The Audi e-tron achieves a range of more than 400 kilometers (248.5 mi) on a single charge in the realistic WLTP test cycle. Behind this value are numerous high-tech solutions. Besides the low drag coefficient of 0.27 with virtual exterior mirrors and effective thermal management, the innovative recuperation system in particular is crucial here. It contributes up to 30 percent of the electric SUV’s range.
Intelligent energy recuperation: recuperation and brake system
The Audi e-tron can recover energy in two ways: by means of coasting recuperation when the driver releases the accelerator, or by means of braking recuperation when the brake pedal is depressed. In both cases, the electric motors function as a generator and convert the kinetic energy of the Audi e-tron into electric energy. Up to 0.3 g, the SUV recuperates energy solely via the electric motors, without using the conventional brake – that covers well over 90 percent of all deceleration. So, energy is returned to the battery in practically all normal braking maneuvers. The internally ventilated, 18-inch wheel brakes do not come into play until the driver uses the brake pedal to decelerate with more than 0.3 g.
The driver can select the degree of energy recovery in three stages using paddles on the steering wheel. In the lowest setting, the Audi e-tron coasts with no additional drag torque when the driver releases the accelerator pedal. The Audi e-tron continues to roll forward. No electricity flows to or from the electric motor while the vehicle is moving. In level 1 (balanced – minimal deceleration) and level 2 (strong – high deceleration), the electric motors generate regenerative brake torque and produce electricity. The electric SUV reduces the speed noticeably – the driver can decelerate and accelerate using just the accelerator pedal. This creates the one-pedal feeling. There is no need to use the brake pedal in this case.
In addition to manually adjusting the recuperation level with the steering wheel paddles, the driver can also select automatic mode in the MMI. The predictive efficiency assist then regulates the deceleration as needed and predictively, for example in relation to the route or vehicles in front. The driver can adapt the deceleration effect by selecting the desired recuperation level via the shift paddles. It remains active until the driver operates the accelerator pedal again.
The Audi e-tron taps its maximum recuperation potential using the integrated electrohydraulic brake control system. Audi is the world’s first carmaker to use such a system in an electrically powered series-production vehicle. Depending on the driving situation, the electric SUV decides whether to decelerate using the electric motor, the wheel brake, or a combination of the two acting on each axle individually. The transition between electric and hydraulic braking is smooth and homogeneous, so the driver does not even notice it. Brake forces remain constant. It is in the rare cases that the Audi e-tron uses its wheel brake, for example during maximum full-stop braking, that the powerful properties of the integrated brake control system really stand out. The new electrohydraulic actuation allows it to build up brake pressure for the wheel brakes with great precision and roughly twice as fast as a conventional system. When automated emergency braking is performed, there are only 150 milliseconds – slightly more than a blink of the eye – between the initiation of braking and maximum brake pressure between the pads and disks. Thanks to this rapid pressure buildup, the electrohydraulically integrated brake control system shortens the braking distance by up to 20 percent compared with a conventional brake system.
In case of a brake application at a speed of 100 km/h, for example, the Audi e-tron can recuperate electric power with a maximum of 300 Nm (221.3 lb-ft) and 220 kW; that corresponds to more than 70 percent of its operating energy input. No other series production model can achieve such a value. Overall, the Audi e-tron attains up to 30 percent of its range through recuperation. The system is the most variable and thus the most efficient on the market.
Every thousandth counts: drag
With an electric car, weight influences energy consumption far less than in the case of a car with a combustion engine. In town it is generally efficient because it can recover a large part of the energy, which it uses to accelerate, when rolling up to the next red light. The situation is totally different on long journeys, however, where the Audi e-tron is perfectly at home: Here from speeds of around 70 km/h (43.5 mph) the rolling resistance and the inertia take second place to aerodynamic drag irrespective of the type of car. The energy required to overcome that drag is lost. This is why the engineers were so focused on the aerodynamics during the development of the Audi e-tron. When equipped with the standard exterior mirrors, the Audi e-tron achieves a drag coefficient of 0.28. Even with the optional virtual exterior mirrors the value is 0.27 – a top result in the SUV segment. Customers benefit directly from this low figure as drag contributes decisively to the high range of more than 400 kilometers (248.5 mi) in the WLTP cycle. A hundredth of the drag coefficient figure represents a range of around five kilometers (3.1 mi) driving under everyday conditions.
