported by research grade test and simulation results will be a key enabling technology for the next generation of high performance natural gas engines.

【会议名称】

第27届CIMAC会议

【会议地点】

上海

【下载次数】

1

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