Keywords: hydrogen, exhaust gas recirculation, EGR, internal combustion engines, lean burn, combustion strategies, PFI, port fuel injection
Combustion strategies for PFI hydrogen IC engines
Hydrogen is a very interesting fuel when it comes to load control. The high flame speeds of hydrogen mixtures and its wide flammability limits permit very lean operation. Where possible, Wide Open Throttle (WOT) operation is used, which regulates the load with the mixture richness (qualitative control) instead of using volumetric regulation (quantitative control) and, thus, avoiding pumping losses. In a search for high power densities, high efficiencies and most importantly, low NOX emissions, several combustion strategies have to be considered. One can choose a lean burn strategy where quality-based mixture control is used, but in this case, NOX emissions are problematic for intermediate to high loads. Using stoichiometric mixtures with external Exhaust Gas Recirculation (EGR) is a solution, but its effect on efficiency and power output has to be considered. This paper presents the experimental results on a single-cylinder engine. Tests were carried out for lean hydrogen-air mixtures and stoichiometric hydrogen-air mixtures with the addition of a variable amount of EGR. A comparison is made of the two strategies; the influence of EGR and lean combustion on the indicated power output, indicated efficiency and NOX emissions and the conversion efficiency of a Three-Way Catalyst (TWC) is examined.