Understanding of Intake Cam Phasing Effects on the Induction and Fuel-Air Mixing in a DISI Engine

Variable Cam Timing (VCT) has been proven to be a very effective method in PFI (Port Fuel Injection) engines for improved fuel economy and combustion stability, and reduced emissions. In DISI (Direct Injection Spark Ignition) engines, VCT is applied in both stratified-charge and homogeneous charge operating modes. In stratified-charge mode, VCT is used to reduce NOx emission and improve combustion stability. In homogeneous charge mode, the function of VCT is similar to that in PFI engines. In DISI engine, however, the VCT also affects the available fuel-air mixing time.

This paper focuses on VCT effects on the induction process and the fuel-air mixing homogeneity in a DISI engine. The detailed induction process with large exhaust-intake valve overlap has been investigated with CFD modeling. Seven characteristic sub-processes during the induction have been identified. The associated mechanism for each sub-process is also investigated. Based on the physics of the induction process, the experimentally observed effect of cam phasing on engine manifold pressure is well understood. Both optical engine experiments and CFD modeling showed that retarding the intake cam improves the fuel-air mixing homogeneity over a wide VCT range. Dynamometer engine testing confirms the improved mixing as the CO emission decreases as the intake cam retards. Modeling revealed that the mixing improvement is not due to the overall large-scale in-cylinder flow structure, but due to the local small flow structures.