Abstract
To efficiently control auto ignition of CAI engines, intake-air temperature is increased using the recycled exhaust gases, called internal EGR. Because the distribution of the internal EGR affects the mixture formation as well as the overall mean temperature inside a combustion cylinder, the method to control the intake and exhaust valve timing is often used to achieve the negative valve overlap condition. In the present study, transient numerical simulations have been carried out to predict the mixing characteristics and flow field inside the cylinder with respect to the variation in the valve timing: The 3D unsteady Eulerian- Lagrangian two-phase model is used for the interaction between the intake-air with the remaining internal EGR during the under-lap operation while varying the EVO/EVC timing. Computational results show that retarding EVC motion is more effective than advancing EVO to control the pressure, temperature and distribution of the internal EGR. When the EVC timing is retarded, the mean pressure inside the cylinder during the under-lap period is decreased because of the following two reasons: First, less amount of residual gas remains inside the cylinder. Second, the cylinder back-flow motion, re-circulated into the intake-port at an early stage of the intake-stroke, is substantially mitigated. Overall, the mean pressure remains nearly unchanged during EVC retardation and EVO advancement because of the increased intake-air, induced by the back-flow mitigation. On the other hand, the mean gas temperature and mass fraction are decreased because the removal rate of the high temperature residual gas is accelerated while retarding EVC motion. It is also observed that the mixing characteristic of the residual gas, intake-air, and fuel tends to be homogeneous due to mixing intensification during EVC retardation.
Keywords: CAI (Controlled Auto Ignition), HCCI (Homogeneous Charge Compression Ignition), Under-lap, Internal EGR (Exhaust Gas Recirculation), Uniformity