Numerical Study And Thermodynamic Analysis On Methanol/Diesel Reactivity Controlled Compression Ignition(RCCI)Engines

Due to the aggravated energy shortage and environmental problems,developing a more efficient and cleaner internal combustion engine fueled with alternative fuels is profound significance.In this study,genetic algorithm,energy and exergy analysis are used to comprehensively discuss the engine performance of methanol/diesel reacitivty controlled compression ignition(RCCI).Moreover,the impacts of exhaust gas recirculation(EGR)and fuel property on the engine performance of RCCI are also intensively investigated.(1)The fuel efficiency,nitrogen oxide(NOx)and soot emissions are chosen as the optimized objectives,total five important parameters are optimized simultaneously by non-dominated sorting genetic algorithm-?(NSGA-?),including premixed methanol fraction,start of injection(SOI)of diesel,EGR rate,initial temperature and pressure.After continuous revolution,all the three objectives are improved.Medium EGR rate,higher premixed fraction,advanced SOI and higher initial temperature and initial pressure are preferable for the Pareto optimum solutions.Through analyzing the sensitivity of each parameter,it is found that EGR rate and initial temperature are the most significant parameters for engine performance.Both SOI and premixed fraction considerably affect the distributions of fuel concentration and reacitivity,so they can be used to effectively control the NOx and soot emissions.The impact of initial pressure on engine performance is relatively slight,hence it can be used to adjust the combustion phasing(CA50)without significant influence on engine performance.In addition,increasing methanol fraction can retard CA50 and smooth the in-cylinder gradient of fuel concentration and reactivity,consequently,resulting in the reduction of heat release rate(HRR)peak and ringing intensity(RI).(2)When engine is operated at middle load and speed,the effect of methanol fraction and EGR rate on methanol/diesel RCCI is investigated under various initial temperatures.When CA50 is kept constant,the engine performance keeps unchanged by adjusting EGR rate and initial temperature simultaneously.However,the combustion becomes intensively when both initial temperature and methanol fraction are increased,which improves the fuel efficiency at the cost of unacceptable RI.At middle loads,the necessity of introducting EGR depended on the level of initial temperature.When initial temperature is below the critical value,satisfactory engine performance can be achieved by adjusting methanol fraction without the introduction of EGR.Once the initial temperature is increased beyond the critical value,the introduction of EGR was indispensable.At high loads,the employment of EGR is always necessary.(3)Since the octane number of methanol is higher than that of gasoline,and methanol supresses the low temperature heat release(LTHR)behavior,the combustion phasing(CA50)of methanol/diesel RCCI is later than that of gasoline/diesel RCCI with the same operating parameters.When the premixed fraction and SOI are kept the same,the combustion rate of methanol/diesel RCCI is faster than that of gasoline/diesel RCCI,which is ascribed to the more homogeneous distribution of fuel,as well as the smaller and more compact molecule of methanol.The faster combustion rate and the more retarded CA50 of methanol/diesel RCCI are beneficial to achieve the better fuel efficiency than gasoline/diesel RCCI.The sensitivity of engine performance to the variation of initial temperature is associated with the position of CA50 and the intensity of LTHR.And the sensitivity of gasoline/diesel is low due to the obvious LTHR and early CA50..(4)For conventional diesel combustion(CDC),homogeneous charge compression ignition(HCCI),and RCCI,the position of CA50 determines the RI and fuel efficiency.When CA50 is kept constant,extending the period from CA10 to CA50 is beneficial to alleviate RI.The position of CA90 shows a significant effect on fuel efficiency.The intensive heat transfer of CDC is attributed to the obvious temperature gradient in heat transfer layer,while the heat transfer of HCCI or RCCI is dominated by the 'heat transfer area,which can be reduced by optimizing the geometry of piston and reducing the surface-to-volume ratio of chamber.The exergy-to-energy ratios of heat transfer and exhaust gases in CDC are higher,which indicates the higher utilization efficiency for the energy from heat transfer and exhaust gases.While the exergy-to-energy ratios of heat transfer and exhaust gases in RCCI are almost not affected by the variation of CA50.(5)Chemical reaction(Deschem)is the largest reason for the exergy destruction in internal combustion engines.When engine is operated at the same loads and speed,considerable exergy destruction is accumulated in HCCI and RCCI during the transition from LTHR to high temperature heat release(HTHR),which leads to the higher Deschem.However,the obvious inhomogeneities of temperature and fuel concentration in CDC cause the larger exergy destruction due to heat conduction(Desheat)and mass transfer(Desmass),resulting in its larger totoal exergy destruction.Although increasing equivalence ratio itself leads to the increase of exergy destruction,the combustion temperature increases accordingly with the larger equivalence ratio,which is benefical to the reduction of exergy destruction.When equivalence ratio is increased towards the stoichiometric ratio,the combustion temperature plays the dominant role on exergy destruction,consequently the reduction of the Deschem/released energy(RE)ratio is found.When the equivalence ratio further increases beyond the stoichiometric ratio,the impact of equivalence ratio itself on exergy destruction is enhanced which outweighs the impact of combustion temperature,so the Deschem/RE ratio increases inversely with the increase of equivalence ratio.(6)When different fuels are adopted in RCCI engines,different operating parameters are required to achieve the same CA50 due to their different fuel properties.Thus,different combustion temperatures as well as various exergy destruction fractions are achieved.The impact of fuel on exergy destruction stems from the LTHR behavior and chemical reaction path.It is found that the main heat release reactions are among the small molecules and radicals,the reacions of " H+O2+M(?)HO2+M","CO+OH(?)CO2+H",and "HCO+M(?)H+CO+M" are the most significant reactions that affect the exergy destruction.Overall,the methanol/diesel RCCI generates lower exergy destruction fraction than gasoline/diesel RCCI at the same CA50.In summary,in order to reduce exergy destruction,higher premixed fraction and advanced SOI of diesel should be employed.Meanwhile,the equivalence ratio should be controlled below the stoichiometric ratio,and higher combustion temperature,the fuel with single-stage HRR(no LTHR)as well as low exergy destruction generation from reaction paths,are desirable.