On average, the Audi drivers refueled only every 14 laps, considerably less often than the petrol-powered entries. At one point a record-setting 16 laps were driven on a single 90-liter fuel load. By the end of the 24-hour race, the winning R10 was on it's 380th lap, a new record for the event.
Unveiled late in 2005, the R10 is Audi Motorsport's most expensive project ever, costing an estimated 70 million Euros per year. It is an all-new design that nonetheless looks remarkably like it's predecessor, the massively successful gasonline-fueled R8. It is powered by a turbocharged all-aluminum 90° V12 with common-rail direct injection that generates 650bhp and 811 lb-ft of torque in competition form. The power band lies between 3000 to 5000 RPM, a range so low as to be virtually unheard of in modern race cars. But its primary weakness is weight: the engine is rumored to weigh upwards of 200 kilograms, about 50% more than a comparable petrol-powered engine.
This is not the first diesel to enjoy racing success. In 1931 Dave Evans became the first driver to complete the Indianapolis 500 without refueling. BMW and VW have raced touring cars, the former winning the 24 hours Nürbergring event based primarily on the extended range afforded by its diesel powerplant. Nor is this the first diesel to run at Le Mans. That honor belongs to a Lola powered by a Caterpillar-badged VW V10 that ran in 2004. But there is little doubt that both Audi's TDI brand and the Diesel cycle scored a major coup last weekend.
But does this race represent the Fosbury Flop of endurance racing, or was it an artifact of this year's rules? The answer, more than likely, is a bit of both.
The concessions afforded diesel-powered cars at Le Mans this year are numerous. Compared with a turbocharged gasoline-fueled car, the diesels enjoy a 50-percent larger displacement limit, a 52-percent larger intake restrictor, and an absolute boost pressure limit nearly twice as high. Additionally, the diesels are allowed variable nozzle turbines in their turbochargers. It is also rumored that Audi successfully lobbied to raise the minimum weight to accommodate the R10's massive powerplant.
While diesels in the wild enjoy significant fuel savings over equivalent petrol cars, the differences largely diminish when operating at full throttle (the so-called pumping losses incurred by a petrol engine sucking air past a partly closed throttles goes away when that throttle is wide open). The differences in fuel consumption nearly vanish when one takes into account the higher energy density of diesel fuel: gallon-for-gallon diesel packs about twelve percent more heat energy.
At the same time, the lean-burn character of a diesel engine requires it to pass more air through the engine for every unit of energy sent out the crankshaft. Coupled with the extremely low maximum engine speed (limited by the speed of combustion inherent in compression-ignition engines), a corresponding increase in displacement and/or boost pressure is needed to achieve the same power output as a smaller, higher-strung petrol powerplant. As for the rule against VNTs in turbo-petrol cars, it is likely a cost-saving measure (their much higher exhaust gas temperatures make engineering a reliable variable-nozzle arrangement an expensive endeavor). So from a first-order numbers perspective, the rule changes are perfectly fair, except for that twelve percent number: we might well see eighty-liter fuel tanks on next year's slate of diesel-powered entries.
In any case, it could be argued that this year's race is as much an underestimation of diesel's potential by the rules-making committee as it is an affirmation of diesel's capabilities by Audi's win. But to draw a meaningful conclusion one really has to go back to why racing events have rules, and what results the rule-setting bodies are attempting to achieve.
Broadly speaking, racing organizers aim to hold a safe and affordable event (by their own admittedly skewed standards) that is either technically interesting, compelling for spectators, or (ideally) both. To keep Le Mans technically interesting, a wide variety of technologies are allowed. At the same time, some "correction factors" have to be applied to ensure that the race isn't a field day for certain technologies leaving others in the dust; in other words, to keep the race somewhat interesting to watch1. In any case, the addition of diesels to the mix certainly made this year's race noteworthy.
Audi, for its part, is using this race to cement its TDI brand of assorted turbocharged direct-injection diesel engines as not just an economy brand, but a performance one as well. And evidently they are willing to spend upwards of $100 million to do it. Presumably any technological advances that fall out of the project are gravy.
At one point in automotive history it was believed--perhaps rightly so--that racing enhanced the state of the art in such a way as to be applicable to road cars. While this is ever less the case, it is still a secondary aim of many racing events that the race cars should, first and foremost, be cars. In Europe, where 55% of new cars are diesel, it is only logical that such cars be fielded, and that the Le Mans event should put the pinnacle of diesel technology to the test in the way that only the premier 24-hour endurance race can.
1This is the genius of NASCAR, which almost universally elicits wrinkled noses from automotive technical enthusiasts: while the cars are restricted to 1950's-state-of-the-art configurations, they are so evenly matched that "turning left 1000 times" becomes more fun to watch than many Formula One races.