Investigations On The Exergy Loss Mechanisms Of The Controllable Chemical Reaction Pathway Combustion Of Advanced Compression Ignition Engines

Currently,improving thermal efficiency is the biggest challenge of the IC engine field.Although the brake thermal efficiency of IC engine could be higher than 50%under certain operating conditions,the most important factor restricting the further improvement of thermal efficiency is still the fuel exergy loss caused by the inherent irreversibility of"unconstrained combustion".To reduce the irreversibility loss,there is need to make the combustion more"constrained".The controllable chemical reaction pathway combustion can control the combustion heat release,but the study of its exergy loss is still very limited.To this end,based on classical equilibrium and non-equilibrium thermodynamics,this study conducts an in-depth exergy analysis on the controllable chemical reaction path combustion mode using numerical simulation combined with theoretical analysis as the main research method.The splitting combustion with low temperature reforming（LTR）/high temperature heat release（HTR）is proposed,and its potential to control the combustion process and improve thermal efficiency is explored,which provides a theoretical basis for the development of high efficiency IC engines.Firstly,an exergy analysis method suitable for different combustion modes of internal combustion engines based on the classical equilibrium thermodynamic energy balance and exergy balance principles is established.Combined with the experimental result,the effects of three combustion modes of HCCI,PPC and RCCI,as well as EGR and fuel types on the exergy destruction loss and thermal efficiency are analyzed.The results show that the effects of different combustion modes on thermal efficiency and exergy destruction loss are mainly controlled by the combustion efficiency,heat transfer loss and injection strategies.Increasing combustion efficiency can increase exergy destruction losses,but reducing heat transfer loss and fuel direct-injection can decrease exergy destruction loss.The combustion mode has no obvious influence on the exergy destruction loss.The main factor restricting the further improvement of the thermal efficiency of the new combustion mode is still the irreversibility of the engine working process.When introducing external intercooled EGR,increasing the EGR rate will reduce the limit thermal efficiency of the three combustion modes,and reduce the irreversible exergy loss.In addition,fuel type can also significantly affect exergy destruction loss.Secondly,in order to reveal the sources of exergy loss in different combustion flame mechanisms of internal combustion engines,the entropy generation and exergy loss characteristics of one-dimensional premixed flame,autoignition and flame regime conversion under IC engine conditions are studied by using the non-equilibrium thermodynamic entropy generation analysis method.In order to analyze the energy conversion from the perspective of the chemical reaction pathway,a complex reaction kinetic analysis method based on the entropy generation flux path was also proposed.The results show that the chemical reaction kinetic analysis method proposed in this paper can identify the main global reaction paths of autoignition and flame propagation under different conditions.When the flame regime changes from premixed flame propagation to autoignition,the total irreversible exergy loss does not change much,but the source of exergy destruction source changes significantly.The entropy generation from heat and mass transfer decreases,and gradually changes to chemical reactions.Meantime,the fuel consumption path also changes accordingly.The Gibbs formation enthalpy difference between the product and the reactant is the driving force of the chemical reaction.By changing the initial conditions,the Gibbs formation enthalpy difference between the product and the reactant can be changed,which in turn changes the direction of the reaction and deviates from the chemical equilibrium.LTR/HTR splitting combustion mode,that the low-temperature reforming of rich premixed gas to obtain small molecular products followed by the high temperature combustion of lean reaction product/air mixture,can simultaneously avoid high exergy loss and NOx and Soot generation areas,has the potential to become a high efficiency and clean combustion mode.Reforming changes the fuel activity and molecular structure.In this regard,the theoretical calculation formulas for the irreversible exergy loss of the ideal combustion process and the thermal efficiency of the Otto cycle are first proposed.The influences of ten classes of fuel containing different functional group and H₂ and CO on the exergy destruction loss of combustion process and thermal efficiency are analyzed.The results show that,without considering the influence of the combustion time scale,the difference in exergy destruction loss between different fuels is mainly caused by the difference in the fuel chemical exergy/LHV ratio（ε）and the non-dimensional entropy increase（α_S）of the combustion process.The exergy destruction loss of small molecule hydrocarbon fuels and oxygen-containing intermediate products is lower than that of macromolecular fuels.The influence of air and EGR dilution on exergy destruction loss is mainly caused by temperature effects and dilution effects.Regardless of fuel volatility,flame propagation speed and self-ignition characteristics,the first law efficiency has the greatest dependence on the product specific heat ratio（γ_c）of the combustion stage,and it increases with the increase ofγ_c.Small hydrocarbon fuels has higher second law efficiency,while oxygen-containing intermediate products and CO have both lower first and second law efficiency.Alkanes are located at the ideal area of both high first and second law efficiency.The small exergy destruction loss of small alcohols,aldehydes and ketones often does not bring the improvement of thermal efficiency,but increases the exhaust exergy.Under the condition of air or EGR dilution,the increase of exergy destruction loss will cause the reduction of exhaust gas exergy.Therefore,the relationship between exergy destruction loss and thermal efficiency is not a trade-off relationship.When reducing irreversible exergy loss,it is necessary to increase the fuel exergy/work conversion efficiency.Finally,the effect of LTR/HTR splitting combustion on engine combustion and energy conversion efficiency is studied numerically and experimentally.The results show that low-temperature fuel reforming can achieve flexible control of the engine combustion process.From the perspective of fuel exergy/work conversion,increasing the reforming equivalent ratio and temperature is conducive to the production of small molecular hydrocarbon products which have higher work potential.When the fuel undergoes deep reforming,the combustion phase of the engine is delayed,and the overall thermal efficiency can be increased.Further increasing the compression ratio can reduce the irreversible exergy loss,and the thermal efficiency（exergy/work conversion efficiency）can be increased compared to the non-reforming benchmark conditions.Therefore,it can be inferred that compared with the traditional low-temperature combustion method,LTR/HTR splitting combustion has the potential to achieve flexible control of the combustion process and further improve thermal efficiency.