Bi-Turbo: Two turbos, working together - if you have 4 cylinders - one turbo fills only 2 cylinders. So you can take two smaller turbos instead of using one which is bigger. The advantage is the seize, a smaller tubo is able to react faster, so you have a smaller "turbo hole" (i don't know if thats the right translation.
Twin-Turbo: You have also two turbos, but a small one for low revs, which reacts really fast and a big one for higher revs which has more boost instead of reacting fast.
It is a PR stunt because with a similar budget they, or any other works team could produce a faster petrol car with no real disadvantages.
Audi has been choosing what wins Le Mans ever since it launched the R8, it's never lost it since the launch of the R8, and ffs the Bentley was an Audi PR stunt, atm the R8s are semi-works cars based on a 2 or 3 year old chassis and there is currentley no opposition from other cars. If Audi would allow fully independant customer teams and we'd see some great RACING like under the Porsche 956/962s.
of course it's a PR stunt, they just want to sell more diesels, especially in USA
...another few years of dominance in LeMans with same FSI engine won't give them as much in terms of publicity, as thay had before...
but i heard some rumours that audi is developing another car with conventional FSI twin-turbo V8 as a backup, in case the diesel car will not perform as expected...
Errr.....it's not a PR stunt. Of course it looks like it because it's true that they of course want to sell more diesels. However, the ACO regulations give an advantage to vehicles powered by "other" fuels (diesel, bio-ethanol).
the advantage is a larger restrictor and engine capacity - 5.5 l for tdi, 5.0 for petrol n/a, but without it the diesel won't be competitive at all, because it is always heavier than petrol engine with similar power...
first audi had problems to get r10 under minimum weight (solved now, as ACO increased minimum weight for all cars by 25 kg)
then there are wider front wheels which are needed because of engine's weight - that reduces aero efficiency
and the torque, the biggest advantage of diesels, has to be electronically limited in lower gears, otherwise gearbox will fall to pieces...
so the advantage of diesel isn't so big as it may seem...
yes, R10 is a great achivement of engineering, and i'm glad that a car like this exists, but the reason why diesel r10 was created is not as noble, as audi marketing people want you to believe...