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).
Study on tank mileage, examining the impact of wind resistance, streamlining, gear reduction, and chassis friction on fuel economy.
| Identifier | ExFiles\Box 149\3\ scan0003 | |
| Date | 1st January 1939 | |
| January, 1939 TANK MILEAGE 31 increase obtained at low speeds. For example, the effect of a 10 per cent reduction in wind resistance at 40 m.p.h. is only 2 per cent in economy; whereas, at 78.5 m.p.h., it is 8 per cent. Actual road tests confirming in a general way these calculated results have been made and are interesting in this connection. A conventional four-door sedan was fitted with removable sections to give it a streamlined form. These sections consisted chiefly of a tail, underpans, redesigned fenders, radiator grille, hood, cowl and windshield. The total effect of the changes resulted in a 49 per cent reduction in the wind-resistance coefficient from 0.00120 to 0.00061. The road-load economy, as shown in Fig. 6, was increased 56 per cent at 80 m.p.h. and 17 per cent at 60 m.p.h. In order to accomplish these results, however, it was necessary to increase the overall length of the car 44 in. This would make a sad tail indeed for traffic driving and parking. Fig. 5 – Increase in miles per gallon with reduction in wind-resistance coefficient when the gear ratio is reduced numerically so as to maintain the same power available for acceleration at each speed (Chart text: Y-axis: % INCREASE IN ECONOMY; X-axis: WIND RESISTANCE COEFFICIENT % .00127 = 100 %; Lines: 78.5 MPH, 60 MPH, 40 MPH) It seems reasonable to conclude, therefore, that streamlining alone will not result in an improvement in existing tank mileage, but will have the effect of permitting higher speed operation without seriously penalizing existing tank mileage. Chassis Friction Unlike wind resistance, the effect of chassis friction and rolling resistance is more uniform over the entire speed range. Although no solution is offered for accomplishing any reduction, it would result in an appreciable increase in economy as shown by the curves in Fig. 7. Here the possible increase in economy of our Model X car has been plotted against the per cent reduction in chassis friction which shows, for example, that a 20 per cent reduction in chassis friction would increase the average road-load economy 1.1 miles per gal., or 5.5 per cent. These figures are based on normal operating conditions with well-broken-in cars. The effect of initial stiffness on new cars as they are delivered to owners is a matter of painful experience to most of us. Car No. SERD 481 Displacement 241.5 Gear Ratio 3.89 Dir. .704 O.D. Engine RPM/MPH Dir. 45.7 O.D. 32.2 Tire Size 6.50x16 6 Ply Table Data: (Column Headers: Conventional, Streamlined) Road Weight - lbs.: 3537, 3627 Projected Frontal Area - sq. ft.: 27.2, 27.2 Overall length without bumpers - inches: 178, 223 Wind Resistance Coefficient: .00120, .00061 Equivalent Flat Plate Area (WT): 9.7, 4.5 Fig. 6 – Road-load economy with conventional overdrive and streamlined overdrive (Chart text: Y-axis: MILES PER GALLON; X-axis: MILES PER HOUR; Lines: CONVENTIONAL OVERDRIVE, STREAMLINED-OVERDRIVE) Gear Reduction The principal consideration given to the selection of final drive ratios is usually that of accelerative or hill-climbing ability. This ability, of course, requires a great deal of excess power, and the resulting reduction in load factor is responsible for the high specific fuel consumption at road loads throughout the normal driving range. Since all engines develop their maximum brake thermal efficiency at or near full load, the most desirable gear reduction for maximum fuel economy would be infinitely variable. Fig. 7 – Increase in mean road-load economy with reduction in chassis friction (Chart text: Left Y-axis: % INCREASE IN MEAN ROADLOAD ECONOMY; Right Y-axis: MPG INCREASE; X-axis: % REDUCTION IN CHASSIS FRICTION; Lines: % ROADLOAD ECONOMY INCREASE, MPG INCREASE) | ||