There’s a lot to be said for maximum torque at zero rpm, something internal combustion engines (ICE) can’t match. That said, one company is working on a variable drive supercharger that could offer significant gains in both ICE power and fuel economy.
Rotrak is a joint venture between Torotrak, a traction drive company, and Rotrex A/S, a centrifugal supercharger manufacturer. Rotrak has produced a prototype centrifugal supercharger that also employs a continuously variable transmission (CVT) drive.
To understand the benefits, it’s first necessary to understand the drawbacks of forced induction, which includes both turbocharging and supercharging.
Small turbocharged engines, unless they employ complex twin-turbo systems, often suffer from turbo lag when tuned to make top-end power. Turbo engines tuned for low-end performance can’t supply the boost needed for high-rpm benefit.
Superchargers are generally better at providing low-end torque, but their fixed drive ratio means that they produce too much boost at higher engine speeds. In other words, they’re wasteful above a certain engine speed.
An ideal supercharger system would have an infinitely variable drive ratio, which would allow it to vary compressor speed based on throttle setting and engine load. Such a system would provide enough boost at low end without creating too much boost at high engine speeds.
As Green Car Congress explains, that’s where the prototype Rotrak system comes into play. The CVT drive allows the supercharger to maximize efficiency across an engine’s entire range, which allows smaller engines to drive like bigger ones.
While no data on real-world performance or fuel economy gains is available yet, the system has a tremendous amount of promise. Ultimately, it would allow further downsizing of engines from today’s small-displacement turbos, like Ford's EcoBoost line.
A drivable prototype is expected in the next few months, which (hopefully) means we’ll be hearing more about the Rotrak system later this year.