Fuel injection rate and its variation of a GDI injector operated in engine motoring conditions

The cylinder head assembly from a GDI engine was separated to build a GDI fuel injection system capable of controlling the rail pressure and fuel injection levels. The GDI fuel injection system is driven by a three-phase AC motor with a direct connection between the camshaft and the motor. The operating conditions of the GDI fuel injection system are equivalent to those of actual engine driving. The AC motor drives the GDI high-pressure fuel pump by rotating the engine camshaft. In the GDI high-pressure fuel pump (HPFP), fuel is pressurized on the upward stroke of the plunger and pumped to the fuel rail. Fuel rail pressure (FRP) control is possible by controlling the opening/closing timing of the HPFP’s pressure control valve (PCV), the fuel injection duration and the camshaft speed. In order to calculate the fuel injection rate characteristics of the GDI injector statistically under fixed conditions of the camshaft speed, FRP, and injection duration, the measurements of the fuel injection rate were repeated at regular time intervals. To calculate the variation of the fuel injection rate, the S/N (signal-to-noise) ratio, which is the average FRP divided by the FRP standard deviation, was calculated. A fuel injection rate was obtained under 88 experimental conditions combining the three conditions of the HPFP’s PCV opening/closing timing, fuel injection duration, and camshaft rotational speed. The smaller S/N ratio shows the larger FRP variation. Also, it was identified that the S/N ratio is related to the variation of the fuel injection rate.