Advertisement

BMW Pairs Electric And Traditional Turbos To Boost Efficiency

Follow John

Turbocharger

Turbocharger

Enlarge Photo

To improve fuel efficiency, engines are gradually shedding accessory drives. Electric power steering is now commonplace, and electric air conditioning compressors are used on hybrid cars to keep them cool while the engine's off.

Now, BMW has taken a new tack to electrifying ancillary equipment: It's created a twin turbocharger setup in which one half, the compressor that operates at lower engine speeds, is run electrically rather than mechanically.

From idle to acceleration

The news came to light in a patent application that was covered on the F30Post forum, which said it might be used in a potential future "tri-turbo" BMW model.

The patent filing highlighted the design's superior engine response--especially during the transition from engine idle to full acceleration, in part because no mechanical "waste gate" valve needs to open and close to allow exhaust gases into the turbine that powers a turbocharger.

Instead, the compressor is powered by a simple electric motor. In fact, you could make the argument that the "electric turbo" is really more of an electric supercharger, since a turbocharger contains a turbine (hence its name) powered by hot exhaust gases--and the BMW invention doesn't.

BMW N54 turbocharger

BMW N54 turbocharger

Enlarge Photo

Boost vs responsiveness

Traditional single-stage turbochargers always traded off boost and responsiveness, leading to the dreaded "turbo lag" phenomenon.

Early turbocharged engines might take a second or more to spool up the speed of their turbos before they gathered enough momentum to pack more air into the combustion process, allowing more gasoline to be mixed with the greater volume of air and thus increasing power.

The smaller the turbo, the quicker it could respond--but the less power it could generate once up to speed.

The solution to this was twin turbocharging, with a lower-volume, more responsive turbo for lower engine speeds, and a higher-volume, more powerful turbo that would take over as engine speed increased.

Low-volume boost ASAP

But turbos still inherently contain some lag. So BMW proposes to power the smaller compressor electrically, with an electric motor that may already be running at some speed and can be quickly accelerated while it's clutched to the compressor drive.

Volkswagen ‘twincharger’ 1.4-liter TSI engine

Volkswagen ‘twincharger’ 1.4-liter TSI engine

Enlarge Photo

That means it should respond almost instantly to a demand for power, while the larger turbo it's paired with spools up to speed.

But this isn't the only approach to the problem. Volkswagen built for several years a "Twincharger" 1.4-liter TFSI engine that used a small supercharger for boost at lower engine speeds.

BMW's proposal might be viewed simply as an adaptation of that idea, replacing the mechanically-driven supercharger with an electrically-driven one.

All of this innovation is driven, of course, by automakers' need to meet corporate average fuel economy standards that will increase to 54.5 mpg by 2025.

+++++++++++

Follow GreenCarReports on Facebook and Twitter.

Advertisement
 
Follow Us

 

Have an opinion?

  • Posting indicates you have read this site's Privacy Policy and Terms of Use
  • Notify me when there are more comments
Comments (3)
  1. Wonder how many electric motors are needed to fix the problems with ICE engines. The need a motor to start the car. They need a motor (generator) to create electricity the car needs to run. I know they have a couple of motors used to compensate for idle mixture problems at cold. Fuel injectors could be viewed a small linear motors.

    And now they want to add an electric motor driven supercharger.

    Maybe one large electric motor to drive the car is a better idea.
     
    Post Reply
    +1
    Bad stuff?

  2. I like your thinking, John!

    The aero drag caused by cooling internal combustion engines is non-trivial.

    Electric motors are about 2-3X more efficient than most internal combustion engines -- TWO to THREE TIMES more efficient!

    When you consider that it takes *more electricity* to make the gasoline to run an internal combustion car, than it does to run an electric car the same distance -- it becomes blindingly obvious what we need to do.

    Neil
     
    Post Reply
    Vote
    Bad stuff?

  3. A correction to the article - BMW's electric turbo system doesn't eliminate the turbine.

    The compressor, turbine and electric motor are coupled and uncoupled via clutches depending on what the engine needs.

    For example, at very low engine speeds just the electric motor and compressor work together. But at high engine speeds, the electric motor bails and the compressor and turbine go at it like a normal turbo sized for high-RPM power.
     
    Post Reply
    Vote
    Bad stuff?

 

Have an opinion? Join the conversation!

Advertisement
Advertisement

Get FREE Dealer Quotes

From dealers near you
Go!

Find Green Cars

Go!

Advertisement

 
© 2014 Green Car Reports. All Rights Reserved. Green Car Reports is published by High Gear Media. Send us feedback. Stock photography by izmo, Inc.
Advertisement