| For the dominant role internal combustion engines(ICEs)have played in transportation and serious energy and environment crisis we have encountered,technologies that could improve engine efficiency,as well as reduce greenhouse gas emissions,attracting more and more popularity in recent years.The highest peak brake thermal efficiency(BTE)of current passenger vehicle engines is only slightly above 40%,and the maximum BTE for slider-crank engines can hardly exceed 60% without radical changes to present engines structure and combustion strategies.A consensus opinion why the maximum BTE for slider-crank engines is limited under 60% is the irreversible ?unrestrained combustions‘ in ICEs,which occur far from thermodynamic equilibrium states that would loss approximately 20–25% of the fuel‘s working potential.Thus,understanding the mechanism of the inefficiencies and seeking for ways to reduce the irreversibility are significant to realize further improvement of the engine efficiency.Both the essential characteristics of ?unrestrained combustion‘ exergy loss process and control approaches to minimize the combustion irreversibility as well as maximize the work extraction efficiency were investigated by experiment and simulation in this study.To better describe the exergy loss mechanism of the combustion process,which is well known as a non-equilibrium physical and chemical process,a new exergy loss computational code on the basis of the entropy generation equation,aiming to quantify the exergy loss behavior of each individual elementary reaction,is innovatively developed according to the local equilibrium hypothesis of non-equilibrium thermodynamics,which provides a theoretical basis to explore the main intrinsic irreversible sources of the non-equilibrium combustion process.The present study investigated the intrinsic exergy loss sources of a primary reference fuel(n-heptane)by detailed chemical kinetics.Three apparent peaks in the overall exergy loss rate during combustion process,with each peak caused by very different element reactions,were observed based on the microscopic exergy loss events.Then,we detailed the effects of various parameters on exergy loss events within the overall exergy loss rates,exergy loss rates of individual reactions,and loss distributions.A high temperature and no oxygen atmosphere fuel reforming has been proposed for the purpose of exergy saving by theoretical analyzing the detailed exergy loss events of combustion process,the correctness and feasibility of this fuel reforming have been verified through experiments.The exergy behaviors of high temperature and no oxygen atmosphere fuel reforming have been extensively studied,and many benefits had been observed including:(1)simplify the reforming device;(2)improve the total chemical exergy while effectively converting large moleculae to small moleculae;(3)improve the mixture‘s ratio of specific heat capacity for higher work-extraction;and(4)Longer ignition delay for better mixing process.Furthermore,a promising high efficiency RM-HCCI(Reformed molecule HCCI)combustion principle was proposed.In a RM-HCCI engine,large hydrocarbon fuels were reformed into small molecule fuels under high temperature and no oxygen atmosphere before injected into the cylinder when the exhaust gas enthalpy to a certain extent was recovered,further improving the engine efficiency.The RM-HCCI has many advantages:(1)less exergy losses during combustion processes,(2)longer ignition delays and shorter but controlled combustion durations,(3)improved ratio of specific heats,all these advantages show us a bright further of improving engine efficiency by RM-HCCI combustion principle. |
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