Gasoline Direct Injection engines are efficient devices which are rivaling diesel engines with thermal efficiency approaching the 40% threshold at part load. Nevertheless, the GDI engine is an important source of dangerous ultra-fine particulate matter. The long-term sustainability of this technology strongly depends on further improvement of engine design and combustion process.
This work presents the initial development of a full-cycle CFD model of a modern wall-guided GDI engine operated in homogeneous and stoichiometric mode. The investigation was carried out at part-load operating conditions, with early injections during the intake stroke. It included three engine speeds at fixed engine-equivalent load. The spray model was calibrated using test-bed and imaging data from the 7-point high-pressure fuel injectors used in the test engine. Experimental data on combustion were also used for calibration purposes, whereas measurements of engine-out soot number density from a Differential Mobility Spectrometer formed the basis and motive of the investigation.
Following the ECU controller, as the speed is increased at fixed engine load, the fuel injection is advanced to enable longer real-time for fuel-air mixing. In spite of stronger in-cylinder motion, this causes extended liquid spray impingement, potentially leading to the formation of liquid film, a source of soot formation during combustion. At increasing engine speed the mixture appears better prepared at spark timing, and the Air Fuel Ratio approaches correct stoichiometry in the vicinity of spark-plug. While the process of mixing continues after combustion commences, leading to new charge stratification, the higher engine speed case shows greater peak temperature during combustion. These mechanisms are used to explain the increase in soot number density measured at higher engine speed.