Audi’s mild-hybrid systems: 12V for 4-cylinders, 48V for 6- and 8-cylinders
2 June 2017
Audi will begin introducing new mild-hybrid drive vehicles (MHEVs) this year. The next generation of luxury sedans, the Audi A8, will have 48V MHEV systems regardless of engine type.
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.
The new technology is suitable for 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 miles) in the New European Driving Cycle (NEDC), for example. Unlike some 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 mph).
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. (Earlier post.) 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.
MHEV: operation. 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 N·m (44 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 or her foot off the accelerator at a speed between 30 and 160 km/h (19 to 99 mph), the car can coast for up to 45 seconds with the engine off completely. During slow coasting, the start-stop phase already begins at 22 km/h (14 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 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 in the NEDC (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.
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 centerpiece of the eARS 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 moving to introduce 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 such as pumps and compressors to 48 volts. These 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.