Volkswagen will be selling a combination supercharger and turbocharger when its 2006 Golf goes on sale in Europe later this year. Called a "Twincharger" by VW, the system promises to increase power in the 1.4-L gas engine by some 20 percent. Fuel consumption will average 39.2 mpg.
The belt-driven supercharger increases air flow to the engine at low speeds. As engine rpm increases, an exhaust-driven turbocharger joins in, eventually taking the load completely as the engine speed continues to increase and an electronically controlled bypass switches the supercharger out. According to Eaton Engineering Manager Craig Sell, whose company is supplying the units, the system combines the advantages of superchargers and turbochargers while eliminating the disadvantages of each.
For example, a supercharger furnishes excellent low end boost, available from the moment the throttle opens. Its "linear airflow," as Sell calls it, provides a steady horsepower gain without delay. The downside is that superchargers provide too much boost at high engine rpms, which must be dumped, equating to "wasted work," Sell says.
It's here that a turbocharger shines, he says. As exhaust flow increases, more energy is available to spool up the turbocharger turbine.
The system overlaps the outputs from the supercharger and turbocharger until the turbo is up to speed. Then a bypass valve opens and the a clutch depowers the supercharger. A typical operating sequence, beginning at idle, starts around 800 rpm where the engine breathes naturally, the supercharger sits unengaged, and the bypass valve has opened; at around 1200 rpm, the clutch engages the supercharger and the bypass valve closes; at about 2500 rpm, the bypass valve opens partway as the turbocharger begins spooling up; and, near 3500 rpm, the bypass valve opens completely, the supercharger disengages, and boost comes from the turbocharger alone.
Such systems have been deployed in passenger cars before, Sell says, though never on such a scale. Similar systems have been used by the heavy vehicle industry, he adds.
†By enhancing air volume available to an IC engine throughout its operating range, the Twincharger increases power on demand without turbo lag.