Recuperation in cars reduces fuel consumption

When engines work, they convert chemical energy bound in the fuel into mechanical energy: they make the car roll. If the driver brakes, part of this energy is lost again in the form of warmth, the brakes become heavier. But there are technical procedures to prevent energy losses like this one. The technical term for this is recuperation. The potential fuel savings are not insignificant.

The technical term comes from the latin "recuperare" recuperate, which means to regain or regain again. When buying a car, people who think about fuel-saving technology such as an automatic start-stop system often come across the term recuperation brake. Because both methods of increasing efficiency are usually offered together. But recuperation is not new.

The golf city stromer, which was built in a limited edition of 120 units starting in 1993, already had recuperation technology on board to increase efficiency. And even the first electric vehicles around 100 years ago were theoretically capable of recovering braking energy. "Provided that the energy storage devices used were rechargeable", says christian kolf from the institute for vehicle technology and mobility (IFM) at tuv nord.

"In principle, recuperation means that kinetic energy is converted back into electrical energy," says thiemel, explains arnulf thiemel from the ADAC technical center. When a car is rolling, it can drive a generator that produces electricity. The electrical energy is then stored in a battery.

In conventional cars with internal combustion engines, the battery in question is the starter battery, which is charged via the alternator as a generator. As a result, the engine is relieved of the task of maintaining the charge level of the starter battery at a certain level in order to operate the heating or air conditioning system, for example.

For a long time, however, this process did not bring any increase in efficiency, as thiemel explains: "it is often erroneously assumed that a generator is always running anyway, so that the electricity generated in the process is available ‘for free’." In principle, however, the alternator minimizes engine power. "This energy conversion creates a counterforce and thus a braking effect on the shaft", adds kolf.

"A generator can swallow 5 to 7 hp of engine power under full load", thiemel quantifies the effect. This increases consumption. "For every 100 watts of electrical power generated, you have to reckon with about 0.1 liters more consumption by the internal combustion engine", according to a report in the magazine "gute fahrt" (issue 3/11).
But there are ways to still use the alternator to increase efficiency. For this purpose, it is only ever switched on as a generator when the driver is braking anyway or is only coasting in overrun mode and thus using the engine brake. Otherwise the alternator runs along without making it more difficult for the engine. Experts call this process micro-hybrid. "With efficient dynamics, BMW was the first manufacturer to bring it to the mass production stage," explains, says thiemel. Models from other manufacturers with recuperation technology, such as audi or volkswagen, also use this trick.

And this trick is really worth something: "depending on the driving cycle and the number of deceleration phases, fuel consumption can be reduced by 5 to 10 percent", says manfred crampen, who conducts research on recuperation at the institute of motor vehicles at the rheinisch-westfalische technische hochschule (RWTH) in aachen, germany. In addition to reducing driving resistance through low-friction tires, the best possible aerodynamics, engine modifications and lightweight construction, he considers energy recovery processes to be very important in making cars more efficient.

However, recuperation only really comes into its own in hybrid and electric cars. According to thiemel, the technology can even reduce fuel and electricity consumption by up to 20 percent, especially in urban traffic with many deceleration phases. Unlike vehicles powered exclusively by internal combustion engines, however, in these cases it is not the starter battery that is charged, but the drive battery – during rolling and braking.

"When the brake pedal is pressed lightly, the mechanical brakes are not immediately applied, but only the electric motors or generators are activated, which rotate more heavily when they generate electricity, and can thus already brake the vehicle noticeably", so thiemel. Up to one-fifth of the power of disc or drum brakes can be accounted for by electric brakes. In more violent manovers, the mechanical brakes intervene.

Like all electric cars – from the opel ampera to the mitsubishi i-miev – the golf blue-E-motion, announced for 2013, also relies on recuperation. "In the pre-production vehicle, which is currently in test fleet operation, shift paddles on the steering wheel are used for this purpose, as drivers are familiar with from automatic transmissions", explains VW spokeswoman ruth holling. Three stages of recuperation for more or less recoverable energy can be selected.

"In heavy traffic and relatively short braking distances, the strongest stage is selected", holling explains. In this setting, the braking effect of the e-motor is also strongest – an indication that comparatively much energy flows back into the lithium-ion battery of the e-golf. "With a little training you can even move the car without using the brake pedal at all."

Even more efficient than recuperation is so-called sailing: "during energy conversion, efficiency losses occur in the engine, in the traction exchange funnel and in the battery. For this reason, it often makes more sense energetically to let the vehicle roll in sail mode", says tuv expert kolf. This is especially true when there is a change on the highway. On the open road, the engines of the e-golf, the audi A1 e-tron or even the hybrid version of the mercedes e-class, which will be available after the summer vacations, can be switched to idle mode. Thus, the vehicles roll off almost unbraked, which increases their range.

However, sailing requires that the driver recognizes traffic situations in which coasting is possible. And that brings us back to anticipatory driving, which can make a decisive contribution to saving fuel in all cars without special eco-technology.

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