CFD Modeling of Conventional and Pre-Chamber Ignition of a High-Performance Naturally Aspirated Engine

The abatement of carbon dioxide and pollutant emissions on motorbike spark-ignition (SI) engines is a challenging task, considering the small size, the low cost and the high power-to-weight ratio required by the market for such powertrain. In this context, the passive pre-chamber (PPC) technology is an attractive solution. The combustion duration can be reduced by igniting the air-fuel mixture inside a small volume connected to the cylinder, unfolding the way to high engine efficiencies without penalization of the peak performance. Moreover, no injectors are needed inside the PPC, guaranteeing a cheap and fast retrofitting of the existing fleet. In this work, a 3D computational fluid dynamics (CFD) investigation is carried out over an experimental configuration of motorbike SI engine, operated at fixed operating conditions with both traditional and PPC configurations. The employed CFD methodology is based on a unique flamelet-based combustion model, regardless the selected ignition strategy. First, 3D gas exchange simulations were performed to achieve realistic conditions for the power-cycle analysis. A specific care was given to the analysis of the PPC scavenging process, which is crucial for a reliable estimation of the exhaust gases stratification near the ignition position. Then, the combustion process was simulated with both ignition strategies, clarifying in particular the dependency of the hot-gases ejection process from the flow field inside the PPC. Finally, a numerical-experimental comparison was carried out in terms of pressure and heat release trends, demonstrating the reliability of the employed CFD methodology in the design of high-performance SI engines.