June 25, 2005

Last Stop at Le Mans for the Audi R8
By: Chris Ostberg

Nearly perfect. That's the first thing that comes to mind when contemplating the most successful Le Mans prototype car of all time. It does everything so well - without excess drama and flair - much like Audi's road-going cars. It can be fairly labeled a "sleeper" despite its significant achievements. It is quieter than the other prototype cars. It does not belch flames on downshifts like the V10 Judds. With 2005 marking the last year of the venerable and dominant Audi R8 prototype car affectionately known as the "freight train", we wanted to take pause to recognize exactly what makes this car so special.

It all started back in the fall of 1997 when Audi committed themselves to develop a prototype class sports car. Over the following year and a half, Audi spent much time in house designing, testing, and developing what was to become one of the most impressive race cars of modern time. In the early stages of the R8 program, two variants existed; an open top roadster (R8R) and closed top coupe (R8C). While the fixed roof R8C was able to reach higher terminal speeds due to superior aerodynamics, racing class rules allowed the open top R8R to run slightly wider tires which decreased the frequency of tire changes during the race.

The R8R made its debut in March of 1999 at the 12 hours of Sebring. Three months later, Audi scored 3rd and 4th place finishes at Le Mans, a truly extraordinary feat for such a young racing program. That was just the beginning. Except for the Bentley win of 2003, Audi has won every Le Mans race since the year 2000. The victory tally at the end of 2004 had reached 52 with more wins expected during 2005.

When introduced at the tail end of the 20th century, the R8 was nothing short of revolutionary. As a testament to its advanced engineering, the racecar still has the ability to compete - and win - in today's racing environment. Among many technical highlights, Audi had produced the first Le Mans race car to utilize a direct injection gasoline engine. Audi calls its direct injection technology FSI (Fuel Stratified Injection).

A typical fuel injected engine introduces fuel via an injector which is mounted within the intake manifold runner. The incoming air charge then carries and atomizes the fuel into the combustion chamber through the intake valves. This method does not allow the optimal metering or atomization of fuel during many operating conditions, especially low speed engine operation where the intake charge velocity is not sufficient to fully disperse the fuel load.

Audi's FSI system utilizes a high pressure common rail injector mounted directly in the combustion chamber. The mechanics of the system are very similar to a modern diesel engine. Using a much higher injection pressure (100 bar versus a typical 4 bar for non direct injection engines), the FSI system can precisely meter the fuel delivery as well as optimally atomize the fuel mixture. Increasing the injection pressure promotes atomization (the dispersing or "misting" of the fuel charge) which will lead to a more complete fuel burn.

Another benefit of FSI is the cooling of the combustion chamber caused by the incoming fuel charge being sprayed directly into the cylinder. This cooling resists detonation or engine knock. The benefit is that a higher compression ratio, more advanced timing and more boost can be used when compared to an equal indirect injection engine. The low RPM combustion is much improved compared to an IDI (Indirect Injection) engine. This helps the R8 engine produce an impressive 700 NM of torque (516 ft-lbs) at a remarkably low engine speed. Having said that, the FSI benefits can be felt during all points of engine operation.

Last and perhaps most significant to the Audi team, the FSI setup reduces fuel consumption up to 10% during racing conditions. The precise fuel metering, complete fuel combustion and increased compression ratio all contribute to this improvement. By conservative estimates the Audi R8 can drive one more lap on the track at Le Mans between refueling compared to competitors, a clear advantage.

In addition to the success of the R8 on the racetrack, the technologies developed during the R8 program enhance and influence Audi's passenger car lineup. At this time the same FSI technology employed on the track is available on up to twelve Audi vehicles worldwide. The benefits of FSI realized by the R8 race car are even more dramatic in normal passenger car service.

The FSI system uses two different modes of operation depending on engine loading conditions. Under partial loading, the engine will request a "stratified combustion" strategy. This "lean burn" operation can increase the air fuel ratio (from the standard stoichiometric 14.7:1 up to 20:1 and beyond) by precisely centering the combustion process over the spark plug region. This is where the largest gains in fuel consumption can be achieved. This strategy, however, is not very beneficial to the R8 race car since racing conditions call for predominantly high or full load. The combustion strategy employed under this full load environment is referred to as homogeneous combustion and does not vary all that much from typical combustion.

Although "lean burn" combustion is not utilized on the track, gains in fuel economy due to FSI are still seen during homogeneous combustion from the elevated compression ratio, precise fuel metering, and optimized injection timing. Unfortunately, US market FSI cars cannot take advantage of stratified or lean burn combustion at this time due to quality issues with US fuel. Perhaps this situation will shift as the quality of US fuel increases over the next few years.

Besides the FSI-related advantages, the R8 is also renowned for being bulletproof reliable and quick to service when necessary. The fact that no R8's have experienced an engine failure during competition is an amazing detail. The six-speed sequential Ricardo gearbox endured more than 50,000 shifts during the 2000 season without a single malfunction. This record was repeated in 2001. Endurance racing, as most people come to realize, operates in an arena where speed may not be the most important asset of a racecar.

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