Study On Spark Ignition Natural Gas Engine Based On In-Cylinder Thermochemical Fuel Reforming(TFR) Concept

Based on a premixed turbocharged 4-cylinder spark ignition natural gas engine,in-cylinder thermochemical fuel reforming combustion mode was realized.The influences of the factors such as TFR circulation mode,compression ratio and spark timings on in-cylinder reforming process and engine combustion as well as emission characteristics were investigated.Based on previously stated investigations,combinations of TFR with stoichiometric combustion and with lean burn was researched.Ultimately,two high efficiency-low emission engine technical strageties,namely“Stoi+TFR+TWC”and“Lean+TFR”were propsed.The influences of EGR strategy on the combustion and emission characterics were firstly investigated in this paper,including analyzing the mechanisms and principles of EGR's impact.To thoroughly investigate the mechanisms that EGR affects engine performances,the dilution,thermal effects of intake charge diluents?CO₂,N₂ and Ar?and chemical effect of CO₂ were studied detailedly through isolation method.Results showed that EGR helps to lower NOx emission significantly while exacerbate engine combustion due the heavy thermal effect of CO₂and N₂ in EGR.To overcome the disadvantages from EGR on engine combustion,tentative examination of H₂ additon based on EGR strategy was conducted.It was proved that EGR+H₂could help to roundly improve engine performances.However,EGR+H₂ strategy could hardly be taken into practical application due to storage problem.So,trials of“in-cylinder thermochemical fuel reforming”?ICTFR,simplified as:TFR?on spark ignition natural gas engine were conducted,aiming to realize an organic online combination of EGR and H₂.The mechanism of TFR mode involves running one of the engine cylinder?multi-cylinder engine?rich of stoichiometric and routine the all the exhaust of this rich combustion cylinder?TFR cylinder?back into the intake manifold.The rich combustion in that cylinder was secured by a port fuel injector?PFI?enriching system in the expriments.In the exhaust of the rich combustion cylinder,except for CO₂ and N₂,which were the main components of EGR,significant amount of H₂ and CO were also validated.Hence,a combination of EGR and H₂ were proved.Whereafter,three TFR circulation modes,Pre-Compressor TFR Loop:PC-TFRL,After-Compressor TFR Loop:AC-TFRL,After-Throttle TFR Loop:AT-TFRL,were investigated and compared.AC-TFRL circulation mode was considered to be the best overally.Under AC-TFRL mode,the influence of TFR on engine performances were thouroughly investigated.Compared with conventional natural gas engine with stoichiometric combustion,natural gas engine coupled with TFR showed an equivalent fuel economy while a remarkably lower BSNOx emission value.However,the chief drawbacks of TFR natural gas engine here related to its unstable combustion in TFR cylinder and narrow equivalence ratio window for stable running,which also led to IMEP discrepancies between normal cylinders and TFR cylinder.However,a further deep analysis showed that the optimization on TFR cylinder was the key for improving global engine performances.To solve previously referred issues,strategies such as liquid fuel?methanol,ethanol,n-heptane?enrichment,increasing engine compression ratio and spark timing optimization were adopted.Ultimately,engine optimization were successfully realized.Results proved that methanol,ethanol as well as n-heptane all could help to improve in-cylinder TFR performances and even engine performances.In compare,methanol is the best enriching fuel over natural gas,ethanol and n-heptane.By using methanol enrichment strategy,previously referred issues were successfully solved.Experimental results also proved that increasing engine compression ratio was also very effective measures for improving engine performances.After increasing engine compression ratio,the combustion quality in TFR cylinder was remarkably improved and the equivalence ratio window for smooth TFR cylinder running was significantly widened.Moreover,engine combustion stability and fuel economy were significantly improved and engine BSNOx was dramatically lowered.Combined with spark timing optimization,a smooth and high efficiency engine running under TFR mode were achieved.Based on previous researching works,combinations of TFR with stoichiometric combustion and with lean burn,i.e.,“Stoi+TFR+TWC”and”Lean+TFR”strategies,were investigated.Moreover,engine performance optimization was conducted by using high compression ratio-methanol enrichment-spark timing-equivalence ratio strategies.In“Stoi+TFR+TWC”strategy,it was proved that controlling on TWC with“dithering”was the key for success.However,equivalence ratios control was kernel for“Lean+TFR”strategy.Compared with original lean burn strategy,engine performances were significantly improved with remarkably higher engine efficiency and significantly lower emissions after being optimized using either“Stoi+TFR+TWC”or“Lean+TFR”strategy.Ultimately,two high efficiency-low emission engine technical strageties,namely“Stoi+TFR+TWC”and“Lean+TFR”were propsed.In terms of these two strategies,in comparison,“Lean+TFR”proved better engine efficiency while“Stoi+TFR+TWC”demonstrated higher engine power potential and lower engine emissions.