Fully charged

Covering hundreds of kilometres each day at unrelenting speeds, extremes of temperature, terrain and the added pressure of competition will all play a part on the Audi Sport teams taking on the Dakar from this weekend. Every member of the teams and every component of the three RS Q e-trons will be tested to breaking point daily, but no other system will be under the intense scrutiny as the electric drive technology powering the e-tron vehicles.

The Dakar presents unique technical challenges for engineers and competitors alike. Compared to the Formula E World Championship, which Audi last contested with a battery-electric drive, the standards at the Dakar Rally are different. Daily stages run to hundreds of kilometres, impossible terrain plus high outside temperatures and a minimum vehicle weight set by the regulations at two tonnes place massive demands on the most proven drive technologies. 

“It is not possible with today’s battery technology to realise an all-electric BEV off-road vehicle for the Dakar Rally under these conditions,” says Lukas Folie, high-voltage battery engineer. So, the engineering team led by Axel Löffler, Chief Designer of the RS Q e-tron, had to define basic benchmarks for the overall concept of the vehicle with electric drive and energy converter essentially from scratch. 

The drive system is made up of a TFSI engine which is mechanically coupled to an MGU (Motor-generator unit) that generates the electricity for the high-voltage battery. It transfers its energy to two other MGU units – one driving the rear wheels, the other the front wheels and when the power flow is reversed – during braking – both units recuperate energy and feed it back into the battery.

The capacity of the high-voltage battery is 52 kWh which is sufficient for the maximum expected requirements on each leg of the rally and the weight of the high-voltage battery including the cooling medium is around 370 kilograms. 

In order to realise maximum efficiency, Audi is also relying on a principle in the desert that has already been used in the Le Mans sports cars and in Formula E, meaning that the RS Q e-tron recovers energy during braking. The MGU units on the front and rear axles can convert the rotational movement of the wheels into electrical energy with the aim of recuperating the maximum energy. The power flow in this reverse direction is not subject to the same power limitations as when accelerating. What sounds so simple requires a complex Intelligent Brake System (IBS) which combines the hydraulic braking function with the electric regenerative brake.

Although it has to move a larger mass due to the regulations, the RS Q e-tron manages with less energy than the competition, its smaller tank volume for the energy converter specified in the regulations testament to its efficiency.

When the rally drivers leave camp in electric drive for each stage, a highly complex control system begins. The Audi RS Q e-tron with its innovative drive must always be prepared for all conditions in terms of distances, speeds, difficulty of the terrain and other factors. The engineers and electronic technicians have programmed algorithms to keep the State of Charge (SoC) – the charge level – within defined ranges depending on the energy demand. Energy extraction and battery recharging are always in balance over defined distances. If, for example, a difficult dune passage with high driving resistance requires maximum energy for a short time, the state of charge drops within a controlled range. This is because the drive power of the engine-generator units on the front and rear axles is limited to a maximum of 288kW under the regulations, but the energy converter can only provide a maximum charging power of 220kW. This means that in extreme cases, consumption is briefly higher than energy generation. “Something like this is possible for a limited time,” says Lukas Folie. “But over a longer distance, it always results in a zero-sum game. We then have to regulate the power consumption down so that the battery’s state of charge remains within a corridor. The absolute amount of energy available on board must be sufficient to cover the day’s leg.”

It’s another factor on top of all the other demands placed on the team and the vehicles in an already high pressure environment. But then it is triumphing over all of these factors that is the motivation to take on a challenge like the Dakar in the first place – after all, if it were easy, what would be the point.