A Study on Combustion and Emission Characteristics of GDI Engine for HEV at Quick Start 2014-01-2709

Gasoline Direct Injection (GDI) engines have attracted interest as automotive power-plants because of their potential advantages in down-sizing, fuel efficiency and in emissions reduction. However, GDI engines suffer from elevated unburned hydrocarbon (HC) emissions during start up process, which are sometimes worsened by misfires and partial burns. Moreover, as the engine is cranked to idle speed quickly in HEVs (Hybrid Electric Vehicle), the transients of quick starts are more dramatically than that in traditional vehicle, which challenge the optimization of combustion and emissions.

In this study, test bench had been set up to investigate the GDI engine performances for ISG (Integrated Starter and Generator) HEVs during start up process. Based on the test system, cycle-controlled of the fuel injection mass, fuel injection timing and ignition timing can be obtained, as well as the cycle-resolved measurement of the HC concentrations and NO emissions. This paper focus on the detailed effects of coolant temperature, fuel temperature, cranking speed, injection timing and total equivalence ratio on the combustion and emission characteristics of the GDI engine under stratified combustion condition during the engine quick start process cycle by cycle.

The results showed that the optimization of the injection strategy and ignition strategy should take the effect of coolant temperature, the cranking speed and the fuel temperature into account. The optimal fuel injection strategy varies with cranking speed, and total equivalence ratio near 1.0 provides good trade-off between combustion and emissions at low cranking speed. The best recommended cranking speed was 600r/min, with the 1^st injection timing range of 90°CA∼120°CA, the 2^nd injection timing near 300°CA for under cold start condition with overall consideration of the combustion and emissions, while the cranking speed should not exceed 800r/min during hot start with advanced injection timing to reduce HC emissions. For the ignition timing, an ignition timing of 350°CA is suitable for the trade-off between combustion and emissions under cold start at a cranking speed of 400r/min and 600r/min. While the recommended ignition timing for hot start is 350°CA and 350°CA∼355°CA at cranking speed of 400r/min and 800r/min respectively. A well fitted fuel temperature and injection timing can promote better combustion and low emissions obviously.