Rudolf Erren, a German engineer who in the 1920s and 30s advanced the use of hydrogen as a transportation fuel in cars, trucks, buses, and submarines. Erren developed a new fuel injection system that allowed the hydrogen to be fed directly into the cylinder, thereby eliminating the carburetor, which was poorly suited to inject a gaseous fuel. The remaining engine components were unchanged, minimizing conversion costs, and the vehicles were able to operate on either hydrogen or other hydrocarbon fuels while in operation with the flip of a switch from inside the vehicle. It has been estimated that Erren and his colleagues converted thousands of vehicles in Europe and the U.S.
Hydrogen Basics (U. S. Department of Energy, National Renewable Energy Laboratory)
Hydrogen, Fuel Cells and Infrastructure Technologies Program (U. S. Department of Energy, Office of Renewable Energy and Energy Efficiency)