| The increase of the global car ownership had a great impact on the shortage of petroleum resources and environment worsening. It is more and more important to develop high efficiency and low emission engines for the passenger cars. Port fuel injection (PFI) gasolines are accounted for the major part of passenger cars in China. Many experimental investigations on gasoline engine cold start were performed by foreign scholars indicated that most of the pollutants are produced during the cold start processes. Relative to the experimental investigation, numerical simulation of the engine combustion process can provide a lot of information, and it is inexpensive, efficient and flexible to compare different design schemes.This work was on a four-cylinder port fuel injection engine for automotive application. The intake port and combustion chamber of one cylinder were modeled by CAD software. In order to provide theoretical guidance for emission reduction at cold start, numerical simulations of the cold starting for the engine was carried out by using the CONVERGE CFD software which is based on the Eulerian-Lagrangian approach. Considering the air-fuel mixing of the PFI engine is largely affected by the evaporation of the fuel film, this paper firstly verified the physics sub-models including the multi-component fuel model, evaporation model and wall film model. The results showed that the models can accurately predict the impinging sprays and fuel evaporation during the cold start process.Secondly, numerical simulation of the air-fuel mixing process during cold start was carried out to explore how to form ignitable mixture in the very first cycle. A three-component fuel model was adopted to study the effect of different parameters on the equivalence ratio of the mixture, fuel evaporation rate and fuel distribution. The parameters include injection operating parameters and inlet design parameters. The results showed that the equivalence ratio increased linearly with the increase of the fuel injection quantity, but the excessive amount of the injected fuel inhibited fuel evaporation. At the Close-Valve-Injection (CVI) mode, early fuel injection makes fuel evaporation rate decrease. The fuel evaporation rate decreased with the advancing of the injection timing. A fast evaporation appeared for the fuel injected towarding the back of the intake valves than that injected on the surface of the intake port. At the Open-Valve-Injection (OVI) mode, it was difficult to generate homogeneous mixture in the cylinder and the equivalence ratio reached its peak when the fuel was injected towarding the inside of the intake valves. Excessive high temperature of the fuel could inhibit the vaporization of the fuel film. The increase of the spray cone angle could increase the film evaporation. Compared with single injection, the equivalence ratio increased and the wall wetting was reduced in a split injection.The air-fuel equivalence ratio decreased slightly when the engine speed increased, but the speed would greatly improve the gas turbulent kinetic energy (TKE) in the cylinder. A counterclockwise tumble appears in the cylinder and the tumble ratio decreases at a low valve lift scheme. The cross tumble ratio and swirl ratio increases by using an asymmetric valve lift scheme. An inlet deflector makes tumble ratio and TKE increase. These studies show that slightly rich mixture can be formed using CVI, and the uniformity of the mixture is better when using OVI. The equivalence ratio increases by retarding the intake valve timing. The effect of the intake valve surface temperature on fuel film evaporation is greater than that of the intake gas temperature.Finally, a detailed kinetic modeling is performed using the SAGE chemical kinetic solver with isooctane mechanism to simulate combustion and emissions of the engine. The effects of different design parameters including equivalence ratio, ignition parameters, intake temperature and swirl ratio on the combustion process and hydrocarbon emissions were studied. |
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