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Better and better
 
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Alexander Hitzinger is the technical director for the Porsche 919 Hybrid

The car just keeps getting better. Constantly. The engineers never stop improving the aerodynamics of the Porsche 919 Hybrid. They keep pushing the limits of what is possible. A key priority: faster lap times. But above all, the knowledge they gain from enhancing the aerodynamics flows straight into the development of Porsche’s production cars—to lower their fuel consumption and enable even better performance.

The edges of the openings in the front wheel arches have been subtly smoothed for the race in Silverstone. For Le Mans the openings have been pulled in toward the tires. And for the Nürburgring they have acquired small sills. These minor alterations have all been made by the aerodynamics department. Its engineers always have a precise objective, but thise objective is constantly changing. The car keeps getting better. To the untrained eye, all the white 919s look the same. But in fact, 80 percent of the Le Mans car’s shell was replaced after the race. Why? Because the second half of the season included tracks like the Nürburgring, whose many curves challenge the car in completely different ways than the long straights of the Circuit de la Sarthe where Porsche won its 17th overall victory on June 14, 2015.

More than 20 aerodynamic experts work on the Le Mans 919 Hybrid prototype. They devote their efforts to two sides of the same coin—downforce and drag. What generates downforce? “Steep contours on the front or rear wing, for example,” says Alexander Hitzinger, technical director of the Le Mans prototype program (LMP1). When air flows faster under the wing than over it, this creates a pressure difference. There is less pressure below the wing, so the vehicle is pressed down onto the road. This is known as downforce. However, increasing the downforce generally comes at the price of increasing the windage, which in turn means more drag. And the greater the drag, the lower the top speed.

And yet the contours of the wings are just one part of a complex whole. Every square inch of the prototype’s carbon-fiber shell, every air intake and outlet, every edge no matter how subtle is subject to the dictates of aerodynamic efficiency. “Most details that affect aerodynamics are not visible because they are underneath or inside the car,” explains Hitzinger. “Air flowing around the car as a whole and air flowing through the car’s bodywork create a complicated interplay that is affected by a variety of driving situations.” He promptly lists some of them: “driving in a straight line, curves, braking, crosswinds, slipstreams, and tricky turbulence patterns caused by vehicles right behind you.”

Because of the conflicting demands placed by different driving situations that all occur in one and the same race, it is not possible to optimize every detail of the car for everything it will face. But race tracks have different characteristics, and therefore require different priorities. And that is why the 919 Hybrid is constantly in a state of flux. The 2015 season was no exception. The car underwent a huge number of aerodynamic modifications, both large and small. The most significant changes were prompted by the unique features of the Circuit de la Sarthe in Le Mans. “Because of the extremely long straights on this course, you want to keep drag to a minimum,” says Hitzinger. “So the idea is to reduce the downforce to the absolute minimum. But for the WEC races before and after Le Mans, we configured the car to increase the downforce.”

Hitzinger’s team started off by developing aero kit number 1, which they used for the 919’s first functional tests in Weissach in December of 2014. By the time the 2015 season started in Silverstone in April, the car was already equipped with kit 2. Kit 3 followed in Spa, and was a precursor to kit 4 for the highlight of the season in Le Mans. For the race at the Nürburgring in late August, kit 5 was developed, a high-downforce package that replaced 80 percent of the car’s shell.

Exhaust unit

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While the 2014 model had a single very high exhaust unit, the two tailpipes on the current version are mounted lower. This change improves how air hits the edge of the engine hood, which generates additional downforce.

Fin

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The regulations mandate this component for safety reasons. It helps ensure stability: if the car banks, a stream of air hits this surface to create a force that helps level the car.

Fascia

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Reducing the surface area of the front fascia of the car helps to reduce drag. The regulations include templates with dimensions for the design of the car body. To comply with these dimensions and still manage to reduce the surface area, the 919 was given a ledge above the cockpit reminiscent of a unicorn—a complicated production process designed to reduce drag.

Wheel arches

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For safety reasons, the wheel arches have to be open at the top. If air enters them in an uncontrolled manner, for example during a spin, the openings allow it to escape. And that lowers the risk of lift in the car. During normal driving, air can flow into, out of, or over the openings. This can be influenced by the contours of the area in front of them. Air hits the front of the car and accelerates over the headlights to the wheel arches. Depending on the angle at which it hits the openings, the flow either pulls air out of the wheel arches or pushes it in. Alternatively, the air flow can also act like a curtain and close the openings. Depending on what happens here, more or less air flows through the car and generates downforce. Air can also exit to the sides behind the starting number or make its way along the underbody to the rear diffuser. The more air that exits the wheel arches, the greater the amount that flows around the front wing and therefore generates downforce. To reduce this for the race in Le Mans, the panels at the front of the wheel arches were pulled down, which essentially prevented air from exiting the openings.

Flick

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Detailed work at the corner: this small add-on part generates less downforce than its significantly larger counterparts used at the start of the season.

Lateral air vents

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The lateral vents help regulate the flow of air entering the car in front, some of which exits from the wheel arches and some of which finds its way through the underbody to the rear diffuser. The fewer the obstacles, the more air can flow through the car and the more downforce can be generated by the front wing. Air vents with a modified design contribute to the overall aerodynamic composition of the 2015 vehicle.

Additional element

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The additional element on the rear of the car helps to increase downforce at the back. The rear is no easy terrain in this regard, because LMP1 regulations specify the main dimensions of its wing and diffuser. The front of the car offers more options for increasing downforce. But because the vehicle has to be aerodynamically balanced, greater downforce can only be generated at the front if the same is also done at the rear.

The engineers run through a series of assessments to determine which tracks require more downforce and which ones more drag. The first indications are provided by the general features of each course, such as its layout, topography, asphalt conditions, and temperature forecasts. As of 2015, the team has also been gathering its own data for the 919 on all WEC courses. Before body components are designed and models are built, CFD (computational fluid dynamics) systems are used to simulate their effects and interplay. The next step is to build models. Porsche’s engineers test a 60-percent model in the wind tunnel of the Williams Formula One team in Grove, England. “Only then do we make full-size components and test them,” says Hitzinger. “If we didn’t have this rapid prototyping process, it would be far too expensive and take too much time to make the components.”

The 2015 race car benefited enormously from the new wind tunnel at the Porsche Development Center, which can be used to test full-sized cars. The Weissach racing department is well aware of this advantage. “Starting in December of last year, we achieved an impressive number of improvements there by comparing the CFD and tunnel results for the models, and doing detailed bodywork with small add-on parts,” says Hitzinger. This also means that the development teams for production cars and race cars are working more closely together on aerodynamics. Their joint efforts spur the engineers on to ever greater heights.

When the prototype won Le Mans in June 2015, the developers were already working on its successor. Not much information is available about this third generation of the Porsche 919 Hybrid. But it is expected to retain its basic concept: a unique powertrain consisting of a future-oriented, downsizing, two-liter, four-cylinder, turbocharged combustion engine with two innovative energy recovery systems. “Generally speaking, that is,” says Hitzinger, doing his best to look nonchalant. But we know what’s going on. Not one piece will remain the same. The car just keeps getting better and better.

By Heike Hientzsch