Mixture formation in a direct injection gas engine: Numerical study on nozzle type, injection pressure and injection timing effects

• Direct gas injection is simulated during an engine compression stroke.
• Injection timing has a profound influence on mixing rate.
• Importance of mixing mechanisms is highly dependent on nozzle type.
• Jet-piston interaction provides a mixture confinement method.
• Phase space visualizations reveal two stages of mixture evolution.

DI (direct injection) gas engines aim at providing clean and efficient combustion. Mixture quality control and hydrocarbon emission reduction are key development challenges in such engines. Here, a CFD (computational fluid dynamics) study of the DI gas injection process is carried out. The aim is to provide knowledge that aids e.g. engine designers in i) extending the lean limit at part load conditions via stratified mixtures, ii) mitigating incomplete combustion by improving mixing and eliminating fuel crevice flow. We investigate the sensitivity of the mixture formation process to nozzle type, injection pressure and injection timing. First, the present CFD method is discussed in free gas jet computations. For reference, we utilize planar laser induced fluorescence measurements and large eddy simulation results. After this, a total of 12 DI cases in moving mesh engine conditions are simulated. The main findings and novel results are listed as follows: 1) injection timing has a considerable influence on mixing rate, 2) efficacy of mixing mechanisms is highly nozzle type dependent, 3) jet-piston interaction may be utilized in the generation of a confining toroidal vortex in the piston bowl, 4) phase space analysis reveals two highly case dependent stages of mixture evolution.