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24/07/2012
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A new type of pressure charger that combines the best of supercharging with the best of turbocharging has taken another big step towards production. Torotrak's V-Charge (formally Rotrak) has been successfully validated in a development vehicle, confirming that the system can boost torque from zero to 95% in less than 400ms at any engine speed – around one third the time required by the latest generation of heavily-downsized small turbocharged engines in the critical low-speed range where poor response is noticed by drivers.
 The company says the results prove that V-Charge currently provides the only cost-effective route to affordable downsized engines that combine low CO2 with the driveability of a larger, naturally-aspirated engine.
"Engine downsizing is the dominant industry trend for reducing emissions but with today's technologies there is a compromise between cost and performance," explains Torotrak product director Andrew de Freitas. "In a downsizing application, a single turbo will deliver the required CO2 reduction but at the expense of significant areas of driveability. A more sophisticated boosting system provides good driveability but with the cost and engineering issues that come with complex technologies. Our test results, and the comments of knowledgeable specialists who have driven the development vehicle, confirm that V-Charge will deliver class-leading driveability at an affordable price."
The most difficult challenge for conventional turbo technologies is that different system characteristics are required at high and low engine speeds. "For a range of reasons associated with the air flow characteristics of the system, it isn't possible to provide both optimum high speed power and good low speed torque with quick response," explains de Freitas. "For a turbo system, the only current solution is expensive technology such as variable geometry turbines, twin turbos or combined turbo and supercharger systems."
Superchargers overcome the lack of immediate low-speed boost by being geared to deliver the necessary flow from a low RPM. "Again there is a compromise; a supercharger with this high gearing wastes energy at high speeds because of the by-passing needed to avoid surplus pressure," says de Freitas. "Conventional superchargers are generally a more expensive solution, which so-far has made them unsuitable for volume vehicles."
Torotrak says that V-Charge solves these problems by combining proven elements from both types of forced induction system. A small, high-efficiency compressor impeller, similar to that used in a conventional turbo, is driven mechanically via Torotrak's proven Compact Variable Drive, a gearless variable speed transmission that can change ratio from 0.4 to 2.5 within 250 ms. This allows high boost to be provided on-demand at low engine speeds, giving instant throttle response, without the inefficiency of providing too much boost at higher engine speeds.
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Author
John Challen
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