Efficiency Improvement of Boosted Low-Temperature Gasoline Combustion Engines (LTGC) Using a Double Direct-Injection Strategy

For lean or dilute, boosted gasoline compression-ignition engines operating in a low-temperature combustion mode, creating a partially stratified fuel charge mixture prior to auto-ignition can be beneficial for reducing the heat-release rate (HRR) and the corresponding maximum rate of pressure rise. As a result, partial fuel stratification (PFS) can be used to increase load and/or efficiency without knock (i.e. without excessive ringing). In this work, a double direct-injection (D-DI) strategy is investigated for which the majority of the fuel is injected early in the intake stroke to create a relatively well-mixed background mixture, and the remaining fuel is injected in the latter part of the compression stroke to produce greater fuel stratification prior auto-ignition. Experiments were performed in a 1-liter single-cylinder engine modified for low-temperature gasoline combustion (LTGC) research. The main objective of this study is to quantify the effects on the HRR and efficiency gains possible by applying this D-DI fueling technique to a near-perfectly homogeneous mixture and to a moderately stratified mixture (all the fuel injected early in the intake stroke). For the D-DI fueling technique, the timing of the late injection and the fuel-fraction split between the early and late injections were independently varied. This study demonstrates that the D-DI fueling strategy is effective at reducing the heat release rate, which allows CA50 to be significantly advanced without knock. Moreover, compared to fully-premixed fueling, with PFS gain in efficiency of as much as 3%-units were measured while maintaining very low NOx and soot emissions. Nonetheless, with the D-DI fueling, efficiency improvement is less than expected from the allowable CA50 advancement. Additional detailed analyses show that this is due to a combination of reduced combustion efficiency and increased heat transfer as stratification is increased beyond a certain level using this fuel D-DI fueling strategy and injector hardware. Overall, this study provides key understanding about the practical use of multiple direct injections to exploit PFS for intake-boosted operation.