Computational analysis of the in-cylinder mixture formation in a direct injection hydrogen spark-ignition engine

Hydrogen is considered a vital solution for decarbonizing road transport due to its ability to operate efficiently at high dilution levels, thereby reducing nitrogen oxide (NOx) emissions and enhancing engine efficiency. This study delves into the fundamental phenomena governing air-fuel mixture formation in a spark-ignition (SI) engine fueled with hydrogen, with a focus on the effects of turbulence and in-cylinder airflow. To achieve this, a bespoke computational methodology was developed, combining constant-pressure test rig and engine simulations. The accuracy of these simulations was validated against experimental data for fuel injection, mixture formation, and combustion in both cases. A comprehensive parametric study was conducted via numerical simulations, exploring the effects of varying start-of-injection (SOI) timings and air-fuel ratios (A) on the mixing process. The results reveal that injection timing has a profound impact on the mixing process, imposing critical constraints on combustion initiation and pollutant emissions. Late injections-SOI timing of-130 CAD aTDC-result in a highly stratified mixture in terms of equivalence ratio (cent), with a difference of almost 85% between the average global equivalence ratio (cent = 0.94) and the target equivalence ratio cent = 0.38 at the onset of combustion. The temperature field also shows some degree of stratification, although for the case of the most delayed injection, the average mixture temperature is reduced by more than 5%. Notably, the hydrogen stratification dominates over the temperature stratification, yet they exhibit a clear correlation that could potentially inhibit NOx production. The enhanced reactivity of hydrogen-rich mixtures may be offset by lower mixture temperatures, an effect that becomes more pronounced as the overall dilution ratio decreases: in the most diluted case (A = 2.2), a reduction of around 3% in the average temperature of the mixture is observed with respect to the A = 3.2 case. This is attributed to the increased mixture reactivity, which leads to higher NOx emissions while preventing engine misfire. Ultimately, this study provides valuable insights into the complex interactions governing hydrogen injection in spark-ignition engines, shedding light on the optimization of injection strategies and operating conditions to minimize pollutant emissions and improve engine efficiency, thereby contributing to the development of more sustainable and environmentally friendly transportation solutions.