Seat of power

In mid-2017, Audi’s new mild hybrid electric vehicles (MHEVs) began to enter the product line-up and with the arrival last week of the all-new Audi A8, Australian customers can now experience first hand not only its extraordinary luxury, but also the mild-hybrid powertrain.

A Mild-Hybrid Electric Vehicle (MHEV) is not, as the name suggests, an electric vehicle in the pure sense, in that it does not offer an electric-only mode. Rather, the MHEV is a combustion engine operating with a motor or generator in parallel configuration.

The new technology is well-suited to both diesel or petrol engines and can reduce consumption in a V6 petrol engine by up to 0.7 litres per 100 kilometres in real-world driving conditions.

The six-cylinder engines in the Audi A8 are equipped with a new 48-volt system that serves as the main vehicle electric system. The lithium-ion battery with a capacity of 10 Ah that supplies this system is housed in the boot, while a water-cooled belt alternator starter (BAS) is mounted to the combustion engine’s front end with a heavy-duty V-ribbed belt connecting it to the crankshaft. The BAS generates a recuperation power of up to 12kW and 60Nm of torque.

The 48-volt MHEV technology offers exceptional convenience and efficiency. When drivers take their foot off the accelerator at speeds from 55 to 160 km/h, the car can coast for up to 40 seconds with the engine shut off completely – not that the driver is even aware of it happening, so smooth is the transition. When coasting at low speeds, the start-stop phase begins at 22km/h. The drive management system utilises information from the navigation map and on-board sensors to evaluate whether coasting is more fuel-efficient or recuperation. It selects recuperation, for instance, if the efficiency assistant detects a vehicle driving ahead. In the braking phases, the BAS assists in braking and can feed the recovered electrical energy into the 48-volt storage unit or use it directly for electrical needs.

When the driver pushes the accelerator pedal again after a coasting phase or a stop, the BAS restarts the combustion engine, again seamlessly. The conventional pinion starter is only used for the initial starting operation when the engine oil is still cold and a higher starting torque is required. The BAS start is controlled based on load demands by the driver, whether the air conditioner is on for example. The same is true if the driver releases the brake pedal in a start-stop phase to rejoin other vehicles in traffic. 

If the driver presses the accelerator pedal further, indicating a need for significant acceleration, the engine start is executed as quickly as possible, and there is what is known as the predictive convenience start. Here, the engine is started as soon as the car ahead starts moving, even if the driver is still pressing the brake pedal.

The functional principal of MHEV technology that utilises a 12-volt electrical system is the same as with 48 volts. However, the coasting phases, recuperation output and the CO2 savings are somewhat reduced.

Additional models will also be equipped with mild-hybrid drivetrains in the future – the new Audi A6 and A7 Sportback for example. The new architectures enable even more power and torque, with innovative functions leading to greater fuel savings. Over the mid-term, the brand plans to convert ancillary units like compressors to 48 volts; they will then lend themselves to more precise need-based control as well as lighter and more compact construction. The same applies to large static convenience consumers such as window heating and sound systems. Small consumers such as control units and lights, on the other hand, will remain in the 12-volt system well into the future.