Considerable emphasis is placed on boost these days, not just in the performance market where it’s a more practical power solution over increased displacement, but also on the OEM production side where it’s a practical power necessity to offset the trend of decreasing engne displacement needed to meet tighter emissions and fuel economy mandates.
Despite the promise of surging horsepower numbers, boost has its tradeoffs–whether it’s generated by a turbocharger or supercharger. That’s why the concept of variable boost is drawing so much attention throughout the industry–and one of the latest campaigns down this road is the rather unique V-Charge from Torotrak, demonstrated in the video above. Just like variable valve timing or even variable displacement oil pumps, variable boost will help the engine adjust to different needs at different rpm and load conditions. When all these variables are in tune at the right time and in the right amount, performance and efficiency will dramatically improve.
In conventional configurations, power adders such as turbos and superchargers generally have peak efficiency restricted at certain rpm range, depending on the setup and engine design. For superchargers, engine speed and the drive mechanism to turn the supercharger are the main controlling factors. While positive-displacement or Roots-style superchargers enjoy a genuine reputation for snappy low-end response and don’t have to spin very fast compared to the engine speed, centrifugal superchargers–which are basically mechanically driven turbochargers and must spin seven or eight times faster than the engine–can be dull on the bottom end if set up for an emphasis on top-end power. That’s acceptable in a racing situation, but for street performance a compromise must be made to achieve drivability.
Variable-speed or variable-boost superchargers designed to maximize both low- and top-end performance are not new concepts and several have been introduced throughout this century. For example, transmission specialist Antonov introduced what was called the world’s first 2-speed supercharger at the 2006 Global Powertrain Congress. It featured an automatic-shifting 2-speed drive system for centrifugal superchargers and was intended for the US tuner market but never broke ground.
A more interesting concept comes from Eaton, which developed an electrically assisted variable speed Roots-style supercharger that could be applied to mild hybrid vehicles utilizing start-stop function. And we also know about a variable speed supercharger drive developed by Fallbrook Technologies that uses a continuously variable planetary drive that could be mated to a conventional centrifugal supercharger. So far, none of these options are in significant production.
ProCharger stunned the performance aftermarket a few years ago when it introduced the i-1 supercharger at SEMA. Using a CVT or continuously variable transmission between the driven pulley and a centrifugal supercharger, the boost can be adjusted according to the rpm–either for all-out performance or fuel economy and drivabilty. The CVT features two adjustable-diameter pulleys connected by a high strength belt. An electric motor is used to alter the pulley sizes, effectively changing the drive ratio between the engine and supercharger. The ProChrager is already built with an internal 8:1 step-up ratio, and the CVT allows 2:1 step up at low rpm to increase boost and improve throttle response, or falls back to a .5:1 at high rpm for the most efficient supercharger speed.
The system is electronically controlled and the user can select from three pre-programmed modes that offer a wider power band, depending on the application and driving situation. Although there is a custom program option, the boost levels are still preprogrammed for rpm. In other words, the boost isn’t adjusted in real time by the engine’s ECU as the computer evaluates a multitude of variables, such as throttle position, fuel delivery, gear selection and more in addition to the rpm.
The Torotrak V-Charge, on the flip side, is designed for OEM applications where the ECU would have complete control over the supercharger with an extremely wide range of rpm and boost–all completely independent of engine speed. Instead of a CVT to vary the supercharger speed, the V-Charge leverages Torotrak’s expertise in gearless traction drives to develop a variable-speed drive that turns a supercharger. Company officials say the V-Charge “delivers from 0 to 95 percent of target torque in just 400 milliseconds and removes boost just as quickly when not required.”
As noted in the illustration above, the traction drive variator can adjust the drive ratio from an underdrive at .35:1 up to an overdrive of 2.82:1. The unit receives a 3:1 overdrive from the engine, and also has a built-in planetary style traction drive epicyclic that produces a fixed 12:67:1 ratio speed increase. At 1,000 engine rpm, the V-Charge could spin the supercharger’s centrifugal compressor from 13,300 up to 107,200 rpm in continuously variable amounts.
ToroTrak has posted videos with excellent graphics to help illustrate the technology behind its V-Charge supercharger. It’s a rather unique solution to help automakers provide both performance and fuel economy as the industry is pressured to please the customer and reduce CO2 output.