For boosting an engine, exhaust gas turbochargers offer efficiency in scavenging waste heat and converting it for more power. Their problem is that they often lack advantage in low-speed conditions before the turbo has a chance to spin-up. Traditionally, superchargers, especially the Roots-type offered by Eaton, more easily provide that power at low-rpm, but at a cost to efficiency since they are mechanically driven by the engine crankshaft, not by its exhaust.
Eaton’s addition of a clutch to its TVS supercharger changes that equation a bit. According to company engineers, the clutched TVS features an integrated electromagnetic clutch located between the drive pulley, connected to the accessory drive, and the supercharger rotors. During idle, light throttle, and cruising situations, the supercharger is disengaged, and air bypasses the unit.
With this arrangement, "the supercharger becomes a true transient response device, providing boost when needed in high power, low engine speed situations,” explained Dan Ouwenga, Engineering Manager for superchargers. “When extra power is not required, the clutch decouples the supercharger from the engine, reducing friction and saving fuel.” OEMs can now select the portion of the engine speed range to operate the supercharger, enhancing control and efficiency.
Perhaps one of the more interesting applications, at the expense of engine complexity, is pairing a clutched supercharger with exhaust gas turbocharger to get the best of both. That is what Volvo did on its T6 Drive-E 2.0-L, direct-injected inline four that powers the 2015 XC90. The engine will transition the turbo for boosting at higher engine speeds, and the clutch disengages the Eaton supercharger to increase efficiency.
Does the clutch add packaging complexity? “The clutch itself is approximately 100 mm in diameter and 50 mm long and weighs 2 kg,” explained Ouwenga. “The impact to packaging is dependent on the size of the clutch but also the other components that may be added to the supercharger.”
It will also increase calibration complexity, since the clutch control has to be integrated into the ECU software strategy. Then, calibrators will need to set locations where and when the clutch should be engaged, and also manage how soft or hard the engagement of the clutch occurs.
“The clutch itself only requires a 12-V signal to activate, similar to any automotive electromagnetic clutch," he remarked. "The challenge is the modulation during the clutch engagement, which requires a modulated signal to allow the clutch to smoothly engage.”