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Saving at the Limits

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Hermann Reil

Photos
Ferdi Kräling
Ulrike Myrzik

The Technology of the Le Mans Racer

The new Audi R18 e-tron quattro is not only one of the fastest cars in the world, it is also among the most technically complex. Four engineers from Audi Sport talk us through a few of the highlights.

Ulrich Baretzky
Head of Engine Development

Ulrich Baretzky is well used to worry. Not because his engines lack power or are too temperamental – definitely not that. The head of racing engine development at Audi Sport is one of the most successful and experienced engineers in his field. And nobody can remember the last time a works race car with the four rings failed to cross the finish line because of an engine defect. It certainly hasn’t happened in the last 20 years.

It’s neither the ignition pressures nor the crankshaft stiffness that has given Baretzky cause for concern in recent weeks, but the FIA – the people who make the rules for the World Endurance Championship (WEC), with the 24 Hours of Le Mans as the absolute climax. Just shortly before the first race of the new WEC season, they once again changed the classification for the amount of energy available for diesel and gasoline engines. As the only competitor with TDI technology, Audi now has less fuel at its disposal per lap, the permissible flow rate is now lower, the tank capacity has been reduced by 0.5 to 54.3 liters and the flow cross section during refill is now smaller for the R18 e-tron quattro. At the same time, the figures for the LMP1 teams with gasoline engines have been raised.

“It’s going to be really difficult for us this year,” says Ulrich Baretzky. Nobody disputes that the TDI – the diesel engine with direct injection and turbocharging – is still by far the most efficienct internal combustion engine. In the last eight years, diesel technology has dominated Le Mans unchallenged. In 2014, however, the TDI must make optimum use of every single molecule of fuel, because the regulators are allowing it considerably less energy than the gasoline engines. The bottom line is that it is permitted to consume 138.7 megajoules per lap, while a gasoline engine in the comparable classification is permitted to use 147 megajoules. This becomes even starker when converted into liters: 3.95 liters of diesel per lap compared with 5.05 liters of gasoline – whereby the higher energy content of diesel is obviously a factor.

“Naturally, we want to win anyway,” says Ulrich Baretzky – and he is certain that the completely new engine that he and his team have developed for the 2014 car will play an important role in that. In its basic architecture, Baretzky is depending on the successful formula used in the victories from 2011 through 2013 – i.e. six cylinders arranged in a V. The large cylinder-bank angle of 120 degrees ensures a low center of gravity. The “hot”, i.e. the exhaust side, lies on the inside between the two rows of cylinders, which is where the variable-geometry turbocharger is mounted. The block is made from cast aluminum, the cylinders coated with Nikasil and the pistons and conrods are in forged steel. Despite this tried-andtested concept, virtually every single screw is new, every detail fundamentally rethought and redesigned.

Consumption

The R10 TDI was already a fuel-efficient race car. But, since 2008, fuel consumption has been reduced by almost 40 percent more – while maintaining roughly the same fast lap times.

1 Cylinder bank angle of 120 degrees, for a flat layout and low center of gravity
2 Extremely compact format and further weight reduction, despite increased displacement
3 Turbocharger with variable geometry, mounted on the “hot” side, between the cylinder banks
4 Engine block and cylinder heads in aluminum; pistons and conrods made from steel.
5 Outward-facing intake manifold

Transmission

The sequential 7-speed manual transmission has a housing made from carbon fiber – one example of absolute perfection in lightweight design at Audi Sport.

4.0 bar charge pressure, 4.0 liter displacement

This year, the engine is finally allowed to breathe freely without a restrictor. Plus, the permissible charge pressure has been increased. This makes conditions for the race engine once again similar to those for production engines.

The new TDI engine is optimized for the full-load profile at Le Mans. Despite its slightly higher displacement, it is considerably lighter than its predecessor.

