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.”
Source: Autosport