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The 2.0 TFSI in the
Audi A6 hybrid has a displacement of 1,984 cc. The engine has an
output of 155 kW (211 hp), with a maximum torque of 350 Nm
(258.15 lb-ft) at 1,500 to 4,200 rpm. In the typical Audi
manner, the four-cylinder engine combines three potent
efficiency technologies: FSI gasoline direct injection,
turbocharging and the Audi valvelift system (AVS). The AVS
further increases power, torque and efficiency by switching the
stroke of the exhaust valves between two stages depending on the
load and engine speed.
For use in the Audi A6 hybrid the
2.0 TFSI underwent modifications in several areas. The
alternator as an ancillary unit has been dropped, while the
crankshaft bearing and the fine tuning of the turbocharger were
adapted to the new requirements. A secondary air system at the
cylinder head makes sure that the exhaust gas treatment cuts in
particularly fast. Integrated in the engine control unit, the
hybrid manager controls the efficient change and smooth
transitions between the operating modes, and was fully developed
by Audi alone.
A largely modified eight-speed
tiptronic serves the power transmission, without the aid of a
torque converter. Its place is taken by the disk-shaped electric
motor. This is combined with a multi-plate clutch operating in
the oil bath and joining or separating the electric motor and
the TFSI. Controlled by intelligent management, the clutch
operates ultra-precisely, gently and fast in all situations.
The highly comfortable and
fast-shifting hybrid transmission, which guides the engine
torque to the front wheels, greatly contributes to the
efficiency of the Audi A6 hybrid – its gears are widely spread.
When the 2.0 TFSI is deactivated, an electric pump maintains the
oil pressure in the hydraulic system to safeguard the convenient
start-stop feature.
The power electronics, in the form of a pulse-controlled
inverter, serve as a controller between the battery, which
outputs direct current, and the electric motor, which operates
on alternating current. Water-cooled by a separate
low-temperature circuit, the component is distinguished by its
low volume and weight. The power electronics include a DC/DC
converter that couples the consumers in the 12V electrical
system with the high-voltage network.
A permanent magnet synchronous
machine serves as an electric motor, as a starter and – during
deceleration – as a generator. It briefly outputs up to 40 kW
(54 hp) of power and 210 Nm (154.89 lb-ft) of torque. The
electric motor is integrated in the cooling circuit of the TFSI
engine.
A lithium-ion battery
system, weighing a mere 36.7 kg (80.91 lb), serves as the energy
store for the electric motor of the Audi A6 hybrid. The compact
battery, which takes up only 26 liters (0.92 cubic ft) of space,
is located in the collision-protected area under the luggage
compartment; the luggage capacity remains an impressive 375
liters (13.24 cubic ft), and can be enlarged to 850 liters
(30.02 cubic ft) thanks to the standard folding rear seat
backrests. The battery consists of 72 cells; at 266 volts its
normal energy is 1.3 kWh and its power 40 kW.
The lithium-ion battery is cooled
by air in two ways, depending on the requirement. At
low-temperature load, it draws temperate air from the vehicle
interior by way of a fan. If its temperature exceeds a certain
limit, a separate refrigerant circuit is activated. It is
coupled to the climate control system of the Audi A6 hybrid and
uses a separate evaporator. This powerful active cooling system
greatly helps the sedan to travel relatively long distances on
electric power alone.
The driver can
decide which operating mode the Audi A6 should use – one of
three driving modes can be selected at the press of a button on
the center console or by means of the automatic selector lever.
The EV characteristic map gives priority to the electric drive,
while the D mode efficiently controls both the engine and the
electric motor. The S mode and the tiptronic gate are designed
for a sporty driving style. The hybrid sedan has five operating
states: Driving with the combustion engine alone, with the
electric drive alone or in hybrid mode is possible, as are
recuperation and boosting.
Both drive sources are deactivated
when the vehicle is stationary. In this convenient start/stop
mode the automatic air conditioning remains active – its
compressor has an electric drive. Once the driver releases the
brake, the A6 hybrid starts rolling. It can be driven up to 100
km/h (62.14 mph) under electric power – and therefore with no
local emissions.
At a constant 60 km/h (37.28 mph)
up to three kilometers (1.86 miles) can be traveled – enough for
many residential areas and inner cities.
When the accelerator is pressed
beyond a certain threshold, the internal combustion engine takes
over the drive function. At lower revs this engine will
temporarily have a heavier load than is required for the drive –
the load point shifts to a higher range and the efficiency
improves. The excess torque benefits the electric motor, which
then serves as a generator to recharge the battery and power the
consumers.
In quick acceleration, or
“boosting,” the electric motor operates together with the TFSI;
at full throttle in the S mode the full system output is briefly
available. The hybrid drive then supplies the propulsion power
of a large, naturally aspirated engine, but with far more
efficiency.
When the driver releases the
accelerator, the electric motor serves as a generator again. In
most situations the TFSI is then decoupled from the drive
system, so as not to cause any drag losses. The electricity
recovered by the electric motor when coasting and during braking
is buffered in the lithium-ion battery.
If the driver only slightly
brakes, the electric motor performs the deceleration itself; in
more forceful braking the hydraulic braking system is
simultaneously activated. Its brake booster has a second power
supply through an electric vacuum pump. A complex rule-based
strategy adapts the braking to the conditions of electric
driving and energy recuperation. As elsewhere in the A6 family,
an electromechanical unit serves as the power steering.
The Audi A6 hybrid comes
with a display concept for making the driving states of the
hybrid drive tangible to the driver. The tachometer on the
instrument cluster has been replaced by a power meter, with a
needle that indicates the total system output on a scale of 0 to
100 percent. A second scale is divided into colored segments –
the green and orange sections clearly indicate the drive mode
currently used by the sedan.
Operational readiness,
recuperation, electric driving, boosting and ignition are
indicated. An additional instrument displays the charge level of
the lithium-ion battery.
At the same time, the energy flow
display clearly depicts the mode of operation and active
components of the hybrid drive. The driver can call up the
graphic via the driver information system and in the MMI
navigation plus display. The MMI screen gives a clearly arranged
bar graph of the average consumption and recuperation quantity
in five-minute increments.
The Audi A6 hybrid is based on the
familiar A6 Sedan and embodies the same lightweight design
technologies. At 1,770 kilograms (3,902 lb), it will be the
lightest hybrid in its competiti.
Photos: Audi
(July 16, 2011)
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