Maybe it's a case of inferior public relations or perhaps it's just that battery-powered cars have better managed to capture the imagination of mainstream motorists, but one just doesn't hear much about the hydrogen-fuelled vehicles that were seen as our salvation just a few short years ago. But at least two companies are taking a much less travelled road to the hydrogen highway, using the lighter-than-air gas to fuel internal combustion engines. BMW is perhaps the better known with its Hydrogen7, the V12-powered 7-Series that is altered to run on both petrol and hydrogen.
But Mazda, too, has developed hydrogen-fuelled vehicles, but in this case it's using the company's unique Wankel rotary engine. It's an odd choice, since the rotary is known as impressively powerful for its displacement but voracious in its appetite for fuel. But, as Akihiro Kashiwagi, Hydrogen RE Development programme manager, points out, the rotary is actually better suited for conversion to hydrogen than the piston engine. The key issue, says Kashiwagi, is that hydrogen is much more easily burnt than petrol. This causes a problem in piston engines because the hydrogen must be injected into the hot combustion chamber, which means that ignition can occur prematurely.
The Wankel engine, on the other hand, has separate chambers for the intake and combustion cycles so that the raw hydrogen is injected into the inlet chamber and only reaches the hot combustion area shortly before it has to burn. Despite this relative efficiency, however, Mazda's rotary still produces about half the power (109hp versus 210) when fuelled by hydrogen rather than petrol because of hydrogen's inherently poorer energy density.
Indeed, when driving the hydrogen-powered RX-8, it's quickly obvious that much of the pizzazz is sacrificed in the quest for (near-) zero emissions. The engine still sounds as raucous as ever but it's obviously lost some of its zoom-zoom. The upside of the conversion, of course, is much reduced emissions. Essentially all CO2 pollutants are eliminated, as the only by-product of the combustion of hydrogen and oxygen is water vapour. The H2 Wankel does emit traces of nitrous oxides, but that's mostly as a result of oil seeping past the seals on the edge of the engine's rotors, always an issue with a rotary engine.
The RX-8 Hydrogen RE is a bi-fuel vehicle that has an 800km range on petrol as well as 160km on hydrogen. Mazda leases out a few of the cars in Japan (where they've accumulated more than 160,000km of real-world testing) and will lease another 30 in Norway. Though the RX-8 version has only been undergoing testing for about three years, Mazda has also developed a second-generation hydrogen vehicle in the guise of its Mazda5 (known as the Premacy in Japan).
Unlike the RX-8, which has a conventional drivetrain, the Premacy uses the rotary engine as part of a hybrid powertrain. Like the Chevrolet Volt, the car is driven only on electric motors while the engine is used only to charge the batteries. In an odd twist, it is the larger minivan that feels sportier than the RX-8. The hydrogen-fuelled rotary engine still only produces 109hp, but it feeds that 147hp electric motor that has a lot more torque than the RX-8.
Though Mazda concedes that the fuel cell is a more appropriate technology for use with hydrogen because it makes more efficient use of the fuel and is completely emissions-free, fuel cell technology remains unproven, unreliable and extremely costly. On the other hand, the conversion of the rotary to hydrogen fuel is relatively simple and cheap, with only the storage of high-pressure hydrogen and the tribulations of locating a second fuel tank for the lighter-than-air gas as complications.