ringo wrote:A servo drive is a DC motor, A generator must be alternating current.
I have to disagree. The MGUs will be AC servo machines with self inducting coils and permanent magnets. They are often referred to as AC servo motors, but they can easily work as generators as well. So your limitation of servo motors to DC technology is not correct. There is a wide spread use of AC servo technology in the automotive and machinery industry. I have given a Wickipedia link to identify the technology. You can check it out there.
ringo wrote:But mostly, a motor/ generator is a load, it is not a relief valve, it cannot control exhaust mass flow through the turbine.
You also put yourself in a compromising position by the flow profile of the turbine and compressor if you cannot have any control of flow through it.
You will have only one compressor flow/ turbine flow profile for the turbine, but all you will able to do is to retard it by loading the turbine with the MGUH, and increase back pressure and heat soak by doing that. When you have a waste gate you will be able to change the profile of the turbine flow relative to compressor flow for various situations.
This is an imperfect world, you never know what situations may arise where you will need to drop the back pressure off the engine or change the turbine performance on a dime.
A load cannot do that, the most it will do is retard and stress things.
A waste gate is the simpler and cheapest form of auxiliary control. It is controlling flow, which is much different to torque therefore the likelyhood of stalling the turbine is much reduced. A generator runs a high risk of stalling. The waste gate is a fail safe and i wont be surprised if all the engines have wastegates on their exhaust systems. It's also another means of modifying engine breaking as well when the driver comes off throttle.
I have not denied that there could be benefits for waste gates in failure situations. You have never addresses the point that we are not likely to get any excessive back pressure in standard operational mode. The turbine will be designed to deal with whatever loads the normal operation is going to put on it including temperatures and pressures at full power setting of the ICE and full load of the MGU-H. Equally the designers will consider thermal and mechanical loads gradients that occur in standard acceleration and deceleration profiles. The MGU-H by merit of it's working principle will have a much higher dynamic capability to handle transients than the ICE is ever likely to produce.
So there is no rational need for a waste gate except for certain failure modes
- like sensor loss,
- mechanical failures and events of similar nature like the
- control system going suddenly off line.
The loss of the rotary encoder from the MGU would be particularly fatal. The angular position of the magnets would be unknown and the machine would immediately go non-operational and into fail safe mode.
In the case of the first and third failure mode you probably do not want the turbo to explode and spill it's guts all over the circuit. Therefore a waste gate would be a sensible fail safe option in my view.
One has to consider that teams will only have five of the units from each of the six elements of the power unit. If you have a sensor failure or a faulty cooling circuit on the power electronics you may not want it to affect the turbo assembly, the ICE or the MGU-H.
