A high-performance sports car with a purely electric drive system

Audi presents the highlight of the IAA 2009 the e-tron, a high-performance sports car with a purely electric drive system. Four motors – two each at the front and rear axles – drive the wheels, making the concept car a true quattro. Producing 230 kW (313 hp) and 4,500 Nm (3,319.03 lb-ft) of torque, the two-seater accelerates from 0 to 100 kmh (0 – 62.14 mph) in 4.8 seconds, and from 60 to 120 kmh (37.28 – 74.56 mph) in 4.1 seconds. The lithium-ion battery provides a truly useable energy content of 42.4 kilowatt hours to enable a range of approximately 248 kilometers.

The concept

Electric drive systems are still very much outsiders. The first vehicles of this type took to the roads around 1900, yet in 2009 no volume car manufacturer has a car powered exclusively by batteries in its lineup. Fewer than 1,500 electric vehicles are currently registered in Germany, corresponding to only 0.035 percent of all registered vehicles.

Yet electric driving potentially offers numerous advantages. Electric cars reduce the dependence of transportation and the economy on the raw material petroleum. They produce no direct exhaust emissions and thus ease the local burden on the environment. Electric drive systems are also significantly more efficient than combustion engines, consequently making them easier on the customers’ wallets. Other strengths include sportiness and the fun they bring to driving. All of the torque is essentially available the moment the driver steps on the accelerator, allowing for breathtaking acceleration.

There is still a lot of work to do before electric cars are ready for volume production, however. The greatest challenge is the integration of the energy storage system. Acceptable range and performance requires a traction battery that is heavy and takes up a lot of space. Audi is taking a new approach to offset these disadvantages – a holistic approach with a specific vehicle package, a systematic lightweight construction concept and an optimal configuration of all components for the electric drive.

Audi e-tron – The Holistic Approach

The most important development related to batteries for electric drives are lithium-ion cells. Numerous experts throughout the world are working on their further development for use in cars, with the primary objectives being to reduce weight and increase capacity and performance. Audi has also opted for this technology, both for use in a hybrid production vehicle, such as the upcoming Q5 hybrid, and in the e-tron test platform.

The requirement specification for the concept vehicle goes far beyond battery technology and the replacement of the combustion engine with an electric drive system, however. The Audi development engineers decided back in the concept phase to design practically every component and technology based on the new requirements of electric mobility. The interaction of all elements has a decisive influence on the factors efficiency, range and practicality.

Design and Package

The caliber of the car is apparent to the observer at first glance. The Audi e-tron has a wide, powerful stance on the road. The car body seems almost monolithic; the closed rear end appears powerful and muscular. The trapeze of the single-frame grille dominates the front end and is flanked by two large air intakes. The top of the grille merges into the flat strips of the adaptive matrix beam headlamp modules with their clear glass covers. High-efficiency LED technology is used for all lighting units – a matter of honor for Audi as the worldwide pioneer in this field.

The headlamps are the core of a fully automatic light assistance system that reacts flexibly to any situation. The new technology recognizes weather conditions and adapts the illumination to rain or fog. The technology at the heart of the light assistance system is a camera that works together with a fast computer to detect oncoming traffic, recognize lanes and measure visibilities, such as in the event of fog.

A new design element unique to the e-tron are the air intakes in the single-frame grille and in front of the rear wheel wells. They are closed flush under normal circumstances and opened by means of flaps when additional cooling air is required. Maximum efficiency is also the reason behind this measure. The concept car has a remarkably low drag coefficient, which gets even better when the flaps are closed.

1.90 meters (74.80 in) wide, just 4.26 meters (167.72 in) long and 1.23 meters (48.43 in) tall – those are the proportions of a supercar. The wheelbase of 2.60 meters (102.36 in) leaves plenty of room between the axles for people and technology. Like with a mid-engined sports car, the cabin of the e-tron is shifted far forward toward the front axle, leaving room in front of the rear axle for the roughly 470 kilogram (1036.17 lb) battery unit, the inverter and the power electronics.

The two electric motors, which have their own cooling system, are mounted behind the rear axle. The front electric motors are mounted on the front axle, with their cooling system arranged in front of them. This special package, which features a 42:58 weight distribution, ensures perfect balance, which contributes to the driving dynamics of the e-tron.

Interior and Control Concept

Optical and functional references to the new drive concept characterize the interior design. They establish an advanced connection between proven Audi genes and new formal hallmarks. Typical for the Audi design language is the reduction of the architecture, controls and flow of information to the essential in favor of visible lightweight construction and a tidy overall impression.

The dash appears to float and has a curve that extends laterally into the door panels. With no need to allow for a transmission, shifter and cardan tunnel, the designers took advantage of the opportunity to create a particularly slim and lightweight center tunnel and center console. The flush gear selector, with which the driver chooses between the modes forward, reverse and neutral, emerges from the tunnel when the vehicle is started.

