Both fuels offer a great potential

Both fuels offer a great potential for efficiency improvements compared to gasoline engines thanks to a variety of favourable properties. However, there is a clear distinction between engines specifically designed for hydrogen or methanol operation and flex-fuel engines, which should also run on gasoline. For dedicated engines, the literature indicates that peak brake thermal efficiencies up to 45% and 42% are possible on hydrogen and methanol respectively. The ability to employ qualitative load control instead of throttling enables relative efficiency improvements compared to gasoline between 10 and 20% due to reduced pumping losses in part load. On our flex-fuel engine, operation on hydrogen using qualitative load control enabled the highest efficiencies, especially at low loads, whereimprovements up to 40% relative to gasoline were possible. At elevated loads, rising NOxemissions necessitated a switch to throttled stoichiometric operation, resulting in efficiencies comparable to those on gasoline. The efficiency benefit of methanol is moremodest (5e10% relative to gasoline), but can be retained over the entire load range. Theseimprovements are mostly due to reduced pumping losses, increased burning velocities anda slight decrease in cooling losses. Future well-to-wheel studies should take this considerable potential for efficiency improvements into consideration and should also distinguish between dedicated and flex fuel engines.