Under the Skin: Will Hydrogen Combustion Engines Become Viable?


Hydrogen has so far struggled to deliver on its initial promises as an alternative fuel for road transport, but it’s a topic that simply refuses to go away.

Its use in hydrogen fuel cells to produce electricity without emissions still has enormous potential, but the impetus behind the operation of internal combustion engines has weakened. Interest remains, however, one example being Toyota’s development of a three-cylinder hydrogen racing engine taken from the GR Yaris and used to power a specially developed Corolla Sport entered in the Fuji 24 Hours.

Although hydrogen is a clean fuel compared to gasoline or diesel, it is only completely emission free when converted in a fuel cell system to generate electricity. When burnt in a combustion engine, it’s not quite. Although no unburnt hydrocarbons (HC), carbon monoxide (CO) or CO2 are produced, nitrogen oxides (NOx) are. Air is 78% nitrogen, and upon combustion it is oxidized to produce toxic NOx, but the amount depends on the temperature of the combustion chamber, and this is where hydrogen engines may have an advantage.

Hydrogen is much less difficult than gasoline or diesel, mixing and burning fully and efficiently in a much wider range of air-fuel ratios. As a result, a hydrogen engine can run very poorly (more air, less fuel) and still produce much lower levels of NOx than gasoline or diesel. Exhaust emissions can be reduced to minute levels using existing exhaust emissions technology.

These attractive facts depend on a number of things. Although hydrogen carries a large amount of energy by weight, it is much less dense than liquid fuels, hence orifice injection engines, in which fuel is injected into the intake manifold and mixed with the air outside the cylinders, produce much less power running on hydrogen than they do on gasoline. Direct injection makes things better and, in combination with variable geometry turbocharging, makes hydrogen combustion engines more viable.

There is still a compromise. Modern direct injection turbocharged hydrogen engines can produce more power than an equivalent gasoline engine by increasing the proportion of hydrogen in the air-fuel mixture, but NOx levels increase. Otherwise, hydrogen engines are essentially modified gasoline engines which in production form would have stronger components and direct hydrogen injection systems. Hydrogen can be stored in the same proven 700 bar compressed gas tanks that are used in fuel cell vehicles.

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