Open & Closed Case

Le Mans sports car racing will never be the same. The year 2006 marked the first time that a diesel-powered sports car had ever won the historic 24 Hours of Le Mans. The same car – Audi's R10 Turbocharged Direct Injection (TDI) – also achieved overall wins in every American Le Mans (ALMS) event it started, putting to rest any doubt that diesel engines could compete against the fastest gasoline-powered cars.

There are four classes of sports cars that compete in any given Le Mans event: Le Mans Prototype 1 (LMP1), Le Mans Prototype 2 (LMP2), Grand Touring Class 1 (GT1) and Grand Touring Class 2 (GT2), and all four classes are on the track at the same time.

Audi's motto is "Vorsprung durch Technik," meaning "leadership by technology." Audi pulled out the stops with the R10 TDI project: A clean sheet of paper was brought out as no template existed for a diesel-powered sports car for competition at that level.

THE DIESEL ADVANTAGE

High torque output and fuel efficiency have always been the strong points of the diesel engine. However, these advantages have traditionally been offset by the diesel's smoky exhaust and noisy operation.

The heart of the R10 TDI is a 90-degree V12 all-aluminum diesel engine with twin turbochargers. A high-pressure common rail injection system is used, with injection pressures exceeding 1,600 bar (23,500 psi).

The R10 TDI creates 650 hp with a driveshaft-twisting 810 lb.-ft. of torque. The immense torque output of the R10 TDI created a challenge for Audi engineers, who were tasked with designing a drivetrain that could transfer this torque to the ground reliably and efficiently.

Fuel efficiency is an attribute that is not generally associated with high-performance race engines. If anything, racecars are viewed by most as being designed for speed, with little consideration given to the amount of fuel consumed. The R10 TDI gained a major advantage over its gasoline-powered competition by simply going further on a tank of fuel.

Diesel fuel has a higher density than gasoline; therefore, it weighs more for a given volume. However, diesel fuel also has a significantly higher heat content (BTUs) than gasoline. Combined with a diesel engine's higher efficiency, this leads to more miles traveled between fills. Less time in the pits equals more time on the track.

At the beginning of the 2006 race season, the ACO rules stated that both diesel and gasoline-powered LMP1 cars would be allowed to carry 90 liters of fuel. This allowed the R10 TDI to make two more laps (17 miles) on a tank of fuel at Le Mans than its predecessor, the R8.

With the resulting overall win at Le Mans and early dominance of the American Le Mans series, a "balance of performance" adjustment was announced by IMSA on Aug. 1, 2006 that allowed the gasoline-powered cars to increase their fuel capacity to 95 liters. Prior to that announcement, IMSA had also allowed the gasoline cars to run with less weight (860 kg instead of 925) and had reduced the size of the restrictor in the R10 TDI team's fuel fill hoses from 38mm to 33mm. Despite these rule changes, the Audi team adjusted their strategy and continued to dominate, registering overall wins in every race they started in the 2006 season.

A SILENT REVOLUTION

In order to compete in a Le Mans event, all classes of cars must comply with ACO regulation 5.5.3: "The engine must not produce visible exhaust emissions under race conditions." Aside from the obvious safety issues, it goes without saying that any manufacturer attempting to impress the public with the state of their engine technology would be embarrassed by the presence of smoke trails behind their entries.

To mitigate this problem, Audi fitted Dow Automotive diesel particulate filters (DPFs) in the R10 TDI's exhaust system. This had a twofold effect on its performance. First, it eliminated particulate emissions from the car's exhaust and made it as transparent as that of the gasoline cars. A secondary benefit was realized in that it virtually eliminated exhaust noise at high speeds.

Patrick Hespen, media representative for Audi North America describes it this way; "Above 110 mph, you couldn't hear the R10's engine. This created a challenge for the drivers, as they now had to rely more heavily on their instruments to determine shift points, etc."

The Audi drivers were able to quickly adjust to this lack of stimuli, and found that the relative quiet of the R10 TDI was far less fatiguing than the gasoline-powered sports cars they had driven in the past.

The R10 TDI's quiet operation left an impression with the spectators as well, because now they only heard its tires on the pavement as the car sped past. This is in stark contrast to the gasoline LMP1 cars, whose exhaust noise is intense enough to cause the ACO to study the problem and institute new regulations to limit it.

