BMW CleanEnergy.

(Taken from BMW web site)

Hydrogen will gradually replace petroleum as the key automotive fuel, leading to a sustainable reduction in CO2 emissions. Sheer driving pleasure - without emissions: this is the inspiration behind BMW CleanEnergy. The BMW Group has developed hydrogen engines that perform with the typical BMW characteristics: dynamics, efficiency and smooth power delivery - & all without harmful emissions.

Hydrogen, which can be won from water using regenerative energy sources such as solar and wind energy, reduces our dependence on fossil fuels and combusts to produce energy and water - and zero CO2, the main cause of the greenhouse effect. The result is the "hydrogen loop": water converts to hydrogen and then back to water, with no harmful emissions and the promise of limitless supplies.
As an automotive fuel, hydrogen is used in two different ways: either in fuel cells that produce electricity to power electrical engines, or as BMW has developed it, in combustion engines.
A key advantage of the combustion engine approach is a factor known as bivalence: both hydrogen and petroleum can be used to fuel such engines, enabling an efficient use of existing infrastructure. This supports a smooth transition from fossil fuels to regenerative energy.
The BMW CleanEnergy power units are bivalent, enabling drivers to use petrol when, for example, no hydrogen fuelling station is available. These vehicles have both a hydrogen and a petrol fuel tank. When one is empty, the bivalent engine switches automatically to the other.