BMW recently filed a German patent for its electric turbocharger design and implementation wise, it may take a while before we get to see it being used on their production vehicles. But at least, we know that someone or some company is developing such technology.
Before I continue on how the electric turbocharger works, let me explain a little about turbochargers and also on small turbo vs. a big turbo, for those who don’t know how it works. Below is a diagram of how a conventional turbocharger works.
The turbine in the turbocharger spins at speeds of up to, roughly around 150,000 rotations per minute (rpm) -- that's about 30 times faster than most car engines can go. And since it is hooked up to the exhaust, the temperatures in the turbine can be very high. That is why it needs an intercooler.
A small turbo spool up quickly and it is good for low end power but they run out of boost at higher RPMs. A bigger turbo takes a while to spool up and this creates lag but their larger compressors create more power at higher RPMs. A twin turbo set-up usually uses both of these turbos to create the optimal performance needed.
BMW’s electric turbocharger set-up is going to be similar to a twin turbo set-up. When the accelerator is pressed, the electrically controlled and powered compressor spools up to provide instant boost at the lower RPMs and when the turbine reaches a pre-determined operating speed, it’s coupled to the compressor to provide boost from the engine’s exhaust gases (just like how a conventional turbocharger works).
In order for the system to have a smooth operating performance, under all load conditions, the system design uses a series of clutches which automatically engage and release both the compressor and turbine. When the maximum boost is achieved, the electric motor will act like an alternator and generates electricity to charge the battery.
BMW also states that the design will give an excellent engine response especially from the idling to acceleration stage.