Hydrogen mixing and combustion in an SI internal combustion engine: CFD evaluation of premixed and DI strategies

In this study, the effect of start of injection (SOI) timing is investigated on a single cylinder spark ignited (SI) internal combustion engine (ICE) fueled with hydrogen, using Computational Fluid Dynamics (CFD) simulations. Mixing fields and combustion behaviors at different relative air-fuel ratios (λ) and injection strategies are discussed. Direct injection (DI) cases with three different SOI timings for each λ are compared against port-fuel injection (PFI) cases modeled assuming perfectly premixed charge at the intake plenum. Results for the investigated cases show that PFI cases at λ = 2 and λ = 3 have very high combustion efficiency while at λ = 3.5 partial burn occurs (only 70% of the fuel is oxidized). In DI cases, combustion efficiency decreases compared to corresponding premixed situations, as expected, but significant improvements are obtained by advancing the SOI timing towards the end of the intake stroke. In addition, early SOI timings tend to reduce the duration of initial combustion stage, CA0-10, which correlates with ignition repeatability, therefore suggesting potential benefits on combustion stability under very lean conditions. The method and results of this work can be used as a guideline for developing efficient hydrogen injection strategies.