| The energy crises and environmental problesms promote the development of energy-saving and emission-reduction theories and technologies for internal combustion engines(ICEs).High-efficiency,low-carbon,and low-emissions are the common goals for the future advanced engines.Utilizing low-carbon alternative fuels is a feasible pathway to achieve their goals for ICEs.Natural gas is a promising low-carbon alternative fuel for conventional automotive fuels.In this work,a comprehensive study was carried out to investigate the combustion,emissions,and performance of natural gas/diesel dual-fuel engines by using advanced numerical simulation and optimization method.Firstly,a parametric study was conducted to study the combustion and emission characteristics in a natural gas/diesel dual-fuel engine based on multi-dimensional computational fluid dynamics(CFD)simuations.Two combustion strategies,diesel pilot ignition(DPI)and reactivity controlled compression ignition(RCCI)were examined and compared by analyzing the combustion,emissions,and engine performance.A discussion on controllability of natural gas/diesel dual-fuel combustion was carried out after obtaining a deep understanding of the effects of fuel system parameters and air system parameters.Secondly,a genetic algorithm(GA)numerical optimization platform was developed by coupling a micro GA code with a CFD code.Based on this optimization platform,natural gas/diesel dual-fuel combustion was systematically optimized at medium speed and at low,medium,and high loads,respectively.This effort was made to explore the possibility of obtaing high-efficiency and clean combustion at multiple operating conditions in a heavy-duty engine.The results showed that high-efficiency and clean combustion can be achieved by optimizing the seven parameters including diesel injection timing,injection pressure,injection split ratio,percent energy substitution(PES),EGR fraction.The engine is able to obtain higher than 45%thermal efficiency while keeping the peak pressure and peak pressure rise rate(PPRR)within the constraints.In the meantime,the NO_x emission can be suppressed below 0.4g/kW.h,which is the Euro VI NO_x emission standard.Especially,although the EGR fraction employed at the low and medium load was very low,the NOx emissions were much lower than the NOx emission standard.This indicates that the engine can operate at these conditions with high-efficiency and low-emission combustion even without introducing EGR.Moreover,a parameteric study based on the optimal solution was performed.It showed that the injection parameters have a wide flexible variation range to achieve high efficiency and clean combustion.Thirdly,the effects of diesel injection strategy and pistion shape on the combustion and engine performance were evaluated.Additionally,the effect of compression ratio was also examined to find the possibility of further improvement of engine thermal efficiency.The results showed that higher than 45%thermal efficiency and lower than 0.4g/kW.h NO_x emission can be avhieved at the low and medium load by optimizing the four parameters including PES,EGR fraction,injection timing,and injection pressure for diesel single injection strategy.However,the thermal efficiency can only reach to 35.5%at the high load while keeping the NO_x emission and peak pressure within the constraints.At the high load,to obtain higher thermal efficiency and lower HC and CO emissions,diesel double injection strategy is necessary to tailor desired in-cylinder reactivity gradient distribution to improve combustion process.Regarding the effect of piston shape,it was found that at a fixed condtion(same intake pressure and temperature,engine speed,and equivalent diesel mass),the requirements of PES and EGR fraction to ensure high-efficiency and clean combustion were not varied with the change of pistion shape.At the optimum point,the in-cylinder flow motion and mixture formation process are different for different pistion configurations,thus,the detailed information of ignition and combustion development are different.However,the bulk combustion characteristics are similar;combustion phasing and combustion duration are close.The differences in thermal effiency between different piston confgurations are derived from the differences of heat transfer.The increase of compression ratio has a slight improvement for the thermal efficiency of natural gas/diesel dual-fuel engine.With the further increase of compression ratio,the level of improvement weakened.This is due to the fact that the improvement potential of thermal efficiency is supressed when considering the requirement of controlling NO_x emission,PPRR and heat transfer.The attempt of increasing compression ration was not an effective way to improve the thermal efficiency of natural gas/diesel dual-fuel engine,another method needs to be further explored.Finally,the influence of natural gas composition on natural gas/diesel dual-fuel combustion was numerically evaluated using computational fluid dynamics(CFD)simulations.The results showed that the ignition delay enlarged and the combustion duration shortened when the methane number is decreased.For RCCI combustion,the retarded trend of ignition is more significant than that of DPI,while the decreased trend in combustion duration is less significant.To understand this trend,chemical kinetics studies of ignition delay characteristic and temperature A-factor sensitivity studies were conducted.Furthermore,the responses of emissions to methane number were also investigated.The results show that for DPI the HC and CO emissions decreases with the decreased methane number.However,for RCCI the variations of HC and CO emissions with the methane number are not so obvious as for DPI combustion.For both DPI and RCCI combustion,the NOx emissions show a strong dependence on combustion phasing rather than natural gas composition.Overall,to control DPI combustion,the methane number should be considered together with other parameters.However,attention should be paid to other control parameters for the RCCI combustion.The engine performance of RCCI is not sensitive to the variation of natural gas composition,so the RCCI engines can be more adaptive to natural gases from different sources. |
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