With its 919 Hybrid, Porsche has developed a new field of technology at racing speed. The German automaker exhausted all possibilities in designing the two-time Le Mans winner, especially in terms of the drive concept. It consists of the 2-litre V4 turbocharged petrol engine, the most efficient combustion motor that Porsche has built up to now, and two different energy recovery systems.
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 percent, with the remaining 40 percent 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 Le Mans regulation changes, the power from the combustion engine is just under 500bhp (368kW) and the output from the electric motor is well over 400bhp (294kW).
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 – 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 by 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 919 Hybrid, a very close look at the individual alternatives was taken. There was no question that Porsche would use the braking energy at the front axle, as this means a huge amount of energy from areas already partially developed combined with massive progress. For the second system, brake energy recuperation at the rear axle or through utilising exhaust gas was considered.
Two aspects pointed in favour of the exhaust solution: weight and efficiency. With brake energy recovery, the system has to recuperate energy within a very short space of time, which means coping with a lot of energy, but at the expense of weight. The acceleration phases, however, are much longer than the braking phases, which allow a longer period of recuperation and make the system lighter.
Furthermore, with the combustion engine, the 919 already has a drive system on the rear axle. Even more power at the rear would have generated more inefficient wheelspin. Moreover, this leads to heavy tyre wear.
Arguably Porsche’s bravest decision for the hybrid system of the 919 was opting for 800 volts. Establishing the voltage level is a fundamental decision in electric drive systems. It influences all else – battery design, electronics design, e-motor design and charging technology. Porsche pushed this as far as technically possible.
It was difficult to find components for this high voltage, particularly a suitable storage medium. Flywheel generator, supercapacitors or battery? Porsche chose a liquid-cooled lithium-ion battery, with hundreds of individual cells, each enclosed in its own cylindrical metal capsule seven centimetres high and 1.8 centimetres in diameter.
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.
Here’s 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 in 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 roadgoing electric car therefore are different. But with the 919, Porsche has advanced into regions of hybrid management that were previously unimaginable.
The 919 served as the testing lab for the voltage level of future hybrid systems. Important basic knowledge was discovered during the LMP1 programme, such as cooling for the energy storage (battery) and the electric motor, the connection technology for extreme high voltage as well as the battery management and the systems’ design.
From the 919 Hybrid Le Mans Prototype racecar (which won the 2016 Le Mans 24 Hours endurance race), a series product will appear by the end of the decade to become the first purely electric-driven Porsche. 
Lexus has created a high-performance hybrid coupe.
