Study On Exergy Loss Of Combustion And Approach For Achieving High Thermal Efficiency Under Gasoline HCCI Lean-Burn Combustion

It is of significant meaning to improve the thermal efficiency of the internal combustion engine(ICE)for reducing CO₂ emission and mitigating energy crisis.The combustion process optimization plays an important role in improving the ICE's thermal efficiency.It has been proved that homogeneous charge compression ignition(HCCI)is able to realize clean combustion.Thus,the attention is paid to further improve the HCCI engine thermal efficiency.The objective of this study was to improve the thermal efficiency of the gasoline HCCI engine by using the numerical simulation combined with the engine experiment.The mechanism of the exergy loss during the non-equilibrium combustion process and the critical factors of exergy-work transformation process were studied in terms of the second law of thermodynamics.The model engine was designed to further optimize the combustion,and the approach to achieve high and super-high thermal efficiency was explored and confirmed.The exergy loss during the combustion process of a piston-type ICE is inevitable.The exergy loss during HCCI combustion is mainly produced by the chemical reaction.Therefore,the mathematical model based on the non-equilibrium thermodynamics and detailed chemical kinetics was established to study the exergy loss mechanism of the gasoline and n-butanol combustion.The results indicated that the exergy loss source of the two fuels could both be classified as three stages:Stage1was the process of large molecule fuels converted into small molecule fuels;Stage2was the process of small molecule fuels through the H₂O₂ loop reactions converted into CO;Stage3 was the oxidation processes of CO,H,and O to CO₂ and H₂O.Different chemical characteristics of the fuels could lead to different features of the exergy loss source and the exergy loss rate in Stage1.However,it had the same effects on the exergy loss of the thermodynamic parameters at combustion boundary for the two fuels.The exergy loss could be reduced by increasing the combustion rate and temperature based on the synergistic control of thermodynamic parameters(temperature,pressure,equivalence ratio,oxygen concentration)at combustion boundary.Meanwhile,the combustion temperature needed to avoid over high for suppressing the chemical dissociation loss.The combustion of low total exergy loss could be achieved by adopting HCCI lean burn combined with the technologies such as boosting,exhaust gas recirculation(EGR)and high compression ratio.The thermal efficiency can be improved not only by reducing the exergy loss of combustion but also by improving the transformation efficiency of exergy to work.Therefore,the transformation process of exergy to work was studied.The results indicated that the critical factors for improving the transformation efficiency of exergy to work were the optimization of combustion phase and combustion rate,as well as the optimization of the specific heat ratio of the mixture.The second law of thermodynamics thermal efficiency of the gasoline HCCI lean-burn engine could exceed 50%through the combustion optimization,which demonstrated that the high efficiency of the exergy transfer to work could be achieved by adopting the gasoline HCCI lean-burn combustion.In addition,the gasoline reformed molecule HCCI(RM-HCCI)was proposed in this study:the gasoline was reformed into the small molecules(C?4)under the high-temperature and no oxygen atmosphere outside the cylinder and were introduced into the cylinder to achieve HCCI combustion.Gasoline reforming was conducive to reduce the exergy loss and retard the ignition of the mixture which could mitigate the dependence of EGR contributing to the specific heat ratio improved.Based on the above features,the gasoline RM-HCCI had the advantageous on expanding the highest load and improving the thermal efficiency.A model engine was designed to optimize the gasoline HCCI lean-burn combustion for further improving the thermal efficiency based on the split cycles and the variable compression ratio(?_e)combined with the technologies for reducing the exergy loss of combustion and improving the transformation efficiency of exergy to work.The numerical simulation method was used in this chapter.The sectionalized compression and expansion could be achieved by the low-pressure cycle(consisted of the turbocharger system)and the high-pressure cycle(the synergetic controlling by the?_e,EGR and?)respectively.The design advantages of the model engine were that it could recover the exhaust energy fully to provide high charge density for HCCI lean burn.Furthermore,it could adjust the thermodynamic parameters flexibly to control the combustion phase and combustion rate precisely through the dual variable cycle design of high-pressure and low-pressure cycles.Load(L)-?(F)-EGR€-?_e€coordinated control strategy was proposed in this study,or LFEE strategy for short.Based on the LFEE strategy,it could realize the simultaneous optimization of combustion phase,combustion rate and combustion temperature,as well as the optimization of?and EGR rate to improve the specific heat ratio under the rough combustion restriction.The highest mean effective pressure of 16 bar could be achieved and the highest brake thermal efficiency of 50%could be reached by adopting the LFEE strategy.The approach for achieving the super-high thermal efficiency was explored.Increasing the engine load and the peak cylinder pressure was found to improve the thermal efficiency effectively,especially within the indicated mean effective pressure range of 8?15bar.To improve thermal efficiency greatly,heat insulation for the engine was proved necessary.It showed the potential to achieve super-high brake thermal efficiency over 60%and maintain clean combustion by adopting the LFEE strategy in high-strength and adiabatic model engine.