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 petrol consumption figures based on the tractive resistance of hard versus soft tyres.
| Identifier | ExFiles\Box 160\2\ scan0026 | |
| Date | 19th June 1923 | |
| X457P 19.6.23. PETROL CONSUMPTION FIGURES. From Canadian Dunlop Cord figures - Tractive resistance at 20 m.p.h. (hard tyres) = 34 lbs. per ton. " " " " (soft tyres) = 48 lbs. per ton. Weight of car 2.1 tons. H.P. absorbed by tyres and car at 20 m.p.h. Hard tyres = (34 X 2.1 X 20 X 88) / (550 X 60) = 3.8 HP. Soft tyres = (48 X 2.1 X 20 X 88) / (550 X 60) = 5.36 HP. Now the HP. required to turn the engine at 20 m.p.h. = 3.25 (from our previous T.R. tests). Total work done turning the engine over and driving the car at 20 m.p.h. - Hard tyres 3.25 + 3.8 = 7.05 HP. Soft tyres 3.25 + 5.36 = 8.61 HP. Increase in total HP. consumed due to soft tyres = 1.56 on 7.05 or 22.1% One would therefore naturally expect the petrol consumption to be about 20% higher but when tried on 6-EX it is only 6 1/2 % higher. As we had not carried out our tyre tests for T.R. on 6-EX, and our figures for Canadian Dunlops were much above Lockwood's figures, it appeared that the low % increase in petrol consumption was due to the increase T.R. of the S.S. Dunlop Cords with deflation being unusually small. We have since observed, however, in a May number of the 'Auto car' that the Dunlop Co. disagree with Lockwood'in the same direction and to the same extent that we do. We suggest therefore that the tyre T.R. tests taken with the Hawk are repeated with 6-EX, the car on which the petrol consumption test was run. If they agree to any extent with those of the Canadian Cord, it appears to us that the only factor remaining to explain our difference of 15% is that the thermal efficiency of the engine might vary with the throttle opening, i.e., that although the throttle has has to be opened a little wider to maintain the same speed with soft tyres, the thermal efficiency of the engine increases so that the petrol consumption does not rise as much as might be anticipated. We rather think, however, that a Tyre Test will much reduce the apparent discrepancy. (Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson}) | ||