Evaluation and Design of Injector Hole Patterns Using CFD with a Fuel Tracer Diagnostic for Gasoline Direct Injection (GDI) Engines 2011-01-0840
A CFD code is enhanced with a fuel tracer diagnostic to track the liquid and vapor fuel mass separately from individual spray plumes of a multi-hole injector and the wall film. The approach works by solving a set of additional scalar transport equations for fuel vapor generated from individual nozzle hole and the wall film. The diagnostic tool is first validated against experiments from a 4-valve, wall-guided spark-ignition direct-injection (SIDI) engine. A CFD analysis is carried out to understand the experimentally observed trade-offs in combustion stability and smoke emissions between a 70degree hollow-cone swirl injector and a 40 degree, 5-hole, circular-type multi-hole injector at a lean, stratified idle operating condition. Engine tests show that the multi-hole injector results in lower COV of IMEP than the hollow-cone swirl injector at the expense of significantly higher smoke emissions. The CFD model predictions illustrate that the multi-hole injector resulted in richer fuel mass in the piston bowl and greater amount of liquid wall-film mass than the hollow-cone swirl injector. In particular, there exists more fuel vapor mass surrounding the spark gap for the multi-hole injector that correlates with the experimentally observed improvement in combustion stability. The majority of this fuel vapor mass originated from the spray plume targeted directly at the spark plug. On the other hand, the increased smoke emissions with the multi-hole injector agrees with the larger computed rich fuel vapor and liquid wall-film mass. The primary source of the liquid wall-film mass originated from the two spray plumes with the shortest penetration path length to the piston bowl surface. These findings are used to design a crescent-type multi-hole injector. Predictions using the redesigned injector show reduced liquid wall-film mass and a sufficient mixture distribution at the spark gap to suggest lower smoke emissions while maintaining good combustion stability.