Certainly, these were not the diesel engines that the North American public had turned up their noses at in the past.

RULE CHANGES

The 2007 Le Mans race season will be ushered in with various new rules being instituted. The LMP1 class will not be untouched by these changes, as the ACO regulations state that fuel capacity for gasoline-powered cars will be kept at 90 liters, but diesel cars will be limited to 81 liters. This is to ensure that all competitors receive essentially the same quantity of energy in each tank of fuel and helps level the playing field between the gasoline and diesel-powered cars.

A longer-term trend in Le Mans racing is the ACO's push to move the prototype cars away from the traditional open cockpit configuration. In fact, the ACO has given no-tice that open cockpit prototypes will not be allowed to race as of the 2010 race season.

"The ACO maintains that closed cockpit sports cars add to race excitement and are more popular overall with the fans," says Doug Robinson, executive director of the International Motor Sports Association (IMSA). "This is not to discount the investment that some race teams have put into open cockpit prototypes, and for the time being, open and closed cockpit cars will both be competing in the prototype classes."

There may be other reasons for changing the Le Mans prototypes to a closed cockpit. Earl Fannin, media manager for Road Atlanta, states that while closed cockpit prototypes have never raced at their track, "the new regulations may be a move to make Le Mans racing more distinct from the Indy Series or Formula One."

There are two primary technical advantages that the closed cockpit enjoys over an open configuration – not the least of which are better aerodynamics. It also is possible to build a more rigid chassis using a closed cockpit, which can lead to better stability and handling. However, the drawbacks with this design are numerous: a higher center of gravity, increased weight and increased construction costs.

Another significant design issue with closed cockpit cars is keeping the temperatures inside the car at a level that does not overheat the driver. Much of a human's body heat is dissipated from the head, so an open car has a natural cooling effect as outside air flows over the driver's helmet.

ACO regulations state that temperatures around the driver of a closed car cannot exceed 32°C (90°F), and strongly recommend that an air conditioning (A/C) system be installed in the car to limit these temperatures. As an incentive for adoption of an A/C system, the ACO helps make up for any lost performance by slightly increasing the size of the engine air intake restrictors.

The regulations do allow for a natural ventilation system to be used instead of A/C, but the performance of the system must be verified and the drivers monitored for heat stress. Any evidence of excessive cockpit temperatures will result in "the car being stopped until the problem is resolved."

LOOKING AHEAD

On June 14, 2005, Peugeot announced that they would again compete at the 24 Hours of Le Mans race, this time with a diesel-powered LMP1 car. The car is unique in that it is built in a closed cockpit configuration and thus will adopt the latest related ACO regulations early.

Since that time, the first public showing of a scale model of the Peugeot 908 was given at the Paris Auto Show in September 2006, and the automaker has announced that the 908 will compete at the 2007 24 Hours of Le Mans.

The Peugeot 908 will be powered by a 100-degree V12 twin turbo diesel engine that is expected to deliver 690 hp and 885 lb.-ft. of torque. Bosch is one of Peugeot's partners in this project, playing a key role in the design of the fuel and engine management systems.

Like the Audi R10 TDI, it will displace 5.5 liters and its exhaust system will be fitted with DPFs. The first tests of the car took place in January 2007, and it has been announced that further testing will take place at Sebring before its inaugural race at the Le Mans series race at Monza in April.

THE IMPORTANCE OF SEBRING

Timing is everything. In March 2006, Audi raced its R10 TDI for the first time at the 12 Hours of Sebring race. While Sebring is a racetrack with very different conditions from the circuit at Le Mans, the purpose was to "smoke out" any issues the car may have prior to competing in the 24 Hours of Le Mans. Any problems that surfaced could then be dealt with effectively during the three months before the 24 Hours of Le Mans event.

Another ALMS event that is likely to see increased participation from European race teams is Petit Le Mans, which is held at Road Atlanta in October. Petit Le Mans serves as a qualifier for the 24 Hours of Le Mans race to be held the following year.

2007 promises to be a particularly exciting year for Le Mans racing, both in Europe and the United States. With new high-profile competitors and advanced technology being showcased, race fans have more reasons to watch than ever before.