The drivetrain concept: Maximizing efficiency

The future is electric. This applies to Audi as a whole and also to Audi Sport. When the vision was born to tackle the world’s toughest rally with an electrified powertrain, a pioneering spirit was required.

The drivetrain of the Audi RS Q e-tron is electric. The front and rear axles are not mechanically connected and are each powered by a motor-generator unit (MGU) from Formula E. Because there are no charging opportunities in the desert, Audi has chosen an innovative charging concept: There is an energy converter on board of the Audi RS Q e-tron that charges the high-voltage battery while driving. The energy converter consists of the highly-efficient TFSI engine from the DTM, coupled to a generator (another drivetrain unit from Formula E).

“Even before the Dakar project, we asked ourselves what a future drivetrain system in motorsport could look like,” says Stefan Dreyer, Head of Development at Audi Sport racing. “We wanted a drivetrain that is efficient and performs well, while at the same time and can also handle long distances. That’s how the energy converter concept came about. It’s great that we can do our part in motorsport to develop sustainable drivetrains that will keep the world moving forward.”

It is quite important for Audi that the battery is not a standardized part in the Dakar Rally. “As engineers, we basically see development potential in every component,” says Dreyer. “The MGU and the inverter were both developed completely in-house by Audi Sport for competing in Formula E and they already achieve a system efficiency of about 97 percent. There’s not much more leeway there. The situation is completely different with the battery and energy management. That’s where the greatest development potential lies in electromobility in general. As always, we are working closely with our colleagues from production car development on this project.”

The high-voltage battery of the Audi RS Q e-tron was designed from scratch by Audi Sport and manufactured together with an external partner in less than a year. “Because of the short time, we had to make a few compromises, so there is still more potential,” says Dreyer. “But, it’s a great achievement to already have a battery that meets all our requirements.”

The high-voltage battery weighs 370 kilograms and has a capacity of 52 kWh. It is charged by the energy converter while the car is being driven. In principle, the process runs automatically so that the driver and co-driver can concentrate on driving and navigating.

Energy management is much more complex than on the circuit. “On the circuit, you have a good idea of all of the parameters,” says Dreyer. “The track. The race format. The competitors. You spend a lot of time in the simulator to prepare as optimally as possible. In the Dakar Rally, we have to deal with a lot of unpredictable situations. The software is therefore our biggest task.”

But, the extreme stresses on the hardware should not be underestimated either. “In the Dakar Rally, there are jumps, you sink into the sand, you hit obstacles. The components of the drivetrain were basically not developed for this particular application,” says Dreyer. “It’s a big challenge to make sure that those components can hold up to these loads.”

The electric drivetrain is identical on the front and rear axles. All of the components (MGU, inverter, 1-speed transmission, differential and slip clutch) are housed in an aluminum enclosure. The inner workings of the MGU and inverter have been reinforced, and cooling has been optimized. The front and rear axles are not connected to each other. The function of the center differential is taken over by the software.

The two-liter TFSI engine from the Audi RS 5 DTM has been optimized for use in the desert. “The DTM engine is a perfect fit,” says Dreyer. “But, it obviously operates under different conditions at the Dakar Rally. We reduced the output from 450 to about 200 kW. It operates within the narrow optimized rpm range of between 4,500 and 6,000, instead of 9,000 rpm. The engine has to run as efficiently as possible in the heat, cold and at altitude. We optimized the pistons, injectors and turbocharger for this and spent a lot of time on the test bench. We are proud of the specific consumption that is well under 200 grams per kWh.”

The advantages of the electric drivetrain in the Dakar Rally are obvious to the head of development: “The drivetrain is extremely efficient. There is hardly any power loss. You don’t have to expect temperature problems while you’re driving slowly in the sand. But, above all, the electric drivetrain is much easier to control in all of the given situations. The maximum torque is available from zero revolutions. You can regulate the drivetrain very precisely, whether the car is going over jumps or driving in the desert. The electric drivetrain does not react to external influences such as air pressure, temperature and humidity. And you can use it at full power throughout the rally. It’s all much more consistent than with a combustion engine.”