Who will win Le Mans?

Porsche is back at the sharp end of the Le Mans 24 Hours grid, and Toyota returns with its most concerted assault yet since rejoining the prototype ranks in 2012. They are both attempting to end a run of Le Mans victories for Audi - a marque that's so far struggled in this year's World Endurance Championship - that stretches back to 2010.

That's why Le Mans this year is the most eagerly anticipated in years.

New rules for the LMP1 category, based on energy consumption, were crucial in tempting Porsche back for the first time since 1998, but they are also going to play a central role in this year's French showpiece. The playing field in the diesel versus petrol battle has finally been levelled, though the debate over the regulations isn't entirely over.

Last year's undeniable power advantage enjoyed by Audi's turbodiesel powerplant at Le Mans has been removed thanks to regulations that limit the amount of energy a P1 car can use over the course of the 8.47-mile Circuit de la Sarthe.

Each car is given a set amount of fuel it can use per lap, according to whether it runs petrol or diesel and its level of hybrid power. There are a series of stop-go penalties - from 10 seconds to two minutes at Le Mans - for exceeding the limit.

The first two rounds of the WEC (of which Le Mans is a double-points round) have gone the way of Toyota and its new TS040 HYBRID. Audi had a nightmare at Silverstone and then wasn't on the pace at Spa with either high or low-downforce versions of its latest R18 e-tron quattro, while Porsche's 919 Hybrid wasn't consistently fast nor was it reliable. Which all begs the question...

IS THIS TOYOTA'S YEAR?

Toyota is better prepared for Le Mans than the previous two years. It started testing its 2014 challenger earlier, had two cars in Le Mans configuration out for the Belgian WEC round in May and has completed a total of four endurance simulation tests.

"Last year we were reasonably prepared," says Pascal Vasselon, technical director of the Toyota Motorsport GmbH squad masterminding the Japanese manufacturer's WEC assault. "This year we have taken one more step, not as big as between '12 and '13, but compared with last season we are one step ahead."

Toyota had the quickest car at the opening two WEC events: its TS040 was faster over one lap than the Audi and faster over a stint than the Porsche. That was at two circuits very different from Le Mans, but the evidence of the test day on June 1 suggests that Toyota will have an edge around the Circuit de la Sarthe too.

The two Toyotas headed the times at the test, but Audi was within a second of the TS040s. Porsche, meanwhile, was just over 1.5s behind.

"We are happy to be slightly faster, but we are not surprised or disappointed," says Vasselon. "The Audis are reasonably close, not miles away, but we were not expecting to dominate."

HAS AUDI GOT IT WRONG?

Audi makes no secret that it is at a disadvantage to Toyota, as well as Porsche, out of the corners as a result of its choice to run in the two-megajoule sub-class of energy return. Its rivals both run at 6MJ.

"The difference is there," says Audi Sport boss Wolfgang Ullrich. "In the first four to five seconds of every straight, there is a big difference in available power - more than 200bhp, I think. It is difficult to catch that up."

The lack of hybrid power is a double whammy for Audi, reckons Ullrich. He believes the R18, which is likely to have more downforce than its rivals at Le Mans, will be faster in the corners, but points out that they might not be able to exploit that advantage in a race situation against a car that is faster in a straightline. And that's not factoring traffic into the equation.

Ullrich insists that Audi hasn't made the wrong choices on technology, even though it opted to abandon a Formula 1-style 'Motor Generator Unit - Heat' energy-retrieval system driven by the V6's turbocharger.

The system was shelved because it didn't bring the expected gains and also compromised the weight distribution under a set of regulations in which the minimum has been reduced by 45kg from 2013.

Ullrich says Audi "didn't have any other choice", but also argues that it was the correct decision based on the table of fuel figures that existed at the time. Those values were subsequently adjusted ahead of the start of the season in favour of the petrol-powered cars. That brings us on to another factor at Le Mans this year...

FUEL MILEAGE AND STINT LENGTH

Audi clearly wasn't happy about the pre-Silverstone changes. It was upset that it now has less fuel available, and therefore power, but reserved its biggest criticism for the modifications to the fuel capacities of the cars.

It argues that these potentially give the Toyotas and Porsches the ability to go a lap longer on a tank of fuel.

The German manufacturer claims that this is contrary to the Equivalence of Technology, the foundation-stone of the new rules. That's something disputed by the FIA and the ACO. But they stress that a potentially longer stint length for Audi's rivals is not as a result of the fuel they use, but because they are running a larger hybrid system.

