In the continuing development of a hydrogen fueled IC engine optimized for application to a generator set or hybrid vehicle, experiments were performed at Sandia National Laboratories on two engine configurations. The intent is to maximize thermal efficiency while complying with strict emissions standards. The initial investigation was conducted utilizing a spark ignited 0.491 liter single cylinder Onan engine and has progressed to a spark ignited 0.850 liter modified for single cylinder operation Perkins engine. Both combustion chamber geometries were “pancake” and achieved a compression ratio of 14:1. The engines were operated under premixed conditions. The results demonstrate that both engines can comply with the California Air Resources Board's proposed Equivalent Zero Emission Vehicle standards for NOx during operation at an equivalence ratio of 0.4. The Onan engine achieved an indicated thermal efficiency of 43% at 1800 RPM, as determined by integration of the pressure-volume relationships. Initial experiments with the lager displacement Parkins engine have realized a gain, relative to the Onan engine, in indicated thermal efficiency of 2% at 1800 RPM, and 15% at 1200 RPM.