To achieve this value, the Audi engineers used a wide range of aerodynamic measures in all body areas. Some of these technical solutions are evident at first glance, while others fulfill their purpose hidden away from sight. Thanks to these solutions, the drag coefficient for the Audi e-tron is almost 0.07 less than for a comparable, conventionally powered vehicle. With a typical usage profile this set-up increases the range by around 35 kilometers (21.7 mi) per battery charge in the WLTP cycle. To achieve this additional range simply by cutting weight, the engineers would have had to shed a half a ton (1,102.3 lb).
The optional virtual exterior mirrors, which in the Audi e-tron are making their world debut in a series-production automobile, are both a visual and aerodynamic highlight. Compared to the standard mirrors, they reduce the vehicle width by 15 centimeters (5.9 in) and, thanks to their new shape, not only reduce drag, but also noticeably cut the already low wind noise. Their flat supports incorporate a small camera with a resolution of 1,280 x 1,080 pixels. The captured images appear on OLED displays in the transition between the instrument panel and door.
The standard adaptive air suspension, an air suspension system with controlled damping, also plays a major role in reducing drag. From speeds of 120 km/h (74.6 mph), it lowers the body of the Audi e-tron by as much as 26 millimeters (1.0 in) from the normal height of 172 millimeters (6.8 in). As a result, the rectangle that the tires represent for the airflow and opposes the slipstream partially disappears into the wheel arch. This measure also improves handling.
Also contributing to the low Cd value is the fully lined underbody of the all-electric SUV. The front and rear sections are completely covered. Underneath the passenger cell, an aluminum plate protects the high-voltage battery against damage from below, such as stone chipping or curbs. Its bolting points come with bowl-shaped indentations, similar to the dimples on a golf ball. They make the air flow much better than a totally flat surface.
Because the wheel arches and wheels are typically responsible for one-third of the drag, Audi uses air curtains at the front of the electric SUV. Integrated into the side air inlets and easily visible from the outside, they direct the slipstream so that it flows to the outside past the standard, aerodynamically optimized 19-inch wheels. Their design is flatter than with conventional wheels. The 255/55 size tires stand out with their ultralow rolling resistance.
Another measure at the front of the car to help lower drag is the adjustable cooling air intake – a frame behind the Singleframe with two electrically operated louvers. When shut, the air in this area flows with virtually no swirl. As soon as the drivetrain components need cooling or the air conditioning condenser requires ventilation, the top louver opens first and then both louvers. When the hydraulic wheel brakes are subject to high loads, the controllable cool-air inlet opens and releases two ducts which channel the cooling air into the front wheel arches to the brakes.
Important performance factor: thermal management
The effective thermal management system in the Audi e-tron guarantees fast DC charging with up to 150 kW, long battery life and reproducible road performance even under heavy load. For the customer, this means high performance at all times.
The thermal management system comprises four circuits that can be connected in various ways as required. It cools the electric motors including their rotors, the power electronics and the charger. It also cools and warms both the interior and the high-voltage battery. The rotors, which reach up to 13,300 revolutions per minute during real vehicle operation, consist of magnetically conductive electrical sheets and lightweight, high-purity aluminum. Coolant flows through the inside of the shafts to ensure that the temperature does not exceed 180 degrees Celsius. The stators and end shields of the electric motors are also water-cooled. This solution indirectly benefits the gearboxes mounted on the end shields. Effective cooling, particularly of the coaxial electric motor on the rear axle, presented the developers with new challenges. The solution is to route coolant through a double-walled tube with a ceramic seal at the rotor of the electric motor. A total of 22 liters (5.8 US gal) of coolant flow through the roughly 40 meters (131.2 ft) of coolant lines in the Audi e-tron. Being the hottest components in the powertrain, the electric motors provide the thermal management system with a large quantity of heat.
The standard heat pump uses their waste heat – up to 3 kW of actual power losses are efficiently used for heating and air conditioning the interior. Depending on the outside temperature, that can boost the Audi e-tron’s range by up to ten percent in customer operation.
The thermal management system also ensures that the battery is kept within its optimal efficiency range of 25 to 35° Celsius in all situations, from a cold start in winter to fast highway driving on hot summer days. This also contributes to the long service life. During DC charging with 150 kW, which is possible for the first time ever in a series production automobile with the Audi e-tron, cold coolant dissipates the heat from electrical power loss. If the battery is still cold when charging in winter, it is heated with warm coolant.
The equipment, data and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.