Numerical analysis of a high-performance motorbike engine: from conventional to pre-chamber combustion strategy

The reduction of pollutants and CO2 emissions is a compelling topic even for small size, high-performance engines for motorbikes. The turbulent jet ignition (TJI) combustion strategy is a noteworthy solution for such powertrains, especially employing a pre-chamber in its passive configuration, thanks to the minor engine modifications needed and the low additional cost. The passive pre-chamber ignition allows to achieve a significant reduction of the combustion duration, hence opening to a possible increase of engine efficiency and performance. In this work, a 3D computational fluid dynamics (CFD) investigation is carried out over an experimental configuration of a naturally aspirated spark ignition (SI) motorbike engine running at several different operating points employing both the conventional and the TJI combustion strategies. The purpose of the research is to achieve a deeper understanding of the experimental data by means of the CFD analyses, clarifying the advantages and drawbacks of the retrofit of a motorbike engine with a passive pre-chamber. The investigations on the pre-chamber scavenging show that the exhaust gas residuals (EGR) concentration inside it mainly depends on the EGR in cylinder, rather than the pre-chamber geometry. The impact of the flow field development during the pre-chamber filling process on the subsequent combustion for the various tested operating conditions is presented. The flame front propagation in both combustion chambers is differently affected, showing a greater asymmetry at higher engine speeds. The turbulence introduction in the main chamber provided by the jets ignition is demonstrated and quantified by means of the turbulent combustion regimes diagram. The wall heat transfer investigation highlights the greater heat losses experienced by the passive pre-chamber engine layout. The higher heat loss contribution for each of the combustion chambers surfaces is demonstrated and quantitatively estimated, emphasizing the differences among the various operating conditions. The impact of the increased heat losses on the engine indicated mean effective pressure (IMEP) is assessed.