Computational optimization of the performance of a heavy-duty natural gas pre-chamber engine

Abstract

Optimization of the performance of a heavy-duty natural gas active pre-chamber (PC) engine was conducted using computational fluid dynamics (CFD) simulations. Seven different piston geometries were evaluated for their impact on engine performance. Unlike the narrow-throat PC, various piston geometries yielded a significant impact on the PC flow motion and fuel–air mixing process using the large-throat PC. Compared to flat-bottom pistons, ω-bottom pistons promoted the reverse flow from the MC and induced a broader lean-mixture distribution in the PC. For various ω-bottom pistons, enlarging the inner piston radius resulted in slower flame propagation within the PC, delaying the jet issuing and slowing down the turbulent flame propagation within the MC. The flat piston effectively reduced jet flame-piston interaction and promoted turbulent flame propagation within the squish region during the late combustion stage, but it generated higher wall heat transfer loss from the cylinder liner. A composite of ω- and flat-shaped pistons maintained the benefits of relatively high flame propagation speed within the squish region but low wall heat transfer loss, yielding the highest thermal efficiency of seven piston designs. The PC fueling ratio (PCFR) was found to be a critical factor in influencing the jet issuing process. A lower PCFR to establish a near-stoichiometric mixture within the PC promoted flame propagation and led to a faster jet ejection. An increase in compression ratio yielded higher engine efficiency due to the advanced combustion phasing and larger expansion ratio, it also resulted in higher wall heat transfer loss and nitric oxide (NOx) emission. The introduction of exhaust gas recirculation was able to effectively inhibit NOx formation, while also resulting in higher engine efficiency due to the lower wall heat transfer loss.