Research On Improving Fuel Economy Of GDI Engine Using Miller Cycle

With the increase of car ownership, more and more fossil fuels are consumed by the car. At the same time, a large amount of carbon dioxide is discharged into the atmosphere, which has intensified the greenhouse effect. In this context, the car’s energy saving has become an important aspect of the current automotive research. Automobile energy saving is essentially the automobile internal combustion engine outputs more power with less fuel oil. In recent years, more and more energy saving technologies had been applied in the engine. As one of the most potential energy saving technologies, Miller cycle has been concerned by more and more researchers. Miller cycle(LIVC) in the application of GDI engine, the engine can be allowed to use a greater geometric compression ratio, reduce the engine pump gas loss and exhaust loss, thereby improving the fuel economy of the engine.In this paper, a GDI 1.4L engine as the prototype to research application of the Miller cycle to improve the fuel economy of the GDI engine. The research of this paper is divided into two parts:GT-POWER is used in the first part to explore improvement effect of the engine fuel economy with Miller cycle(LIVC). The first research project is to use the Miller cycle to improve the fuel economy of the original machine under 2200r/min,75% load、2200r/min,50% load and 2200r/min,25% load. By comparison, the application of Miller cycle compared to Otto cycle in the above three conditions, respectively, to obtain improvement effect of the fuel economy of 1.19% and 4.11% and 6.15%.Another research project of the first part is application of Miller cycle to improve fuel economy of the engine which improved compression ratio. There are two ways to increase the engine’s compression ratio, the first one is to keep the volume of combustion chamber unchanged, increasing piston stroke of the engine(CVC). Second is the engine piston stroke remains unchanged, reducing combustion chamber volume(CVS). The 120 N.m、90N.m、60N.m and 30 N.m of 2200r/min are chosen as the research conditions, and compression ratio is increased from 10.5 to 11.5 and 12.5, respectively. By comparing the effect of two kinds of increasing compression ratio ways find that CVS way is better than CVC. Using CVS way in The 120 N.m、90N.m、60N.m and 30 N.m condition fuel economy respectively, to enhance 2.06%, 2.52%, 2.76% and 2.04%, however, CVC in the same four loads fuel economy only to enhance 0.89%, 0.4%,- 2.43% and-9.77%. Therefore, it is a wise choice to choose the CVS method to increase the compression ratio.In the second part, with the AVL FIRE software, the three-dimensional intake-chamber model of the engine is used to study influence of Injection parameters on formation of mixture gas in the cylinder. The experimental condition is 2200r/min, 50% load, the closing time of the intake valve is 674°CA ATDC, the ignition advance angle is-13°CA ATDC, the average equivalence ratio of mixture is 1. The engine simulation process includes intake process and compression process. Injection duration is set to 20°CA, 35°CA and 50°CA. Under each fuel injection duration, respectively, corresponding to 380°CA ATDC, 420°CA ATDC, 460°CA ATDC and 500°CA ATDC four different fuel injection timing. Analysis of velocity field shows that air flow in the cylinder is in a counter clockwise direction, and the more late injection time, the better formation of counter clockwise roll flow in the cylinder.The the velocity field shows that flow direction inside the cylinder is counterclockwise. Short duration of injection with late injection time or long duration of injection with moderate injection time can form large size and high strength tumble. When the injection timing is same, short duration of injection can always form stronger tumble, flow direction also tend to be more consistent.The whole model and cylinder equivalence ratio curve shows the average equivalent mixture gas in the intake manifold is always less than 1, and varies smaller with the injection timing is delayed. Therefore, to ensure that the average equivalent ratio of mixture in the cylinder is 1, must reduce the fuel injection quantity. And the later injection time is, the less fuel injection quantity is. When the injection time relatively early, the amount of fuel in the intake manifold changed slightly with the injection duration. When the injection time delay, the amount of fuel in the intake manifold decreased with extending the injection duration. Therefore, when the injection timing is relatively late and the injection duration is longer, it is necessary to further reduce the appropriate fuel injection quantity to guarantee the average equivalent ratio of cylinder is 1.Analysis of concentration field shows that: high concentration of mixed gas along the left side of the cylinder, the top surface of the piston, the right side of the cylinder, the center of the cylinder line diffusion, in general, is to follow a counter clockwise direction. The mixture which return into the intake manifold is gradually changed from dilute to concentrate. With injection timing advance and injection duration time shorter, the mixture uniformity in the mode at the ignition time is getting better and better, and the uniformity of the mixed gas is also getting better and better and the concentration becomes more and more high in the period before intake valve closes.