Goodbye turbo lag!

Posted on July 02, 2018 by Rolf Behrens, Images by Roland Witsch, Robert Hack

Electrically assisted turbocharging to develop engines that deliver increased agility and lower consumption.
Friedrichshafen, Germany

If you drive a turbocharged automobile, you probably know all about ‘turbo lag’. You step on the gas at low revs and for a while – not much happens. In future, for mtu engines at least, this turbocharger design problem will be consigned to history, thanks to new technology that combines conventional turbochargers with an electric motor. 

The two prototypes undergoing tests on the mtu test stand in Friedrichshafen look like nothing more than a couple of upturned cook pots, each with a round hole in the middle. But beneath the unremarkable-looking cover there lurks cutting-edge technology – possibly even the ultimate answer to some of the most complex questions ever faced by engine designers: How do we get rid of turbo lag? How do we meet increasingly strict emissions regulations without losing out on agility and consumption? And how can we get turbochargers to deliver maximum performance across the engine’s entire operating range?

Beneath the unremarkable-looking cover there lurks the cutting-edge technology electrically assisted turbocharging.

At mtu, the magic concept is electrically assisted turbocharging. And this is what is hidden beneath the two cook pot-type aluminum covers fitted in front of the turbocharger compressor on a 10-cylinder diesel engine. mtu has acquired exclusive rights to this new technology from the G+L innotec company, and the partners will now be jointly developing the product through to series maturity. Market debut for the first engines with electrically assisted turbocharging is scheduled for 2021. 

“Electrically assisted turbocharging is a significant milestone on the road to hybridization,” explained Dr Johannes Kech, Director, Turbocharging and Fluid Systems at mtu. “This technology will allow us to develop engines that deliver increased agility and lower consumption at the same time as enhancing ecological performance,” he added. All this is made possible by using an electric motor to smooth out weaknesses in turbocharging systems.

Linked to a conventional turbocharger, the electric motor ensures that extra fresh air is available when-ever the engine needs it. As a result, delays in performance ramp-up are now a thing of the past.

Increased demands on turbochargers 

Turbochargers are intended to increase performance and efficiency. They utilize the energy contained in engine exhaust gases to drive a turbine coupled with a compressor and deliver more oxygen to the combustion chambers in the diesel engine. However, the physical constraints of the technology mean that the additional performance only cuts in at higher engine speeds because at lower speeds, there 
is not enough exhaust gas to drive the turbine fast enough. Consequently, there is always a delay until the necessary speed is reached. In the past, designers tried to solve the problem with ever more complex (and therefore increasingly expensive) constructions, including sequential and alternating switching concepts and adjustable turbine blades. 

Today, these technologies have just about reached their limits because the demands made on turbochargers are constantly increasing. Customers operating in more and more applications are demanding even greater acceleration across the operating spectrum. At the same time, turbochargers are increasingly expected to achieve reductions in emissions by helping to prevent the generation of diesel particulates and nitrogen oxides during the combustion process. Maximum performance across wider speed ranges with simultaneous suppression of emissions – expectations have now almost surpassed the possibilities open to conventional technology. 

Electrically assisted turbocharger squares the circle

Turbocharging technology has now turned to electrification to square the circle. “With electrically assisted turbocharging we marry a conventional mtu turbocharger with an electric drive motor,” explained Joachim Thiesemann, who is responsible at mtu for integrating the new system in the standard turbocharger landscape. It was as part of this process, that his coworkers mounted the ‘cook pots’ described earlier externally, in front of the compressor wheel. Each cook pot houses a permanent magnet, the rotor of the electrical unit, and the electrical winding that is integrated in the compressor casing. “The electric motor makes it possible to virtually decouple the operating point of the turbocharger from the speed of the diesel engine,” explained Rudi Rappsilber, who is in charge of testing the new system at mtu. The result is that significant delays in performance ramp-up are now history, and optimum turbocharging can be achieved in almost every operating state. For development engineers and users alike, that represents the fulfillment of a dream. An additional advantage is that the technology can be implemented with existing turbochargers without excessive complications. The additional installation space required is limited.

On the test stand, Rappsilber and his coworkers completed a final check on the cabling for the measuring sensors before leaving the room. At the press of a button, the engine started up, and after a short idling run, the throttle was fully opened. The steep rise in the speed curve brought a smile of satisfaction to the faces of the engineers: “The engine is accelerating much faster than without electric support,” reported Rappsilber, adding: “That promises to translate into a significant improvement in agility for operation in the field later on.”

The new technology is expected to be available to owners and operators of ships, gensets and land-based vehicles from 2021. That said, Thiesemann, Rappsilber and their coworkers will still have to spend many hours at the drawing board and the test stand before their work with electrically assisted turbocharging reaches the point where emergency gensets in the field ramp up to full speed faster, motor yachts accelerate quicker and their engines power them up to top performance with less fuel and lower emissions. Nevertheless, the process is likely to involve increasingly frequent smiles of satisfaction.

?Technology development in this connection is publically subsidized as part of the MethQuest research project by the Federal Ministry for Economic Affairs and Energy 

Point of contact

Dr Johannes Kech
+49 7541 90 2153

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