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The turbocharger revolution - using electric motors

The phenomenon of turbo lag could eventually become a thing of the past if current developments come to fruition. The inherent characteristic of a turbocharged power unit - referring to the initial delay between pressing the throttle pedal and the arrival of power at the driven wheels - can be of great concern, and is often one of the obstacles to a wider uptake of the technology. It is somehow easy to forget that when turbochargers last ruled the roost in Formula One, many OE engine manufacturers wouldn't look at gasoline pressure charging, and those that did preferred the responsiveness of the mechanical supercharger and its ability to keep heat in the exhaust gas to 'light off' the ubiquitous three-way catalyst.

For much of the time since then, turbochargers seemed to be the almost exclusive domain of the diesel engine, and since transient response isn't a diesel engine's greatest attribute, turbo lag didn't feature high up in the priorities. These days, however, the pressure on fuel economy is forcing gasoline engine manufacturers to downsize their products, so to retain performance, the spectra of turbocharger lag in gasoline engines is once again concentrating minds.

The traditional approach is to use multiple turbocharger units. Operating in parallel or (more likely) some kind of sequential arrangement, such systems are bulky and difficult to package, not to mention extremely complex and therefore potentially unreliable. The requirement for complex lubrication systems in addition to water-cooled turbine housings also adds further expense.

However, developments in electric motor technology and onboard electricity storage are coming to the rescue, and instead of having the compressor and turbine of a turbocharger together on the same shaft, in this latest application the turbocharger is effectively split into two, with each part - compressor and turbine - attached to a high-speed electric machine. So while the turbine powers the generator to create electrical energy, which is then stored, the electric motor subsequently takes that energy and powers the compressor, delivering boost to the engine when it most needs it. In this way, and by using some kind of buffer electrical storage (batteries or supercapacitor), the traditional turbocharger failing of turbocharger lag can be avoided.


induction-system compressor

But that isn't all. The physical separation of compressor and turbine gives the option to run the compressor at its most efficient operating point for longer, and the technology of the wastegate will perhaps become a thing of the past. Also, to the engineer responsible for packaging the unit in the engine bay, the ability to split the turbo, placing the compressor towards the intake side and the turbine next to the hot exhaust, must have obvious benefits.

Another benefit might be the move away from the fully floating bearings. Replaced by, at this stage of development, ceramic roller bearings, gone is the need for the lube oil feed and along with it all the affects of 'key-off' heat soak on the engine oil.

But after more than three years in development, at long last interest in the technology is being shown by the racing world but not in the way you might think. With the loss of 'Kinetic' from the KERS of Formula One, under the new regulations energy can be scavenged directly from the exhaust and at least one programme using this approach is being developed in time for 2014. Operating at just under 500 V and with currents of the order of 150-plus A, officially this is for Le Mans, but once the control module is condensed to a more reasonable 6 x 4 in (150 x 100 mm) many more manufacturers will begin to take note.

And when installed, once again together with its compressor, turbo lag will become a thing of the past.

Fig. 1 - Turbocharger compressor and electric motor

Written by John Coxon

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