Comparison of the renewable transportation fuels, hydrogen and methanol formed from hydrogen, with gasoline – Engine efficiency study

The use of hydrogen derived methanol in spark-ignition engines forms a promising approach to decarbonizing transport and securing domestic energy supply. Methanol can be renewably produced from hydrogen in combination with biomass or CO2 from the atmosphere and flue gases. From well to tank studies it appears that hydrogen derived methanol compares favourably with liquid or compressed hydrogen both in terms of production cost and energy efficiency. Since existing well to wheel studies are based on outdated technology, this paper tries to provide efficiency figures for state-of-the-art hydrogen and methanol engines using published data and measurements on our own flex-fuel engine.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, where improvements up to 40% relative to gasoline were possible. At elevated loads, rising NOx emissions necessitated a switch to throttled stoichiometric operation, resulting in efficiencies comparable to those on gasoline. The efficiency benefit of methanol is more modest (5–10% relative to gasoline), but can be retained over the entire load range. These improvements are mostly due to reduced pumping losses, increased burning velocities and a 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.

► Well-to-tank energy efficiency and economics of hydrogen and methanol are comparable.
► Well-to-wheel studies underestimate engine efficiency improvements enabled by these fuels.
► A clear distinction must be made between dedicated and flex-fuel engines.
► We provide published efficiency values for dedicated methanol and hydrogen engines.
► We provide efficiency values for flex-fuel engines measured on our own test engine.