Study Of Knock In A High Compression Ratio Natural Gas Engine

With the rapid development of the automobile industry and increasingly serious environment pollution caused by automotive pollution,natural gas as an alternative fuel is an important way to reduce oil demand and alleviate the problem of environmental pollution.Due to the existing differences of natural gas composition in different regions,its antiknock is not the same,so the compression ratio of engine has to be designed based on lower antiknock quality to avoid the occurrence of knocking combustion,which also limits the increase of compression ratio of natural gas engine and influences the improvement of economic performance.Hereby,based on the investigation of the evaluation index of knocking combustion in natural gas engine,the effects of different methane concentrations(i.e.99%methane concentration and 93%methane concentration)on the increase of compression ratio are studied.Moreover,by adding n-butane to natural gas,this paper studies the antiknock quality and influence degree of natural gas with the different proportions of n-butane.The main contents and results of this paper are as follows:(1)In order to investigate the evaluation index of knocking combustion characteristics of spark-ignition natural gas engine,based on the comparison and analysis of combustion and knocking of natural gas engine under different speed,load and compression ratio conditions,the evaluation indexes of knock intensity and occurrence probability are summarized.The cycle is considered as a knocking cycle when its knock intensity is not lower than 0.3MPa.Also,when knocking occurrence probability index is greater than or equal to 10%,the condition is considered as knocking condition.(2)The effects of different methane concentrations in natural gas on combustion process,anti-knock performance,economic and emission performance of natural gas engine under different speeds,loads and compression ratios were studied.Low methane concentration of natural gas has higher content of high carbon alkanes,which results in lower antiknock quality and easily leads to engine knock and abnormal combustion.The study found that with the utilization of low methane concentration of natural gas,under the compression ratio of 14 and high-speed conditions(i.e.2000r/min),and compression ratio of 16 and low-speed conditions(i.e.1000r/min),there exists knocking combustion,in which knocking occurrence probability index is far greater than 10%,The pressure rise rate is also significantly higher than that with the utilization of high methane concentration of natural gas's.At the same time,it was found that the NOx emissions from the above-mentioned working conditions were significantly increased,while the NOx emissions with the high methane concentration of natural gas were significantly lower.By adjusting the ignition advance angle,it was found that with the use of high methane concentration of natural gas,the ignition advance angle corresponding to knock occurrence is earlier,which also shows its better antiknock performance.(3)The effects of n-butane concentration in natural gas with high content of alkane on combustion process,antiknock performance,economy and emission performance of natural gas engine was studied.With the increase of the concentration of n-butane,maximum explosion pressure increases,the heat release rate is accelerated,the center of combustion is closer to the TDC,and the combustion duration is shortened.When the concentration of n-butane is increased to result in knocking combustion,the indicated thermal efficiency and the indicated mean pressure decreased significantly.At the same speed,with the increase of the concentration of n-butane,the knocking tendency of engine increases;with engine speed increasing,the concentration of n-butane which leads to knocking combustion decreases.The research of this paper has a certain theoretical significance and realistic value for the realization of combustion with high efficiency and low emission.