Performance Study On Lean Burn Natural Gas Engine Combined With In-cylinder Thermochemical Fuel Reforming

With the aggravation of energy crisis and environmental pollutions,seeking for alternative fuels for traditional oils is becoming extremely urgent.Due to its clean combustion property and abundance in reservation,natural gas is considered as one of the most promising alternative fuels.While,natural gas engine generally suffers from poor performances such as low power output and low thermal efficiency due to some disadvantages such as low volumetric energy density and laminar flame speed of natural gas.Accordingly,improving the performances of natural gas engine is of great significance for solving energy and environment problems.In this paper,following works were accomplished based on a four-cylinder,turbo-charged,spark ignited lean burn natural gas engine coupled with in-cylinder thermochemical fuel reforming?TFR?:1)exploring the performances of the lean burn natural gas engine when coupled with TFR,2)optimizing engine performances by using methanol as enriching fuel instead of natural gas itself,3)studying the influence of equivalence ratios on engine performances while enriching TFR cylinder with methanol,4)researching the effects of spark timings on engine performances under methanol enriching condition.Based on these works,optimization has been finished on TFR lean burn natural gas engine by using reforming fuel,equivalence ratios and spark timings collaborative strategies.As demonstrated by the experiments,H₂ and CO generated through in-cylinder TFR contributed a lot to accelerating flame propagation,extending lean operation limit,increasing in-cylinder pressure and heat release rate,shortening flame development and rapid combustion durations,enhancing engine stability and thermal efficiency,and decreasing NOx and HC emissions.However,in-cylinder TFR of natural gas fuel led to disadvantages such as incomplete reforming and worsened engine stability.To overcome these problems,a trial of methanol enrichment instead of natural gas enrichment has been conducted.Results proved enhanced H₂ and CO production,bettered thermal efficiency,decreased NOx and THC emissions and even improved engine stability for methanol enrichment strategy.Under methanol enrichment case,the influences of the equivalence ratio of TFR cylinder(?_TFR)on engine performances have been further studied.With the increase of?_TFR,increased H₂ and CO concentration,improved engine combustion,enhanced engine operation stability and thermal efficiency were all demonstrated.Besides,a decrease of HC emission without increases of NOx and CO emissions was proved.Besides,the effects of spark timings of conventional and TFR cylinders,ST_N and ST_TFR,were also studied.With the advance of ST_TFR,in-cylinder TFR process was improved,resulting in higher H₂ and CO concentrations in the reformate.However,no remarkable influence of ST_TFR on engine CO emission was found.The effect of ST_TFR on NOx and HC emission was unapparent under high engine load.While,advanced ST_TFR lead to decreased NOx and HC emissions under low engine load.The advance of ST_N improved engine combustion quality,stability,thermal efficiency and increased NOx emission.However,it did not notably affect TFR process and engine CO and HC emissions.Lastly,an optimization on the natural gas engine has been conducted through reforming fuels,equivalence ratios and spark timings collaborative strategies.Simultaneous reductions of NOx emission and specific fuel consumption were successfully achieved.All these experimental results signified that TFR was an effective measure for realizing high efficiency-low emissions natural gas engine.