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July 31, 2011

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The best of two worlds – the Audi A6 hybrid

Ingolstadt- Power like a V6, fuel consumption like a four-cylinder car – the Audi A6 hybrid, coming on the market in 2012, combines the best of two worlds. Its gasoline engine, a 2.0 TFSI, and the electric motor can briefly generate 180 kW (245 hp) of system power and 480 Nm (354.03 lb-ft) of torque. Despite the high and spontaneous generation of power, the average rate of fuel consumption in the standard cycle is below 6.4 liters per 100 km (36.75 US mpg). The sedan can travel up to a speed of 100 km/h (62.14 mph) on electric power alone, and cover around three kilometers (1.86 miles) with zero local emissions at a speed of 60 km/h (37.28 mph).

Performance Directly mounted behind one another, the 2.0 TFSI and the electric motor of the Audi A6 hybrid form a parallel hybrid system. Together the two units have a brief system output of 180 kW (245 hp) and a system torque of 480 Nm (354.03 lb-ft).

The sedan accelerates from zero to 100 km/h (62.14 mph) in 7.3 seconds and has a top speed of 238 km/h (147.89 mph). In the European standard driving cycle, the Audi A6 hybrid consumes less than 6.4 liters per 100 km (36.75 US mpg) – that equals CO2 emissions of less than 146 grams per km (234.96 g/mile). The 73-liter (19.28 US gallon) fuel tank ensures a long range. (These performance and consumption data are provisional).

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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