Numerical Investigation to Fuel Injection Strategy and Thermal Condition Impacts on GCI Combustion at Low and Medium Loads Using CFD

This research numerically investigated the combustion process and exhaust emissions from a light-duty Gasoline Compression Ignition (GCI) engine operating at low load as well as medium load conditions using a commercial computational fluid dynamic (CFD) software Converge. The fuel injection strategies and thermal boundary conditions effects were examined to produce locally stratified and globally lean partially premixed compression ignition (PPCI) combustion. The effects of fuel injection pressure, number of injections, and the quantity of fuel injected in each pulse were examined and optimized for emissions and fuel consumption (FC) under the design constraints of 180 bar peak cylinder pressure (PCP) and 10 bar/° CA maximum pressure rise rate (MPRR). At 1250 rpm/3 bar indicated mean effective pressure (IMEP), the desired cylinder temperature and residual gas fraction at intake valve closing (IVC) was achieved by using exhaust gas rebreathing, uncooled exhaust gas recirculation (EGR), and heated intake air via an external heater. The simulation results showed that a high level of EGR dilution up to 55 % was required to control the combustion phasing, the MPRR, and the oxides of nitrogen (NOx) emissions. The optimized dilution effectively slowed down the combustion process and led to the reduced combustion temperature without combustion stability concerns. The CFD simulation revealed the importance to contain the combustible fuel-air mixture within the combustion bowl in reducing combustion losses and soot, CO emissions. At 2000 rpm/12 bar IMEP, PPCI-diffusion combustion was achieved by employing a split injection strategy. Various fuel quantities for each injection were evaluated. It was concluded that GCI combustion with proper thermal boundary conditions was able to achieve 44 % closed-cycle indicated thermal efficiency and improved fuel consumption at the cost of NOx, HC, and CO at low load. At medium load, GCI showed encouraging engine and emissions performance over the baseline diesel engine.