Optimization of the engine cold start is critical for gasoline direct injection (GDI) engines to meet increasingly stringent emission regulations, since the emissions during the first 20 seconds of the cold start constitute more than 80% of the hydrocarbon (HC) emissions for the entire EPA FTP75 drive cycle. However, Direct Injection Spark Ignition (DISI) engine cold start optimization is very challenging due to the rapidly changing engine speed, cold thermal environment and low cranking fuel pressure. One approach to reduce HC emissions for DISI engines is to adopt retarded spark so that engines generate high heat fluxes for faster catalyst light-off during the cold idle. This approach typically degrades the engine combustion stability and presents additional challenges to the engine cold start.
This paper describes a CFD modeling based approach to address these challenges for the Ford 3.5L V6 EcoBoost engine cold start. A Ford in-house developed CFD code MESIM (Multi-dimensional Engine Simulation) was applied to study the effect of injection parameters and piston designs on the fuel preparation process under various engine speed and engine temperature conditions during the engine crank, run up and cold idle operations. It was found through the modeling studies that the formation of a robust fuel-air mixture distribution around the spark plug was the key to improving combustion stability and reducing the emissions. The factors that could directly affect the fuel-air mixture distribution are the injection timing, fuel pressure, and piston bowl design. Modeling results provided the physical insight of the fuel preparation mechanism of various injection strategies, and helped to optimize the split-injection strategy to improve the fuel-air mixture distribution around the spark plug. The engine test data confirmed that the optimized split injection strategies reduce HC emission by about 30% with lower fuel consumption and substantially improved combustion stability during engine cold-start.
