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).
Method for calculating a car's performance from clutch horsepower, detailing various power losses.
| Identifier | ExFiles\Box 124\2\ scan0169 | |
| Date | 14th May 1940 | |
| RM{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden}6/JMc.14.5.40. APPENDIX. Method of calculating the performance of a car given the clutch horse power. (1) From open exhaust test bed figures for the engine unit in question deduct the correct exhaust and air silencer losses. From the resulting figures corrected to the underbonnet temperature of the car, may be obtained a CLUTCH HORSE POWER CURVE. The fan loss is rather complicated, since part of it is absorbed in helping air to get through the matrix. Its effect is thus to reduce the frontal area of the car by a rather unpredictable amount. The resulting problem in loss and gain is best solved by ignoring fan loss completely. (2) POWER FOR ACCELERATION is what remains when clutch horse power has been spent on three losses - tyres, transmission and windage. a.{Mr Adams} Tyre loss. The losses for most types of tyre may be deduced from figures obtained on the dynamometer rig. Front and rear tyres have different losses and correction must be made for the axle loading in each case. Tyre losses on the road are taken as being 2/3 of those obtained on the 4 ft. dia. steel drums. b. Transmission loss. This loss is very small compared with tyre or windage loss. The assumption of 1 h.p. for every 20 m.p.h. appears to be satisfactory. c. Wind resistance. Windage horse power KAV³/375 where A=frontal area of car in sq. ft. V =road speed in m.p.h. K = wind resistance coefficient. For a Bentley type of body this is approximately .0017. For Phantom III it was nearer .002 and for the first Paulin Bentley B.27.LE it was .00121. (cont'd) | ||