Impact of flame kernel formation process on ultra-lean hydrogen combustion in spark-ignition engines: modeling and experimental validation

Ultra-lean operation of hydrogen-fueled spark-ignition (SI) engines is a promising solution for the ongoing energy transition, but simulation of their performance poses a major challenge due to the complex interaction between thermo-diffusive (TD) flame instability and in-cylinder turbulence. This interaction influences even the flame kernel (FK) formation process, whose significant impact on the engine performance has so far been estimated primarily through experimental evidence. Here, this estimation is conducted by integrating a FK model recently proposed by the authors into a 0D/1D engine simulation framework formulated within a commercial software package. The FK model provides a 1D description of an expanding spherical kernel based on the transient thermo-diffusive theory through laminar flame speed, activation energy, and Lewis number. From the FK model outputs, a kernel duration is defined and used as a delay between spark timing and start of combustion in the 0D/1D simulation. Consolidated literature sub-models account for TD instability and turbulence effects during both FK formation and in-cylinder combustion, simulating the latter with a phenomenological approach. The model predictions are compared with experimental outcomes from a single-cylinder hydrogen-fueled SI engine operated at 1500 rpm, different loads (3–8 bar BMEP) and multiple equivalence ratios (0.25–0.71). The instability-affected FK delays are captured well in all 21 operating points tested, leading to substantial improvements in the prediction of pressure cycles, burn rates, combustion angles, BMEPs, and NOx emissions. The accuracy of the proposed 0D/1D simulation makes it suitable for ‘virtual engine’ applications aimed at fast calibration and testing of alternative designs.