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Audi A8 range 48V system: The Knowledge

Audi's next-generation A8 will feature a 'range-wide' adoption of 48V circuitry, for the very first time. Engine Technology International details the new electrical subsystem

 

The all-new Audi A8 range, which is set to make its debut next month, will be the first in the brand’s history to feature an electrified drivetrain as standard. Audi is keen to highlight that this represents another 'important milestone on the e-mobility pathway'.

The range's internal-combustion engines will be equipped with mild-hybrid technology based around a 48V electrical system which features as the primary electrical system for the first time. The technology will work in favour of the running refinement and performance that are so crucial at this level, and also in the interest of greater efficiency.

The mild hybrid drive comprises two main components: first, the watercooled 48V belt alternator starter (BAS), which complements the conventional pinion starter. The latter is only required for cold starts in the new Audi A8. Then there is the lithium-ion battery in the luggage compartment to act as the energy accumulator for the MHEV (mild hybrid electric vehicle) with a 10Ah charge carrier capacity and a 48V system.

The advantage of the BAS becomes particularly clear when the driver approaches a red traffic signal or a roundabout. If the traffic signal turns green during braking while the vehicle is coasting to a stop or if a gap appears for the driver to move into, and the driver releases the brake, the combustion engine is started immediately. The car accelerates without any delay. This is all thanks to the belt alternator starter, which is permanently connected to the combustion engine.

The new drive combines efficiency with comfort in a very special way: The fourth-generation A8 can enter the noiseless coasting mode within the speed range of 34mph to 99mph, and can then coast with zero emissions for up to 40 seconds with the engine off altogether. As soon as the driver depresses the throttle pedal again, the BAS prompts a swift, very smooth restart. In addition, start/stop operation is actually active from 14mph.

The convenient start/stop function in the new Audi A8 differs markedly from previous systems. The high-connectivity luxury saloon can distinguish between different traffic situations. When the road ahead is clear, the BAS starts the combustion engine conveniently and promptly, letting the Audi A8 drive off swiftly. After long standstill phases and if the customer wants to turn up the air conditioning, the combustion engine is started extra-smoothly and quietly. There is the new feature of predictive convenient starting: As soon as the vehicle in front of the Audi A8 moves, the engine starts even if the brake is still pressed. In this scenario the control strategy concludes that the driver will fall in with the flow of traffic.

To achieve this level of efficiency, the new Audi A8 comes with 'sophisticated' powertrain management: It processes both route data and information from its highly networked set of sensors, which include the front camera. The powertrain management uses this equipment to decide whether the A8 is in coasting or recuperation mode. It recovers energy not just from brake applications or when coasting; the Audi A8 also does so if it is in danger of getting too close to a preceding vehicle while coasting. All these measures are beneficial not just to comfort, but also at the fuel pumps: In the A8, the mild hybrid drive reduces fuel consumption during customer operation by up to 0.7l/100km (62.1mpg).

 

27 June 2017

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