The hydrogen engine is a spark ignition engine running on hydrogen fuel. Conventional gasoline engines used in cars can be converted into hydrogen engines by some modifications.

Main exhaust emission is water. As far as energy efficiency and applicability in ground transportation are concerned, the main challenges lie in hydrogen generation and on-board storage.

Hydrogen storage

The following table shows the heat value of hydrogen compared to gasoline.

Source: Buchner 2000

Due to the low volume-specific heat value of H2 (about 3000 times smaller than that of gasoline), on-board storage in both cars and railways is a challenge.

The following methods exist:

• as compressed gas
• in liquid form
• in a hydrogen-absorbing metal alloy (metal hydride).

The benefits of metal hydride storage include the fact that a relatively large volume of hydrogen can be stored per tank, and the fact that it is almost impossible for large volumes of hydrogen to be released into the air, which gives a safety advantage. The disadvantage of this method are tank weights. In order to combat this, R&D (e.g. at Mazda) is putting effort into the development of metal alloys with higher absorption capacity.

Hydrogen generation

The most common way of obtaining hydrogen is water electrolysis according to the reaction:

2 H2O -> 2 H2 O2

Another promising approach lies in direct conversion from fossil sources. Options discussed are:

• Reforming from carbon gas
• Partial oxidation of heavy hydrocarbons
• Steam reforming from natural gas