Audi is pressing ahead with the electrification of its drive systems across a broad front. In mid-2017, the new mild-hybrid drive vehicles (MHEVs) will start joining the product line. The next generation of luxury sedans, the Audi A8, will have them on board – in the 48-volt version – regardless of engine type.
The new technology is suitable for the interplay with either diesel or gasoline engines and can reduce consumption of a V6 gasoline engine by up to 0.7 liters per
100 kilometers (0.2 US gal per 62.1 mi) during customer operation, for example. Unlike other efficiency technologies within the engine, the MHEV drives increase ride comfort, since they allow silent coasting within larger speed ranges up to a maximum speed of 160 km/h (99.4 mph).
Audi offers the MHEV drives in two variants. For the four-cylinder engines they are based on the familiar 12-volt electrical system. The six-cylinder and eight-cylinder engines, as well as the W12 cylinder units, will receive a new 48-volt system generally serving as the main vehicle electric system. In particular, this technology offers many ways for making driving more efficient, sportier and more comfortable in the future.
At the 2017 Geneva Motor Show the brand presented the potential of its new technologies in the form of the Audi Q8 sport concept showcar. Its 48-volt electrical system integrates a further developed MHEV system as well as an electric-powered compressor (EPC). Together the two components provide a new level of dynamics. The efficiency is also significantly increasing – at low speeds as in parking, the showcar can even be driven electrically.
The mild-hybrid drive from Audi in the new A8 comprises two central components. One of them is a water-cooled belt alternator starter (BAS) on the front side of the engine. A heavy-duty V-ribbed belt connects it to the crankshaft. The BAS yields a recuperation level up to 12 kW and 60 Nm (44.3 lb-ft) of torque.
The second component is a lithium-ion battery with 10 Ah charge capacity and a constant voltage of 48 volts. In the new large sedan, the newly developed 48-volt power system serves as the main vehicle electrical system. The 12-volt system is connected to the main electrical system via a DC/DC converter. Located in the luggage compartment, the lithium-ion rechargeable battery has about the size of a large lead battery. Controlled air cooling provides the thermal management.
The 48-volt-based MHEV technology is particularly more comfortable and efficient. If the driver takes his foot off the accelerator at a speed between 55 and 160 km/h (34.2 to 99.4 mph), the car can coast for up to 40 seconds with the engine off completely. During slow coasting, the start-stop phase already begins at 22 km/h (13.7 mph).
Once the driver accelerates again – whether from a stop or while driving – the vehicle restarts quickly and very comfortably: the BAS revs up the internal combustion engine to the target speed, then injection occurs again and, in the case of a gasoline engine, ignition. While the conventional pinion starter remains on board, it practically comes into play only at the initial starting, if the engine oil is still cold and viscous. In such a situation, the belt of the BAS could slip through.
In many situations, recuperation – recovery of energy during deceleration – is more efficient than coasting. To decide between the two, the drive management system in the new Audi A8 uses the front camera and, optionally, data from the predictive efficiency assistant, the route data stored in the navigation system and other data supplied by the highly networked sensor set. The bottom line is that the mild-hybrid drive achieves fuel savings of up to 0.7 liters per 100 kilometers during customer operation (with the V6 TFSI).
Audi also offers the new MHEV technology with the conventional 12-volt electrical system. In this configuration, it interacts with the 2.0 TFSI engine. The functional principle is the same as with 48 volts, although the coasting phases, recuperation output and the CO2 savings are somewhat smaller.
Broad base: 48-volt vehicle electrical system
In a different layout – without MHEV – the 48-volt constant voltage system already entered volume production in 2016 as the Audi SQ7 TDI. In this vehicle, the alternator still operates on a 12-volt basis, and a DC converter couples the 48-volt electrical subsystem. It in turn supplies the electric-powered compressor (EPC) for the V8 diesel as well as the electromechanical active roll stabilization (eARS).
The EPC supports the two turbochargers of the 4.0 TDI engine with up to 7 kW of power whenever they cannot draw enough energy from the exhaust stream. The power is immediately available when the driver accelerates – the experience is particularly impressive when starting off. The eARS is another innovation from Audi. Its centerpiece is an electric motor that uncouples the two halves of the stabilizer when driving straight ahead. The result is excellent ride comfort. During sporty driving along bends, the electric motor turns the tubes towards one another, for greater tautness in handling.
Audi is now taking great strides in introducing the 48-volt and MHEV technologies into volume production. In a few years, other Audi model series will also be receiving the new mild hybridization scope. The new architectures allow the realization of even greater power and torque, and innovative features will enable greater fuel savings. In the medium term, the brand plans to convert ancillary units like pumps and compressors to 48 volts, they will then be able to be more precisely controlled according to requirements, as well as them having a lighter and more compact construction. The same applies to large static convenience consumers such as window heating or sound systems. Small consumers such as control units or lights will remain in the 12-volt system well into the future, however.
Electrical coasting, powerful boosting: Audi Q8 sport concept
The brand has demonstrated the great potential of MHEV systems with its Audi Q8 sport concept car, which made its debut at the 2017 Geneva Motor Show. Located between the crankshaft and transmission, the starter alternator outputs 20 kW and 170 Nm (125.4 lb-ft). During deceleration, the powerful MHEV system can recover a high measure of energy and feed it back into the lithium-ion battery. At low speeds, it can drive the sports SUV by itself. Boosting by the internal combustion engine, a 3.0 TFSI, affords a total of up to 700 Nm (516.3 lb-ft).
The 48-volt system of the Audi Q8 sport concept features an electric-powered compressor (EPC) in addition to the integrated starter alternator. It closes the turbo lag and allows for a large and powerful mono twin scroll turbo. With a system output of 350 kW (476 hp), the showcar accelerates from zero to 100 km/h (62.1 mph) in 4.7 seconds, and presses ahead to a top speed of 275 km/h (170.9 mph). The MHEV system lowers fuel consumption of the concept car by approximately one liter per 100 kilometers.
Further information on official fuel consumption figures and the official specific CO2 emissions of new passenger cars can be found in the “Guide on the fuel economy, CO2 emissions and power consumption of all new passenger car models,” which is available free of charge at all sales dealerships and from DAT Deutsche Automobil Treuhand GmbH, Hellmuth-Hirth-Str. 1, 73760 Ostfildern-Scharnhausen, Germany (www.dat.de).
* Figures depend on the tire/wheel sets used and the engine/transmission variant
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.