Numerical Simulation And Experimental Study On The Low-Temperature Combustion With Fuel Reforming In IC Engine

Fuel reforming strategy is one of the important ways to improve engine thermal efficiency and reduce harmful emissions by alter the chemical reaction pathway of fuel,which has a good application prospect.Therefore,based on the theories of homogeneous compression combustion and low-temperature combustion,a new concept through control the chemical reaction pathway of fuel to realize high-efficient and clean combustion in engine was proposed,namely flexible cylinder engine(FCE)reforming mode.In this paper,the fuel reforming process,cooling process,and in-cylinder combustion process in FCE mode were studied and analyzed,respectively.Moreover,the feasibility of the reaction pathway control strategy in engine was verified experimentally.In the FCE mode,the engine mainly consists of normal cylinder and flexible cylinder.The fuel is reformed in the flexible cylinder,and then mixed with the same fresh fuel,and then the mixture is introduced into the normal cylinders.The whole process consists of three key sections,i.e.,low-temperature reforming of the fuel,cooling of the reformed products,and combustion of the mixture including the fresh fuel and the reformed products.The results of low-temperature reforming model study show that the intake uniformity coefficient is 4.80%in the case of four-cylinder engine with“1 for 3”strategy,which the reformed products from one cylinder should feed to the other three cylinders.However,the intake uniformity coefficient is 2.06%in the case of six-cylinder engine with“1 for 2”strategy,which the reformed products from one cylinder should feed to two cylinders,and there are two reforming cylinders in this case.Therefore,the“1 for 2”strategy in six-cylinder engine is the more suitable plan for FCE mode.The results of fuel reforming process study show that the effect of initial intake air temperature and equivalence ratio on the reforming process is greater than that of initial intake air pressure;The fuel H-abstraction-the first oxygen addition-the first isomerization-the second oxygen addition-the second isomerization,accounts for most of the formation of low-temperature reformed products;As the reforming temperature increase,the formation pathway of reformed products was altered from the fuel typical low-temperature oxidation pathway to the coupling of low-temperature oxidation and pyrolysis pathway;The results of cooling process of reformed products study show that there is great influence on the low-temperature reformed products in the initial cooling process(<0.01s),and the concentration of reformed products tends to be stable in the subsequent stage of cooling process(>0.01s);The delayed,linear,and natural cooling strategy have different effects on the concentration of low-temperature reformed products,and the effect of cooling strategy on reformed products is in the order of delayed cooling>linear cooling>natural cooling.To revealing the mechanism that combustion between reformed products and fresh fuel,an investigation was conducted to study this mechanism fueled with n-heptane,PRF50,and PRF90 in this paper,respectively.It indicated that the addition of reformed products could promote or inhibit the formation of OH which has an important influence on the combustion state in the cylinder.Generally,OH is mainly from the typical low-temperature reaction process in the fuel ignition stage.However,the effect of short-chain alkanes,olefins,alkynes(such as CH₄,C₂H₂,C₂H₄,etc.),short-chain peroxides(such as CH₃O₂H,C₂H₄O₂H,etc.),and ketones peroxides(KETs)on the formation of OH was improved with the addition of reformed products;High-reactivity reformed products could improve the laminar flame speed of fuel;The mole fraction of related species with close relation to emission(C₂H₂,C₂H₄,C₃H₄,C₃H₆,1,3-C₄H₆,CH₂O,CH₃CHO,etc.)also decreased with the addition of reformed products.The feasibility of the controllable reaction pathway strategy in engine was verified experimentally fueled with PRF50.Results show that when the reforming temperature was in the range of lower temperature,the reactivity of reformed products increased gradually as the reforming temperature increased in the variable reforming temperature strategy.Moreover,after the reforming temperature raised to a certain value,the reactivity of reformed products would decrease as the reforming temperature continue to increase.In addition,the largest reduction of CO and UHC was 17.02%and 37.98%,respectively,and the indicated thermal efficiency of engine could be improved by 4%after the addition of reformed products;The longer reforming time is,the stronger or weaker reactivity of reformed products will be.However,this principle is not applicable when the reforming temperature is near to the transition point of reformed products reactivity,since there is a timing for reformed products with strongest or weakest reactivity,and the reactivity of reformed products does not change linearly as the reforming time.In this paper,600K is the transition point of reformed products reactivity,and COV could be reduced to 3%by altering the reforming time;When the reforming temperature was in the range of generating high-reactivity reformed products,the reactivity of reformed products increased first and then decreased as the reforming ratio increased,and when the reforming temperature was in the range of generating negative-reactivity reformed products,the reactivity of reformed products decreased as the reforming ratio increased.The combustion efficiency could be improved to 98.5%by altering the reforming ratio;In this paper,the effect of reaction time between reformed products and fresh fuel on combustion was conducted by changing engine speed.Results show that fuel reforming strategy has greater influence on the fuel combustion process at low engine speed.Since the reaction time between reformed products and fresh fuel becomes shorter as the engine speed increase,weaken the ability of reformed products to alter the engine combustion phasing,and the range of combustion duration.