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The passive prechamber concept, known as jet ignition (JI), represents an effective way to promote mixture ignitability, reduce combustion duration and extend knock limits in spark ignition engines. These aspects allow the adoption of a higher compression ratio and the operation in lean conditions, thus increasing thermal efficiency. Despite the potential benefits, the literature typically shows that in port fuel injection (PFI) engines at full load a shorter combustion duration does not necessarily translate in a growth of IMEP. Despite this issue has been frequently observed, the causes have not been fully explained. In previous works some of the authors supposed that the gain in indicated efficiency could be counterbalanced by the lower adiabatic efficiency, as a result of the higher heat exchange coefficient and the additional heat transfer from the prechamber surface.

In the last years, the engineering in the automotive industry is revolutionized by the continuous research of solutions for the reduction of consumptions and pollutant emissions. On this topic maximum attention is paid by both the legislative bodies and the costumers. The more and more severe limitations in pollutant and CO2 emissions imposed by international standards and the increasing price of the fuel force the automotive research to more efficient and ecological engines. Commonly the standard approach for the definition of the engine parameters at the beginning of the design process is based on the wide-open throttle condition although, both in homologation cycles and in the daily usage of the scooters, the engines work mainly at partial load where the efficiency dramatically decreases. This aspect has recently become strongly relevant also for two wheeled vehicles especially for urban purpose.