One-pedal driving in electric cars

IN 1894, WHEN Louis Antoine Krieger started making electrically powered horseless carriages (pictured above), he introduced a feature that had appeared earlier on some electric trains. The motors that drove the front wheels of Krieger’s landaulet could operate in reverse, to work as generators when the driver slowed down. That let them recover kinetic energy from the vehicle’s forward motion, turn it into electricity and use this to top up the battery. But there was another benefit. The harvesting of this otherwise-lost energy also produced a handy braking effect, helping slow the vehicle without the driver having to apply the somewhat dodgy mechanical brakes.

Regenerative braking, as the technology Krieger used is now called, pretty much disappeared from road transport when electric power gave way to the internal-combustion engine. But, with sales of petrol- and diesel-powered vehicles peaking, and scores of new electric and hybrid cars appearing on the market, it is staging a comeback. Its principal advantage is that it increases the distance a vehicle is able to travel between charges. For example, according to its makers Audi, regeneration contributes 30% of the 400km maximum range of the firm’s e-tron SUV. But regen braking, as it is known for short, also promises to do for the brake pedal what automatic gear boxes did for the clutch, and thus to make driving a one-pedal experience.

As Martin Tolliday, an automotive expert at Ricardo, a British engineering consultancy, observes, regenerative systems already encourage a different way of driving. Some cars, such as the Tesla Model 3, permit the driver to choose levels ranging from mild to aggressive. Mild lets the car coast more freely. Aggressive pulls the car up more sharply and recovers the most energy. Carmakers put sensors in such vehicles to turn on the brake lights once the force of regen braking reaches a level similar to that of conventional braking.

Regeneration also permits braking methods to be blended. When a driver lightly touches the brakes in some models, it is not the friction brakes that are applied initially, but regen braking instead. Nissan, a Japanese firm, has taken this idea furthest. The latest version of its Leaf electric car features a switch that activates what it calls the ePedal. This combines acceleration and braking into a single action. As with other electric cars, when the driver relaxes pressure on the pedal, regenerative braking takes over. With an ePedal, though, if he takes his foot off altogether, the friction brakes are eventually applied as well.

The Leaf still has a brake pedal, but it is there for use only in extremities. In normal road conditions Nissan reckons the ePedal can meet 90% of a driver’s deccelaration needs and, particularly in heavy traffic, it avoids his having to shift his foot constantly from one pedal to another. Once he has got used to it, he rapidly learns how to play the ePedal when approaching junctions or stationary traffic ahead, and can bring his vehicle to a standstill without ever pressing the brake. He need not do so even going downhill. On inclines, the car will hold itself stationary until the ePedal is pressed, at which point it will set off again.

Nor are drivers the only people who will be affected by the rise of regen braking. So, too, will those who service cars and who make components for them. Conventional brakes work by clamping a set of friction pads onto a disc on the wheel hub. The result of this friction is wear and tear, so pads and discs need to be replaced at frequent intervals. Regen braking means those components may last the lifetime of the vehicle. Some taxi drivers of elderly Toyota Prius hybrids reckon they are running on their vehicle’s original set of brake pads, even with a million kilometres on the odometer.

Whether and when the brake pedal will disappear completely remains to be seen. Mr Tolliday thinks it is likely to happen eventually. Already, a number of cars use auto-braking, in which radar sensors apply the brakes if a vehicle gets too close to the one in front. In such a situation, if he is driving at all, a motorist with a single pedal could simply remove his foot and let the sensors work out how best to brake without the vehicle losing control of itself.