Overview: Audi A8 Hybrid

Audi presented the A8 hybrid as a ‘near-production concept’ at the Geneva Motor Show. The car’s layout is that of a series-parallel hybrid, in which a combustion engine and an electric motor can power the car together or separately.

The A8’s combustion engine is a four-cylinder TFSI petrol unit of 1984cc. Working together with the electric motor, total system output is substantial: 245PS and 480Nm. Despite its aluminium construction, the A8 hybrid is a heavy car — 1885kg empty — so the claimed performance figures are a tribute to the substantial combined torque that this powertrain delivers. Standing start acceleration to 100km/h in 7.6s, together with a maximum speed of 146mph, should satisfy any rational driver. The overall fuel return in the European test cycle is 45.6mpg (6.2l/100km), yielding 144g of CO2 per kilometre. Of course this is a creditable figure, though as usual we are left to speculate what the returns would look like if a TDI had been substituted for the petrol engine.

Audi’s 2.0 TFSI powerplant combines direct fuelling, turbocharging, and the AVS Audi valvelift system. This regulates the valve lift in two stages. In combination with the adjustable intake camshaft, the Audi valvelift system improves cylinder charging and gas-flow to fatten the torque curve at lower engine speeds. Working by itself, the 2.0 TFSI delivers 211PS and 350Nm; the torque value remains constant from 1,500 to 4,200rpm. Two balancer shafts in the crankcase offset second-order inertial forces, reducing four-cylinder vibration.

A hydraulically-operated wet-plate clutch links the petrol engine with the synchronous electric motor. Interposed between the electric motor and the eight-speed tiptronic gearbox is an additional cut-out clutch, functioning as a torque converter. In the usual fashion, the motor also serves as a generator to capture kinetic energy during overrun and braking. The motor’s solo output is 45PS and 211Nm, the torque being delivered from the first revolution.

The energy storage system of the Audi A8 hybrid is a lithium-ion battery pack mounted, together with its cooling system, in the rear of the car. The boot capacity of 400l is 100l down on the standard car.

Several systems that operate on engine power in the normal A8 have been modified for the hybrid. The compressor in the climate control system runs entirely on electric power; the steering system is electromechanical; and the brake servo is supplied by an electric vacuum pump which works only when required. During electrical braking — when kinetic energy is captured by the motor, retarding only the driving wheels — a control system ensures that the ABS and ESP functions continue to operate.

As is normal on a hybrid car, a power electronics unit manages the interaction of the car’s systems. Its pulse control inverter regulates the interplay of the battery and the electric motor, while the DC-DC converter supplies power to the low-voltage onboard network. The power electronics unit, which is connected to the battery and the electric motor by high-voltage cables, is located in the engine compartment.

The A8 hybrid’s ‘technology architecture’ of power and control systems provides a glimpse of the forthcoming Q5 hybrid, which will be introduced later this year.

On the road

Being a series-parallel hybrid, the A8 is capable of running on electric power alone. Its maximum speed without the engine is 40mph, with a range of two kilometers. The Audi’s driver can switch the car to electric operation manually.

This range is impressive for a heavy car, and better than a Prius could manage, but the operating range of hybrids needs to improve substantially if they are to operate entirely without their combustion engines for trips into urban centres. Two kilometres will barely get you from Tower Bridge to Liverpool Street and back. This is clearly not Audi’s fault, but it is an indicator of how far we must travel before the ‘zero emission’ modes of our hybrids really count for anything.

It is not really surprising, given our love of gadgets and things that light up, that the dashboards of this first brave cohort of hybrids present their drivers with a lot of information that, to be charitable, is of only marginal interest and of questionable real value. At worst, we can imagine a driver, hypnotised by an animated diagram of the energy flows in his car’s complex powertrain, driving straight into the back of a bus. Audi’s press release for the A8 hybrid announces proudly that ‘Both the display screen of the instrument cluster, and the large monitor of the MMI operating system on the dashboard, present all momentary power flows in brilliant graphics.’ By the prevailing standards, the Audi’s displays are actually the height of good taste.

When the Audi A8 hybrid pulls away from rest, both the combustion engine and the electric motor work together, mixed and matched according to the driver’s demand for power. Above 40mph, the combustion engine alone drives the car, while the electric motor acts as a generator.

When the driver demands less power, the combustion engine is disconnected from the drivetrain, and the Audi ‘coasts’. In braking and overrun, the electric motor functions as a generator to recharge the battery pack. This produces negative torque at the front wheels.

If the driver abruptly demands a lot of power, the electric motor assists the petrol engine. In this situation, the system’s maximum outputs of 245PS and 480Nm are delivered to the wheels. The sprint between 37mph and 75mph (60km/h to 120km/h) in fifth gear takes 7.5 seconds, according to Audi, so this car has a very useful day-to-day performance.

Audi A8 hybrid drivetrain.
The instrumentation and displays of Audi’s A8 hybrid are the model of restraint compared with some.
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