Numerical evaluation of wall-guided spark assistance for cold operations in a heavy-duty gasoline compression ignition engine

Abstract

This study focused on conducting three-dimensional (3D) computational fluid dynamics (CFD) simulations to evaluate the impact of wall-guided spark plug ignition assistance on gasoline compression ignition (GCI) combustion performance during cold operations. The numerical simulations were performed on a 6-cylinder, 15 L heavy-duty diesel engine operating at a compression ratio (CR) of 16.5 using RON92 E0 gasoline. Spark assistance was simulated by incorporating an ignition model with the necessary spark energy to ensure stable combustion during cold operations. Initially, closed-cycle 3D CFD combustion simulations, based on a design of experiments (DoE) campaign, were employed to investigate a lean spark-assisted CI strategy during the cold-to-warm transition. A numerical optimization approach, based on a Response Surface Model (RSM), was adopted to optimize injector spray patterns, fuel injection strategies, and in-cylinder swirl motion to achieve the best performance of wall-guided spark assistance combustion. The optimized injector spray pattern, fuel injection strategy, and swirl ratio were predicted to provide 12 percentage points increase in combustion efficiency over the baseline, while meeting constraints for maximum pressure rise rate (MPRR) and NO_x emissions (MPRR < 5 bar/CAD and NO_x < 2 g/kWh). Building on the aforementioned optimal design, an open-cycle 3D CFD analysis was subsequently carried out to simulate engine starts at various cold ambient temperatures using a stoichiometric, spark-initiated flame propagation strategy. The results showed that with the optimized design, reasonable GCI combustion during engine starts was achieved at cold ambient temperature of 300 K. Notably, as the ambient temperature decreases from 300 to 243 K, the emergence of a noticeable fuel film was observed, presenting an undesirable potential for increased engine-out unburned hydrocarbon and smoke emissions. Considering the specific emission requirements at lower ambient temperatures, further investigations are warranted to assess the significance of the observed fuel film in relation to these criteria.