From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Continued analysis of blower and supercharger performance characteristics, including boost, efficiencies, and engine power.
| Identifier | ExFiles\Box 140\1\ scan0060 | |
| Date | 23th July 1934 | |
| -2- Hs{Lord Ernest Hives - Chair}/Aln.1/WJ.23.7.34. Cont'd.{John DeLooze - Company Secretary} The curves attached show the features which would be expected in the absence of a diffuser, namely, no fall of pressure as the delivery is progressively reduced and a much more gradual fall as it is increased, than is the case with diffuser type blowers. We were unable to discern any surging at all thoughout the tests. The maximum boost is just about equal to the value we calculate for the purely centrifugal pressure set up in the rotor showing that there is no further gain due to kinetic energy after leaving the rotor. There is no flattening off in the rate of rise of the boost with increase of speed due to the proportioning of clearances or other features. The tip speed of the rotor in the 3000 R.P.M. case is the same as in one of our Goshawk blower tests and while the maximum boost with the Graham is 5.5" in the case of the Goshawk, it is 8.0". The efficiencies shown are for the unit with carburetter (where fitted) and somewhat unexpectedly increase with speed. The highest value reached for the complete unit is 46% at 3500 R.P.M. It was not convenient to measure the torque taken to drive the blower, but instead, horse-power curves have been derived from the air horse-power divided by the efficiency and allowing 90% efficiency for the skew-gear drive. Estimated working points on the engine are shown on the curves. These are obtained, making allowance for the absence of petrol, from an engine boost curve given by the makers, which appears reasonable. This indicates an efficiency above 2000 R.P.M. of from 32% to 35% and boosts of a little over 60% of the maximum. From the figures of engine horse-power quoted, this gives .150 lb. of air per minute per horse-power. Taking the axial width at the minimum casing width which is a little short of the rotor tip radius, and including the clearance, the leaving angle for the gas at these working points is only 1° 50 so that it appears that the gas flows smoothly into the volute in spite of the absence of a diffuser. As regards engine power we should estimate an increase over the unsupercharged condition, allowing for power absorbed by the blower, of about 4% at 2000 R.P.M., 10% at 3000 R.P.M., and 12% at 3500 R.P.M. The makers give about 30% at 3000 R.P.M. for instance but their own curves of boost and carburetter depression do not account for this amount. The working points deduced on our curves give engine volumetric efficiencies reckoned from the induction pipe of around 95%, a figure which might easily be reached and exceeded as boost increases. There is reason to think that this is rather high however, and it may be that the real working points are a little higher up the boost curves which would mean less air per horse-power, higher blower efficiency and less power absorbed to drive | ||