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The 24 Heures du Mans is the world’s most popular endurance race event, held annually in France near the town of Le Mans. Alongside this race, there are a number of other endurance race series, with very similar regulations, such as the American Le Mans Series (ALMS). As the length of these endurance races is significantly longer than in Formula One, the requirements for the races are different and therefore the design of hybrid systems will also be different.

Endurance racing appeared to be taking the lead in hybrid motorsport when in 1993, Chrysler designed a hybrid prototype sports car which consisted of a 559kW AC traction

motor, a 373kW electrically coupled flywheel, a 373 kW liquefied natural gas powered turbine and a 9000 µF capacitor bank. The intention of this car was to compete in the 24 Heures du Mans. However, issues surrounding the flywheel containment meant that this car was never raced [42-45]. The failure of this vehicle may explain why there was no further development of hybrid vehicles in endurance racing immediately after this.

In 2009, shortly after the change in Formula One regulations, the sporting regulations for the 24 Heures du Mans introduced provisions for hybrid cars to be able to compete in the race [46]. Whilst no hybrid cars competed in the 24 Heures du Mans in 2009, Zytek did develop a hybrid electric hybrid which competed in the Petit Le Mans stage of the ALMS series, finishing 2nd in the GT1 class and 12th overall [47]. This served to prove the concept that hybrids in endurance racing were now able to compete with conventionally fuelled vehicles.

In October 2010, Porsche raced their 911 GT3 R Hybrid race car in the Petit Le Mans stage of the ALMS series. This vehicle has two 60kW motors driving the front wheels, using energy provided by a Williams Hybrid Power electromechanical flywheel. The car finished 18th out of 41 entries [48]. The power flow around the vehicle is shown in Figure 13, which shows how the ICE and electric flywheel/electric motor interact to form parallel hybrid architecture.

Figure 13. Power flow around the Porsche 911 GT3 R Hybrid race car

In 2011, further sporting regulations for the 24 Heures du Mans were developed to encourage hybrid vehicles to compete. The regulations allow for hybrid systems to reduce fuel consumption and CO2 emissions and dictated that the system must not be aimed at obtaining additional power [49] unlike in Formula One. This indicates that the motivation behind allowing hybrids into the 24 Heures du Mans are primarily environmental and not for performance or competitive reasons. This is explicitly stated in the regulations and is achieved by having the hybrid system not under the control of the driver, meaning that Formula One style push to pass systems are not allowed. This fundamental difference means that the hybrid systems used in Le Mans cars will be significantly different to those used in Formula One. This change is likely to make the hybrid systems designed for Le Mans, closer to hybrid systems designed for road vehicles.

Table 6 shows a breakdown of the regulations for the 24 Heures du Mans relating to hybrid drivetrains. It can be seen that the regulations are less restrictive than those enforced in the Formula One World Championship. The hybrid system is only limited by the amount of energy it can release between two braking events and can be coupled to either the front or the rear wheels. However the system may only be controlled through the accelerator pedal

and must also be capable of propelling the vehicle on electric only power along the pit lane (400m) at 60km/h.

Maximum Power (In or Out) Unrestricted Maximum Storage Capacity Unrestricted

Maximum Energy Released 500kJ between 2 braking events Control System Accelerator pedal

Connection Point Either front wheels or rear wheels

Hybrid Type Electrical or mechanical/electromechanical flywheels – Must be capable of propelling the car along the pit lane (400m) at a speed of 60km/h using only the power generated by the hybrid system

Table 6. 24 Heures du Mans Hybrid Regulations, adapted from [49]

It can be seen that the power and energy capacity of the systems permitted are not restricted and energy release is only restricted between two braking events and not on a per lap basis, as in Formula One. The regulations specify that the system must connect at either the front or rear axles, with the aim of recovering energy from the brakes, therefore any systems developed will be a parallel hybrid. The power flow allowed in the system is therefore identical to that shown in Figure 12 for the Formula One KERS power flow.

Peugeot indicated that they would be entering the 24 Heures du Mans with a diesel electric hybrid drivetrain in 2011. However this car was later withdrawn from the competition due to technical difficulties. MIK Corse developed a car with an electric hybrid drivetrain and successfully tested this vehicle in the Le Mans Series and the 24 Heures du Mans test day. However this car was also withdrawn from the 24 Heures du Mans competition due to technical difficulties [50]. The only car to be entered into the 2011 24 Heures du Mans competition as a hybrid was the Oreca Swiss Hy Tech-Hybrid. This is the first time that a hybrid car has competed in the 24 Heures du Mans. The car has a flywheel based mechanical hybrid drivetrain from Flybrid Systems, originally designed for the Formula One World Championship. The car completed 115 laps before it retired with electrical

In comparison to the Formula One World Championship regulations, the 24 Heures du Mans regulations allow for more freedom in the design of the system. It may be that this freedom, whilst allowing the teams to make their own decisions on the design of the system, also makes it more difficult to develop a reliable hybrid system. However it is also true that the budgets for endurance racing teams are significantly lower than that of Formula One teams and that the teams competing in the 24 Heures du Mans and ALMS championships do not have the resources or the budget required to develop hybrid drivetrains to the same standard as Formula One.

In 2012, two teams entered the 24 Heures du Mans race with hybrid vehicles. Toyota competed with two petrol/electric hybrid vehicles with the hybrid system providing additional electric power to the rear wheels. Audi competed with two diesel hybrid vehicles, utilising the hybrid electric flywheel system from Williams Hybrid Power, with electric motors acting on the front wheel. While the Toyota cars both failed to finish the race, the Audi cars finished in 1st and 2nd [51]. This has shown that if sufficient resources are available, it is possible to develop, compete and win in Le Mans with a hybrid vehicle.

One of the drivers of the Audi R18 e-tron Quattro, Alan McNish has been quoted as saying that one of the major advantaged of the Audi system is that it allows the vehicle to behave as if it has traction control [52], otherwise not permitted in the regulations. What this demonstrates is that some of the advantage of having a hybrid system comes not from the reduction in fuel usage, but from the ability to tune the system to behave like traction control.