The cockpit of the e-tron is also oriented toward the driver – a further characteristic Audi trait. Instead of the classic instrument cluster, the concept car is the first Audi to be equipped with a large, fold-out central display with integrated MMI functions. It is flanked by two round dials. The MMI is controlled via a scroll pad with a touch-sensitive surface on the steering wheel (“MMI touch”) – an element inspired by modern smartphones.

While an analog speedometer on the right provides speed information, the instrument on the left tells the driver how much power is being drawn. The central display shows the range in the status bar and presents all key information from the infotainment and navigation systems. It also provides the driver with relevant data from the vehicle’s communication with its surroundings. The instruments combine the analog and the digital worlds into a single unit.

Characteristic for the concept of the Audi e-tron is the near total elimination of switches and small components such as the ignition. The climate control unit is located to the right above the steering wheel. The display provides temperature and ventilation information. Again drawing inspiration from a smartphone, the system is controlled by means of a touch-sensitive sliding control.

Drive System and Energy Supply

Four asynchronous motors with a total output of 230 kilowatts (313 hp) give the Audi e-tron the performance of a high-output sports car. The concept car can accelerate from 0 to 100 km/h (0 – 62.14 mph) in 4.8 seconds if necessary, and goes from 60 to 120 km/h (37.28 – 74.56 mph) in 4.1 seconds. The torque flows selectively to the wheels based on the driving situation and the condition of the road surface, resulting in outstanding traction and handling.

The top speed is limited to 200 km/h (124.27 mph), as the amount of energy required by the electric motors increases disproportionately to speed. The range in the NECD combined cycle is approximately 248 kilometers (154 miles). This good value is made possible by the integrated concept: technology specially configured for the electric drive system combined with state-of-the-art battery technology. The battery block has a total energy content of roughly 53 kilowatt hours, with the usable portion thereof restricted to 42.4 kWh in the interest of service life. Audi uses liquid cooling for the batteries.

The energy storage unit is charged with household current (230 volts, 16 amperes) via a cable and a plug. The socket is behind a cover at the back of the car. With the battery fully discharged, the charging time is between 6 and 8 hours. A high voltage (400 volts, 63 amperes) reduces this to just around 2.5 hours. The Audi engineers are working on a wireless solution to make charging more convenient. The inductive charging station, which can be placed in the garage at home or also in special parking garages, is activated automatically when the vehicle is docked. Such technology is already used today in a similar form to charge electric toothbrushes.

The battery is charged not only when the car is stationary, but also when it is in motion. The keyword here is recuperation. This form of energy recovery and return to the battery is already available today in a number of Audi production models. During braking, the alternator converts the kinetic energy into electrical energy, which it then feeds into the onboard electrical system.

The Audi e-tron, which is slowed by four lightweight ceramic brake discs, takes the next large step into the future. An electronic brake system makes it possible to tap into the recuperation potential of the electric motors. A hydraulic fixed-caliper brake is mounted on the front axle, with two novel electrically-actuated floating-caliper brakes mounted on the rear axle. These floating calipers are actuated not by any mechanical or hydraulic transfer elements, but rather by wire (“brake by wire”). In addition, this eliminates frictional losses due to residual slip when the brakes are not being applied.

This decoupling of the brake pedal enables the e-tron’s electric motors to convert all of the braking energy into electricity and recover it. The electromechanical brake system is only activated if greater deceleration is required. These control actions are unnoticeable to the driver, who feels only a predictable and constant pedal feel as with a hydraulic brake system.

Driving Dynamics

The normal distribution of the tractive power is clearly biased toward the rear axle in accordance with the weight distribution of the e-tron. Similarly to a mid-engined sports car, roughly 70 percent of the power goes the rear and 30 percent to the front. If an axle slips, this balance can be varied by means of the four centrally controlled electric motors. The electric vehicle from Audi thus enjoys all of the advantages of quattro technology.

The four individual motors, which in the interest of greater traction are installed behind the wheels as wheel drives, also enable the e-tron’s lateral dynamics to be intelligently controlled. Similar to what the sport differential does in conventional quattro vehicles, torque vectoring – the targeted acceleration of individual wheels – makes the e-tron even more dynamic while simultaneously enhancing driving safety. Understeer and oversteer can be corrected by not only targeted activation of the brakes, but also by precise increases in power lasting just a few milliseconds. The concept car remains extremely neutral even under great lateral acceleration and hustles through corners as if on the proverbial rails.

The chassis has triangular double wishbones at the front axle and trapezoidal wishbones made of forged aluminum components at the rear axle – a geometry that has proven in motorsports to be the optimal prerequisite for high agility, uncompromising precision and precisely defined self-steering behavior. A taut setup was chosen for the springs and shock absorbers, but it is still very comfortable.

The direct rack-and-pinion steering gives finely differentiated feedback. Its electromechanical steering boost varies with speed, so that the e-tron only has to provide energy while steering, and not while driving straight ahead.

As befitting its status, the Audi concept car rolls on 19-inch tires with a new blade design. 235/35 tires up front and 295/30 tires in the rear provide the necessary grip.

SOURCE: Audi