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 analysis of vehicle fuel economy in relation to gear ratios, engine speed, and vehicle weight, illustrated with graphs.
| Identifier | ExFiles\Box 149\3\ scan0004 | |
| Date | 1st January 1939 | |
| 32 S.A.E. JOURNAL (Transactions) Vol. 44, No. 1 Fig. 8 - Optimum economy, ratio for optimum economy, and engine r.p.m. for optimum economy compared with values based on a 4:1 ratio [Graph 1 Labels] Left Y-Axis: MILES PER GALLON Right Y-Axis: OVERALL RATIO, ENGINE RPM - HUNDREDS X-Axis: MILES PER HOUR Lines: 4:1 RATIO, RATIO FOR OPTIMUM ECONOMY, OPTIMUM ECONOMY, ECONOMY WITH 4:1 RATIO, ENGINE RPM WITH 4:1 RATIO, ENGINE RPM FOR OPTIMUM ECONOMY to permit full-load operation at all speeds. Although we do not, as yet, have a transmission that fulfills this requirement, it is interesting to consider this condition since it represents the optimum or ideal economy that can be obtained from a car of given weight and with a given engine. These ratios can easily be established graphically, and the corresponding optimum economy calculated. This calculation has been made in connection with Model X, and the optimum ratio and economy are as shown in Fig. 8. A speed of 400 r.p.m. has been assumed as the minimum engine speed, and the optimum economy is based on part-throttle operation below that speed. The overall ratios shown are those which would be required with 6.00-16 tires. The average optimum ratio in this case from 20 to 60 m.p.h. is 1.32:1 whereas in Fig. 3 the economy was based on the ratio of 4.1:1. The optimum mean road-load economy is increased 8.2 miles per gal. from 21.4 to 29.6. In other words, the economy obtained with the normal gear ratio is 72 per cent of the optimum. A similar calculation made on a representative group of 1938 sedan models shows that the actual mean road-load economy on these cars runs from 68 per cent to 89 per cent of their respective optimum values. Semi-Automatic Transmissions With the gear ratio reduced to produce maximum economy, there would be no power available for acceleration and, for this reason, some type of automatic transmission would be necessary. The first commercial step toward realizing some of this gain is the semi-automatic or overdrive transmission. These units usually are built with fourth-speed ratios of about 0.7:1 which, if applied to Model X, would result in a 4.3 miles per gal. or 20 per cent increase in mean road-load economy. This improvement is shown in Fig. 9, which also shows the comparative specific fuel consumption at road load with the two ratios. In actual practice it is necessary to compromise the ratios somewhat for the sake of accelerative ability and, in several cars which furnish such units as optional equipment, the final drive ratios in fourth or overdrive are from 21 per cent to 26 per cent lower than the direct-drive ratios in the corresponding standard models with three-speed transmissions. Car Weight If we assume that a certain acceptable level of high-gear performance must be maintained, a reduction in car weight Fig. 9 - Improvement in mean road-load economy and road-load specific fuel consumption with overdrive as compared with direct drive [Graph 2 Labels] Left Y-Axis: MILES PER GALLON Right Y-Axis: LBS. OF FUEL PER BHP. HR. X-Axis: MILES PER HOUR Lines: OVERDRIVE, 2.87 OVERALL RATIO, DIRECT DRIVE, 4.1 OVERALL RATIO, MILES PER GALLON, ROADLOAD SPECIFIC FUEL CONSUMPTION Fig. 10 - Calculated increase in mean road-load economy with reduction in weight [Graph 3 Labels] Left Y-Axis: % INCREASE IN MEAN ROADLOAD ECONOMY Right Y-Axis: MPG INCREASE X-Axis: % REDUCTION IN WEIGHT, CAR WEIGHT - LBS. Lines: % ECONOMY INCREASE, MPG INCREASE | ||