A Quantum of Power
It’s happened again:
Courage is a question of imagination. Alexander Hitzinger, Technical Director of the LMP1 program, has plenty of imagination, as well as the confidence to make the most out of everything that seemed possible in the Le Mans–winning
When braking, kinetic energy is converted into electrical energy on the front axle. In the exhaust tract, in addition to the turbocharger there is a second turbine that also converts excess energy into electrical energy. The brake energy accounts for 60 percent of the total energy created, and the exhaust tract energy for 40 percent. The electrical current is buffered in a lithium-ion battery that powers an electric motor as needed. In this case, “needed” means the driver wants to accelerate, and can utilize the energy for this purpose at the touch of a button. Hitzinger describes the output of the combustion engine as “significantly over 500 hp.” And the output of the electric motor: “significantly over 400 hp.”
The interaction of these two power sources requires a sophisticated strategy. What that means on the race track is this: in every braking phase, the battery stores energy—and the energy is recuperated. On a 13.6-kilometer lap at Le Mans, that happens 38 times—before every corner. Sometimes a bit more, sometimes less. It depends on the intensity of the maneuver, that is, the speed at which the driver comes barreling toward the corner and how tight the following corner is. The driver brakes and recuperates energy up to the apex of every corner, and then re-accelerates. And it is precisely at this moment that as much energy as possible should be available.
The driver both floors the gas pedal—applying fuel energy—and boosts output by calling up electrical energy from the battery. While the combustion engine powers the rear axle, the electric motor drives the front axle. Thus, the 919 Hybrid zooms out of the corner with all-wheel drive—while already beginning to recuperate energy again. On the extremely long Mulsanne straight in particular, where the 919 Hybrid reaches over 330 km/h, the turbine in the exhaust tract is a workhorse. So far, so simple. Yet both energy sources are subject to limits: the car may not use more than 4.65 liters of gasoline, and 2.22 kilowatt-hours of electrical energy, per lap.
The driver, then, must manage his resources carefully to ensure he’s on course with the race plan and has not used one iota more energy than allowed at the end of the lap, but ideally not any less, either. It’s a balancing act. If he consumes more, he’ll be penalized. If he uses less, he’ll lose performance. The trick is finding precisely the right instant to stop boosting with electrical energy, and easing off the gas at the right moment. The 2.22 kilowatt-hours of electrical energy correspond to 8 megajoules—the highest energy class permitted by the regulations.
“The concept selection was driven by our detailed examination of the individual alternatives,” says Hitzinger of the decision-making process. It was immediately clear that braking energy from the front axle would be used. This was a “no-brainer”—a huge energy score drawn partially from previous development work and extensive new development. “As a second system, we were looking at brake energy recuperation on the rear axle or exhaust recuperation.” Two factors spoke in favor of the exhaust solution: weight and efficiency. “With brake energy recuperation, the system has to recuperate the energy within a very short amount of time and handle a great deal of power, which comes at the cost of added weight. The acceleration phases, by contrast, are much longer than the braking phases, so recuperation takes place over a longer period of time, which makes the system lighter. Moreover,” adds Hitzinger, “we already have a drive unit on the rear axle with the combustion engine. If we had added more power in the rear, we would have generated more slip.” Slip is essentially the opposite of efficiency, and increases wear on the tires to boot.
Arguably the most courageous fundamental decision was Hitzinger opting to use 800-volt technology for the hybrid system. “Defining the voltage range was a key decision in designing the electric drive system,” he emphasizes. “It impacts everything—the design of the battery, the electronics, the electric motors, charging technology, and the charging infrastructure. In the process, we went as far as we possibly could.”
It was difficult to find components for such high voltage, and in particular a suitable storage medium. A flywheel, supercondensors, or a battery? Hitzinger opted for a liquid-cooled lithium-ion battery comprised of hundreds of individual cells, each enclosed in its own cylindrical metal capsule, 7 centimeters high, and 1.8 centimeters in diameter.
With road-going vehicles as well as race cars, developers have to strike a balance between power density and energy density. The higher a cell’s power density, the faster it can be charged and the faster it can deliver the energy. The other parameter, energy density, indicates the amount of energy that can be stored. In racing scenarios, the cell, metaphorically speaking, has to have a huge opening. As soon as the driver applies the brake, a huge amount of energy has to go in instantaneously, and when he uses that energy, it has to go back out just as quickly. To take an everyday example: if the empty lithium-ion battery in a smartphone had the power density of the 919 battery, it would be fully recharged after about 20 seconds of charging time. The drawback: a short call would be enough to completely drain it again. To ensure that smartphones can run for days, the energy density, or storage capacity, is the priority. Translated to electric cars, storage capacity means greater range. “In this respect, the requirements of a race car and an electric car for use on the road are different,” says Hitzinger, “but with the 919, we forged ahead into regions of hybrid management that were previously unthinkable.”
Author Heike Hientzsch
Photography Frank Kayser/