Building a PSRU is not all that difficult
The main difficulty is isolating the power impulses from the engine.
I have been talking about it for a long time here. It has to be engineered. Here is the thread I started and explained a lot of this stuff:
The whole topic of PSRU's and drive shafts and torsional vibration has been talked around and I keep seeing the same difficulty in the thinking of the involved folks. Since we on HBA tend to be an intelligent bunch, I got to thinking about how much I had to work to get my arms around the whole...
Now I make my point .... is an automatic transmission available for the Mazda rotary ?? ... if so use the torque converter between the engine and gearset which will produce a very smooth power transfer.
Also been talked about. I have 23 years doing automatic transmissions for Ford and then Chrysler/FCA, so I KNOW this stuff. Listen carefully.
An open torque converter will throw away roughly 20% of the engine power that goes in. That power gets thrown away as heat. We have to turn over the oil in the converter several times a minute and circulate it through a suitably large cooler. We need not only the torque converter, but the sump, oil seals, oil pump, cooler circuit, and pressure controls. They are part of the automatic tranny, so it is doable.
How to avoid the big losses? Well, we could do what the car companies have been doing since the 1980's (and Packard first did in the 1940's) - Put in a clutch to bypass the hydrodynamic drive. These clutches add some more requirements. We have to also keep the valves and controls to turn the clutch on and off, pressure control to run the clutch at a proper pressure for the rest of the torque converter. These also come with auto trans built since the mid 1980's.
The things is, once we have the clutch, we still have to get the spring rate and travel and torque capacity right so that we isolate the vibration from running the engine. And all the other parts of the torque converter and its control and cooling become irrelavant - all we really need is the spring set between engine and gearset... So, we are back to where I was in the article cited above. You have to engineer the gear set, shafts, bearings, and pick isolation (the spring set) appropriately for the torque and inertia on both ends of the system.
And therein lies the rub...