The IMSA SportsCar Championship underwent a profound shift with its adoption of new rules for 2023. Teams now had a spec hybrid system to optimise, while the move from DPi to LMDh (known Stateside as GTP) also meant a new philosophy for calculating energy usage – falling in line with the World Endurance Championship, allowing for the platform to be equalised against Le Mans Hypercars.
The consequence of stretching a tank of fuel too far is unchanged. But no longer is stint length dictated entirely by fuel numbers. Instead, the primary concern is now how a car uses energy from a virtual tank – think of it as a combination of fuel used by the internal combustion engine and torque produced by the hybrid system – measured by sensors attached to the rear axle (or both axles for LMH cars). The importance to the rulemakers of tracking the so-called Maximum Stint Energy (entirely separate to the actual fuel load) was underlined at the WEC’s 6 Hours of Portimao, where the #7 Toyota was heavily delayed while a malfunctioning torque sensor was fixed.
In both series the stint energy (measured in megajoules) prescribed to each car falls under the Balance of Performance, so differs for each car and track. While fuel flow rates are now unrestricted, the BoP also determines a replenishment rate for the virtual tank (also in MJ). A subtle difference, but a significant one.
“We’re governed by energy; we want to make sure that we have to pit because of virtual energy, not because we’re running out of fuel,” says Iain Watt, technical director for Cadillac squad Action Express Racing. “There’s a sensor on the side of the car that picks up when we’re adding fuel and that’s the trigger for adding energy, so it’s purely virtual. There’s no add-in electrical energy.”
Stint energy isn’t a new concept. It’s formally been part of the WEC since the Hypercar class replaced LMP1 in 2021; LMP1 cars had been subject to caps on fuel usage per stint and energy deployment over a lap. But for IMSA stalwarts and newcomers to the platform alike it has taken some getting used to.
“It’s more complicated, because it’s another layer,” concedes Watt. “It took the first race for us to really see how this was all going to work because it was new for everybody.”
BMW racer Philipp Eng says his simulator work as a Formula E reserve “definitely helped” his adaptation, but finds that the extra parameters to keep tabs on mean that engineers are heavily relied upon. “You definitely need that guidance on what to do,” he says.
Exceeding the stint energy limit, which was 920MJ at the start of the year before creeping down to 902MJ for BMW and Cadillac at the most recent Indianapolis round, carries a heavy penalty. IMSA regulations stipulate that the first violation incurs a 100-second hold in addition to the pitstop, increasing in increments of 100s for each further offence, so teams must log both the virtual and real tank figures.
Complicating this, the virtual tank doesn’t begin to deplete until the race has started, while the pitlane is exempt too. Fuel used for pace laps and driving through the pits therefore means it’s not entirely in sync with the virtual energy, but it would take several lengthy cautions for fuel to become a limiting factor.
"Say we’re adding 25s of energy – I don’t want to fill the car in 25s. But I want to have something that’s close to that amount, maybe two thirds of a tank, plus a little bit of fuel" Iain Watt
Since teams “can pretty much pick how much fuel we actually put in the car,” explains Chip Ganassi Cadillac driver Sebastien Bourdais, there’s no reason to brim the tank at each stop since that means carrying extra weight unnecessarily. There’s nothing to gain from holding more fuel than virtual energy capacity.
“Say we’re adding 25s of energy – I don’t want to fill the car in 25s,” Watt outlines. “But I want to have something that’s close to that amount, maybe two thirds of a tank, plus a little bit of fuel.”
As such, cars can remain stationary in the pits with the fuel probe attached after the actual process of fuelling has been completed. “You can be ‘plugged in’, not take fuel, and replenish the virtual tank,” says Bourdais.
As Eng notes, for drivers this makes little difference to their process as they still have to wait for the nozzle to be removed and the lollipop to drop, “but for the engineers it’s quite a big challenge to always get the energy right”.
This was doubly so for Action Express at Sebring, a race it went on to win after being delayed when the sensor to replenish energy wasn’t triggering during fuelling. “We ended up having to poke a finger in the proximity sensor hole,” recalls Watt.
Knowledge of how much energy you start a stint with “opens up tricks in races when you have a lot of yellows and you’re not actually going to be limited by fuel but limited by energy to get to the end,” according to Bourdais.
Fortunately, techniques to save fuel are hand-in-glove with saving energy, lifting and coasting being the primary method. “To save energy is basically the same thing as to save fuel,” confirms Eng.
But while monitoring fuel levels is still necessary, Bourdais says drivers “mostly have to keep an eye on the energy you have left to finish the race”. And just like that, fuel mileage in and of itself is no longer an endurance racing kingmaker.
