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Old 5th Nov 2017, 00:36
  #6239 (permalink)  
Join Date: Nov 2015
Location: Farnham, Surrey
Posts: 1,243
Go back more than half a century and you find a lot of aero-engine companies working on a concept called "turbo-compounding" in which an exhaust-driven turbo-supercharger had its rotor mechanically connected to the crankshaft so that the excess exhaust energy in the cruise could be extracted to do work rather than being "dumped" through wastegate systems. Many of the larger 1940s piston engines had turbocompounded versions (the 4360 twin wasp major being a good example), but the idea was obsoleted by the arrival of jets. The idea was also played with in cars and trucks where the mechanical coupling also eliminates turbo-lag, but it's difficult and expensive to do it right in a way that is reliable enough to be worth the effort.

Scroll forward to around 2010 when the hybrid engine formula was going through its concept stages. The fundamental idea of the formula was to use the vast development budgets of F1 to find ways to make infernal combustion engines more efficient. They'd elected for turbocharging (because that's an essential part of an efficient IC engine), and the idea of turbocompounding was attractive for making it even more efficient, so they decided to do it in a "modern" way by compounding it "electrically" via the hybrid system rather than with mechanical clutches and gears.

So the MGU-H is a Motor/Generator Unit which is on the shaft of the turbo. It can extract energy to charge the battery, and it can also be used to keep the turbine spun-up at low throttle to eliminate turbo-lag using power from the battery. In principle it can also be used to drive the compressor directly (becoming a mechanical supercharger), but this isn't very efficient and I understand it's rarely done (although I'm told by one source that this is also part of the Mercedes "magic qualifying mode" but I don't know if that's true).

Almost all of the energy extracted by the MGU-H (and there's a LOT of it) is "used" as mechanical drive, delivering torque into the gearbox via the MGU-K. A failed MGU-H is a big deal, losing a net 150-200bhp from the peak power available to the driver. The MGU-H also has a secondary function in that it is used to "regulate" the turbo's rotor speed. This allows them to run the turbo well into its surge island with impunity, making the whole thing even more efficient. So the MGU-H is an important part of this >900bhp, 50% thermally efficient power system - essentially you couldn't have the power or the efficiency without it.

The FOM proposal is to eliminate the MGU-H and recover the lost power by allowing more fuel and increasing the rev limit. The engine manufacturers don't want to do this because increasing the fuel consumption (reducing the efficiency) is the opposite of the whole reason why they're doing this F1 malarky anyway, and because the increased rev limit will require a complete redesign of the engines to cope with the extra stresses. They also don't want to add the weight of extra fuel to what they already regard as overly heavy cars.

Personally I think removing the MGU-H would be a silly thing to do. They will be starting to appear in hybrid road cars next year anyway, and they are a key part of the whole PU concept - they are what makes them technically seriously cool!

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