Influence Mechanism Of Gas Composition On Combustion Performance Of High Compression Ratio Natural Gas Engine

In order to cope with global climate change and increasing energy demand,it is of great significance to find clean and low-carbon alternative energy to solve environmental problems and energy crisis.As a kind of clean energy,natural gas has the advantages of low carbon content,high octane number and abundant reserves.It is one of the best choices in the transition stage from fossil energy to non-fossil energy under the"dual carbon"goal,and has a good application prospect in heavy engines.Natural gas consists of methane,ethane,butane and other hydrocarbon and non-hydrocarbon components.The composition and content of natural gas are different according to the place of origin,age or the processing mode in the production and transportation process,which also leads to the change of the physical and chemical properties of natural gas from different sources,and further affects its combustion performance in the engine.In order to adapt to the variation of natural gas composition in different regions,the compression ratio of heavy-duty spark ignition natural gas engines is often low,resulting in a decrease in thermal efficiency.Therefore,in order to further improve the fuel efficiency of natural gas engines and reduce carbon emissions,a higher compression ratio is needed.However,with the increase of compression ratio and load,the cycle variation of combustion in cylinder may intensify and the possibility of knocking combustion increases.And the antiknock performance of gas varies with the composition of gas.The aggravation of combustion cycle variation and knock will seriously affect the combustion characteristics of the spark ignition engine,resulting in a significant impact on the economy,power performance and emission performance of the natural gas engine.Therefore,this paper mainly studies the influence of natural gas composition on combustion mechanism of spark ignition natural gas engine with high compression ratio through experiments,focusing on the influence mechanism of methane and n-butane concentration in natural gas coupled with high compression ratio on combustion,cycle variation and knock.It has important theoretical significance and engineering practical value to improve the thermal efficiency and combustion performance of spark ignition natural gas engine.At the same time,this paper also studies the influence of zero-carbon fuel hydrogen blending combustion on combustion cycle variation of high compression ratio natural gas engine at low-load,which can not only provide theoretical guidance for improving combustion stability of spark ignition natural gas engine,but also provide reference for further energy saving and carbon reduction of natural gas engine.The main contents and conclusions of this paper are as follows:（1）Based on engine thermodynamic bench experiments,the combustion performance of a spark ignition natural gas engine at different speeds and loads was studied.The results show that with the increase of the load,the gas consumption rate and combustion duration of the natural gas engine decrease,the maximum peak pressure,pressure rise rate and NO emission increase,and the mean indicated pressure（IMEP）cycle variation coefficient(COV_IMEP)decreases.Under the condition of small loads,there are obvious misfire and partial combustion problems in engine combustion,and the corresponding cycle variation increases greatly.The variation of combustion cycle will also increase under heavy loads.COV_IMEPdecreases with increasing engine speed.COV_IMEPis closely related to the cyclic changes of the maximum peak pressure and the maximum pressure rise rate.With the increase of speed and load,the cyclic variation of the maximum peak pressure decreases,and there is an obvious linear correlation between the corresponding crank angles,and the greater the linear correlation coefficient,the smaller the cyclic variation of IMEP.（2）The influence mechanism of different compression ratios（i.e.,11.6,14 and16）on combustion characteristics and cycle variation of natural gas engines was studied.The results show that the higher the compression ratio,the lower the gas consumption rate,the higher the thermal efficiency,the lower the carbon dioxide emission（CO₂）,but the higher the corresponding maximum peak pressure and the maximum pressure increase rate,the higher the NO emission.In addition,the combustion duration is prolonged with the increase of compression ratio under medium and small load conditions,and the variation of combustion cycle may also increase.Combined with three-dimensional numerical simulation,it is found that the turbulent motion in cylinder,especially the turbulent kinetic energy near the spark plug,is the main influencing factor.For different compression ratios,the more concentrated the pressure rise rate is near TDC,the lower the cyclic change is.The higher the compression ratio is,the crank angle corresponding to the maximum pressure rise rate is more concentrated,but COV_IMEPis not necessarily the lowest.（3）The influence mechanism of the above compression ratios combined with methane concentration（i.e.,93%and 99%methane concentration）on the combustion performance of natural gas engines was further studied.The results show that high compression ratio can obviously improve fuel efficiency and reduce gas consumption,but high compression ratio will bring high knock tendency.However,the knock of high compression ratio natural gas engine can be suppressed by using 99%methane concentration.Compared with 93%methane concentration,99%methane concentration has higher maximum peak pressure,longer combustion duration,higher gas consumption rate and higher NO emission.However,the IMEP cycle variation of natural gas with methane concentration of 93%is low as a whole due to the faster flame propagation rate.（4）The effects of n-butane in natural gas on combustion performance and knock of high compression ratio natural gas engines were studied.With the increase of the concentration of n-butane,the maximum peak pressure in the cylinder increases,the combustion heat release rate accelerates,the combustion duration shortens,the average maximum pressure rise rate（MPRR）increases,and the corresponding average knock intensity（MAPO）increases.MPRR and MAPO knock indicators were used to evaluate knock conditions and knock severity quantitatively,and the validity of the method was analyzed and verified.The results show that the increase of n-butane concentration may increase the effective thermal efficiency under the condition of slight knock,but it will lead to engine damage under the condition of severe knock.The n-butane concentration corresponding to knock decreases with increasing engine speed.（5）The influence mechanism of hydrogen adding on combustion characteristics and cycle variation of natural gas engines was studied.With the increase of hydrogen concentration,the maximum peak pressure increases,the heat release rate increases,the ignition delay period and combustion duration shorten,and the combustion center advances,but the pressure rise rate and combustion temperature increase correspondingly.The effective thermal efficiency is not affected by hydrogen concentration.NO emission increases with the increase of hydrogen concentration,while CO₂emission decreases significantly.With the increase of hydrogen concentration,the cyclic variation of the indicated mean effective pressure decreases obviously,and the cyclic variation of the maximum peak pressure and the corresponding crank angle change linearly,and the whole advance with the increase of hydrogen concentration.The crank angle corresponding to the maximum pressure rise rate of each cycle gradually concentrates near TDC.