Mazda RX-8 Hydrogen RE (2007) CAR review

How does a hydrogen-powered RX-8 work – and no technobabble please...

It's simple. The RX-8's rotary engine can run on either petrol or hydrogen, which is stored in liquid form in a second fuel tank. The engine is fitted with a second set of hydrogen injectors that are situated above the intake chamber, and the driver has a switch in the cabin for selecting the fuel of choice. A rotary engine is far better suited to burning hydrogen than a conventional piston engine – which goes some way to explaining Mazda's persistence with the rotary engine when all other manufacturers left it behind.

Why is a rotary engine better suited to hydrogen?

Hydrogen, as you know, is more than a touch volatile – its 'flame front' – the point at which it bursts into flame – is almost seven times that of unleaded petrol. Which means keeping it away from heat until actual combustion is crucial. Unlike a piston engine where intake and combustion occur in the same chamber, a rotary engine has separate intake and combustion chambers, so the temperature in the intake chamber is always going to be much lower than in a piston engine. Indeed, in converting it to hydrogen, the only changes Mazda's engineers made to the RX-8's twin rotor engine are the hydrogen injectors, a gas recirculating system to further boost combustion efficiency and a second engine management system to look after the engine when it’s running on hydrogen.