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Audi demonstrates fuel cell vehicle with electric quattro

At the LA Auto Show, Audi lifted the lid on the A7 Sportback h-tron quattro – a fuel cell technology demonstrator with a range of more than 480km that emits only water. The A7 Sportback h-tron quattro uses a powerful electric drive with a fuel cell as its energy source, in combination with a hybrid battery and an additional electric motor in the rear. The vehicle has 170kW of power at its disposal, but has no mechanical connection between the front and rear axles. As an e quattro, the h-tron features fully electronic management of torque distribution.

“The A7 Sportback h-tron quattro is a genuine Audi – at once sporty and efficient. Conceived as an e quattro, its two electric motors drive all four wheels,” explains Ulrich Hackenberg, member of the board of management for technical development at Audi. “The h-tron concept car shows that we have mastered fuel cell technology. We are in a position to launch the production process as soon as the market and infrastructure are ready.”

In visual terms, the technology demonstrators that Audi brought to the Los Angeles Auto Show resemble the production models. As the h-tron label (for hydrogen) suggests, this concept car takes its place alongside the other Audi models with alternative drive principles – the e-tron and g-tron. Externally, there is no other evidence of the fuel cell that converts hydrogen into electrical power on board the vehicle.

 

The crucial differences are beneath the hood of the A7 Sportback: The fuel cell is installed at the front, mirroring the conventional A7 Sportback’s combustion engine. Because the exhaust system only has to handle water vapor, it is made of weight saving plastic.

The fuel cell itself comprises over 300 individual cells that form a stack. The core of each of these individual cells is a polymer membrane. There is a platinum-based catalyst on both sides of the membrane. Hydrogen is supplied to the anode, where it is broken down into protons and electrons. The protons migrate through the membrane to the cathode, where they react with the oxygen present in air to form water vapor. Meanwhile, outside the stack, the electrons supply the electrical power – depending on load point, the individual cell voltage is 0.6-0.8V.

The entire fuel cell operates in the voltage range of 230-360V. The main auxiliary assemblies include a turbocharger that forces the air into the cells, the recirculation fan – which returns unused hydrogen to the anode, thus increasing efficiency – and a coolant pump. These components have a high-voltage electric drive and are powered by the fuel cell.

 

There is a separate cooling circuit for the fuel cell. A heat exchanger and a thermoelectric, self-regulating auxiliary heating element maintain pleasant temperatures in the cabin.

The fuel cell places higher demands on the vehicle cooling than an equivalent combustion engine, but achieves efficiency of up to 60% – almost double that of a conventional combustion engine. Its cold-starting performance is guaranteed down to -28°C.

A special feature of the A7 Sportback h-tron quattro is its plug-in hybrid functionality – this represents a logical evolution from the Audi A2 H2 and Q5 HFC test cars. The vehicle contains a lithium-ion battery with an 8.8kWh capacity, which can be recharged from a power socket – the battery has been adopted from the A3 Sportback e-tron. It is located beneath the boot and has a separate cooling circuit for thermal management.

This high-performance battery makes the ideal partner to the fuel cell – it can store energy recovered from brake applications and supply considerable power for full load boosting. This paves the way for impressive acceleration, making the A7 Sportback h-tron quattro truly live up to quattro standards. Both the front and rear axles have no mechanical connections for the transmission of power. In the event of slip, the torque for both driven axles can be controlled electronically and adjusted continuously.

On battery power alone, the Audi A7 Sportback h-tron quattro can cover up to 50km. The battery in the rear of the plug-in hybrid can be recharged via a cable. Depending on the voltage and current rating, a full recharge takes between two hours (using an industrial power socket at 360V) and four hours (using a domestic power socket at 230V).

The battery operates at a different voltage level to the fuel cell. For that reason, there is a DC converter (DC/AC) between the two components. This tri-port converter is located behind the stack. In many operating statuses it equalizes the voltage, enabling the electric motors to operate at their maximum efficiency of 95%.

The power electronics in the front and rear of the vehicle convert the direct current from the fuel cell and battery into alternating current for the electric motors to drive the front and rear axles separately.

 

The two electric motors, which are cooled by a low-temperature circuit together with the voltage converters, are permanently excited synchronous machines. Each motor has an output of 85kW (or 114kW if the voltage is temporarily raised). The peak torque is 270Nm per electric motor.

The electric motors’ housings incorporate planetary gear trains with a single transmission ratio of 7.6:1. A mechanical parking lock and a differential function round off the system.

The Audi A7 Sportback h-tron quattro offers the full appeal of electric drive in conjunction with the new e quattro. The silent propulsion is fully available from the off, and the fuel cell reaches its maximum output within one second at full load – a more dynamic response than a combustion engine, as the entire drive system involves only a few mechanical components.

With 540Nm of propulsive power at its disposal, the A7 Sportback h-tron quattro – which tips the scales at approximately 1,950kg, races from a standstill to 100km/h in 7.9 seconds. Its top speed is 178km/h. The e quattro concept requires precise coordination of the electric motors – the technology demonstrator offers a dynamic, stable and high-traction drive that is comparable to a production car with mechanical quattro drive.

 

A power meter, in place of the revolution counter in the instrument cluster, informs the driver of power flow. The outer sections show the fuel level in the hydrogen tank and the level of battery charge. Graphics on the MMI monitor visualize the energy flow. When the driver presses the EV button, the vehicle drives solely on battery power.

Switching from automatic transmission mode D to S increases the level of energy recovery when braking, charging the battery effectively during sporty driving. Brake applications, too, are almost always accomplished electrically. The electric motors then act as alternators and convert the car’s kinetic energy into electrical energy that is stored in the battery. The four disc brakes only become involved if more forceful or emergency braking is required.

 

The vehicle’s tank flap conceals a filler connector for the hydrogen. Fully refueling with H2 takes around three minutes, roughly as the same as a conventional automobile. The tanks communicate with the refueling system by infrared interface and equalize the pressure and temperature levels.

The vehicle’s four hydrogen tanks are located beneath the base of the trunk, in front of the rear axle, in the centre tunnel. An outer skin made from carbon fiber reinforced polymer (CFRP) encases the inner aluminum shell. The tanks can store around 5kg of hydrogen at 700 bar – enough to drive for around 480km. According to the NEDC cycle, fuel consumption is roughly 1kg of hydrogen per 100 kilometers – a figure equivalent to 3.697l/100km combined.

The h-tron quattro always travels with zero local emissions. By using renewable hydrogen, it can also be used globally as a zero emissions vehicle: Since 2013 Audi has been operating a pilot plant in which renewable wind power is used to produce hydrogen by electrolysis. At present, this hydrogen is still used in an additional production process to obtain synthetic methane (Audi e gas). A future move to feed this hydrogen into a hydrogen supply and filling station network would make it available for refueling fuel cell vehicles. This is a sound option for sustainable mobility with no emissions.

26 November 2014


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