News
Technology
This weekend the Le Mans Prototype
With the 919 Hybrid,
During braking, a generator at the front axle converts the car’s kinetic energy into electrical energy. In the split exhaust system, one turbine drives the turbocharger while another converts surplus energy into electrical energy. The braking energy contributes 60 per cent, with the remaining 40 per cent coming from exhaust gas. The recuperated electrical energy is stored temporarily in a lithium-ion battery and feeds an electric motor on demand. “On demand” means: the driver wants to accelerate and calls up the energy at the press of a button. In accordance with the latest regulation changes, the power from the combustion engine is just under 500 HP (368 kW), and the output from the electric motor is well over 400 HP (294 kW).
The use and interplay of these two energy sources require a sophisticated strategy. In every braking phase, energy is won – that is, recuperated. On the Nürburgring’s 5.148-kilometre Grand Prix circuit this happens 17 times per lap, before every corner. The amount of recovered energy depends on the severity of the braking manoeuvre, or in other words, the speed at which the driver arrives at the corner and how tight it is. Braking and recuperation last until the apex of every corner, the driver then accelerates again. In this moment, the aim is to utilise as much energy as possible. Hence, the driver steps on the throttle pedal using fuel energy, and also “boosts” electrical energy from the battery.
While the combustion engine drives the rear axle, the electric motor takes care of the front axle. The 919 catapults out of the corner without any loss of traction using all-wheel drive – and in the process recuperates energy again because on the straights the extra turbine in the exhaust tract is hard at work. At constantly high engine speeds, the pressure in the exhaust system increases rapidly and drives the second turbine connected directly to an electric generator. Both energy sources, however, are limited by the regulations: a driver may not use more than 1.8-litres of fuel per lap and no more than 1.3 kilowatt hours (4.68 megajoules) of electricity. He must calculate this carefully so that at the end of the lap he has used exactly this amount – no more, no less. He who uses more is penalised. He who uses less, loses performance. He must stop “boosting” and lift his foot off the throttle at exactly the right moment.
Converted to the 13.629-kilometre lap of Le Mans, which is the scale model for the regulations, the amount of electrical energy allowed is 2.22 kilowatt hours. This corresponds to eight megajoules – and that is the highest energy class stipulated in the regulations.
For the concept choice for the
Arguably
It was difficult to find components for this high voltage, particularly a suitable storage medium. Flywheel generator, supercapacitors or battery?
In both a road and racing car, power density and energy density must be balanced. The higher the power density of a cell, the faster energy can be recharged and released. The other parameter, energy density, determines the amount of energy that can be stored. In racing, the cells – figuratively speaking – must have a huge opening. Because as soon as the driver brakes, a massive energy hit comes in, and when he boosts it must leave at exactly the same speed. An everyday comparison: If an empty lithium-ion battery in a smartphone had the same power density as the 919, it would be completely recharged within a lot less than a single second. The downside: A brief chat and it would be empty again. So that the smartphone lasts for days, the energy density has priority, and that means storage capacity.
In an electric car for everyday use, storage capacity translates into range. In this regard, the requirements of the racing car and a road-going electric car therefore are different. But with the 919
Tous les rapports de course de la saison précédente peuvent être trouvés dans nos archives.