The rulemakers' calculations suggest that a car running a 2MJ hybrid system will be able to manage 13.2 laps of Le Mans on a tank of fuel and a 6MJ car 13.9. That raises the prospect of Toyota and Porsche saving fuel to go a magic extra lap.

Vasselon reckons that one more lap between stops is not quite as magical as some suggest.

"People always make a big thing about doing one more lap at Le Mans, but it's a simple calculation," he says. "To do 14 laps we need to do some fuel saving, and we need to make sure that over the race distance it is a gain."

Toyota has yet to reveal its hand and didn't do a 14-lap run at the test day. Nor did Porsche with its low-drag 919, although its fourth-placed car at Spa did manage some 25-lap stints. The winning TS040 mostly went 24 laps between stops, and the second-placed Audi, a high-downforce R18, 23 laps.

Just to confuse the issue, the low-downforce Audi that finished sixth did a couple of 25-lappers in fuel-save mode before the finish.

IS PORSCHE A REALISTIC CONTENDER?

In terms of outright lap speed and a bid for pole position, probably, but a victory is a long shot even for the returning giant of Le Mans.

The rhetoric from the German manufacturer hasn't changed in the better part of a year now, despite a podium first time out for the 919 at Silverstone and pole at Spa. It has consistently stressed that this is a learning year and that expectations are low.

Porsche knows it's up against it in the reliability stakes after encountering problems during its pair of endurance runs at Ricard and Aragon, as well as in the opening two WEC races.

The front-driveshaft issues that affected the #20 car at Spa, and also during testing, are claimed to have been fixed, but "new weaknesses" emerged, according to team principal Andreas Seidl. Those weaknesses were not car-stoppers, however.

"I don't want to go into detail about the problems at Aragon. They wouldn't have prevented us from finishing the race," says Porsche LMP1 technical director Alex Hitzinger, "but they did mean time spent in the garage."

WILL PORSCHE'S RIVALS BE RELIABLE?

There's an air of confidence in the Audi and Toyota camps, which are returning under the new rules with largely proven technology, that they can get through the 24 Hours without problems.

Audi and Toyota have undertaken two and four endurance simulations respectively, and both are going into the race with the belief that they have fundamentally reliable cars.

"We can say that we have not found any issues," says Vasselon of Toyota's four endurance simulations. "All the systems of the car, I would say, are able to do 24 hours. What could happen are what I would call second-order reliability issues, which could be a quality issue with a part or a handling issue [he's talking 'finger trouble' here - ed]."

Audi Sport Team Joest, meanwhile, is "as well prepared as we can be", according to technical director Ralf Juttner. "We have covered a lot of miles," he says, "but you always worry because at Le Mans things can pop up out of nowhere."

CAN AUDI WIN WITH A SLOWER CAR?

Of course. It's done it before - 2005, 2008 (pictured) and 2010 most readily spring to mind - and it can do it again.

"Le Mans is not just about having the fastest car," insists Ullrich. "The level of competition is that high this year that there is really no place for small errors."

The question is how much slower will the Audi be? It was happy with its performance at the test day, which backed up its simulations. Juttner reckons the marque is "not facing an impossible task" going into the race.

"The test has validated the confidence in our ability to compete we had before we came here," he says. "The Toyotas are still quick, but we are where we expected to be. We are better than at Spa, which we also expected.

"We know what we can do and we can see that we are not going to be bashed."

Tactics always play a part at Le Mans, and certainly did in Audi's famous victory over Peugeot in 2008, though there should be fewer safety cars this year. A 'Code 60' rule, which limits the car to 60km/h [37.28mph] under localised yellow flags has been introduced for 2014.

TOYOTA MUST BE FAVOURITE

Toyota was inevitably the favourite for honours in a race it arguably should have won in both 1998 and '99 with its TMG-developed GT-One.

The events of Silverstone, Spa and the Test Day make the marque the pre-event favourite. And that's something Toyota doesn't have a problem with.

"We are quite happy about that," says Vasselon. "If you are the favourites, it means you have done something to deserve that status. Based on the data, yes we have to be favourites."

Vasselon is quick to stress that the tag of favourite doesn't mean the result of the 82nd running of the 24 Hours is a foregone conclusion.

"There is no guarantee of success at Le Mans," he says. "Yes, we are favourites and we are happy with that, but it doesn't mean we are going to win."