| Gas fuel has the advantages of rich resources and clean combustion performance as an alternative fuel.It can solve the energy conservation and environmental protection issues about international combustion engine.The electrically controlled fuel injection system of the high-power gas-fuelled engine is taken as the object of this paper,based on three main aspects:system scheme design,mixing process in the intake port and cylinder,individual cylinder balance control.In order to improve the air fuel ratio control accuracy and the in-cylinder mixing performance,the three-dimensional computational fluid dynamics(CFD)software FLUENT and the one-dimensional modeling software for engine GT-power are used to study the effects of different inj ection schemes for the in-cylinder mixing efficiency and the individual cylinder balance control respectively.(1)Firstly,the overall scheme of the electrically controlled fuel injection system of the high-power gas-fuelled engine is outlined.The functional analysis of the gas fuel injection system is performed.Three kinds of fuel injection methods,including the in-cylinder direct inj ection,the out-cylinder multi-point injection and single-point injection method,are evaluated with the consideration of air fuel ratio control accuracy,emission performance and system complexity.The intake port multi-point injection scheme is chosen after the evaluation process.(2)The conventional gas fuel injection device(GID)driven by the solenoid has the disadvantages of small injection mass flow,large seating speed and big knock noise.Based on the GID of large-bore gas-fueled engine for power generation,which is designed by our research group before,the steady-state CFD model is built to analyze the effects of valve shape and valve opening direction on the steady injection characteristics of the GID.Two evaluation criteria,effective injection pressure and injection efficiency are proposed.According to the steady injection results,new GID with smaller injection mass flow is designed for automotive engine.The moving-coil electromagnetic linear actuator is taken as the driver and the pull-open non-metallic disc type valve is taken as the execution part of the new GID.It has rather larger mass flow rate than conventional GID.And so adequate gas fuel can be injected to each cylinder of the large-bore engine during the intake process with the intake port multi-point injection scheme.(3)The theoretical basis of the turbulent jet-to-cross flow between the gas fuel jet and the air flow in the intake port under the mode of out-cylinder port fuel injection is described firstly.Based on the theory of wall impact jet,the effects of turbulence model and wall function on the CFD calculation accuracy of transient turbulent cross flow and wall impact j et are analyzed.In order to quantitatively reveal the effect of different injection conditions on the mixing performance in the intake port and cylinder,several evaluation criteria are compared.At last the standard deviation of fuel mass fraction and the probability distribution frequency(PDF)of different fuel mass fraction domains are chosen.(4)The transient CFD engine model incorporating the GID's motion was established to analyze the effects of inj ection angle,poppet valve opening direction and injection position on the mixing homogeneity in the intake port and finally the in-cylinder mixing performance were compared.The results indicate:the pull-open GID injects more momentum to the intake port than the push-open one,and the mixing degree both in the intake port and cylinder is higher;the streamwise vortex excitation mechanism is beneficial to improve the radial mixing degree of the jet.While for the turbulent jet-to-cross flow,the length of the jet core zones will be shorten because of the excitation mechanism,and so the mixing performance is deteriorated;for high injection angle condition,the effects of unstable factors such as wake vortices on the mixing process are enhanced,resulting in the periodical distribution of fuel mass fraction in the intake port;the intake flow of two adj acent intake ports,the helical intake port and the tangential intake port,will interfere with each other near the cylinder head during the intake stroke.For the helical port injection case,the gas fuel approaches to the cylinder center and concentrate on the cylinder head near the end of the compression stroke.While for the tangential port injection case,the gas fuel flows to the piston top and concentrate on the piston top near the end of the compression stroke.(5)In order to improve the air fuel ratio(AFR)control accuracy of gas-fuelled engine,based on the common Kalman observer for estimating the individual cylinder AFR of engine with symmetrical exhaust runners,the observer added by the coefficient matrix update step for engine with asymmetrical exhaust runners is designed.The individual cylinder AFRs are estimated with a single universal exhaust gas oxygen(UEGO)sensor located on the exhaust manifold.At last the estimated individual cylinder AFRs are used to feedback control the injection duration and to fix the individual AFR imbalance. |
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