Audi’s A7 h-tron is mean but clean

Time of article published Nov 20, 2014

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By: Dave Abrahams

Los Angeles, California - This may look like a standard Audi A7 Sportback - that's deliberate, says Audi - but the A7 h-tron is good for 500km on a tankful of fuel and you can drink what comes out of its (plastic) tailpipe.

Audi's hydrogen fuel cell-powered A7 is incredibly simple in concept, dauntingly complex in execution. Let's work backwards, starting with an 85kW/270Nm electric motor on each axle, connected directly to the wheels via a 7.6:1 planetary gear with a differential function.

The motors get their juice from a stack of more than 300 polymer-based cells producing a total of 230-360 volts, depending on the load, and because there is no mechanical connection between the front and rear axles, the car's dynamic stability and traction control functions decide how much of that gets directed to each axle for optimum traction under any given conditions.

That voltage is generated by combining hydrogen from the fuel tanks with oxygen from the air to produce water vapour; simple, no?

NOW IT STARTS GETTING COMPLICATED

There's an electric turbocharger to push air into the stack, a recirculation system to return unused hydrogen to the anode to increase fuel efficiency, and an electric coolant pump. The fuel cell generates more heat than a combustion engine, so it has its own high-temperature cooling system.

The hydrogen comes from four tanks - two under the boot, one in what used to be the transmission tunnel and one in front of the rear axle. Each tank has an aluminium inner shell sheathed in carbon-fibre composite and can store about 1.25kg of hydrogen at 700bar which, with an NEDC fuel-consumption rating of one kilogram per 100km, gives a theoretical tank range of about 500km.

Refuelling, at one of the 15 hydrogen stations in Germany, takes about three minutes.

EQUALISING THE VOLTAGES

Then there's an 8.8kWh lithium-ion battery pack, borrowed from the A3 Sportback e-tron, under the floor of the boot, that can be recharged using 220V domestic or 380V industrial power - also with its own low-temperature cooling system.

Fully charged (four hours on 220V, two hours on 380V) the battery will propel the A7 h-tron up to about 50km, given an educated right foot. However, the two electric motors also turn into generators on the overrun, recharging the battery and acting as brakes at the same time - Audi says the four disc brakes are only used for hard braking or emergency stops.

Since the battery operates at a different voltage from the fuel cell, there's a DC converter next to the fuel cell to equalise the voltages, while power electronics at the front and rear axles convert that DC voltage to AC current to power the electric motors.

But DC converters, AC inverters and electric motors also get hot under their respective collars so they share - you guessed it - yet another low-temperature cooling circuit.

BUT DOES IT WORK?

The results are actually quite exciting; under full load, the fuel cells go from standby to maximum power production in one second flat and the full 170kW and 540Nm is available at practically zero revs.

Even at nearly two tons fully fuelled, the A7 h-tron gets off the line like it's doing the Time Warp, hits 100km/h in less than eight seconds and tops out at 178km/h, making it one of the fastest electric cars around and, since its electronic power-distribution system is analogous to a mechanical quattro system, its handling characteristics are very similar.

A power meter in place of the rev counter in the instrument cluster displays power flow in real time, while the outer sections show the fuel levels in the hydrogen tanks and the level of battery charge.

There's even a drive mode switch; switching from D to S tightens up the electric power steering and throttle response and sharpens the brakes - which actually increases the rate of brake energy recovery.

READY TO GO

This is not a concept; Audi says all the technology is production ready and the A7 h-tron could go into production tomorrow, if there were enough hydrogen stations around to make it worthwhile.

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