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Motors – the Le Mans regulations

Formula One appears to be leading the way with some very daring regulations in 2014, with exhaust turbine-driven motor/generators giving a significant boost to the performance of cars whose ‘bare’ internal combustion engine is much less powerful than its predecessor. The ‘pain’ being suffered by some of the engine manufacturers is not necessarily due to the added complexity of the turbo motor/generators (known as an MGUH), but also by problems with their engines and energy storage units. The rate of kinetic energy recovery and use is limited to 120 kW, which is twice the power of the 2013 KERS system and is the first ‘stake in the ground’ in terms of motor and energy storage design. In addition, the maximum amount of energy that may be harvested or deployed during a single lap is 4 MJ

However, the new Le Mans regulations have, as is typical, been written in a way that is much more liberal, leaving the users free to decide what suits their application best. Although there are various choices as to the amount of energy that may be harvested during a single lap (up to 8 MJ per lap), however, there is no limit on the rate at which the energy may be harvested or deployed. Of course, there are some sensible limits as to what we might want to do with the motor: it is pointless designing a motor capable of harvesting or deploying energy at a rate that would overwhelm the tyres. Even in the case that the tyres can cope with a lot of additional torque, perhaps the gearbox would be made very much heavier to cope.

The Le Mans regulations also allow recovery of heat energy, which most easily applies to those teams using turbocharged engines. While there have been schemes for recovery of heat energy from naturally aspirated engines, those using a turbocharger can use the same turbine-driven motor/generator technology that will be used in Formula One. Porsche is thought to be using heat energy recovery on its adventurous new LM P1 power unit, which uses a V4 gasoline engine at its heart.

So, even though the Le Mans regulations are less constricting in terms of the design of the motor, they require more thought at the start of the process than in Formula One – a mistake in calculating a sensible power output can result in the motor, power electronics and energy storage being too heavy and difficult to package. Those choosing very high motor powers will force themselves into using either very high voltages or very high currents. High voltages pose problems for insulation in the motor, and high currents cause problems with heat. Using high current densities – carrying large currents per unit cross-sectional area of conductor, in order to keep the motor physically small – adversely affects efficiency. In conductors with high current density, heat losses are large and the cooling requirements are increased.

It is probable that all the Le Mans hybrid motors will have to be liquid cooled. Air-cooled motors can only carry comparatively low current densities and are therefore physically much larger for an identical power output and operating speed.

Written by Wayne Ward

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