Investigation of supercharging strategies for PFI hydrogen engines

Hydrogen-fueled internal combustion engines (H2 ICEs) are an affordable, practical and efficient technology to introduce the use of hydrogen as an energy carrier. They are practical as they offer fuel flexibility, furthermore the specific properties of hydrogen (wide flammability limits, high flame speeds) enable a dedicated H2 ICE to reach high efficiencies, bettering hydrocarbon-fueled ICEs and approaching fuel cell efficiencies. The easiest way to introduce H2 ICE vehicles is through converting engines to bi-fuel operation by mounting a port fuel injection (PFI) system for hydrogen. However, for naturally aspirated engines this implies a large power penalty due to loss in volumetric efficiency and occurrence of abnormal combustion. The present paper reports measurements on a single cylinder hydrogen PFI engine equipped with an exhaust gas recirculation (EGR) system and a supercharging set-up. The measurements were aimed at increasing the power output to gasoline engine levels or higher, while maximizing efficiency and minimizing emissions. Two strategies were tested: one using stoichiometric mixtures, with or without EGR, where a three way catalyst (TWC) was relied upon for aftertreatment of oxides of nitrogen (NOx ); and a second one using lean mixtures limiting engine-out NOx emissions so that aftertreatment was not needed. Test results are reported for varying supercharging pressure and engine speed, it is shown that both strategies allow power outputs exceeding gasoline levels. The brake thermal efficiencies for both strategies are compared, to derive the best operating strategy as a function of torque demand and engine speed. It can be concluded that the lean burn supercharged strategy is best overall