BMW CleanEnergy vehicles have both a hydrogen and a petrol tank. If one of the tanks is empty, the bivalent BMW engine unit switches over automatically to the other fuel system. One decisive advantage of the combustion engine is bivalence. It enables both hydrogen and petrol to be used, thus creating the ideal conditions for a transition from non-regenerative to regenerative drive energies. During electrolysis water is split into its components of hydrogen and oxygen. The hydrogen is stored and the oxygen is needed in the vehicle for the combustion of the hydrogen. The combustion process generates energy and water as a waste product, which returns to the natural water cycle. And so with the aid of regenerative energy suppliers, an emission-free fuel is obtained.