The first thing you notice is the larger displacement, which has grown from 3.7 to 4.0 liters. No application here of the downsizing principle that Audi is applying very successfully to its production models. “Production and racing are not comparable in this respect,” explains Baretzky. “The customer on the road drives largely under partial load. In Le Mans, on the other hand, we almost never enter partial load. We spend 73 percent of our time under full load, with the rest made up of braking, shifting and coasting. And, under full load, the specific consumption of the 4.0-liter is up to 30 percent lower than for its predecessor.” It has to be. At the end of the day, the regulations for 2014 permit 25 percent less energy than last year.

On the other hand, the engine can now breathe freely. In previous years, the intake air was always limited by a restrictor. “This makes driving incredibly inefficient,” says Baretzky, “because, to make use of every molecule of oxygen, you usually have to inject a bit more fuel. This year, it’s the other way round. And that brings us back a good deal closer to the requirements of a production engine.” The basis for optimum combustion is fuel injection at the highest possible pressure, with pressures of more than 2,800 bar having long been achieved. “This is where racing has advanced production- engine development a great deal over the last few years.”

At 4.0 bar, the permitted charge pressure is also a lot higher this year. And this comes in very handy for Ulrich Baretzky. “One of the efficiency drivers for diesel is ignition pressure – and this is based on compression and charge pressure. Here, too, we are now again a lot closer to the requirements for production engines.”

Completely untypical for a diesel, however, is the range of the current Audi R18 e-tron quattro. On the road, the TDI has a distinct advantage compared with the gasoline engine. At Le Mans, in contrast, the 54.3-liter fuel tanks specified for the Audis in the regulations means they will always have to come into the pits one lap sooner than the competition with their 68.3-liter gasoline tanks. “This flies in the face of any real-life situation,” complains Ulrich Baretzky. Furthermore, the R18 will be filled with standard commercial diesel fuel, while the gasoline engines are allowed to use a “designer fuel” with special additives.

The diesel has always had one disadvantage based on its fundamental concept: It is heavier than a comparable gasoline engine. So Baretzky and his engineers have shaved every last gram out of their new racing unit. “We are now a good deal lighter than 200 kilograms,” says Baretzky. In terms of its power-to-weight ratio, this engine is certainly the best diesel of all time. And we have used absolutely no exotic materials. We want to stay as close to series-production as possible, in order to make sure that the technology transfer from track to road works in future, as well.” Nevertheless, the weight of the TDI impacts the challenge of selecting the hybrid system and other vehicle components. At the end of the day, the minimum vehicle weight of 870 kilograms is the same for all – 45 kilos less than a year ago.

When it comes to efficiency, this high-performance race car would presumably do well in a comparison with road cars. However, the question on how much the Audi R18 e-tron quattro would consume in the EU cycle for series-production cars, of course, remains unanswered. Baretzky: “Our car would never drive as slowly as required by the standard cycle.”

Thomas Laudenbach
Head of Electrics, Electronics and Energy Systems

“As engineers, we are always looking for the best solution. And we obviously want to show that our technology package is better than the competitors.” Although he may not be happy about some details of the new Le Mans regulations, Thomas Laudenbach is satisfied with its basic idea. “The different manufacturers have a great deal of freedom to go head-to-head with a very diverse range of concepts.”

As Head of Electrics, Electronics and Energy Systems at Audi Sport, Laudenbach is responsible for the hybrid concept in the Audi R18 e-tron quattro. And, like his colleague Baretzky with the TDI engine, he has also turned to a concept that has proven victorious in previous years: the combination of motor generator unit (MGU) at the front axle and flywheel accumulator in the cockpit next to the driver. During braking, the kinetic energy is converted into electrical energy, stored in the flywheel and then converted back into propulsion by the MGU during acceleration. During these phases, the R18 e-tron quattro runs on all-wheel drive.

So far, it all sounds familiar from last year. In reality, however, everything is, of course, new – further developed, optimized, adapted to the regulations and thus a very different operating strategy. The new MGU now has an electric motor with more than 170 kW (230 hp) that is connected to the front wheels via a differential. The MGU is water cooled and has integrated power electronics.

