Improving the breed: Audi’s diesel Le Mans race car
It’s been said that “racing improves the the breed. “Diesel engines are perhaps not as developed as gasoline engines partly because they have not had to meet the rigors of high-speed, endurance racing. Automakers from Chevrolet and Honda to Ferrari and Porsche have used motorsports as a way to quickly develop engines. Now, Audi is doing it with its diesel-powered R10 that will be campaigned at the 24 Hours of Le Mans this month. The R10 will compete in LM P1 (Le Mans Prototype 1) Category, the top class at Le Mans offering the greatest opportunity to develop new technology, especially engines.
The R10 has already made racing history, as in March it won the Mobil 1 Twelve Hours of Sebring, becoming the first diesel car in the world to win a major sports car race. Audi used Sebring as a test for the 24 Hours of Le Mans race.
Prior to the R10’s debut, the most successful racing diesel to date was perhaps the Cummins Special that won the pole position at the 1953 Indianapolis 500. Although a factor in the race, the car did not finish.
Another reason Audi is going racing with a diesel is the “race on Sunday, sell on Monday” philosophy. Like many other European automakers, 50% of the Audis sold today have diesel engines. Finally, the 4.2 L gasoline V8 engine used in Audi R8s that have dominated Le Marts for the past six years have been made obsolete because of rule changes. Thus, a new engine was needed.
Incidentally, that V8 racing engine played a significant role in the development of the Audi’s industry-leading FSI, direct fuel injection, spark-ignition technology now offered in Audi’s latest 2.0 L turbocharged and 3.2 L naturally aspirated FSI gasoline engines.
The R10’s 5.5 L, twin-turbocharged and intercooled, high-pressure, direct-injection V12 diesel includes some leading-edge technology. For starters, the engine has both a lightweight aluminum engine block and aluminum cylinder heads, a rarity for diesels. It features twin overhead camshafts, four valves per cylinder and a cylinder bank angle of 90[degrees]. The displacement of 5.5 L is the maximum displacement currently allowed by Le Mans’ rules.
As with the case of the turbocharged direct fuel injection TFSI gasoline engine, Audi worked with Bosch on the R10’s TDI (turbocharged direct injection) engine. Engine management is performed by the Bosch Motronic MS14 system, which controls the latest generation Bosch common rail fuel injection system. Though some engine details are proprietary for competitive reasons, the injection pressure easily exceeds the 23,000 psi typically used in production cars.
Racing rules limit the boost of the twin Garrett turbochargers to 42.6 psi, while the diameter of both engine air intake restrictors is also restricted, to 2 x 39.9 mm. The dry sump lubrication system uses Shell oil.
The engine produces in excess of 650 hp and more than 811 lb.ft. of torque. “This engine is specifically the (for its size) most powerful diesel there is in the world and, up until now, the biggest challenge that Audi Sport has ever faced in its long history,” explained Ulrich Baretzky, head of engine technology at Audi Sport. “There has never been anything remotely comparable.”
The engine’s usable power band lies between 3000 and 5000 rpm, an unusually low range for a Le Mans racing engine that typically runs at 10,000 to 12,000 rpm, but less than the screaming 18,000 rpm of Formula 1 cars.
Because of the very favorable torque curve, gear changes in the R10 occur less frequently than in the R8. The huge amount of torque produced places a tremendous demand on drivetrain and transmission. Power is transmitted to the rear wheels via a ZF Sachs ceramic clutch; X-trac pneumatically actuated, sequential race gearbox and viscous-mechanical locking differential. Bosch’s Acceleration Slip Regulation (ASR) traction control reduces the high loads created by the torque fed to the wheels and helps the driver modulate the V12 TDI engine’s power delivery, particularly under wet conditions. This represents a new challenge even for experienced drivers.
Something else unfamiliar will be the low noise level and, unique for a racing engine, the smooth running of the engine. At high speeds, the engine cannot be heard from the open cockpit and there is hardly any vibration. Indeed, the new R10 can only be recognized as diesel-powered during the warm-up or in the pit lane. Its exhaust will also be invisible since a pair of diesel particulate filters are fitted. And unlike spark-ignition racing engines, there are no flashes of flame from the exhaust created by unburned fuel.
The longer R10 engine required radical changes to the chassis. The R10 has a significantly longer wheelbase than the R8, yet weighs in at 2039 lb.