There was almost an inevitability about it given the dominance in both qualifying and the race and the amount of resource that had been made available for the program, but that should not detract from what has to be termed a “historic” win for the diesel-powered Audi at the 24 Hours of Le Mans race this year. However, it was far from a foregone conclusion despite the win it notched up in the 12 Hours of Sebring in March when the car and engine won on their debut. The bare facts show that the Audi R10 TDI won the Le Mans 24 Hour race just 201 days after being publicly rolled out for the first time in Italy on 29 November 2005. Altogether, the winning car completed 380 laps or 3,223 miles in the 24 hours—one lap more than the previous record set in 2004 around the current circuit layout. Including qualifying, warm-up and the information laps before the race, the engine reeled off 469 laps (3,978 miles) during the Le Mans week. The average speed over the entire distance was 133.848 mph. Its fuel consumption averaged out at 5.73 U.S. mpg.
Such statistics, though, are but just the tip of the iceberg for a car that completed 30,000 test kilometers before the start of the race and an additional 1,500 hours on the engine dynamometers which had to be specially adapted to withstand the unusual forces. Immediately after the Sebring race, the cars were subjected to another 12-hour test run at the Florida circuit and a 30-hour endurance run at Jerez in Spain. Rain was simulated at high-speed tests at the Paul Ricard circuit in France and finally there was the pre-test at Le Mans on 4 June, the only opportunity to test on the track. In between, the three R10 TDI prototypes built to date were frequent visitors to the Audi wind tunnel center.
“At the beginning of the diesel project the first main issue was to figure out the principal design direction,” says Wolfgang Appel, head of vehicle technology at Audi Sport. “However, the shape was also important as it had to resemble the R8—the gasoline-engined car that has dominated racing sports car racing for the last few years—which has become fixed as Audi Sport’s design signature for its Le Mans cars, but nothing from the R8 has been passed on.” The Audi R8 had a hat trick of wins in 2000, 2001 and 2002, there was a brief interruption when a Bentley won—with a development of the Audi engine in 2003—before the R8 got back to its winning ways in 2004 and 2005. The R8 never recorded a single engine failure in the 79 races it contested to date. “When we started the project we did not want to design a diesel engine in an existing sports car as we could then have considered the R8, but that was not our goal. We wanted to make a real diesel sports car and so we started planning the car in parallel with the engine development. However, we couldn’t draw upon any data and had to learn it as we went along. In fact, I can feel now as the Americans did when they started the moon project. In the beginning they did not know what was going on, nor did we!”
“When we started this project in the summer of 2003 we considered every option but quickly found out that one of the key problems was going to be the combustion pressure,” says Ulrich Baretzky, head of engine technology at Audi Sport. “We believed that in order to decrease the stress in the piston, which for sure is the most stressed part in this engine, it was better to divide their load as much as possible which is why we decided on a 12-cylinder configuration.” The R10 TDI is the first Audi diesel with an aluminum cylinder block—the German manufacturer generally uses compacted graphite iron (CGI) on its high-performance production versions. However, as far as Baretzky, was concerned, it had to be the lightest weight ever, and that meant the block had to be constructed out of aluminum. “It meant we had to have some clever ideas and some clever design to make it work the way it does now. It was also clear to us that we would set the standard for our production people. Even the aluminum cylinder block being a fully stressed member on the car was never an issue. It was one of our biggest worries at the start but it was one of the first that went away.”
The engine ran for the first time on the test bed in July 2005. “It was incredibly interesting because we really have explored completely new territory with this engine,” says Baretzky. “Previously, together with our colleagues from the production car development, we had made basic tests with modified production engines and a single-cylinder model. That was all.”
