SINCE the operation of the positive-displacement supercharger is characterized by a fixed displacement of air per revolution and delivery is affected only to a small amount by pressure variations, the supercharger continues to deliver extra air to the engine at the lower speeds. For applications requiring a high torque at the lower speeds, this characteristic is advantageous; it imparts the desired “lugging” ability to the engine in contrast to centrifugal units where the pressure, being proportional to the square of the rpm, falls off rapidly as the speed is reduced.
In spite of the fact that the theoretical efficiency of the Roots type of positive-displacement supercharger is not as high as that of the type which compresses the air within the blower before delivery, the Roots type has the simplest design, having but two moving parts; requires no lubrication in the blower air chambers, the only points of friction being the bearings, seals, and gears.
This paper presents a preliminary means of estimating the performance of a positive-displacement supercharged engine and offers a yardstick of the practical limitations for the performance of the units with present-day equipment. It is shown in a discussion of valve overlap, intercooling, and scavenging that the combined effect of the inter cooler and the use of scavenging permits the highest increase in output of any supercharging arrangement discussed - reaching a value of 166% at 10 psi. Intercoolers are much better adapted for use in marine engines where cool sea water is available to absorb the heat than for automotive engine use.