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).
The factors to consider when comparing car efficiency, focusing on engine displacement and tractive resistance.
| Identifier | ExFiles\Box 160\2\ scan0055 | |
| Date | 12th June 1925 | |
| R.R. 493a (50 H) (D.D. 21. 12-6-25) J.R.D. EXPERIMENTAL REPORT. -5- Expl. No. REFS/ R m2/LG121025 Before comparing two cars for efficiency it is as well to ascertain :- Engine displacement. Direct drive ratio. Weight of the car. From this data the displacement per ton mile can be calculated. This figure will show immediately if two cars are likely to have the same sort of performance. It is xxxxxxx almost impossible to compare the relative engine efficiencies of two cars on the road when the displacement per ton mile of one is 50% greater than the other. The max. speed must also be taken into account, otherwise an under-geared car will create an over favourable impression. (2) TRACTIVE RESISTANCE. As we have previously pointed out, this can be split up, for simplicity, into two main portions :- Tyres and bearings. As these are now practically a constant for every make of car, cord tyres and ball bearings being used, they can be neglected in judging the performance of a car. If freak petrol consumptions at low speeds are desired however, they are points which must be considered. The oil used in the engine, gearbox and axle, is also capable of making a small difference to the petrol consumption if its viscosity is very high. For all normal comparative tests, however, we should say that these factors can be disregarded. contd :- | ||