A hydrogen internal combustion engine vehicle is powered by combusting hydrogen in a combustion chamber similar to today's petrol engines, which also produces water as emissions. The expansion of the gases caused by this combustion then turns the rotor (in the case of the RX-8 Hydrogen RE) to drive the vehicle.



Hydrogen combustion: how does it work?
Hydrogen: a clean, recyclable and abundant fuel source

Hydrogen is one of the most widely available fuels: it accounts for 75% of the matter in the universe on its own. It is found in abundance in the stars and gassy giant planets.

On Earth, however, it is very rare in its natural state (its very low mass means that it escapes gravity's pull). It is, however, found in river and ocean water. Water electrolysis is needed to extract it, a process which splits the H20 water molecule in two: this produces dioxygen 02 on the one hand, and dihydrogen H2 on the other. This electrolysis uses electricity, which can be produced in a variety of ways (wind, solar, hydro-power, etc.).

Hydrogen can also be extracted from organic fossil matter (coal, petrol, natural gas), and is a by-product of a number of industrial processes (chemical, welding, etc.).

Hydrogen combustion follows the simplest of chemical formula: two H2 molecules combine with an 02 molecule to form two H20 molecules in vapour form, while at the same time releasing a very large quantity of energy. This reaction produces extremely few NOx nitrogen oxides and gives off no C02 (a greenhouse gas) whatsoever.

Hydrogen is considerably more combustible than conventional petrol. It can cope with very low concentrations (a mere 4% in air is sufficient for combustion to occur), and the flame front spreads much more quickly (approximately 265 cm/sec in stoechiometric conditions as opposed to 40 cm/sec for petrol). However, hydrogen does possess less energy at an equivalent volume. It is a fuel which is used widely in rockets, including the Space Shuttle. And it can be used as a fuel for an internal combustion engine as on the RX-8 Hydrogen RE.

Unlike fossil fuels, hydrogen forms part of a perfectly-balanced cycle: the hydrogen extracted from water by electrolysis is liberated after use in the form of vapour, which returns to the natural water cycle, while the C02 given off by the combustion of fossil fuels is in excess of the quantity usually absorbed by plants.

Burning hydrogen is not the only way of releasing the energy required to power a vehicle: hydrogen can also power a fuel cell, inside which it reacts with oxygen to produce an electrical current. However, despite the fact that the fuel cell has advantages (high energy output, zero nitrogen oxide released), it is also complex and expensive to
manufacture, and requires a total redesign of the powertrain and its incorporation into the vehicle. This technology is not yet sufficiently mature, therefore, for use in everyday life.



The rotary engine: ideally suited to hydrogen fuel

Mazda's decision to opt for the rotary engine rather than a piston engine as the basis for its hydrogen vehicle designs is not attributable solely to the brand's unique experience with this technology. The rotary engine is particularly suited to the specific requirements of hydrogen fuel.

As we have seen, hydrogen is astonishingly explosive, which can produce problems in the combustion chamber of a piston engine (abnormal combustion). In a traditional engine, the fuel-air mixture is injected directly into a high-temperature combustion chamber which is sealed by very hot exhaust valves. These are hardly favourable conditions, and make hydrogen less attractive as a fuel for a reciprocal engine.

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