The IMSA SportsCar Championship has undergone a profound change with the adoption of the new rules for 2023. Teams now have a specific hybrid system to optimize, while the switch from DPi to LMDh (known in the United States as GTP) has also involved a new philosophy for calculating energy consumption, in line with the FIA WEC, allowing the platform to be equated with Hypercars.
The consequences of excessive fuel consumption remained unchanged. But the length of the stint is no longer dictated solely by its numbers. Instead, the main concern is now how the car uses energy from a virtual tank (think of a combination of fuel used by the internal combustion engine and torque produced by the hybrid system), measured by sensors connected to the rear axle (or both axles for LMH cars). The importance for judges to track so-called Maximum Stint Energy (completely separate from actual fuel load) was highlighted at the WEC 6 Hours of Portimao, where the #7 Toyota suffered a severe delay while a fuel sensor was repaired. malfunctioning couple.
In both series, the stint energy (measured in MegaJoules) prescribed for each car is part of the Balance of Performance, so it varies for each car and for each track. While fuel flow rates are now unlimited, the BoP also determines a refueling rate for the virtual tank (also in MJ). A subtle but significant difference.
“We’re governed by energy; we want to make sure we pit because of virtual energy, not because we’re running out of fuel,” says Iain Watt, technical director of the Cadillac Action Express Racing team. “There’s a sensor on the side of the It’s a car that senses when we’re putting in fuel and that’s the trigger to add power, so it’s purely virtual. There’s no addition of electrical power.”
Energy per stint is not a new concept. It has formally been part of the WEC since the Hypercar Class replaced LMP1 in 2021; the latter were subject to limits on fuel consumption per stint and energy consumption on a lap. But for both IMSA veterans and GTP newcomers, it took some getting used to.
“It’s more complicated, because we’re on another level. It took the first race to really understand how everything worked, because it was new for everyone,” admits Watt.
Virtual energy allocation isn’t depleted by driving down the pitlane, one example of how virtual and real fuel levels don’t move in total lockstep
Photo by: Michael L. Levitt / Motorsport Images
BMW driver Philipp Eng says his simulator work as a Formula E reserve has “definitely helped” his adaptation, but finds the extra parameters to keep under control leave him subject to the engineers’ decisions: “You definitely need a guide on what to do,” he says.
Exceeding the stint energy limit, which was 920MJ at the start of the year before dropping to 902MJ for BMW and Cadillac in the final race at Indianapolis, carries a heavy penalty. IMSA regulations stipulate that the first violation carries a 100 second penalty in addition to the pit stop, with 100 second increments for each further violation, so teams must record both virtual and real tank data.
To complicate matters, the virtual tank doesn’t start to run out before the race starts. The fuel used for the deployment laps and for the passage to the pits is therefore not entirely in line with the virtual energy, but it would take several long cautions for the fuel to become a limiting factor.
“Since teams can choose how much fuel we actually put in the car, there’s no reason to fill the tank at every stop, as that means carrying extra weight unnecessarily. There’s nothing to be gained by keeping more fuel compared to virtual energy capacity,” explains Cadillac-CGR driver Sébastien Bourdais.
“Let’s say we’re adding 25” of energy – I don’t want to fill the car in 25”, but have something close to that amount, maybe two-thirds of a tank, plus a little bit of fuel,” Watt explains.
For this reason, cars can remain stuck in the pits with the fuel probe attached even after the refueling process is complete. “You can be ‘plugged in,’ not putting in fuel and refilling the virtual tank,” says Bourdais.
Eng adds: “For the drivers this doesn’t make much difference, as they still have to wait for the nozzle to be removed and the lollipop to come down, but for the engineers it’s a big challenge to always have the right energy.”
Teams can pick how much fuel to run and don’t always need to brim the tank in the pits so long as they have enough virtual energy
Photo by: Art Fleischmann
This was doubly true for Action Express Racing at Sebring, a race it won after being delayed when the fuel-refueling sensor failed to activate during refueling. “We ended up having to stick a finger in the sensor hole,” Watt recalls.
“Knowing the amount of energy with which you start a stint allows you to exploit tricks in races where there are many neutralizations and you are not limited by fuel, but by the energy to get to the end,” highlights Bourdais.
Fortunately, fuel-saving techniques go hand in hand with energy conservation, with lifting and speed reduction being the primary methods. “Saving energy is basically the same thing as saving fuel,” confirms Eng.
But while monitoring fuel levels is still necessary, Bourdais says: “The drivers must above all keep an eye on the energy they have left to finish the race.” And so, fuel consumption per se is no longer a determining factor for endurance racing.
Fuel saving techniques also preserve energy in the virtual tank
Photo by: Richard Dole / Motorsport Images
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