Centenary of the 24 Hours of Le Mans – A diesel engine under the hood

CENTENARY OF THE 24 HOURS – PERPETUAL INNOVATION ⎮ Diesel engine technology really began to make its mark at the 24 Hours of Le Mans in the second half of the early 2000s (with a first victory in 2006), but it appeared for the first once when racing resumed in 1949 following a nine-year hiatus due to World War II.

Refined gasoline has been used to power combustion engines since their inception. As a result of the process of refining crude oil into gasoline, a large amount of residue such as kerosene, oils and greases remained. German inventor and mechanical engineer Rudolf Diesel (1858-1913), who had filed patents for running vegetable oil engines, turned to these petroleum residues as a very cheap fuel for heavy oil engines.

The first automobile engine was promoted by Diesel during the Paris Exhibition of 1900. With a displacement of 1,960 cc, it developed around 20 hp. He also sold patents to major car brands as diesel engines would become increasingly efficient and powerful thanks to work by Sulzer and Bosch on injection pumps and turbochargers.

It will be necessary to wait until 1936 to see the first car with diesel engine marketed, namely the Mercedes 260 D. After the war, Mercedes, Fiat, Austin and Peugeot equip all their vehicles with diesel engines.

The main difference between a gasoline engine and a diesel is the very high compression ratio of the latter. The compression ratio is the volume ratio measured between the piston and the cylinder head at bottom dead center, then at top dead center. For a gasoline engine it is between 8 and 10, but for a diesel it is between 18 and 25. The purpose of the diesel is to auto-ignite the air/fuel mixture from the high pressure and of the temperature accumulated in the explosion chamber. where the gasoline engine needs a spark provided by the spark plugs. Consequently, engine technology must take this overpressure into account: pistons, connecting rods, cylinder head gaskets and valves must be sized to withstand this doubling of the compression ratio. High pressure fuel injection is essential in the combustion chamber (pressures up to 300 bp on modern engines and even up to 2000 bp for common rail engines). Injection pumps and injectors capable of generating these pressures are therefore necessary, which explains the higher price of diesel engines.

Although the power of a diesel is lower than that of an equal displacement gasoline engine, it generates much higher torque while running slower. The use of turbochargers began to become widespread in the 1980s. Finally, thanks to the fact that there is more energy in a liter of diesel than in a liter of gasoline (about 10%), a diesel engine consumes much less.

The pioneers of 1949-1951

Diesel’s first wave of popularity in the 24 Hours began in 1949 when the ACO, in its effort to revive the motor industry after the war, created new regulations for its inclusion. The brothers Jean and Jacques Delettrez seized the opportunity to enter a car, under their own banner, equipped with a 4.4-litre 6-cylinder engine of GMC origin. It developed 70 hp with a top speed of 175 km/h on the Mulsanne straight. The car completed 123 laps before running out of diesel at the 20th hour.

On the strength of this relatively successful experience, the brothers returned in 1950 with the same car. She managed to stay on track until the last hour until her engine failed, but persistent problems had meant that the car had completed fewer laps (120) than the previous year despite being three hours longer.

Also in 1950, the Manufacture d’Armes de Paris, manufacturer of a diesel agricultural engine, engaged a MAP equipped with 4 compressed 4,960 cm3 cylinders which earned it a corrected displacement of 9,948 cm3 developing 180 hp. History will remember that it was also the first rear mid-engined car to compete in the 24 Hours and that the illustrious 1939 winner Pierre Veyron joined forces with François Lacour at the wheel. Unfortunately, the adventure ended with a failing cooling system after 39 laps.

The following year, the Delettrez brothers returned after making some modifications to the cylinder head of their car. Unfortunately, these same modifications forced his retirement after 24 laps.

More than 50 years will pass before a new diesel attempt at the 24 Hours opens the doors to the triumphant years between 2006 and 2012.

The 2000s | The diesel engine finally gets its victory

The turbo diesel has experienced a boom in the automotive industry since the end of the 1980s, encouraged by favorable taxation on diesel, and manufacturers, particularly French ones, are riding this wave.

In 2004, as at the dawn of a new era, the Taurus team installed a Volkswagen bi-turbo V10 (from a Touareg SUV) prepared by Caterpillar in a Lola B2K chassis. The car released plumes of black smoke and proved to be underdeveloped, forced to retire in the fourth hour, after numerous pit stops, as the gearbox failed to handle the huge torque of its V10 diesel.

The idea of ​​running a prototype with this type of engine, however, appealed to race car designers. But the smell of diesel and the black fumes didn’t seem very much in line with the idea of ​​the competition: enter the DPF (DPF or Particle Filter), to save the day.

The FAP (DPF) is an element installed in the exhaust circuit to trap residual combustion particles, and reburn them during a regeneration cycle. This consists of significantly raising the temperature inside the DPF (DPF) by injecting unburnt diesel to ignite with the temperature and burn the particles.

In 2006, Audi was looking for new challenges after its five victories in 2000, 2001 and 2002, then in 2004 and 2005 with the R8 TFSI (direct injection), and the Bentley interlude in 2003 (based on Audi technology).

The German marque continued its dominance at the 24 Hours with an all-aluminum 5,500cc V12 twin-turbo diesel. The common rail injection allowed a pressure of 1,600 bars to the injectors, and it went from 650 hp to 780 hp over the years with a torque of more than 1,100 Nm. The Audi R10, then the R15+ and finally the R18 TDI took five victories from 2006 to 2011, leaving the Peugeot 908 HDi FAP its only victory in 2009.

In 2007, Peugeot entered the race with a 908 HDi FAP. The engine was also a 5,500cc twin-turbo V12 producing 700-800bhp with 1,200Nm of torque. advertising for its production models. Even if Audi chose not to mention as much in the name of its prototype, it was indeed equipped with a Particle filter (Particle filter).

PHOTOS (Copyright – ACO Archives): LE MANS (SARTHE, FRANCE), CIRCUIT DES 24 HOURS, 24 HOURS OF LE MANS. From top to bottom: the Audi R10 TDI, the first diesel prototype to win, in 2006, 2007 and 2008; the Delettrez brothers’ first diesel-powered car, seen racing in 1949 and 1950; at the wheel of the Audi R15 TDI+, Timo Bernhard, Romain Dumas and Mike Rockenfeller set the current distance record in 2010; the 908 HDi FAP’s victory in 2009 was the third for Peugeot and the last for a French manufacturer to date.

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