Fuel cell ancillaries, however, do create a little noise, so there’s a very slight background hum from the air compressor, which forces air into the stack. But the vacuum pump that feeds the brake servo is silent.
The other noise is tyre rumble and suspension clonk, which in a conventional Equinox would mix in with engine noise to be less noticeable. Production fuel cell cars will need quieter tyres and suspension; this is a well-known engineering challenge.
It was difficult to judge performance on such a short test, although the figures suggest supermini-type acceleration. At least the step-off from a standing start was sufficiently sharp to seamlessly slot into flowing traffic.
Our London test drive also showed how expertly GM has integrated the regenerative brake with the mechanical, the transition from one to the other being much smoother than some we’ve encountered.
An optional transmission setting that extracts more charge during regenerative braking by boosting the braking effort, does exactly that — just as it does on the Volt/Ampera.
The rest of the H4 is less remarkable. This Equinox is the previous generation model, which is reflected in the dated plastics, trim, ride and steering.
Judging the H4 on these obsolete features would miss the point, though. This is a technology demonstrator and future production cars promise up-to-the-minute trim and dynamics.
Technical progress in fuel cells is rapid, too. Since the H4 was created, GM says it has made significant breakthroughs in fuel cell design, so the production stack will be half the size, half the volume and half the weight.
This opens up the possibility of a fuel-cell powered Astra-sized car, although for global marketing reasons, it looks like the 2015 design will be an SUV, possibly the next-generation Antara.
The green credential of zero tailpipe emissions is exciting and there’s no range anxiety like an electric car, because a fuel cell car can travel a couple of hundred miles between fill-ups.
But until the government ‘road map’ comes up with a vision for a nationwide network of hydrogen filling stations, there’s an over-riding anxiety – where to refuel.
The price is also unclear, although there’s some suggestion that around £40k might be a sensible guess.
Depreciation, servicing costs, insurance and taxes are also unknowns that the ‘road map’ will have to address. Given favourable outcomes to all those imponderables, a fuel cell car would be a fascinating, practical and encouragingly green, daily-driver.