Yeah it seems to me that the clutches are key to why this is such a good electric turbo application. Btw, it's not just old airplane engines, but many auto component companies have been exploring electric turbos for a few years now. Some of them completely separate the turbine from the compressor so they are only connected by electrical wiring, some use an electric motor to help spin up a traditional turbocharger.
The specific benefits of this BMW design seem to be:
-disengaging the turbine during coasting, idling, and launch means the exhaust can spin the turbine to a higher RPM and leave more energy for possible further turbos in a sequential setup to also spin to a higher RPM because it doesn't have to waste energy spinning the compressor (which wouldn't even create boost at that low engine speed, nor would boost be desirable even if it could).
-disengaging the compressor (along with the turbine) during coast and idle means that the electric motor can pre-spool to very high RPM while using relatively little energy because it doesn't have to waste any energy spinning the turbine or compressor around.
-Compared to the purely electric turbo concepts, the BMW design only requires one motor/generator versus having a dedicated motor and a dedicated generator, and maintaining the mode where you have a direct mechanical link between the turbine and compressor should be more efficient than having to convert pneumatic energy>mechanical energy>electrical energy>mechanical energy>pneumatic energy.
Originally Posted by captbilly
The idea of an electric turbocharger goes back at least several decades. The Russians used an electric motor/alternator coupled to a standard turbocharger on an aircraft engine (diesel) back in the 80s. I don't believe that BMW has the patent on the idea of en electric turbo, just the specific setup using the various clutches. It may even be that the only reason for the clutches was to get around the Russian invention, or to allow BMW to give the impression that they had some important IP for electric turbos.
Ultimately the idea goes all the way back to the turbo compound aircraft engines used on post WW2 piston engine airliners. In these engines the turbo is actually used to convert some of the waste energy in the exhaust power back to the engine/propellor (and of course also to power the turbosupercharger). In the turbo compound engines they used gears to feed the power back to the engine rather than the motor generator used on the Russian and BMW systems. These gears were the Achilles heal of the turbo compound engine because it was very difficult to build gearing systems that could match the very high RPM of the turbo to the very low RPM of the big radial engines of the day. A single backfire could tear up the gears, but even barring such a failure the gears were constantly getting beaten by the huge power pulses of the slow turning engine. The Russian application was the first I have seen that used a motor generator to harness some of the waste energy (energy not used by the turbo itself). I am not sure why BMW needs all the clutches and other stuff but perhaps they found that they got somewhat better efficiency under the varying loads of a car vs. the much more constant load in an airplane that the Russian system was meant for.