Experimental Assessment of the Heat Losses Due to the Adoption of a Passive Prechamber in a Jet Ignition 4-Stroke Engine

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. This paper thus aims at explaining the thermal losses of a single-cylinder four-stroke PFI engine, both in baseline and passive JI configurations, by means of the experimental measurement of the thermal power released to cooling system and exhaust gas. Results show a decrease of IMEP in the JI engine at low and medium speed, caused by the high heat transfer to the cooling medium which prevails on the lower heat flux released to the surrounding ambient. At high speed instead the JI engine shows an improved IMEP, but the raise of frictions due to the higher peak pressure leads anyway to a BMEP reduction. The sum of these features thus leads to a slightly lower engine power in the whole rpm range, explaining the limits of the investigated technology.