In a 2010 paper, Roger Stone, Engineering Director, Torotrak noted that conventional turbo- and supercharging technologies are more than capable of delivering the required peak power and torque levels from downsized engines but suffer from a number of compromises:

• Small turbocharged engines suffer from “lag” under transient conditions and have great difficulty in achieving sufficient low speed torque and responsiveness to provide the launch feel of a larger engine.

• Positive displacement superchargers are much more able to provide the bottom end response and torque levels but the fixed drive ratio means that they are effectively over-sized and therefore wasteful at higher speeds.

• Both solutions involve part load losses which compromise some of the fuel consumption savings made by downsizing.

The combination of the Rotrex centrifugal supercharger with a Torotrak full-toroidal traction drive CVT (continuously variable transmission) provides a compact pressure charging solution that overcomes many of these problems, Stone noted. The wide ratio spread provided by such a CVT allows much greater control over the available boost pressure even at low engine speed. Furthermore, a significant part of the speed/load map can be exploited by adjusting supercharger speed in order to control charge air mass flow while leaving the throttle wide open, minimizing pumping loss.

Additional benefits in comparison with turbocharger solutions include simpler transient air fuel ration (AFR) control, and reduced thermal inertia in the exhaust, enabling faster catalyst light-off and reduced under hood temperatures.

At low speeds, engines with only two or three cylinders find it harder to give the turbo enough exhaust energy to generate the torque drivers need to pull away quickly. The Rotrak variable drive supercharger isn’t limited in this way. Instead we’re taking energy from the crank, passing it through our variable drive and into a centrifugal compressor to boost combustion. It gives the next generation of downsized powertrains ‘big-engine’ response.

—Andrew de Freitas, Rotrak Product Director

The Rotrak system is performing well, said James Shawe, the senior engineer responsible for the calibration project, as the team learns to control and apply the technology’s very fast torque-delivery.

We’re among the first engineers to calibrate a variable drive supercharger. It’s uncharted territory and a lot of work, but the effort we invest in exploring the technology’s operating envelope is establishing new areas of engine performance for manufacturers. The intellectual property and know-how we’re generating provides a new variable in engine control strategies with which manufacturers can increase performance and reduce CO₂ emissions.

— Andrew de Freitas

Work to develop a drivable prototype for interested Tier Ones and vehicle manufacturers will continue on the rolling road for the next few months. At the same time, Torotrak will gather data on the system’s efficiency to validate fuel economy simulations.

The Rotrak prototype is undergoing testing on one of two single axle rolling-road dynamometers at Torotrak. The dyno is capable of testing vehicles at speeds of up to 250 km/h (155 mph) and the roller has a maximum tractive effort of 6000N. In-cell weather stations monitor atmospheric pressure, relative humidity and ambient air temperature, which can be controlled from 10-30°C.

Resources

• Roger Stone (2010) Exploitation of Full-Toroidal CVT Technology to a Centrifugal Supercharger for Radical Engine Downsizing Applications