Hybrid and high voltage technology
By way of the 919 Hybrid, Porsche has developed a new field of technology at racing speed. For the concept study “Mission E” (project name “J1”), the brand’s first fully electric road-going sports car unveiled in 2015, the designers adopted the 800-Volt technology from the LMP1 racer.
Porsche has exhausted all technical possibilities for the two times Le Mans winning car. This goes especially in terms of the drive concept. It consists of a two-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 into two 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 pressure. 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 under 500 hp (368 kW), and the output from the electric motor is well over 400 hp (294 kW). The result is a system power of more than 900 hp (662 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 13.629-kilometre (8.4 mile) lap of Le Mans this happens 38 times, before every corner. Sometimes more, sometimes less – depending 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 a significant amount of extra power – and in the process recuperates energy again. The Porsche 919 Hybrid is the so far only existing Le Mans prototype that recovers energy not just under braking but also when accelerating.
This is done by the extra turbine in the exhaust tract. Particularly on the extremely long Mulsanne Straight, where the 919 exceeds 330 km/h (205 mph), the turbine is hard at work. At high engine speeds, the pressure in the exhaust system increases rapidly and drives the turbine. It revs more than 120,000 times per minute and is connected directly to an electric generator. Thanks to VTG technology – this means variable turbine geometry, able to adapt to current exhaust gas pressure – the turbine also works efficiently at low engine revs and accordingly low gas pressure. This aspect makes the technology relevant for road going cars and Porsche is still the only manufacturer to utilise turbochargers with variable turbine geometry in petrol engines.
Both energy sources, however, are limited by the WEC regulations: a driver may not use more than 4.31-litres of fuel and no more than 2.22 kilowatt hours of electricity on a lap in Le Mans (13.629 kilometres / 8.4 miles). 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.
The 2.22 kilowatt hours of electrical energy correspond to eight megajoules – and that is the highest energy class stipulated in the regulations. Porsche was the first manufacturer who dared to push the limits so far.
For the concept choice 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. For the second system brake energy recuperation at the rear axle or through utilising exhaust gas were considered. Two aspects pointed in favour of the exhaust solution: Firstly weight, and then 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. Plus 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 wheel spin. When the rear axle struggles to deal with too much power and cannot convert it into drive, inefficient slip evolves. Moreover, this leads to tyre wear and can even badly damage the rubber.
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 – the battery design, electronics design, e-motor design and charging technology. Porsche pushed this as far as 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.
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 second. The downside: A brief chat and it’s 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 Porsche has advanced new levels of hybrid management. 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 this experience, the colleagues in production development gained important expertise for the four-door concept car Mission E with 800-Volt technology. From this concept, a series production road car will appear by the end of the decade.