Numerical Study Of The Combustion And Optimization Of Syngas Reactivity Controlled Compression Ignition (RCCI) Engines

Energy crisis and environmental pollution are two major problems faced in the development of the internal combustion engine(ICE).The combination of the efficient and clean combustion technology and the alternative fuel is a new technical route for internal combustion engine to save energy and reduce emissions.In this paper.the syngas is selected as the alternative fuel,and based on the reactivity controlled combustion ignition(RCCI)mode,the potential of the syngas/diesel RCCI engine in energy saving and emission reduction is investigated.The KIVA-3V code is coupled with Non-Dominated Genetic Algorithm(NSGA)to comprehensively optimize the operating parameters of the syngas/diesel RCCI engine,and the effect of the key operating parameters on the combustion and emission performance is systematically studied.At the same time,the optimal control strategies are carried out to realize the efficient,clean and stable combustion of the engine under the wide loads.Base on different engines,the performance of syngas/diesel RCCI and syngas/dimethyl ether(DME)RCCI is optimized by the NSGA-?.The effect of engine configurations,piston shapes and directly-injected fuel properties on the RCCI engine performance is comparatively analyzed.On the other hand,the regulation of the corresponding control strategies is also studied.Based on the technology of preparing syngas by DME reforming,the concept of DME onboard reforming RCCI engine system is proposed.The simulation model of the DME reformer is established by the COMSOL Multiphysics,which is coupled with the computational simulation of the syngas/DME RCCI engine,and the performance of the engine system is fully evaluated.(1)In order to study the potential of the syngas RCCI engine,the effect of syngas compositions,initial intake temperature and the H2/CO ratio on the syngas/diesel RCCI engine(Engine A)is studied,when the engine is operated at middle speed and low load.By applying the syngas with higher HZ ratio,the higher premix ratio and the adaptable initial temperature,the efficient and clean combustion of syngas RCCI can be achieved.The lean premix combustion of the syngas effectively reduces the in-cylinder temperature,which is able to inhibit the NOx formation.With the high premix ratio,the macrornolecular hydrocarbon fuel is reduced and the soot emissions can be improved.Furthermore,the combustion phasing(CA50)is simultaneously affected by the initial temperature and the premix ratio.However,with the restriction of the ringing intensity(RI),the initial temperature is restricted in a narrow range.With the optimization of the H2 ratio in the syngas,the fuel efficiency and the NOx emissions can be improved,and the RI can be controlled in a satisfactory range.(2)Based on Engine A,the syngas/diesel RCCI combustion is optimized under different engine loads to improve the equivalent indicated specific fuel consumption(EISFC),ringing intensity(RI)and NOx emissions.The results of the optimization indicate that the high premix ratio with early injection timing is able to realize the low-temperature combustion of syngas/diesel RCCI.With the double injection strategy and late main injection timing,the combustion efficiency of the engine can be improved at the low load(Indicated Mean Effective Pressure,IMEP=4.7 bar),and the RI can be reduced at the middle/high loads(IMEP?7.7 and 16.7 bar).When the engine load is increased,the lower initial temperature-higher initial pressure and EGR rate are suggested to control the RI.Based on the optimization of the operating parameters,the efficient and clean combustion of the syngas/diesel can be realized under wide engine loads,and the EISFC,RI and NOx emissions can be simultaneously improved.In order to evaluate the effect of the diluent gases on the syngas combustion process,a diluent factor C is presented.Based on the results of the optimization under different engine loads and the preparation technology of the syngas,a specific syngas is proposed to realize the syngas/diesel RCCI efficient and clean combustion under full loads,in which the H2/CO ratio is 75%and the diluent factor C is 0.8.(3)To extend the syngas RCCI application range,the optimization results of the syngas RCCI with different engines(Engine A and Engine B)and different directly injected fuel(dimethyl ether and diesel)at low load are contrastively analyzed.The potential of the syngas RCCI applied in different operating conditions is also discussed.The fuel-supply strategies should be modified with the variation of the engine size and the piston shape.The premix ratio and the injection pressure are mainly dominated by the engine size,and the injection mode(i.e.single injection or double injection)is decided by the piston shape.Furthermore,the higher initial temperature and pressure are suggested with the decrease of the engine size.For the syngas compositions,the variation of the engine size and the piston shape has little effect on the optimization results.On the other hand,the optimizations of the syngas/dimethyl ether(DME)RCCI and syngas/diesel RCCI(Engine C)indicated that,the higher cetane number(CN)and better evaporation properties of DME leads to the accumulation of the high reactivity region in the cylinder with the high premix ratio.he ignition of the DME is mainly started in the center region of the piston bowl and then the combustion propagates to the cylinder wall.Thus,the higher H2 ratio is needed to expand the combustion limit of the premixed syngas,and further improve the combustion efficiency of the syngas near the cylinder wall.Compared with the syngas/diesel RCCI,the combination of syngas and DME is able to realize the efficient and clean combustion of the RCCI mode with wider premix ratios,which is also more potential to the practical application.(4)The COMSOL Multiphysics and KIVA-3V code are employed to respectively calculate the DME reformer and the syngas/DME RCCI engine,which is coupled in an engine system.In the system,the heat energy required for DME reformer is provided by the engine exhaust,and the product of the reformer is regarded as the premix fuel of the engine.Compared with the CDC and PCCI combustion modes,the syngas/DME RCCI engine has significant advantages in RI and NOx emissions,which solves the problems of the rapid combustion of DME and excessive local temperature.At the same time,with the cooperation of the advance combustion mode and the engine exhaust energy recovery,the EISFC of the engine system is obviously improved,and the thermal efficiency can be increased 1?2%due to the energy recovery.In order to further improve the combustion and emission performance of the engine system,the premixed DME for the reformer is suggested around 70%and the H2 energy ratio of the produced syngas is suggested around 30%.