Effect of Fuel Injection Timing on the Mixture Preparation in a Small Gasoline Direct-Injection Engine 2018-32-0014

Gasoline direct-injection (GDI) engines have evolved as a solution to meet the current demands of the automotive industry. Benefits of a GDI engine include good fuel economy, good transient response, and low cold start emissions. However, they suffer from problems, like combustion instability, misfire, and impingement of fuel on in-cylinder surfaces. Therefore, to highlight the influence of fuel injection timing on in-cylinder flow, turbulence, mixture distribution and wall impingement, a computational study is conducted on a small-bore GDI engine. Results showed that air motion inside the engine cylinder is influenced by direct-injection of fuel, with considerable variation in turbulent kinetic energy at the time of injection. Due to charge cooling effect, mixture density and trapped mass were increased by about 10.8% and 9.5%, respectively. A significant drop in mean in-cylinder temperature (about 100 °C) was observed with direct-injection of fuel as compared to the case without injection, with further variation based on injection timing. Fuel distribution near the spark plug and fuel impingement on in-cylinder surfaces are carefully evaluated. An early injection at 80 crank angle degree after top dead center of intake provided the best fuel distribution and minimum wall impingement. At the time of spark, in-cylinder turbulence was also found to be higher for this injection timing. Experimental results also indicated relatively higher brake thermal efficiency and lower emissions with early fuel injection timings, located around the mid of intake.