MGU-H (heat energy collection) question - backpressure?

[dud]

So the MGU-H units sit in the exhaust flow and harvest "heat". In F1 they sit inside the turbocharger (between compressor and turbine), in the Porsche 919 it sits just in the exhaust. Obviously they do not really "harvest heat", they work mechanically on the gases.

Question is: why is this not creating a backpressure problem back toward the engine?

 

[dud]

Also, the phrase "harvesting heat" implies that the procedure cools down the gases and the unit gathered the energy difference. It sounds hard to believe that this is actually happening in this constantly pressurized system. I don't see any kind of gas expansion going on, much less condensation or other physical processes that would cool.

 

[Terry Rock]

MGU-H...and its mystery in racing.
Am I missing something here?
Motor/Generator unit-Heat suggest...an electrical unit common to the shaft-spool between the compressor and turbine.
Motor mode for electronically spinning the assembly in low flow situations, thereby increasing spool up rapidly and reduced lag.... and generation mode to harvest electrical energy for charging batteries for MGU-K use.
We use a very similar principal for the starter/generator on the business jets I fixed for twenty-five years.
The only difference is the energy source.
In the car it is exhaust gases.
In the airplane it is electrical energy to start and then a monopole to sense the drop-out and RCR (reverse current relays) and GCU (generator control unit) to generate electrical output.

 

[Richard Dastardly]

It is no different to a turbocharger from the standpoint of the exhaust flow, so has the same issues. Of course it harvests heat, heat energy is how combustion engines work - add heat to gas and it becomes more energetic, which for a fixed volume raises pressure. Making it do work like turning a turbine removes energy which is going to lower *something*, what depends on the system.

 

[dud]

OK, so in summary:

• There are two impellers in the exhaust flow (or one impeller connected to two driveshafts): one for driving the turbocharger, one connected to a reversible electric motor
• When the extra power from the turbocharging is not needed (gas foot lift, braking) then they tell the electric unit to use the exhaust flow, more or less cutting off or cutting out the turbo. That charges the battery.
• Backpressure indeed increases but it doesn't matter since you don't want full power right now.
• At least in the F1 case the electric motor can be used to spool up the turbo at a point in time earlier than exhaust gases can (due to compression of gases, inertia of the ICE components etc).
• If I read that correctly the 919 cannot do the latter since the electric unit sits after the turbocharger. Would like to obtain confirmation of that.

If that is all correct I am surprised that there is enough energy to harvest from exhaust flow after the driver releases the gas pedal. Of course it is possible that the cars drive the ICE longer than the gas pedal is pressed to deliberately charge up both the exhaust electric motor and to put more energy into the unit that is harvesting kinetic energy.

Too bad there really isn't much information available. What's the point of F1 being a bragging point for future car technology when the technology used is kept under cover. I wonder whether I can find anything in the actual F1 regulations.