Numerical Analysis of a Six-Stroke Gasoline Compression Ignition (GCI) Engine Combustion With Continuously Variable Valve Duration (CVVD) Control

Engine performance and emissions of a six-stroke Gasoline Compression Ignition (GCI) engine with wide range of Continuously Variable Valve Duration (CVVD) control were numerically investigated at low engine load conditions. For the simulations, an in-house 3-D CFD code with high fidelity physical sub-models was used and the combustion and emissions kinetics were computed using a reduced kinetics mechanism for a 14-component gasoline surrogate fuel. Double injections were employed to effectively form the local fuel/air mixtures with optimal reactivity. Several valve timing and duration variations through the CVVD control were considered under both positive valve overlap (PVO) and negative valve overlap (NVO) conditions. Effects of intake-valve re-breathing between the first expansion and the second compression strokes were also investigated. Close attention was paid to understand the effects of two additional strokes of the engine cycle on the thermal and chemical conditions of charge mixtures that alter ignition, combustion and energy recovery processes. Double injections were found to be necessary to effectively utilize the additional two strokes for the combustion of overly mixed lean charge mixtures during the second power stroke (PS2). It was found that combustion phasing in both power strokes is effectively controlled by the intake valve closure (IVC) timing since it affects the effective compression ratio. Engine operation under NVO condition with fixed exhaust valve opening (EVO) and IVC timings tends to advance the ignition timing of the first power stroke (PS1) but has minimal effect on the ignition timing of PS2. Re-breathing was found to be an effective way to control the ignition timing in PS2 at a slight expense of the combustion efficiency. The operation of a six-stroke GCI engine could be successfully simulated and the operability range of the engine could be substantially extended by employing the CVVD technique. In addition, the control of valve timings could successfully control the thermodynamic and compositional conditions of in-cylinder mixtures that enable to control the combustion phasing.