Faster, more powerful, more comfortable: the competition is fierce, also in the electric car market. All this luxury leads to ever-increasing energy consumption, a trend that is now being sought to combat.
At the beginning of the year, Mercedes demonstrated with its EQXX Vision prototype that a consumption of less than 10 kWh per 100 kilometers is possible. During a long-distance test, the concept vehicle consumed 8.7 kWh per 100 kilometers. For comparison: a compact Volkswagen ID.3 consumes, according to the global measurement protocol WLTP, at least 14.9 kWh/100 km on average; a BMW i3 reaches 15.3 kWh/100 km. With the air conditioning or heating on, consumption skyrockets above 20 kWh.
The highlight of the Mercedes EQXX is a particularly aerodynamic body. 62 percent of a vehicle’s propulsion energy is used to counteract the effects of the air. 20 percent of the energy falls on the tires and only 18 percent corresponds to the weight. “When it comes to energy consumption, it is important to improve aerodynamics first, then weight and finally the tyres. Only then do the electric motor and battery follow,” explains Malte Sievers, Mercedes Development Engineer.
The Mercedes concept car’s drag coefficient of just 0.17 was arrived at after extensive testing and analysis in the wind tunnel. For production reasons, this attention to detail cannot be implemented in a production vehicle.
As in a car with a combustion engine, a smaller frontal area and an aerodynamic bodywork guarantee a low drag coefficient and, therefore, lower consumption. “Teardrop-shaped bodies with the roof dropped at the rear, like the Tesla Model Y or the Mercedes EQXX, are very efficient,” says Haydar Mecit, professor of Urban Energy and Mobility Systems at the Institute of Electromobility of the University of Applied Sciences of the German city of Bochum.
For this reason, fluid forms are becoming a trend to reduce consumption. Aerodynamic efficiency also includes, among other things, the design of the rims: “Visually closed wheels minimize turbulence and thus also fuel consumption,” says Mecit. Manufacturers rely on special tire compounds to achieve the lowest possible resistance and achieve greater autonomy.
“The less the vehicle weighs, the less kinetic energy it has to apply when starting,” adds the academic. Aside from the lighter steel or the use of aluminum, most of the potential lies with the battery. The batteries of modern vehicles weigh, in the case of electric vehicles, an average of 700 kilograms. “Over the next five years, energy density will continue to increase and battery weight will likely drop by 20 to 30 percent,” Mecit said.
Another savings potential is the use of silicon carbide in the chips. This silicon (Si) and carbon (C) composite material reduces losses at high operating voltages. In current electric vehicles, 90 percent of the energy reaches the wheel, in the Mercedes EQXX the approximate percentage is 95 percent. A 5 percent increase in efficiency guarantees a greater autonomy or allows the use of a smaller and lighter battery. In future, the lithium-ion battery will be equipped with liquid electrolytes and thus supply 20 percent more energy, as in the EQXX.
What is the development potential? Professor Mecit believes that in vehicles produced in the course of this decade, consumption of less than 10 kWh per 100 kilometers can be achieved. “There is still a lot of potential for development in electric motors, while gasoline and diesel have long since reached their zenith,” he says. Mecit estimates that the efficiency of electric drives can be improved by more than 30 percent.
Savings measures include optimization of the powertrain electronics. Car manufacturers currently use two types of electric motors: synchronous and asynchronous motors. The permanent magnet synchronous motor is the most efficient, but because rare earths are required for its construction, it is also the most expensive. An asynchronous motor is cheaper, but works less efficiently.
“In the future, the electrical steel strips and motor windings will be optimized. This will ensure more power and less energy consumption,” says Mecit. Compared to other manufacturers, the American Tesla has a technical advantage of about five years in the case of the battery and at least seven years in the electric motor.
Jens Obernolte, Volkswagen Division Manager Energy & Weight Management, is responsible for the energy consumption of the VW ID series. Together with his team, the executive focuses all his efforts on implementing measures to reduce consumption. For the successor to the current ID.3, Volkswagen is aiming for a weight reduction of around 5 percent and a consumption reduction of up to 20 percent.
The measures considered to achieve this goal aim to increase the efficiency of the powertrain, aerodynamics, thermal management, the braking system and energy recovery. “We achieve savings through many small changes throughout the vehicle, and not just by increasing the efficiency of one component,” says Obernolte.
Although thermal management does not count towards WLTP consumption, drivers perceive its importance on a day-to-day basis, when using air conditioning or heating. In the next generation of vehicles, Volkswagen plans to increase the energy density of the battery. The German automaker’s expert also considers an energy consumption of less than 10 kWh per 100 kilometers to be realistic over the next ten years, but only in the case of sedans. These have a smaller frontal area than SUVs and can be aerodynamically optimized.
However, low power consumption can also be achieved by reducing the speed. From 60 km/h, air resistance contributes significantly to increased consumption, and from around 70 km/h, every additional 10 km/h translates into 10 percent more consumption. “With electric cars, speed is everything. Drivers can reduce consumption over long distances by driving more slowly, instead of 130 km/h only 120 km/h,” Obernolte recommends. If this allows them to avoid an intermediate stop to recharge the battery, they will also reach their destination faster.
Assistance systems, such as the navigation function, also help save energy. “With an efficient driving style, drivers don’t brake, but instead use inertia or regenerative braking,” explains Obernolte. Since the rolling resistance coefficient has a great influence on consumption, drivers of electric cars should drive with optimized summer tires for as long as possible. Correct tire pressure is another key to practicing an efficient driving style.