BMW has unveiled the hydrogen technology used in its upcoming "7" series sedan. The vehicle will be powered by its standard V-12 engine, which has been modified to accept both hydrogen and gasoline as fuel.

In addition to its standard gasoline tank and fuel system, a large, double-walled stainless steel tank or "dewar" is mounted in the trunk. Liquid hydrogen is carried in the tank at cryogenic temperatures. A layer of thermal insulation as well as a vacuum between the inner and outer tanks provides the necessary thermal protection needed to keep the liquid hydrogen chilled.

Credit: BMW & NHA Annual Hydrogen Conference 2006

When hydrogen is selected as the desired fuel, the chilled hydrogen gas present at the top of the tank is drawn off. As this chilled gas leaves the tank, it is drawn through piping looped between the inner and outer tank walls. By this method, the super cold vapor absorbs any heat present in thermal insulation. The vapor is then passed through a valve unit where it is warmed by engine coolant. At this point the hydrogen gas pressure is at 80 PSI and the hydrogen is sent to the gaseous injectors on the engine.

To provide additional safety, there is no venting of hydrogen from this system. All chilled hydrogen dewars must vent some gaseous hydrogen as they warm up, otherwise the gas pressure could become excessive and rupture the tank. However, when any venting of hydrogen occurs in the BMW system, the excess hydrogen is sent to a small "combustion unit" located underneath the car. Within this unit, the hydrogen is mixed with air, ignited by a glow plug, and allowed to burn off leaving nothing but water.

BMW representatives did not quote specific figures, but it appears that the H2 "7" can travel about 120 miles on a fill of liquid hydrogen. When the hydrogen fuel is exhausted, the engine management system will automatically switch over to gasoline.