The 2015 factory MotoGP teams are limited to 20 liters (5.28 gallons) of fuel per race. But you may recall from high school Chemistry class that the volume of a liquid is affected by the liquid's temperature. A liquid will generally expand as its temperature rises, all the way to a gas state if allowed to get hot enough. If cooled, the liquid will condense according to its chemical make-up, and the fuel used in MotoGP is no exception. So, the teams cool their fuel before each race. But how much cooling are they allowed to do, and what steps do they take to affect the fuel's expansion once it's removed from the cooling process?
The 2015 MotoGP regulations have very specific language related to cooling fuel. The fuel can be no more than 15 degrees C colder than ambient temperature when added to the motorcycle's tank. If the ambient temperature is 25 degrees C (77F), the fuel going in the bike must be 10 degrees C (50F), or higher.
There can be no device on the motorcycle to lower the fuel's temperature; this must be done before adding the fuel to the tank. And there's an official Ambient Temperature displayed on the timekeeping monitors one hour before the start of the race so that all fuel temperature measurements are based on the same starting point.
Each team's technical scrutineer tests the fuel and the containers (storage, transfer, and fuel tank itself) used for fueling to make sure that none of these is colder than allowed. The containers themselves have already been examined and approved by the technical inspectors, and each team can have only two containers for each rider. The containers must be unpressurized, and the fuel tank itself "may not be artificially pressurized above atmospheric pressure at any time." Once the fuel has been inspected and confirmed within the temperature limit, then it can be transferred to the bike's tank.
So, when the bike leaves pit lane to join the grid, it should have exactly 20 liters of fuel in the tank, fuel that is 15 degrees C cooler than the ambient temperature. Immediately, the fuel starts warming to match ambient temperature and its volume starts increasing ever so slightly. This is why it's good to have a fuel tank that is slightly larger than 20 liters. You can see on Yamaha's fuel tank that someone has written 20.1 L, which means there's a tenth of a liter extra capacity for fuel expansion, even though the team is allowed to put only 20 liters of fuel inside the tank.
Refueling or changing fuel tanks on the grid is forbidden, so that precious 20 liters much be managed carefully from the moment the bike leaves the garage for the last time before the race. The riders on the grid have umbrellas shading them from the sun, and some teams choose to protect their warming fuel in a similar way. At Mugello, Ducati placed a reflective cover over the exposed section of Andrea Iannone's fuel tank.
The regulations also require fuel tank breather pipes with non-return valves. The breather pipes "must discharge in a suitable container, one per motorcycle with a minimum capacity of 200cc and a maximum capacity of 250cc." As the fuel's temperature rises and it expands, this overflow system can cause a loss of fuel needed for the race. If the expanding fuel exceeds the volume of the tank, it can move into the overflow. Once it's past the non-return valve, there's no way to get it back into the tank to burn.
Yamaha solves this problem by clamping the overflow pipes once the bike has reached the grid.
Alex Briggs, one of Valentino Rossi's mechanics, explained to me at the Sachsenring that this clamp keeps the warming fuel from passing beyond the non-return valve. As soon as Rossi arrives on the grid, these lines are clamped. The clamp is removed at the last moment, and once the bike is moving on the warm-up lap, this overflow ceases to be a concern. I've been told this process of clamping the overflow lines saves around one lap of fuel, depending on the circuit.
