PRUETT: Taking LMDh hybrids from concept to reality

Paul Laguette illustration

PRUETT: Taking LMDh hybrids from concept to reality

Insights & Analysis

PRUETT: Taking LMDh hybrids from concept to reality


This story first appeared in the Fall 2021 issue of RACER magazine. Click here to get your copy, or set yourself up for a full year of stellar motorsport writing and photography with a print or digital subscription.

Matt Kurdock is buried in the minutiae. The life of IMSA’s technical director has been a blur as he charts the course from today’s DPis to tomorrow’s LMDhs, due for their competitive debut in the 2023 Rolex 24 at Daytona.

Next to go under his microscope is establishing parity between the FIA WEC’s already-active LMH prototypes and IMSA’s LMDhs that have yet to turn a single lap in the real world. Talk about an unenviable task.

Kurdock is stuck in a time warp of sorts. He has the well-known performance capabilities of DPi to draw from, and he’s seen how the WEC’s first true LMH chassis, Toyota’s GR010 Hybrid, experienced a troublesome and often underwhelming debut season. But it’s the LMDh part he’s missing that’s making for long days and nights in trying to model how DPi’s replacement class will match up with LMH once the dueling formulas go head to head in a little over a year.

LMDh, with its spec 40hp energy recovery systems gathering power from and firing it back through the rear wheels, is nothing like LMH, which allows its manufacturers to deploy all-wheel drive through powerful and custom ERS units of their choosing, or even to skip hybridization altogether. Another key conceptual difference is found with LMDh’s use of LMP2-based chassis designs from four approved constructors. LMH manufacturers are free to build their own purebred racecars. Or to convert road cars to comply to LMH regulations. It’s in the “ors” where the convergence complications begin to multiply.

The chasm of options and liberties separating the European and American approaches to prototype racing’s future has Kurdock – even during the brief spell between WeatherTech Championship seasons – trying to figure out how IMSA’s upcoming LMDhs will fare against their active and free-wheeling LMH rivals.

“LMDh is a different animal than DPi, for sure,” he says. “The LMDh has gone up in weight from DPi, and this is to align with the increased LMH weight target, as well as to promote convergence between LMDh and LMH. There’s an extra 80 to 100 kilograms (175 to 220lbs) in weight, but also an increase in anywhere between 50 and 70 kilowatts (65 and 95hp) in power, and that’s power at the wheel. So, there’s the power-to-weight ratio changes, and then the aerodynamic performance of LMDh is somewhat similar [to DPi], but with a slight reduction in drag, accommodated with a slight reduction in downforce.”

In terms of how the LMDhs will compare to speeds established by the DPi cars since 2017, current lap records will likely be safe.

“From a fan’s perspective, LMDh performance, particularly at a circuit like Daytona, will not be that different from DPi,” Kurdock adds. “I think where the LMDh makes its lap time will be different. We would expect straight-line performance to exceed DPi. So, greater overtaking speeds between classes, which we’ll have to manage. Certainly, as far as where to watch the cars at Daytona, head to the end of the straights, and in the braking zones it should be a more entertaining show as we see the classes mix it up with each other.”

Along with the new convergence ballast driving up LMDh’s curb weight, outfitting the mandatory ERS system from Bosch and Williams Advanced Engineering brings newfound heft to the formula. But the modest ERS horsepower punch isn’t enough to neutralize the hike in poundage, which is why, despite the overall rise in technology, power, and torque, LMDh is not expected to beat DPi on the stopwatch.

With current DPi minimum weights ranging from 920-945kg (2,030-2,080lbs), the next-generation Acura, Audi (below, Audi’s tantalizing hint), BMW (above, RACER’s take on what the Munich marque’s design might look like), Cadillac and Porsche LMDhs will start out at a chunky 1,030kg (2,270lb), akin to placing an NFL linebacker in the passenger seat.

“The addition of a second powertrain in the car – the stored energy system and the battery, the motor controller, all the other looms and electronics – is not insignificant, and it’s a challenge,” Kurdock says. “It is a significant portion of the added weight over DPi. But there’s other elements of the car that contribute to that weight increase, such as the survival cell with its safety improvements. The greenhouse is bigger, there’s more pedal space, there’s more side impact protection. But certainly, adding a hybrid powertrain to the cars is a good chunk of that weight increase over DPi.”

To preserve chassis balance with the LMDh formula, which will carry the heavy motor generator unit aft in the bellhousing connecting the engine to the transmission, the cars will hold most of the remaining ERS componentry low and forward of the car’s midpoint.

“The battery and the control electronics are all installed in the survival cell from underneath in a false compartment that is forward of the car’s center of gravity,” Kurdock explains. “We don’t think there’s going to be a significant weight distribution shift from DPi. And there’s some latitude on engine installation; we’ve reserved a dimension of 640 millimeters (25.2in) to fit the internal combustion engine, so that gives some freedom of movement between the bellhousing and the back of the survival cell to fine tune. And then there’s multiple ballast locations throughout the car that teams would be free to move ballast around to really target optimal weight distribution.”

Arriving at how LMDhs will put their electric boost to use is another area where convergence with LMH has influenced the regulations.

“This is a joint regulation between IMSA and [WEC rules body] ACO in trying to manage how the performance of hybrid and non-hybrid cars can be balanced to each other,” Kurdock says. “And so, the LMH and LMDh regulations all use a common philosophy on how power is regulated, and that is limited to power curves informed by torque sensors that are required on the rear axles. In the case of an all-wheel-drive LMH, torque sensors are required on all four drive axles.

“For hybrid LMHs, the power split between the internal combustion engine and the ERS is something we’re really leaving open to the manufacturers to come up with their own preferred methods. An LMH ERS could be a 200-kilowatt (270hp) deployment, so it’s a very capable system. It’s capable enough to launch the car on all-electric power in pit lane, which is something we’re exploring.”

IMSA expects its full complement of ERS power – somewhere between 40 and 67hp – to be in play at all times, which is unique compared to other hybridized series, where electrically-generated horsepower is often treated as an added bonus.

“And on the race track, where the current method is to limit the deployment power to between 30 and 50 kilowatts, depending on the circuit – the endurance circuits being 30 kilowatts for our 24 hour races in both championships, and then 50 kilowatts at all the other venues – LMDh will see that ERS power is available continuously,” Kurdock confirms. “So, whenever full power of the car is commanded, our simulation says that we should always have enough stored energy to be able to deliver that 50 kilowatts continuously for every lap, provided the regeneration targets are met.”

As the first LMDhs venture out for testing and prepare to have all aspects of their designs submitted for homologation review and approval, IMSA will be ready to act. Going back to the target of a converged and balanced class of LMDhs and LMHs racing at Daytona and beyond, look for plenty of performance massaging to keep the various models in line for achieving the same lap times.

“What the ACO and IMSA have done is put together a set of regulations that gives us an appropriate window of adjustment,” Kurdock says. “So, once cars are designed, it should give us enough room to be able to align demonstrated data when they start competing against each other. There’s certainly some ongoing work right now to develop the tools necessary to create the parity in year one and beyond, for both LMDh and LMH.

“Certainly, we can’t wait until this time next year to figure this all out. We’re actively working on that, and working on the balance of performance tools and methodologies that will take what’s happening with LMH and work that together with LMDh once they’re up and running. And then we go racing…”

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