The new flywheel accumulator has a usable capacity of more than 600 kilojoules, which can be absorbed and released again within an extremely short space of time. The electrical energy is converted into kinetic energy, bringing the rotor to a speed of up to 40,000 rpm. As soon as the energy is needed again, the generator brakes the rotor and delivers the resulting electrical energy to the MGU at the front axle – with very low losses. “For us, the very high power density makes this solution better than the battery or capacitor concepts used by the competition,” comments Laudenbach.

So much for the hardware. Also critical, however, is the operating strategy. “We have put a great deal of thought into what amounts of energy we use on which parts of the circuit in order to achieve the optimum lap time,” reveals Laudenbach, adding that all considerations are, of course, supported by sophisticated simulation calculations, optimized in detail or refuted. “Last year, we were only allowed to boost with a limited amount of energy in pre-defined zones. In 2014, we have a lot more freedom in the operating strategy and can therefore make more efficient use of the recuperated energy.”

What is limited, however, is the amount of energy that can be supplied by the hybrid system per lap. The regulations allow for several levels up to a maximum of 8 megajoules per lap. Audi has opted for the 2-megajoule class. “More hybrid energy means more weight,” explains Laudenbach. “The accumulator is correspondingly heavier and you then need a far bigger MGU. And we have to be very careful with weight because of our TDI.”

Countless hours of engineering have gone into the programming of the drive control system, with a combined control unit forming the “brain” of the TDI engine and hybrid system. One extremely difficult aspect is calibrating the recuperation profile in the interaction with the mechanical brake. The MGU at the front axle also has a braking effect when it is absorbing energy, sometimes a very powerful one. “That can’t be allowed to have an impact on the dynamics – as the car is constantly traveling at the absolute limits. This determines how much can be recuperated in certain bends, in order to make absolutely sure that the system does not over brake. And the driver mustn’t notice any of this. He needs to have the familiar, reliable braking and steering feel,” says Laudenbach, explaining the complexity of the task. “We have learnt a whole lot in recent years.”

Flywheel accumulator

The energy accumulator is located in the middle of the vehicle, to the left of the driver. Its usable capacity is more than 600 kilojoules. A display in the steering wheel keeps the driver continuously informed of the current energy status per lap.

1 Flywheel accumulator integrated in the middle of the vehicle
2 Energy status display in the steering wheel

In principle, his colleagues from series development are faced with the same task. The new Audi A3 Sportback e-tron, with its plug-in hybrid drive, uses the front axle for energy recuperation. “It’s just that our colleagues there are dealing with much smaller amounts of energy and most operating conditions on the road are far from the edge of the performance envelope.” Nevertheless, the challenges of efficient racing are very similar to those of efficient road cars.

Does the hybrid drive make the R18 any faster? “Very much so,” stresses Laudenbach, “and we’re not just talking about tenths of a second.”

Chris Reinke
Head of Le Mans Prototypes

When Chris Reinke says “new”, he means new – all new, really new. All that has been carried over from the Audi R18 e-tron quattro of last year is the name and the key aspects of the technology concept, and even this has been thoroughly checked, assessed against a wide variety of alternatives and only then confirmed. It is the process of continual evolution that makes motorsport exceptional – the literally daily process of progression in competition. Everything that was stateof- the-art last year – that had proven itself as the undisputed best – must this year go right back on the test stand. “Every day off is a day of standstill, and every standstill means a step backward,” says Chris Reinke, with a perpetually restive look in his eye. Reinke is Head of Le Mans Prototypes at Audi Sport. This is where the strands of technology and team, of planning and strategy all come together.

We have improved many of the car’s individual components through a process of evolution. In the sum of those, however, the result is a revolution,” says Reinke with conviction. If there is one single screw that is identical with one from last year’s car, then that’s because it was determined all over again as the best possible. “This year, for the first time, it is not the power that is limited, but the amount of energy used. This naturally calls for completely new solutions in the technology, as well as demanding a great deal from the drivers.”

