Single‐Stage Superchargers: A Discussion of the Limiting Factors in the Performance of this type of Blower

IN many types of engine the supercharger is built on to the accessory‐drive end, that is the rear of the engine, so that the supercharger shaft lies co‐axially or parallel with the crankshaft. This has the great advantage of permitting the use of simple spur gearing, but suffers from the disadvantage that in passing through both intake and delivery section, the working fluid is subjected to several right‐angled changes in direction. In the case of the Rolls‐Royce Merlin for example, with air intakes on either side, there are 2 × 2 right‐angled bends before the inlet to the impellor is reached and two more at the delivery side. Such bends naturally involve losses in supercharging which should definitely be avoided. Hence in Germany in the two liquid‐cooled inline types the Jumo 211 (Fig. 1) and Mercedes‐Benz DB600 (Fig. 2) the supercharger shaft is built square to the engine crankshaft and the supercharger located at the side of the transmission casing, so that a single right‐angled bend suffices to change the direction of flow of intake air from that of flight and lead it axially to the impellor. If, at the same time, care is taken in nacelle design to ensure an undisturbed flow of air to the intake duct, then the total static head of flight may be utilized in supercharging the engine, resulting in an increased altitude rating. The proportion of this increase due to full utilization of the flight static head is shown in Fig. 3. For example, at a speed of 600 km.p.h. (373 m.p.h.) its value is about 1,400 m. (4,600 ft.)