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).
Car performance, detailing factors affecting petrol consumption and optimum driving speed.
| Identifier | WestWitteringFiles\N\2October1925-December1925\ Scan160 | |
| Date | 12th June 1925 | |
| R.R. 493A (50 H) (D.D. 31, 12-6-25) J.H.D. EXPERIMENTAL REPORT. -5- Expl. No. REF Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}/LG21125 CAR PERFORMANCE. PETROL CONSUMPTION. There is probably no item of a car's road performance which is so susceptible to alteration in engine adjustments and method of driving as petrol consumption. If an owner is desirous of obtaining economical running above everything else, remarkable results can be obtained by bearing the following factors in mind. Optimum driving speed. Since a car's tractive resistance decreases with decreasing speed, it might appear that since least work would have to be done propelling the vehicle at minimum speeds, the slower the car was driven the better would be its petrol consumption. This is not so however, because when a car is driven very slowly, more petrol is used up rotating the engine and so over-coming its mechanical & friction losses, than in doing useful work at the road wheels. Again, when running at very small throttle openings, the pumping losses are considerably increased due to the large depression in the induction manifold. Another way of saying the same thing is to state that the thermal efficiency of the engine drops at reduced loads. The graph below illustrates this :- contd :- | ||