Because, not only do they have to drive fast, precisely and vigilantly, they also have to have a constant eye on consumption. Repeatedly exceeding the limits per lap will be immediately punished by the race management with stop-and-go penalties. “We assist the drivers of course,” says Reinke, primarily with a precise display in the cockpit. The drivers have a constant eye on whether their energy account is in the red or black. If the driver comes up behind a slower vehicle ahead of a bend, for instance, it may be worthwhile to follow him for a moment to save fuel and then overtake after the bend.

The complex technology of the hybrid system itself is something with which the driver ideally should not concern himself. How and where to make best use of the stored energy are things the car simply knows for itself. It knows its exact position on the track and it knows the line of an optimum lap. However, because conditions are in a constant state of flux – the tires wear or the weather changes – the optimum lap stored is constantly being compared against the one just completed.

The team engineers closely monitor the health of the complex race car, as expertly as any hospital intensive care unit could ever do. “Our cars have long been transmission stations on wheels,” smirks Reinke. Via more than 1,000 data channels, the telemetry sends around 20 megabytes of data per lap – especially when it is passing the pit lane. The monitoring systems there are constantly checking the maintenance of all set values, such as pressures or temperatures. The race engineers are also paying close attention at all times to the key parameters. If something is not running at its best, the driver receives a heads-up over the radio. From outside, away from the pits, the team can’t change anything on the car. Any form of “remote control” is forbidden by the rules.

The R18 e-tron quattro may be one of the most complex and technically sophisticated race cars ever built, but only the driver “drives” the car. His experience, his precision, his skill are what counts. Reinke: “That’s racing. The best drivers perfectly master the fastest car.”

Watching You

The Audi R18 e-tron quattro is the most complex race car that has ever been created in Ingolstadt and Neckarsulm. The electronics in the latest LMP1 race car with the four rings are also the most sophisticated they have ever been.

An Audi R18 e-tron quattro generates data on more than one thousand channels, some of it in millisecond intervals. In Le Mans, technicians monitor their race cars uninterrupted for 24 hours. Be it for system functionality, to ensure compliance with the regulations or for drawing strategically important conclusions, the race car is continuously diagnosing its own condition, something like an ECG system in medicine, and transmitting this information to the pit.

The LMP1 sports car has a whole array of CAN bus systems networking a wide variety of control units. Sophisticated sensors measure everything from suspension data to acceleration, temperatures and pressures, as well as a range of parameters in the area of energy management, and this information is used to generate a database for the control units. The R18 e-tron quattro has a master system control unit that primarily handles control of the engine and hybrid system and also communicates with the other control units in the race car.

The race car is directly linked to the computers in the pits via an online connection. It handles high-speed data transmission in real time for operating conditions that don’t require a high transmission rate – such as temperatures. The sports car also gathers the detailed fine data on each lap and transmits this in a package to the pit via burst signal on driving past.

Two-way transmission is forbidden. The car can send data to the pit, but not the other way round. The only possibility for the team to have an influence on the car is the voice radio contact with the race driver in the car. Should data analysis by the engineers indicate a need for intervention, the driver receives this information via the radio – perhaps on the adjustment of brake balance, engine control or the hybrid system.

There is also a telemetry system for officials from the FIA (Fédération Internationale de l’Automobile), who, together with the ACO (Automobile Club de l’Ouest) monitor compliance with the regulations. Is the hybrid system adhering to the permitted amount of energy? Is the race car’s energy consumption within the specified limits? The FIA also uses a GPS system. This year, it will measure whether a race driver maintains the permitted speed in critical situations, such as yellow phases around an accident. Likewise, the activities at marshalling zones – such as securing an accident site – will be displayed in the cockpit. Thus the driver receives assistance that serves to protect the safety of all participants. A modern LMP1 race car is constantly and comprehensively networked with the team and race management.

