A SINGLE-SPEED supercharger with a compression ratio adequate for maximum power at high altitudes will supply more air than the engine can safely use at low altitudes. Two-speed supercharger drives are provided to obtain high power for take-off at low altitudes and maximum power at high altitudes with one blower.
The Wright two-speed drive utilizes a single compound spur-gear system with the intermediate gears connected by a rotating hydraulically actuated friction clutch for the high ratio, and a planetary reduction gear with a clutch in the low ratio. A later model has the intermediate gears coupled by a roller clutch in low ratio and a planetary step-up gear and stationary clutch in high ratio.
The Pratt & Whitney two-speed drive utilizes hydraulically operated cone clutches for both the low and high ratio. A fluid coupling is used to accelerate the impeller to a speed between that of the high and low ratio drives for the shifting operation only. Two parallel units are used to reduce the gear sizes.
Bristol uses three clutch units equally disposed above the driveshaft so that each unit carries one-third of the load. Each unit contains two multiple-disc clutches, one for each ratio. Oil for the hydraulic actuating cylinders is cleaned by a pair of centrifuges before it enters the clutch units.
The Rolls-Royce supercharger drive uses semi-centrifugal mechanically actuated clutches. One clutch is provided for the low ratio and two similar clutches for the high ratio. The mechanical linkage is operated by a hydraulic cylinder with scavenge oil pressure.
In the Junkers two-speed drive, a pair of bevel gears drive a layshaft connected to the impeller shaft by two intermediate gears. The low ratio intermediate gear is coupled to the shaft by a roller clutch. The high ratio gear is connected by a mechanically operated friction clutch. An aneroid mechanism prevents engagement of the high ratio clutch at low altitudes.
The Mercedes-Benz DB-601 engine has a variable-speed supercharger drive consisting of a straightforward gear system driving the final driveshaft at a speed slightly higher than the highest ratio. The speed is then reduced by a fluid coupling. Operation of the drive is entirely automatic, the ratio of the coupling being regulated by a control unit actuated by an aneroid capsule. The slip of the fluid coupling in low ratio at the lower altitudes causes the oil in the coupling to be heated considerably. This would be a great disadvantage in transport or patrol planes intended for operation at low altitudes. However, this might be an advantage for fighter craft as the high heat rejection would aid in quickly warming up the system while climbing to high altitudes.
The DB-601 design has created considerable interest in this country. We can be sure that the fluid coupling will be studied carefully and possibly adapted to our requirements. Further developments of two-speed drives will be made adapting them to two-stage superchargers with intercoolers between the stages.