The Effects of Piston Shape in a Narrow-Throat Pre-Chamber Engine

The current work utilizes computational fluid dynamics (CFD) simulations to assess the effects of different piston geometries in an active-type pre-chamber combustion engine fueled with methane. Previous works identified that the interaction of the jets with the main chamber flow and piston wall are key aspects for the local turbulent flame speed and overall burning duration. The combustion process is simulated with the G-equation model for flame propagation combined with the MZ-WSR model to determine the post-flame composition and to predict possible auto-ignition of the reactant mixture. Four setups were considered: two bowl-shaped and one flat piston, and one additional case of the flat piston with jets at wider jet angles to the cylinder axis. The results show that premature jet-wall interaction impacts the main chamber pressure build-up, turbulence, and burn rate. It was found that during the jet issuing event, the bowl pistons have an initially faster mass burn rate compared to the flat geometry; towards the end of combustion, the flat piston compensates for the initial disadvantage and consumes the overall charge faster due to the absence of squish. Major differences were observed in heat transfer, peak pressure, and nitric oxide emissions. Ultimately, the turbulence-chemistry analysis and classification of the combustion regime according to the Borghi-Peters diagram are found to lie mostly within the thin reaction zone.