To understand the background, a dip into physics may help. Or into personal experience. Anyone who has ever traveled in underdeveloped regions will be familiar with the problem. Despite the existence of water pipes, at times there’s not enough pressure in them to produce more than a dribble of water. It’s similar with electricity, also in the car. Even if the analogy doesn’t quite hold water, let’s just say: Pressure is to water what voltage is to electricity. No voltage means no current. By the same token, higher voltage means no small leap in power. Hugely simplified, of course.
But let’s take a closer look at “small”. A comparatively mild three volts is easily enough for a smartphone, while for years cars have got by comfortably with twelve volts —which is also quite small in absolute terms. But as electrification increases, things are getting tight. More and more electrical components in the car are drawing power from the alternator and battery. In a modern Audi, the electrical system has to power up to 150 electric motors and over 100 controllers.
Often what is known as the static-load components such as headlights, sound system and seat heaters will use up the alternator’s output of up to three kilowatts on their own. When it comes to supporting convention- al exhaust gas turbochargers with an extremely dynamic electric compressor — as in the RS 5 TDI Concept — a 12-volt system finally comes up against its limits, unable to supply the required energy quickly enough.
The developers at Audi have made a virtue out of this necessity by installing a second, 48-volt electrical system alongside the conventional 12-volt system in the Audi A6 TDI Concept and Audi RS 5 TDI Concept. Prof. Dr.-Ing. Ulrich Hackenberg, Board Member for Technical Development at AUDI AG: “This allows us to make available larger amounts of energy and is a prerequisite for new technologies providing greater sportiness, efficiency and comfort.” The e-turbo in the concept vehicles demonstrates the performance gain with meaty torque and spontaneous response — and the efficiency gain likewise comes into play.
This undergoes a further enhancement in the shape of a 48-volt belt starter generator. This efficiency-optimized generator turns the Audi RS 5 TDI Concept into a mild hybrid and has an energy recovery output of 10 kilowatts during deceleration, more than four times the output of current 12-volt systems. It allows a significant reduction in CO2 emissions per kilometer and a fuel efficiency gain of 0.4 liters per 100 kilometers, because the energy recovered by the generator during braking does not cause an increase in fuel consumption. Plus, there are various positive side effects that boost the system’s efficiency still further. As the higher voltage allows smaller cable cross sections, the weight of the wiring harness is reduced as well as the space needed for installation in the car. In addition, the energy generated can be more efficiently used to electrically power various ancillary units currently driven mechanically from the engine, consuming expensive fuel.
Audi is also taking a new approach to energy storage with the 48-volt system. To supply power when the engine isn’t running, the system uses a separate lithiumion battery. The higher voltage also helps enhance safety and comfort. In fast evasive maneuvers, more energy can be made available to assist steering — and in ve- hicles with active suspension systems, body roll control during cornering will in the future be more effective because the actuators will respond even faster and more efficiently to stabilize the car.
Despite all its advantages, 48-volt technology is likely to remain an add-on to the 12-volt system for the time being. As most electrical components get by easily with the smaller voltage, it would make little economic sense to convert them all to 48-volt operation. But the partial shift from 12 to 48 volts remains an important building block in the four rings’ strategy of electrifying the powertrain in stages. For this, the company’s developers have already come up with a scalable platform concept.
In the Audi RS 5 TDI Concept, a 48-volt electrical system means that, on top of the conventional exhaust gas biturbo, an additional electric compressor arranged in series can provide more boost pressure. Instead of a turbine wheel, the compressor has a seven kilowatt motor that powers the compressor wheel to maximum speed within 250 milliseconds, guaranteeing high torque even at low revs.
Hermann J. Müller (copy)