This year, IMSA’s top-tier GTP prototype division got hybrid power, which changed the racing at the 2023 Rolex 24 at Daytona. Four big automakers decided to make hybrid GTPs and support them. This took an extensive engineering team, a lot of computing power, and testing. The new rules that made hybrid cars possible are just as complicated as the cars themselves because they combine real and virtual energy use. The changes meant that the Cadillac, BMW, Porsche, and Acura teams had to think about many possibilities before deciding how to race. At this year’s Rolex 24 at Daytona, hybrid race cars ran for the first time in IMSA’s top GTP Prototype class. GTP systems are as different from streetcar systems as a hard-boiled egg is from an omelet, even though hybrid passenger cars have been on the road for over 20 years. Cadillac was one of four big car companies that sent hybrid GTP cars to the race. The other three were BMW, Porsche, and Acura. We asked their engineers many questions to determine how their complicated powertrain systems worked and how they were used during the race. We liked that they tried to talk slowly and used simple English to tell us about their strange-looking cars. Shared Hybrid Technology The rules set by IMSA, the organization in charge of the series, said that all GTP manufacturers had to use the same hybrid system. The seven-speed Xtrac rear transaxle and the motor-generator unit (MGU) were both made by Bosch. At Daytona, the MGU made 40 horsepower and can make a little more on longer tracks like Le Mans. The system can function with an input voltage of up to 800 volts and is powered by a relatively small battery that has a capacity of 1.35 kWh. The gas engine and electric motor in these rear-wheel-drive race cars can never put more than 500 kW (671 horsepower) at the back wheels. Torque sensors on the rear axles keep track of how much power each team is putting out. IMSA uses the same telemetry as the teams in the pits to measure the power at the back wheels. The vehicles have a regen system that can be changed and charges the battery on the back axle. Each GTP car has a gasoline engine built to the manufacturer’s specs. Cadillac used a 5.5-liter, 32-valve, naturally aspirated V-8 engine, BMW used a 4.0-liter, twin-turbo V-8 machine, Porsche used a 4.6-liter, twin-turbo V-8 engine, and Acura put a 2.4-liter, twin-turbo V-6 engine in the back of its two cars. There were nine of the electric monsters on the grid. Three of them were made by Cadillac, and each of the other manufacturers made two. Hybrid Power that Changes Form Things start to get weird-science weird at this point. The gas engine can make all 671 horses if you don’t use electricity. While power is going through the transmission, the MGU can’t add to that number. When the electric motor is at full speed and adds 40 horsepower, the gas engine is automatically turned down by that amount. In theory, the driver could make a trip last longer and save fuel. In one lap around Daytona’s 3.56-mile road course, the small battery can be completely drained and almost fully charged. Most regenerative braking happens when the car slows down, but it can also occur on straightaways. When regen is used on a straightaway, it slows the car down. To compensate for this, the team can add 40 horsepower to the gas engine. So, when they need to, the gas engines in GTP cars can make more than 671 horsepower. Manufacturers of passenger cars may use hybrid systems to improve the low-rpm torque curves of their vehicles. We’ve been told that putting an electric motor in a GTP doesn’t change how fast it goes, but we’re still determining that. Usually, race teams don’t share the technical advantages they find during development. Life on the Internet As if things weren’t complicated enough already, the new GTP class has a large virtual part. IMSA rules say that during each driving stint, GTP cars can only use 920 megajoules, or 255.6 kWh, of energy. For this, the torque sensors on the rear axle are utilized. IMSA keeps track of how much energy each team uses to ensure they only use what’s allowed during a stint (the sum of gas-engine power and hybrid electricity). If they do, they will get more time as a punishment, putting them further behind. There is a penalty of 100 seconds at the start. Any team would instead not do something like that. IMSA says that when the energy limit is reached, the GTP cars’ “virtual fuel tanks” must be “virtually filled.” Even if there is still gas in the tank, the team brings the car back when the 920 megajoules of total energy are almost gone. Allotted megajoules are replenished at a rate of 23 per second for as long as the gasoline-fueling hose is connected to the vehicle. If the team decides that the megajoules need to be topped off, the car can stay still even with a full gas tank because it takes 40 seconds to fill up the megajoules while it only takes 15 seconds to fill up the gas tank. When cars pit early because of a yellow flag or a flat tire, they only need a shorter virtual fill-up. Because the cars sometimes have enough gas to keep going after the megajoule limit is reached, the pit stop schedule is usually based on how much energy is used, not just how much gas is used. Woof! Overkill on tactics All of this led to so many possible plans for using hybrid electric power and playing the megajoule game that the computers at the car companies started to smoke from being used so much. How do some of these plans work? Like the other GTP teams, the Cadillac engineers didn’t share their thoughts before the race. During a race, a team will put up a sound barrier to protect any edge it thinks it has. Because the new cars are so much more complicated and there are so many different ways the race could go, the Caddy team said they had to bring in four times as many software engineers as they did last year when they raced in the top IMSA DPi prototype class. Alex Sims, who drives a Cadillac, says that the only thing the driver can directly control from the cockpit is the level of regeneration. They have to move aggressively while trying not to hit anything. How and when the hybrid’s power is used is a team secret. It turns on automatically based on which powertrain setting the driver and the team’s engineers choose. Everything is fine. After 24 hours, the top four GTP finishers were all on the same lap and within a few seconds of each other. The second-place Acura ARX-06 had a faster race lap by about three-tenths of a second than the first-place Acura ARX-06. Its best lap was only one-thousandth of a second quicker than the third-placed Caddy V-LMDh. Up until the last lap, the race was great. The best part was that you didn’t need to know anything about megajoules to enjoy it.
