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Audi’s victory over Porsche and Toyota was one of preparation and conservatism, not engineering wizardry.
Front-mounted motor-generator unit is integrated into the axle, and sends and receives its energy from a flywheel storage system. Of the three competitors, Audi chose the lowest energy harvesting limit.
Perhaps the most radical solution, Porsche’s 919 combined a 2.0-liter turbocharged four with a KERS unit on the front axle, a thermal unit driven by the turbo, and water-cooled lithium-ion batteries.
Vehicle electrification is moving forward, driven by regula-tions around the globe requiring greater efficiency and lower emission levels. It has even invaded the realm of motorsport, where automakers have clamored for a move to hybrid powertrains as a way of increasing competitiveness and showing potential customers their technical relevance. Formula 1 has standardized the engine swept volume, energy recovery system, energy storage system, and more, and this has appeal to the manufacturers who play in that sandbox (Ferrari, Mercedes, Renault and—in 2015—Honda). However, the folks at the Automobile Club de l’Ouest had a different idea for their big race (Le Mans), and convinced the World Endurance Championship’s regulatory body to take a different path.
There are no limits on cylinders or engine size at the 24 Heures du Mans, and competitors can choose to run either gasoline or diesel engines in their LMP1-H prototypes. Also free is the type and method of storing retrieved energy. However, there is a catch: constructors have to choose what level of recuperation they will take from the hybrid system: 2, 4, 6, or 8 megajoules per lap. This choice also determines the amount of fuel that can be used each lap, with an upper limit set on the volume (cc/sec) that can flow from the tank to the motor. As expected, there are tire size, vehicle size, weight, and other regulations which all cars competing in LMP1-H must meet, but how they produce that power, recuperate it, and get it back to the ground is relatively free.
As the centerpiece of the World Endurance Championship, Le Mans is the tail that wags the dog, and every competitor focuses its efforts on the series’ most prestigious race. In 24 hours the leading car will travel nearly 3,000 miles, and over each 8.5-mile lap it must not exceed its energy budget. For a racer in the 6 megajoule (MJ) class, this translates to expending no more than 1.67 kWh of electrical energy per lap. This harvesting and disbursement of energy takes place lap after lap, day and night, and rewards the constructor who is fast, efficient and—most importantly—durable.
Three manufacturers squared off this year: Audi, Porsche and Toyota. Each took a different path, and the early leaders were not around at the end to share the victor’s spoils. Audi, which has dominated Le Mans for nearly 15 years, took the trophy, but its victory was by no means a given. Porsche and Toyota might have won had they not encountered problems.
Audi R18 e-tron quattro
Audi stuck with diesel power and the R18 name for its 2014 vehicle, but changed just about everything else. The 4.0-liter V6 turbo diesel is new, not a development of last year’s 3.7-liter unit. It features a wide bank angle, which lowers the center of gravity, and allows the ancillaries to be packaged atop the block. Audi chose to compete in the 2 MJ class, which (with the restriction for diesel cars) meant it could use 4.64 liters of fuel per lap. Audi engineers apparently felt the fuel consumption penalty for each step up the electric power ladder was too great, and concentrated on matching storage capacity and energy use in order to keep the R18 as close as possible to the 870-kg (1,918-lb) weight minimum. The company ran only one kinetic energy recover system (KERS), this a Bosch-sourced motor-generator unit (MGU) driving and harvesting energy from the front wheels. This energy is sent/taken from a flywheel storage unit supplied by Williams Hybrid Power, a division of the William F1 team. The thermal energy of the exhaust also is harvested, and this energy is shunted to the flywheel or used to spin up an electric turbocharger. Despite the team having a car crash out early and the remaining two suffer from turbo failures (they were replaced during pit stops), it won the 2014 edition of the 24 Hours of Le Mans.
This marked Porsche’ return to the top category at Le Mans after a lengthy absence, and it was looking for a fairy tale ending. Perhaps the most radical hybrid, the 919 featured a turbocharged, direct injection, 2.0-liter V4 gasoline nestled close to the cockpit. Capable of transmitting 493 hp to the rear wheels, it is supplemented by a 247-hp MGU on the front axle. This meant that, like Audi, Porsche has just one KERS unit. Its thermal recovery system, however, is unique. Instead of a wastegate, the turbocharger shunts excess exhaust gas to an extra turbine that drives an electric motor. Thus, the 919 is able to recuperate energy under acceleration. The power created by this and the KERS unit is funneled to a water-cooled lithium-ion battery pack from A123 Systems. Though capable of running in the 8 MJ class, Porsche decided the loss of nearly one liter of fuel per lap did not make up for the increase in power. By running at 6 MJ, it was allocated 4.72 liters of gasoline per lap. Unfortunately, the lead car was out by 1 pm on Sunday with a damaged powertrain. The second car followed 30 minutes later with a broken gearbox, and was classified 11th, 31 laps down from the leader. Porsche is, however, planning production powertrains built around the V4’s 500 cc per cylinder, and hybrid systems like that used on the 919.
You’d think the maker of the Prius would have hybrid technology figured out and, judging from the Toyota team taking pole position for the second year in a row while using 30% less fuel and leading the early hours of the race, it looked like it did. Unfortunately, the good times didn’t last. Toyota chose a 513-hp, naturally aspirated 3.7-liter V8 coupled to KERS units at each end of the car. An Aisin AW MGU unit is used up front, while Denso provides the rear hybrid system. It is housed within the transmission case. Together they are capable of adding as much as 474 hp. This gives the TS040 a total of 987 horsepower. Unlike the Audi and Porsche storage units, Toyota chose a Nisshinbo-supplied ultracapacitor system. A Denso inverter is located atop the ultracapacitor storage unit, and sits next to the driver where it controls the flow of electricity. Toyota felt it was the best answer to harvesting the massive energy produced almost instantaneously under braking. It also chose to improve KERS efficiency rather than add a thermal energy unit that might adversely affect engine heat efficiency and, therefore, fuel economy. Despite leading convincingly early on, problems dropped the lead TS040 to third, five laps down, at the end. However, rumors abound that the lessons learned from the TS040 are being applied to an all-wheel drive version of the next generation Prius platform.