Jan Monchaux
Head of Aerodynamics

Jan Monchaux is French and therefore a connoisseur and hobby chef, which is why he likes to compare his profession, race-car aerodynamics, with cuisine. “The ingredients and the herbs and spices are the same for all. The dishes seasoned with them can be very different, yet all delicious. The overall menu just has to be right.” In racing, however, it is not the subjective taste that counts, but the objective lap time. And a major factor in that is the mastering of airflow and drag.

The R18 e-tron quattro must demonstrate not only speed, but also efficiency. What helps for a start is the lowest possible drag. “The width of the cars has been reduced by ten centimeters this year. They have a lower frontal area despite being higher and, above all, the wheels are significantly narrower.” Ultimately, the wheels are always the source of poor airflow – be it a sports car or a Formula 1 car with open wheels. “Our job is to optimize everything around these four great big lumps,” smiles Monchaux. “A car becomes fast when it has the bad airflow around the four wheels well under control – or at least, better than the competition.”

The narrower wheels also have an indirect benefit. They generate less downforce, i.e. less of the vertical force that presses the car onto the track. And less downforce usually also means less drag. “These days, though, everything ultimately has to fit perfectly together,” says Monchaux. Previously, it was possible to use aerodynamics to compensate relatively quickly for performance inadequacies in the overall vehicle concept – you “simply” added more or less spoiler. It’s not that straightforward any more. The more or less downforce and the resulting more or less drag has an immediate influence on drive strategy. At the end of the day, you have to make optimum use of the defined amount of energy on every single lap. “Never before in motorsport have areas like aerodynamics and drive been so closely coupled with one another.”

The fact that the completely new R18 for 2014 looks to the lay person a lot like its predecessor does not surprise Monchaux. “At Audi, we follow a certain philosophy on race-car aerodynamics – even when the basic parameters have changed significantly. And they have. At the front, for instance, it is now permissible to use a real wing with a flap in place of the previous front diffuser. On the other hand, the exhaust stream cannot be used in its previous form for targeted flow along the rear diffuser. The high fin from the driver compartment to the rear spoiler and the four openings above the wheels are specified by the regulations. They are intended to reduce the tendency for uncontrollable “flying” following accidents.

The target parameters and basic philosophy of a new race car are clear from a very early stage. Then, says Monchaux, comes a “very iterative process” – in accordance with the model “test, error and retest”, step by step. Besides plenty of experience, the computer is also a major factor, with the tiniest changes to surfaces and their effects calculated using CFD (Computational Fluid Dynamics) models. “The devil is always in the detail,” says Monchaux. The extensive work in the wind tunnel doesn’t come until later – but is the most important part of the entire development process. The art lies in keeping the number of wind-tunnel tests as low as possible through good advance work on the computer. This means that more positive concepts can be validated in what is always too short a period of time.

“There is no such thing as the single valid optimum,” says Monchaux, aware that all of the various dishes can taste equally delicious. “As an important part of the vehicle menu, the aerodynamic package must simply be as well balanced as possible with the other ingredients like the drive concept.” And there it is again, that difference from the subjective appreciation of cuisine – at the end of the day, what counts in racing is only the objective lap time.

Computational Fluid Dynamics

Computational Fluid Dynamics is what the aerodynamicists use to check many alternatives in detail before they head for the wind tunnel.

WEC

Audi has two aerodynamic versions for the World Endurance Championship. On the shorter tracks, more downforce is helpful. This version is identifiable by its shorter rear end.

Le Mans

For Le Mans, the R18s are optimized for minimum aerodynamic drag. Here, for instance, the rear diffuser ends with the spoiler, the exhaust flow is directed differently and the openings in the front wheel arches specified by the regulations have been moved to the inside.

1 Air outlet, front wheel arches
2 Large fin along the back is stipulated by the regulations for more directional stability in the event of accidents.
3 Exhaust flows onto diffuser from above
4 Air outlet, rear wheel arches
5 LED lights integrated into side section of rear spoiler.