Will BMW's Electric Turbocharger End Turbo Lag?

Yes, apparently there is a replacement for displacement. If recently-uncovered patent filings are correct, BMW is working on an electrically-assisted turbocharger that combines a traditional turbo with an electric motor-generator. If it works, the system will let smaller engines produce more power, while performing virtually lag-free.

Turbochargers have long been a way to employ "lost" energy, in the form of fast-moving exhaust gases, to create more power by stuffing more air into the cylinders to burn more fuel. The traditional trade-off has been between smaller turbos that spool up quickly, providing good transient response and preventing "turbo lag," and larger ones that can jam more air down an engine's gullet at higher RPMs. BMW's existing twin-turbo setup offsets the issue by using both types. But is the company planning to tie its turbos into the beefed-up electricals of its Efficientdynamics gear? The answer is perhaps.

F30post had already reported that the company was considering a tri-turbo system for the next-gen M3. The new patent filings may have something to do with such a system, but we don't know for sure. We do know that other companies like Borg Warner and Garrett have been working on similar electrically-assisted turbocharger systems for about a decade.

To be fair, "electric turbocharger" is a bit of a misnomer, one that might confuse some into thinking BMW's delving into the world of cheap "electric superchargers," long a bane of the aftermarket. BMW's system is apparently a hybrid turbocharger, which combines a turbine with an electric motor-generator.

[See image below: 1) turbo layout; 2) turbine; 2') turbine axle; 3) compressor; 3') compressor axle; 4) electric motor (and alternator); 5) turbine axle; 6) (turbine axle) clutch; 7) (compressor axle) clutch; 8) gearing]

The turbine and compressor spin on separate axles that can be uncoupled by a clutch — with the electric motor constantly running in the middle. During full-throttle starts, the electric motor runs the compressor in an instant-on fashion, virtually eliminating the time it would normally take the exhaust gases to spin up a traditional turbine. When the turbine's up to speed, the clutch engages the turbine shaft to run in conjunction with the electric motor. Naturally, all of this switchery happens at ridiculously high RPMs.

Now here's the cool part. Rather than using a wastegate to hold back the speed of the turbine, the electric motor-generator kicks into generate mode. The resulting electrical current flows back to the battery (or, potentially, to a supercapacitor), while the additional load from the generator regulates turbine speed.

Naturally, all of this will be controlled by copious electronics, the likes of which aren't clear from the filings.

There's no information on the vehicles in which BMW might use such a system, but considering the company's push to up the degree of electric gear in its engines, something like this is a logical next step. [See F30post for more.]