The effects of reduced fuel injection pressure on the cold start performance of a GDI engine have been studied in a single-cylinder, optically-accessible research engine. Two Delphi Automotive Systems DI-G injectors, with included spray cone angles of 60° and 80° respectively, were studied. Both injectors are designed to operate at a nominal fuel line pressure of 10 MPa. For the study they were operated at several fuel feed pressures between 10 MPa and 2 MPa. Two start of injection timings (50° and 100° ATDC) were examined.
Cold start performance was characterized by measurements of the GIMEP, COV of GIMEP, and total engine out UHCs. Simultaneous Planar Laser Induced Fluorescence (PLIF) and Mie Scattering images of the fuel spray were used to observe spray penetration, mixing, and in-cylinder fuel distribution throughout the intake and compression strokes. Ultimately these images were used to explain observed performance differences.
Significant cold start performance changes are not observed until the injection pressure is reduced to below 5 MPa. The PLIF imaging shows that piston crown wetting and cylinder wall wetting have a dominant effect on engine cold start performance. This suggests that spray penetration and trajectory are at least as important to engine cold start performance as mean drop size. This indicates the desirability of developing an injector technology which can decouple injection pressure, drop size, and spray penetration.