Simulation Of The Combustion And Dynamics Of A Direct Injection Gasoline Engine During The Direct-start Processes

Direct-start is one of the effective ways to save energy and reduce the emissionsof the cars powered by direct injection gasoline engines. Simulation of thecombustion and dynamics of a direct injection gasoline engine during the direct-startprocesses is useful to guide the engineer to achieve successful start of direct injectiongasoline engines.With the AVL-Fire software and the Matlab software, a three-dimensionalcomputational fluid dynamics (CFD) model for simulating the formation of mixtureand combustion in the first cylinder to be ignited by a spark plug and aone-dimensional dynamics model for calculating the engine speed during thedirect-start processes are coupled, respectively. The effect of overall fuel-airequivalence ratios, spark ignition timings after fuel injection and the piston positionsat the time of fuel injection for the first/second cylinder to initiate combustion on theformation of mixture, combustion and rotation process characteristics was modeled.The results are as follows:When the fuel injection pressure is5MPa, with the increase of overall fuel-airequivalence ratio in the cylinder, the inhomogeneity of the mixture in the firstcylinder to initiate combustion increases at the same time after fuel injection and themaximum turbulence kinetic energy induced by injection increases and occurs later.The volumetric fractions of the mixture with fuel-air equivalence ratio in the range of0.7~1.4are larger at0.8and1.2overall fuel-air equivalence ratios than those at1.6and2.0overall fuel-air equivalence ratios. However, it decreases with overall fuel-airequivalence ratio greater than or equal to1.6. At a fixed fuel-air equivalence ratio, themixture can burn rapidly when stoichiometric air fuel mixture is formed around thespark plug for the first time and is ignited.For a fixed piston position of the first cylinder to initiate combustion, the rapidburn of the mixture in it results in high reverse rotational speed and large crank anglein the opposite direction of normal crankshaft rotation. In the case of fixed overallfuel-air equivalence ratios and spark ignition timings in the first cylinder to initiatecombustion at a piston position, the engine can reach the maximum rotation speed inthe following normal rotation operation as the mixture in the second cylinder is ignited just at the moment when the piston reaches its top dead center. At this sparkignition timing, the mixture in the second cylinder to be ignited combusts with thehighest burning rate at1.2overall fuel-air equivalence ratio, which helps the engine tostart successfully. At different piston positions of the first cylinder to run, the enginecan run successfully in the case of direct-start only when the piston of the firstcylinder is stopped at60°before top dead center.