Study And Optimization Of Hydrogen/diesel Reactivity Controlled Compression Ignition Combustion

In order to achieve the goal of carbon emission reduction,hydrogen is introduced into reactivity controlled compression ignition（RCCI）engine,and it is fully ignited by diesel injected before the top dead center.Embedded with a reduced chemical mechanism for hydrogen/diesel,the performance and application potential of hydrogen/diesel RCCI engine is comprehensively evaluated using multi-dimensional computational fluid dynamics（CFD）model KIVA-3V.First of all,from the perspective of the second law of thermodynamics,the differences in combustion characteristics,energy distribution and exergy distribution between hydrogen/diesel RCCI and gasoline/diesel RCCI are compared in detail.The results show that hydrogen/diesel RCCI has a greater advantage over gasoline/diesel RCCI in the reduction of exergy destruction due to higher combustion temperature,shorter combustion duration,and better oxidation pathways.Secondly,in order to further tap the potential of hydrogen/diesel RCCI,seven key operating parameters were synchronously optimized for hydrogen/diesel RCCI by coupling the KIVA-3V code with the non-dominated sorting genetic algorithm-II（NSGA-II）.Taking the equivalent indicated specific fuel consumption（EISFC）and exergy destruction(E_des)as the optimization objectives,the maximum efficiency is achieved in both the first law and the second law of thermodynamics.The main factors affecting EISFC and E_des are identified,and the control strategy of the combustion process for reducing E_des is proposed.The findings suggest that EISFC and E_des show an obvious trade-off relationship,which means that it is difficult to minimize EISFC and E_des simultaneously.On the one hand,EISFC varies non-monotonically with the combustion temperature.On the other hand,although E_des is related to both the specific reaction path of fuel and combustion temperature,the combustion temperature shows a greater impact.Although the reduction of the total amount of burnt exergy also benefits to reduce E_des,it is undesirable at the cost of the decreased combustion efficiency.If the recovery of heat transfer and exhaust exergy is considered,when the recovery ratio is larger than 50%,the total energy utilization efficiency and E_des can be improved concurrently.As a result,the elimination of exergy destruction has a higher priority than increasing thermal efficiency during optimization at the recovery ratio higher than 50%.Finally,based on the typical optimization cases,the effects of four initial parameters on the target parameters were discussed,including the premixed energy ratio(R_pre),the fraction of the pilot injection to total direct injection(F_soi1),initial pressure(P_ivc)and exhaust gas recirculation（EGR）rate:Although increasing R_pre will reduce the combustion temperature and prolong the combustion duration,the advantages in the reaction path brought by higher hydrogen fraction make up for the negative effects of the two on E_des.Therefore,it finally shows that E_des decreases with the increase of R_pre,and higher R_pre can achieve lower E_des while maintaining low EISFC;The higher combustion temperature dominates the decrease of E_deswhen F_soi1 is increased.Since EISFC changes in a relatively low range with F_soi1,E_des can be greatly reduced by improving F_soi1 while slightly sacrificing EISFC;Increasing P_ivc or EGR rate will reduce the combustion temperature and prolong the combustion duration,resulting in the increase of E_des.However,thanks to the significantly reduced heat transfer exergy,the proportion of exergy output work increases when P_ivc increases;The effect of EGR rate on E_desis not obvious for the current cases,and EISFC initially decreases and then increases as the EGR rate rises,so a moderate EGR rate can achieve the lowest EISFC while maintaining low Edes.