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 vehicle fuel economy and performance, detailing the effects of speed, load, and wind resistance with supporting graphs.
| Identifier | ExFiles\Box 149\3\ scan0002 | |
| Date | 1st January 1939 | |
| 30 S.A.E. JOURNAL (Transactions) Vol. 44, No. 1 Knowing the specific-fuel-consumption characteristics of the engine and other factors relative to the car, the road-load economy can be determined with a fair degree of accuracy. An example is shown in Fig. 3. Here, an engine having these fuel-consumption characteristics has been applied to a car, and the road-load economy calculated. Other factors such as weight, gear ratio, frictional and other losses, have been chosen so as to make it representative of present-day low-priced cars, and we may call this car Model X. The specific consumption at road load over the speed range from 20 to 60 m.p.h. varies from 1.3 to 0.7 lb. per b.hp-hr., corresponding to brake thermal efficiencies of 10 per cent and 19 per cent. The calculated mean road-load economy of 21.4 miles per gal. compares favorably with an average of 20.8 miles per gal. obtained on actual tests of a large number of cars in this class. If this power is reduced by amounts equivalent to a percentage reduction in the wind-resistance coefficient, the corresponding increase in miles per gallon at any speed can be determined. This increase is shown in Fig. 4. In this case no change has been made to compensate for the reduction in power required and, in addition to improved economy, the car also would have increased accelerative ability. When the gear ratio is reduced numerically so as to maintain the same excess power available for acceleration at each speed, the resulting increases in economy are as shown in Fig. 5. These curves show the optimum economy increase possible by a reduction in wind resistance, assuming no change is made in the efficiency, weight, or performance characteristics of the car. In considering the effect on tank mileage, the most important fact illustrated by these curves is the relatively small Fig. 3 - Calculated road-load economy GRAPH LABELS: MILES PER GALLON, HORSEPOWER, LBS. OF FUEL PER BHP. HR., MILES PER HOUR, MAXIMUM BHP, MILES PER GAL., NET BHP WITH FAN & MUFFLER, WIND RES. HP, ROAD-LOAD SPECIFIC FUEL CONSUMPTION, CHASSIS FRICTION HP. Fig. 2 - Specific fuel consumption vs.{J. Vickers} speed and load GRAPH LABELS: LBS. FUEL PER BHP HR., PERCENT OF LOAD, ENGINE RPM - HUNDRED. Wind Resistance In studying possible design changes to improve economy, wind resistance usually is given first consideration because it is one of the largest single factors involved. Referring again to Fig. 3, the horsepower required for overcoming wind resistance increases from 5.9 at 40 m.p.h. to 44.8 at 78.5 m.p.h. These are average values obtained by checking the car speed at various throttle openings on the road and then determining the power output on a four-wheel chassis dynamometer at the same speeds and throttle openings. This method is subject to some error because of an unavoidable difference in certain operating temperatures and also because of the difference in rolling resistance of the tires on the road and rolls. It is probably as accurate, however, as a scale-model wind-tunnel test. The results quoted for Model X, which has 27.4 sq. ft. projected frontal area, give a coefficient of 0.00127 when substituted in the conventional formula: Hp. = C A S³/375. Fig. 4 - Increase in miles per gallon with reduction in wind-resistance coefficient GRAPH LABELS: % INCREASE IN ECONOMY, WIND RESISTANCE COEFFICIENT - %, .00127 = 100%, 78.5 MPH, 60 MPH, 40 MPH. | ||