[GoingTooFast]

As we could see from the fuel-efficiency formula, the 'magical' formula, something is missing on the above M4 specs and I think this time we won't need SCOTT's help to figure out what it is...

It's obvious that the tiny turbos from the N54T engine mounted in the 1M (picture below) are not able to run enough boost to sustain 422 PS (416 hp) flat from 5400 to 7000 rpm, thus a much BIGGER turbo becomes a necessity:






However, as you all know with big turbochargers comes big turbo-lag. So, the new S55 engine will be the first ever series production engine to be equpiped with an electric actuated turbocharger in a biturbo arrangement where the electrically driven compressor is placed after the main 'BIG' conventional turbocharger and intercooler in the charge air path. The biturbo arrangement uses both a turbine wheel driven by the exhaust gas stream, and an electrically driven turbine that accelerates to very high speeds in an extremely short time. The so arranged new electrically-powered turbocharger would create power more efficiently and also have less turbo lag. When the main turbocharger’s energy output is low, the air is directed into the electric compressor and is compressed there a second time thus boosting the power at low revs. Then, that in combination with BMW's VALVETRONIC technology (absent in the 1M engine) means increased performance and fuel efficiency:

First, here's an explanation of the electric turbo's individual parts (as labeled in the diagrams):
1) turbo layout
2) turbine
2') turbine axle
3) compressor
3') compressor axle
4) electric motor (and alternator)
5) turbo axle
6) (turbine axle) clutch
7) (compressor axle) clutch
8) gearing

We've highlighted and colored the following schematic diagrams to show how the technology works:

Unlike a traditional mechanical turbo layout, (exhaust) turbine and (intake) compressor aren't fixed on the same axle. Via the clutches #6 and #7 both the turbine and the compressor can be uncoupled from the turbine axle (#5). When the engine is idling or coasting, both clutches are open and an electric motor (#4) can operate without any load. Via gearing (#8) the speed of of the e-motor and the turbo axle can be further adjusted.

If the driver steps on the pedal the clutch (#7) closes and connects the electric motor (#4) to the compressor (#3). Due to the inertia of the (running!) electric motor, the compressor spools up quickly and compresses enough air to make for a fast engine response, which results in less lag. The turbine (#2) which wouldn't be able to spool up the compressor quickly enough is decoupled due to the clutch (#6) still being open.

Once the turbine (#2) has reached a certain speed, the clutch (#6) closes and both the turbine and the electric engine are used to run the compressor (#3).

When maximum boost is reached, the electric motor switches to an alternator mode, generates power for the battery, and avoids the turbine exceeding a certain speed. This renders a wastegate needless and ensures sure no energy from the turbine is wasted. If the driver backs off the accelerator, both clutches (#6, #7) open and the electric motor can continue running without any load from the turbine or the compressor.

According to BMW's patent application, this invention makes for a great engine response, particularly during the transition from idling to load. It also renders a wastegate needless and adds efficiency.