The Potential of Hydrogen High Pressure Direct Injection toward Future Emissions Compliance: Optimizing Engine-Out NOx and Thermal Efficiency

By building on mature internal combustion engine (ICE) hardware combined with dedicated hydrogen (H2) technology, the H2-ICE has excellent potential to accelerate CO2 reduction. H2-ICE concepts can therefore contribute to realizing the climate targets in an acceptable timeframe. In the landscape of H2-ICE concepts, pilot-ignited High Pressure Direct Injection (HPDI™) is an attractive option considering its high thermal efficiency, wide load range and its applicability to on-road as well as off-road heavy-duty equipment. Still, H2-HPDI is characterized by diffusion combustion, giving rise to significant NOx emissions. In this paper, the potential of H2-HPDI toward compliance with future emissions legislation is explored on a 1.8L single-cylinder research engine. With tests on multiple load-speed points, Exhaust Gas Recirculation (EGR) was shown to be an effective measure for reducing engine-out NOx, although at the cost of a few efficiency points. Furthermore, the use of EGR was compared to water injection in the intake port. Water injection displayed a substantially weaker NOx reduction sensitivity, owing in part to a relatively low injection pressure applied in these tests, causing poor water atomization and evaporation. Finally, injection timing sweeps showed that the gross indicated thermal efficiency (ITE) still approached or surpassed 50% for most load-speed points with EGR. At higher loads, where peak pressures constraint the injection advance, ITE was lowest. While further development steps are necessary to ensure compliance, this work demonstrates that H2-HPDI has good potential for meeting upcoming NOx legislative levels, with a potential ITE comparable to, or perhaps even better than, that of modern diesel engines.