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).
Analysis of the effects of high altitude on the Autovac petrol supply system and engine performance.
| Identifier | ExFiles\Box 40\4\ Scan061 | |
| Date | 4th May 1923 | |
| To Hs.{Lord Ernest Hives - Chair} from Rm.{William Robotham - Chief Engineer} AUTOVAC SYSTEM OF PETROL SUPPLY AS AFFECTED BY ALTITUDES. At 15,000 ft. the atmosphere pressure is only 53% of that at sea level. We should therefore say that the height at any engine revs. which the induction manifold vacuum will raise the petrol at 15,000 ft., will be reduced to 53% of the normal. On the other hand, owing to the decrease of air density and pressure at the altitude in question, the volumetric efficiency of the engine will be so reduced that the I.H.P will be reduced to about 59% of the normal. The friction losses remaining the same, the B.H.P. will be reduced even more, the actual amount depending upon the mechanical efficiency of the engine at sea level; for an efficient engine we should say 55%. Of course, being unable to draw in a normal weight of air, will correspondingly reduce the weight of petrol consumed at a given throttle setting, but to produce the same effect in tractive effort as at ground level, the throttle will have to be opened wider and therefore the depression in the induction manifold will be less. Again, owing to the drop in B.H.P., there will be considerably more full throttle work at these altitudes than there would be at sea level, consequently reducing the opportunities of recuperation. We therefore have three main points against the Autovac system for use at high altitudes. (1) | ||