【作者】

Joel Hiltner

【摘要】

该论文已在芬兰赫尔辛基举行的第28届CIMAC大会上发表。论文的版权归CIMAC所有。Developers of lean burn natural gas engines have made dramatic improvements to engine performance in the last three decades. This includes nearly doubling the achievable power density and reducing fuel consumption by 25% while operating on a clean, plentiful and low cost fuel source. These developments, which have greatly benefited end users and society in general, have been achieved in conjunction with dramatic reductions in engine emissions. Potential future regulations intended to limit the emissions of unburned hydrocarbons from natural gas engines are a new challenge that is only beginning to be addressed by heavy-duty natural gas engine manufacturers. Emissions of non-methane hydrocarbons from many sources, including natural gas engines, are becoming more strictly regulated due to their contribution to local air quality issues. Increased natural gas engine utilization has the potential to reduce greenhouse gas emissions substantially, but this benefit requires exhaust methane emissions to be contained. Also of significant importance is the loss of engine efficiency that results from the failure of a given combustion system to convert between 1% and 4% of the supplied fuel energy. The challenge of unburned hydrocarbons is further exacerbated by the fact that technologies which reduce NOx emissions and increase thermodynamic efficiency often decrease combustion efficiency. This paper describes research work aimed at understanding the sources of unburned hydrocarbons from lean burn engines and identifying technology to mitigate these emissions. An overview of all potential sources of unburned hydrocarbon emissions, as described in the literature for spark-ignited engines, is provided. For a sub-set of these potential sources, test results from a single cylinder medium speed engine are coupled with zero and one-dimensional cycle simulation analysis to provide a quantitative perspective of unburned hydrocarbon emissions in lean burn gas engines. Engine test data and basic simulation results are provided which quantify the impact of the piston top land crevice on engine out hydrocarbons. Data for a variety of top land geometries and compression ratios is presented. Crank angle resolved exhaust port hydrocarbon measurements and cycle simulation results are used to quantify the impact of fuel short-circuiting during valve overlap in port-injected gas engines. The impact of flame extinction at combustion chamber surfaces is investigated using flame quench thickness relationships available from the literature. Based on this analysis an estimate of the potential contribution of wall quenching to unburned hydrocarbons is given. Finally, measured variations in total heat released per cycle, as extracted from cylinder pressure data, demonstrates the tendency towards bulk flame extinction near the lean limit for a given engine. The in-cylinder conditions that contribute to flame extinction and thus high emissions of unburned hydrocarbons are investigated. The relative importance of these sources of unburned hydrocarbons is compared and various approaches for reducing each source is discussed and, where possible, demonstrated.

【会议名称】

第28届CIMAC会议

【会议地点】

芬兰 赫尔辛基

【下载次数】

2

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