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).
Technical report on supercharging a Bentley engine, discussing power curves, boost levels, and blower efficiency.
| Identifier | ExFiles\Box 140\1\ scan0090 | |
| Date | 18th December 1934 | |
| -2- We have not done any endurance running on the Bentley on the test bed but have run a number of power curves. So far we have experienced no trouble whatever up to 6 lbs. of boost, running with a nickel cylinder head gasket. Our Peregrine experience of course is that it is necessary to have reinforced cylinder studs to ensure that the gasket will not blow. We have such wide experience of supercharging on aircraft engines that it ought not to be difficult for us to achieve mechanical reliability. (4) Power Output. There is no doubt that it is possible to obtain a far better performance with an engine using a centrifugal blower with a given degree of roughness and maximum stress in the engine parts than can be obtained from a naturally aspirated unit. As an example, with a maximum cylinder pressure of 800 lbs/sq.in. we can only get about 126 B.H.P. out of the engine naturally aspirated, whereas we can get 160 B.H.P. when using a blower. Furthermore, the tendency to detonate at low speeds will be less with a supercharged engine. When we come to high power outputs for competition work, the difference is still more marked. Using a 9 to 1 compression ratio without a blower we can get about 140 B.H.P. for a maximum pressure of 1,000 lbs/sq.in. If we use a 6.5 to 1 compression ratio and 9 lbs/sq.in. boost we can get 183 B.H.P. for the same maximum pressure. These quoted figures are for a blower having a greater efficiency than the Graham. With a Graham type blower the advantage is still there but considerably reduced. We attach a curve showing the output on a standard Bentley engine - (a) compared with the same engine fitted with a Graham blower exactly as used on the car we have purchased and (b) as fitted with a blower approaching the efficiency of those we use on our Aircraft engines. Ultimately we anticipate that the curve obtainable will lie between (a) and (b), because some efficiency has to be lost to ensure good cold weather carburetion and snap opening up. | ||