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Power like a big V6, fuel economy like
a modest four-cylinder unit – Audi presents the A8 hybrid as a
technology concept at the Geneva Motor Show. Its two propulsion units –
a 2.0 TFSI and an electric motor – develop a total output of 245PS of
system power and 480 Nm (354.03 lb-ft) of torque. This enables the Audi
A8 hybrid to accelerate in 7.6 seconds from zero to 62mph and to achieve
a top speed of 146mph. Yet average economy amounts to up to 45.6mpg –
the CO2 equivalent is 144 grams per km.
The technology of the Audi A8
hybrid
The combustion engine of the Audi A8
hybrid is the 2.0 TFSI – a high-tech powerplant. This four-cylinder
unit, which has been named “Engine of the Year” five times in a row
since 2005, is an example of Audi’s downsizing philosophy. It combines
direct fuel injection with turbocharging and the AVS Audi valvelift
system, which regulates the valve lift in two stages. In combination
with the adjustable intake camshaft, the Audi valvelift system improves
cylinder charging and ensures spontaneous and powerful torque build-up.
The 2.0 TFSI delivers 211PS and 350 Nm (258.15 lb-ft); the torque value
remains constant from 1,500 to 4,200 rpm.
In the crankcase of the four-cylinder
unit, which has a displacement of 1,984 cc, two balancer shafts offset
second-order inertial forces to ensure virtually vibration-free and
pleasantly quiet operation that goes well with the luxurious character
of the A8. All components are optimized to minimize friction. The
flow-rate-regulated, pressure-controlled oil pump further reduces fuel
consumption.
Electric motor with ample
propulsive power
A hydraulically operated wet clutch links the 2.0 TFSI with the electric
motor. Its control requires the utmost precision, and a great deal of
know-how has been expended to achieve its smooth, precise and swift
operation. The synchronous motor, which is continually excited, also
serves as a generator that develops 45PS of power and 211 Nm (155.63
lb-ft) of torque. Its full power is available practically right from
standstill and contributes to a sportily powerful start-up. Since an
electric motor also develops heat, its housing is furnished with cooling
ducts. Interposed between the electric motor and the wheelset of the
eight-speed tiptronic, which is controlled entirely by electronics, is
an additional cut-out clutch. In conjunction with the electric unit,
this functions as a torque converter. The propulsive power of both drive
systems is applied to the road through the front wheels.
Lithium-ion technology – the
battery
The energy storage system of the Audi
A8 hybrid is mounted in the rear section. This state-of-the-art
lithium-ion battery is more compact and weighs less than other types but
is substantially more powerful. It is protected by a high-strength
housing, and a ventilation module ensures that it always operated within
the ideal temperature range. Yet the A8 hybrid also provides ample
luggage space of 400 litres (14.13 cubic feet)
Several subsystems that operate on
engine power in a conventional car have been modified right from the
start for use in the Audi A8 hybrid. The compressor in the climate
control system runs entirely on electric power. The steering system is
electromechanical as well. The brake booster too is supplied by a
demand-controlled electric vacuum pump. During electrical – i.e.
recuperative – braking actions, a sophisticated control system ensures
that the ABS and ESP functions operate properly.
The power electronics unit manages the
interaction of the systems. Its pulse control inverter regulates the
interplay of the battery and the electric motor. The DC-DC converter
supplies power to users on the 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 smooth interaction of these
components and their high level of integration attest to the specialized
know-how Audi has accumulated. The technology architecture already
provides a glimpse of a future production model – the Audi Q5 hybrid,
which will be introduced later this year.
Driving experience
The Audi A8 hybrid, which weighs 1,885
kilograms (4,155.71 lb), drives as if it had a big six-cylinder gasoline
engine or a TDI under its bonnet. From zero to 62mph the vehicle
accelerates in 7.6 seconds, and it can continue to accelerate up to
146mph. Yet fuel economy of up to 45.6mpg is possible, corresponding to
144 grams CO2/km (231.75 g/mile). In city traffic, the hybrid
drive uses about 20 percent less fuel than a conventional drive. Both
the car’s performance and its fuel economy benefit substantially from a
major advantage of the A8 model series – its lightweight aluminum body
based on the Audi Space Frame principle (ASF).
The hybrid drive is designed for fully
electric-powered driving up to 40mph and for a distance exceeding two
kilometers – especially well-suited for driving in the city or
residential suburbs. When conditions permit, the driver can also select
the electric mode by actuating a separate e-key in the cockpit. 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.
When the Audi A8 hybrid starts moving,
both of its engines interoperate closely to suit varying power demands.
Above 40mph the combustion engine alone does all the propulsive work,
while the electric motor supplies onboard users and recharges the
battery.
When the driver lets up on the gas, the
combustion engine is disconnected from the drivetrain, so the Audi A8
hybrid ”coasts”. In braking and slow-down phases, the e-motor functions
as a generator – recuperating energy efficiently and feeding it into the
battery.
When the driver floors the pedal, the
engine management temporarily controls the entire system power, and now
the electric motor is boosting power. 245PS and 480 Nm (354.03 lb-ft)
endow the big saloon with enormous thrust. The intermediate sprint
between 37mph and 75mph in fifth gear for instance is accomplished in
7.5 seconds. The Audi A8 hybrid is an extremely dynamic car – with a
fuel efficiency that is setting new standards. This is especially true
in everyday user activities. Even its looks betray the power of this
near-production concept car with its big 21-inch wheels and their 265/35
tyres.
The design of the Audi A8
hybrid
This near-production concept car has
21-inch, two-colour wheel rims styled with highly three-dimensional
surface areas. The exterior paint job of the A8 hybrid is in “Prism
Silver” with “Spectra Flair” as an accent colour – a rainbow effect
that’s accentuated in reflected light along creases and seams on the
body. The prominently located “hybrid” lettering on both front wings
leaves no doubt about what kind of car this is. The accentuation of its
width by a prominent low horizontal spoiler edge and a chrome strip
conveys the vehicle’s solid stance on the road.
The engine compartment cover with its
high-gloss finish and the glass cover of the battery in the boot also
provide visual links to hybrid technology. And the illuminated doorsteps
bear the “hybrid” insignia as an elegant touch to underscore the
electric aspect whenever the doors are opened – using LED technology to
minimize energy consumption.
The passengers also see the prominent
hybrid logo displayed on the dashboard. The "Powermeter" in addition
shows the energy flow in the instrument cluster. |
