Porsche has presented the second generation of its Le Mans Prototype 919 Hybrid race car. (Earlier post.) The new racer will compete in the 8 MJ category (8 MJ of recovered energy per race lap)—the highest of the current four energy categories in the LMP1 class. (The amount of liquid fuel that may be used in a lap decreases in proportion to the amount of electrical energy a driver is able to employ.)
Although the basic concept of the Class 1 Le Mans Prototype (LMP1)— a downsized turbocharged direct injection gasoline engine and two different energy recovery systems—was retained in the second generation, virtually every component was refined. System power now totals nearly 1,000 hp (746 kW). The goals for the next evolutionary stage were to make the vehicle more efficient, more rigid, easier to handle, lighter and yet more robust.
|Porsche 919 Hybrid, 2015. Click to enlarge.|
|2015 energy/fuel use formulas for one lap in Le Mans|
|Recovered energy (MJ)||Liters gasoline||Liters diesel|
The 919 is equipped with a 2-liter V4 turbocharged gasoline engine with an output of more than 500 hp (373 kW), which powers the rear axle, and an electric motor with more than 400 hp (298 kW) that drives the front wheels and gets its electricity from two energy-recovery systems. A liquid-cooled lithium-ion battery—developed by A123 Systems in cooperation with Porsche—temporarily stores converted braking energy from the front axle as well as converted exhaust-gas energy.
The combustion efficiency of the now lighter and more rigid 2-liter V4 turbocharged gasoline engine has been increased. The V engine’s load-bearing function (90 degree cylinder bank angle) within the chassis was also optimized by means of geometric adjustments that have led to better overall rigidity. The previously used centralized exhaust tract has been replaced by a twin exhaust-pipe system in order to improve output and optimize the vehicle’s aerodynamics.
The increased rigidity of the monocoque-engine-transmission unit is also due in part to the shifting system, as the casing of the hydraulically operated sequential 7-speed racing gearbox made of aluminium is mounted in a carbon structure. With this component as well, the engineers were able to accomplish the feat of making it both lighter and more rigid and robust. They were also able to further reduce shift times.
Higher output at a lower weight was achieved by a complete redesign of the energy recovery systems. The kinetic energy produced at the front axle when braking is converted into electrical energy—the first energy recovery system. The second ERS is installed in the exhaust tract, where the exhaust-gas stream drives a second turbine (in parallel with the turbocharger) that acts as a generator.
The electricity thus produced—along with that generated by the kinetic energy recovery system (KERS) at the front axle—is temporarily stored in lithium-ion battery cells. The driver can call up power from the cells that is applied to the front axle by the electric motor; this temporarily transforms the 919 Hybrid into an all-wheel drive race car with system power of nearly 1,000 hp (746 kW).
The reworked drive system is more powerful and more efficient than before. The Porsche 919 Hybrid was homologated for the top energy-recovery category (8 megajoules for one lap in Le Mans) for the first time in 2015.
Chassis. As is also the norm in Formula 1, the Porsche 919 Hybrid monocoque is a carbon-fiber sandwich construction that was completely redesigned for the 2015 season. It is now manufactured as a single unit, rather than in two sections. Its weight has been reduced significantly, and it has become more rigid thanks to the improved arrangement of the layers, while retaining its outstanding safety attributes.
A new structure for the chassis in general and the rear-body in particular also helped to increase rigidity and reduce weight. An important development goal with the all-new chassis was to improve handling, particularly in terms of counteracting understeering in corners.
Aerodynamics. The aerodynamic improvements made for 2015 were again focussing on maximum efficiency and had to follow a two-pronged approach again because the long straights of the Le Mans circuit necessitate very low drag, which means downforce must be limited to only what is absolutely necessary. Different requirements apply to the other WEC racing venues—i.e. more downforce is needed on those circuits.
The experience gained during the first season in 2014 sharpened the view for the delicacy of the aerodynamics. The goal was to reduce to the greatest extent possible the 919 Hybrid’s sensitivity to disturbances such as wind, cornering and extreme steering, slip, and roll angles. All of these change the flow of air around the vehicle, and thus the downforce. They also affect the driver’s faith in the vehicle’s stability, which will usually cause him to reduce speed.
The most noticeable changes to the carbon-fiber skin of the Porsche 919 Hybrid involve the new front end and the leaner engine cover in the rear. Changes to the wheel design were necessitated by the regulations. In order to limit aerodynamic benefits, 50% of the wheel surface has to be open now.
Cooperation with production development specialists. Along with battery technology and the major advances made with compact electric motors, the optimization of combustion engines also holds great significance for future production models. The high-pressure turbocharged 2-liter V4 engine, which is a perfect example of downsizing, is equipped with an innovative direct injection system and displays pioneering combustion efficiency.
Our production car development specialists are directly involved in the LMP1 programme in this area. Whether its calculations related to dynamics and charge cycles, or the design of the injector nozzles—our people work hand in hand with series production development teams. We do not even have our own dedicated test stands, but instead make use of the high-performance infrastructure at the research and development centre in Weissach. The two departments are located close to each other.—LMP1 Technical Director Alexander Hitzinger
The Porsche team will use two of its platforms in all eight races of the FIA World Endurance Championship (WEC); the third 919 Hybrid will race at Spa-Francorchamps on 2 May.
This sets us apart from the competition and we plan to grow even stronger in this segment. To this end, we need substantial expertise within the company. The tremendous pressure to perform in the top LMP1 category means our development specialists for production cars learn a lot in a very short time. On the other hand, young engineers receive top-quality training at turbo speed, and this knowledge can later be applied very effectively in series production. Learning from the race track for everyday roads is a tradition at Porsche.—Wolfgang Hatz, board member for research and development at Porsche AG