- Hybrid power came to IMSA’s top-class GTP Prototype division this year, dramatically changing the racing at the 2023 Rolex 24 at Daytona Race.
- Four major automakers chose to build and return hybrid GTPs, an undertaking that required large engineering staffs, large amounts of computing power, and rigorous testing.
- The new rules that ushered in the hybrid era are as complicated as the cars themselves, combining both real and virtual energy consumption. The changes challenged the teams from Cadillac, BMW, Porsche and Acura to sort out numerous scenarios to define their race strategies.
This year’s Rolex 24 at Daytona marked the debut of hybrid race cars in IMSA’s top GTP Prototype class. While hundreds of thousands of hybrid passenger cars have hummed along our roads for more than two decades, the new GTPs’ systems are as different from those on street cars as a hard-boiled egg is from an omelette.
We spent time at the race with Cadillac, one of the big four automakers — along with BMW, Porsche and Acura — to bring hybrid GTP machines. We mercilessly questioned their engineers in an attempt to find out how the complex powertrains work and how they are used in the race. We appreciate that they spoke slowly and tried to use simple English when explaining their wild cars to us.
Shared hybrid technology
According to the regulations set by IMSA, the series’ sanctioning body, the manufacturers in GTP are obliged to use an identical hybrid system. Bosch supplied the motor-generator unit (MGU), which lies between the engine and the seven-speed Xtrac rear gearbox. The MGU peaked at 40 horsepower at Daytona and will be capable of slightly more at longer tracks like Le Mans.
The MGU feeds a tiny 1.35-kWh battery; the system works up to 800 volts. A GTP car’s total system output—gas engine plus electric motor—cannot exceed 500 kW (671 horsepower) at the rear wheels at any time in these rear-wheel drive racing cars. Torque sensors on the rear axles ensure the teams stick to that power output. IMSA monitors the rear wheel power via the same telemetry that the teams use in the pits. The cars have an adjustable regen system that works on the rear axle to recharge the battery.
The design of the petrol engines that power each GTP car is up to the manufacturer. Cadillac developed a 5.5-liter naturally aspirated 32-valve V-8; BMW used a 4.0-liter twin-turbo V-8; Porsche went with a twin-turbo 4.6-liter V-8; and Acura stuck a twin-turbo 2.4-liter V-6 in the tail of its two cars. Nine of the electro monsters were on the grid, three from Cadillac and two each from the other manufacturers.
Shape-shifting hybrid power
This is where things get weird-science freaky. When no electric power is used, the gas engine can provide the full 671 horses. When the MGU feeds power through the transmission, it cannot add to that total. If the electric motor is at full blast, and its 40 horses are added, the gas engine is automatically switched off by that amount. This would theoretically enable the driver to save fuel and extend a driving period. Much of the regen is accomplished while braking, but it can also be done on the straights; at Daytona, the tiny battery can be drained in a lap of the 3.56-mile road course and recharged in almost a lap.
If regen is used while going down straight away and puts the equivalent of 40 horsepower of drag in the rear axle, the team can boost the gas engine by 40 horsepower to compensate. Which means the gas engines in the GTP cars can produce more than 671 horsepower when needed.
Passenger car manufacturers sometimes use hybrid systems to fatten the low-rpm torque curve of their cars for better performance. We’ve been told that a GTP’s electric motor has virtually no effect on the car’s performance, but we’re not so sure. Race teams don’t like to give away any technical advantages they may have discovered during development.
Life in the virtual court
As if that wasn’t complicated enough, there is an important virtual aspect to the new GTP class. IMSA has decided that the GTP cars may not use more energy per driving period than 920 megajoules, or 255.6 kWh. The calculation is made using the torque sensors on the rear axle. IMSA monitors the total energy (the combination of gas engine power and hybrid electricity) to ensure that the teams do not exceed the maximum amount during a shift. If they do, a time penalty is assessed, putting them that much further behind. The initial penalty is 100 seconds. No team wants to suffer that.
IMSA describes the GTP cars as a “virtual fuel tank” that needs to be “virtually refilled” when the energy limit is reached. When the 920 megajoules of total energy are nearly exhausted, the team brings the car in, even if there is gas left in the tank. When the gasoline fuel hose is attached to the car, the clock starts and the megajoule allocation is replenished at a rate of 23 megajoules per second as long as the hose is in the car. The gas tank might refill in 15 seconds, but a full megajoule takes 40 seconds, so the car can sit still even with a full gas tank if the team deems it necessary to top up the megajoules. In situations where cars pit early during a caution period or for a flat tire, a shorter virtual pad will do. In general, the pit stop schedule will revolve around energy consumption rather than pure gasoline consumption, as the cars will sometimes have enough gas to circulate further if the megajoule limit is reached. Woof!
Strategic overload
All of this created so many strategy possibilities—for how and when the hybrid electric power was deployed and how to play the megajoule game—that the car companies’ computers were smoking from overwork. And what would some of those strategies be? The Cadillac engineers did not speak, nor did other GTP teams make their thinking public before the event. This is racing, and any advantage a team thinks it has, it protects with a wall of silence. The Caddy folks did admit that the new cars are now so much more complex, and that the possible race scenarios are so large in number, they had to quadruple the number of software engineers on the team compared to when they ran IMSA’s top DPi prototype class managed. last year
Cadillac driver Alex Sims said the drivers only control the rain level directly from the cabin. Their job is to ride flat and not crash. How and when the hybrid’s juice is deployed is a team secret; it is automatically engaged depending on which drivetrain setting is selected by the driver at the behest of the team’s engineers.
We think we have it all right. But we know the teams did because the top four GTP finishers were on the same lap and within a handful of seconds of each other after 24 hours. The winning Acura ARX-06’s fastest race lap was about three-tenths of a second faster than its second-place sister car. And that car’s best lap was just one thousandth of a second faster than third-place Caddy V-LMDh’s.
It was a great race until the end. And best of all, you don’t need to know anything about megajoules to enjoy it.