Before the roll-out of the R10 TDI on 29 November 2005, the new engine had completed around 1,000 dynamometer hours, including several endurance runs, with a further 500 hours clocked up between then and the start of the race. With the usable power band lying between 3,000 and 5,000 rpm, the 5.5-liter diesel 90º V12 was said to deliver 650 hp and 1,100 Nm of torque, although the real output figures were being kept close to the manufacturer’s chest. Using a common-rail system developed by Bosch, the injection pressure was said to be higher than the 1,600 bar generally seen in production cars, while the ignition pressures reached values never previously seen in an Audi engine. The end result was the most powerful diesel engine in the world, claims Baretzky, when the specifications in the regulations regarding the maximum turbo pressure—two Garrett turbochargers limited to 2.94 bar absolute—and the diameter of the air intake restrictors—2 x 39.9 mm—are taken into account.
Of special interest was the innovative diesel particulate filter (DPF) technology developed by Dow Automotive that features a chemically treated advanced ceramic honeycomb with a unique microstructure to provide high filtration performance. It is a robust, lightweight, high-porosity filter that offers cost efficiencies for both light and heavy-duty diesel engines. “Dow Automotive’s DPF, made from a wall-flow ceramic honeycomb, contains the physical, chemical and mechanical properties to support the management of particulate matter and the reduction of gaseous emissions effectively,” says Gary Lawrey, vice president, diversified products for Dow Automotive. “Our filter enables improved engine performance by reducing the pressure loss in the exhaust stream due to our high porosity structure, lower thermal mass and faster, more efficient soot regeneration versus other competitive materials.”
ZF Sachs’ ceramic clutch was used, Baretzky saying that with the clutch it is always a game between size and the torque that can be transferred from the engine to the gearbox. “Ceramic turned out to be the best solution in terms of size, weight and length. It was subjected to a 30-hour test in the car as well as on our test bench in Ingolstadt, where we can test the gearbox and engine together. We precisely simulated the driving conditions at the circuit, including the pit lanes speeds and stops.”
The size of the new engine brought its own challenges to everyone else as well, says Appel. “People think that because normal road cars have a diesel engine, the logical thing is just to put one in a racing car, but it’s a challenge to do a proper job. Compared with the R8, the engine’s length has grown through the number of cylinders, and because of the engine’s size—with over 50% more displacement—and a diesel’s typical power and strength, we had to react by making everything as light as possible on the chassis side.”
This includes a new construction technique that saw the R10 dispense with the traditional chassis clothed in synthetic bodywork in favor of a carbon fiber composite construction with an aluminum honeycomb core. This led to considerable weight saving which went some way to compensate for the heavier V12 TDI engine compared to the 3.6 liter V8 that powered the R8. “The materials are the same as those used in Formula One,” says Appel, “which is high-strength carbon fiber and Kevlar. What is new for us is that the monocoque is a single piece. Previously they were built in many pieces which were bonded together to build the main chassis structure.” The R8’s modular design and serviceability were retained but further refined. Not only could the rear bodywork of the R10 TDI be quickly removed but also the front section, including the crash structure, being removable to ease access to the front suspension for the mechanics.
A new vehicle data logging system (FDE) was developed for the R10 TDI by Bosch. All data was transmitted by telemetry to the pits and shown in the cockpit on a steering wheel-mounted display, similar to the MMI-System found in Audi production cars. Every time the car crossed the start/finish line, the high-resolution data from the previous lap was transferred by WLAN via a so-called “burst-system”. The most important functions were controlled by buttons mounted on the steering wheel that had been equipped with a micro-processor developed by Megaline. Megaline was also involved in the development of the electro-pneumatic shift mechanism that was also activated by two steering-wheel mounted paddles. The five-speed gearbox itself originates from specialist transmission manufacturer Xtrac. Despite the enormous torque produced by the TDI engine, the new transmission was lighter than that of the R8.
“This engine will form the base from which the next generation of Audi TDI engines will do everything a little better than they do already today,” says Dr Wolfgang Ullrich, the head of Audi Motorsport. “For Audi, motorsport is more than a simple marketing instrument. The technology transfer between motorsport and production is very important to us. Motorsport has often cleared the way at Audi for new technologies. At the same time, Audi Sport benefits from the enormous know-how held by production development. The Le Mans diesel project is the best evidence for this.”