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).
Page discussing vehicle tank mileage, analyzing the effects of weight, temperature, and trip length on fuel economy.
| Identifier | ExFiles\Box 149\3\ scan0005 | |
| Date | 1st January 1939 | |
| January, 1939 TANK MILEAGE 33 will permit a reduction in axle ratio and a consequent improvement in economy. In calculating the effect of weight reduction on Model X, the base values taken were 3000 lb. car weight plus 400 lb. passenger load, with an axle ratio of 4.1:1. Fig. 10 was calculated by assuming that constant performance would result by so proportioning axle ratio to total weight that the same excess drawbar pull, above that required to overcome chassis friction and wind resistance, was maintained. The curves in Fig. 10 show an 8 per cent improvement in economy for a weight reduction of 10 per cent. The other curve gives the same results in terms of miles per gal. and total car weight. Fig. 11 shows test values on two groups of recent model cars, one group weighing between 3300 and 3500 lb. including 400 lb. of passenger load, the other group weighing 4100 to 4700 lb. Mean economy is plotted against average acceleration in ft. per sec. per sec. The calculated results on Model X are included for purposes of comparison. Tank Mileage Up to this point, our discussion has treated the subject of road-load economy in present-day cars, and the effect of possible design factors. In dealing with tank mileage, a group of factors are encountered over which the designer has little or no control and which have a definite effect on economy. In this group are such conditions as climatic variations, traffic operation, cross-country driving, and the individual driver. Fig. 12 illustrates the average economy of a group of 45 cars in fleet operation in comparison with the average monthly air temperature. The maximum decrease in economy in this case from summer to winter was 2.3 miles per gal. or 12 per cent. These cars were kept in a heated garage over night. Where cars are left over night at atmospheric temperatures and driven for short distances, this spread may well become very much greater as may be deduced from Fig. 13. The data for this curve were obtained by starting a car which had stood over night and reached an air temperature of 20 deg. fahr., and measuring the economy each mile for 11 miles. Eight miles of operation were required to bring the economy up to the normal level, although the water and manifold temperatures reached their normal operating level after 3 miles. The average economy for the first mile was only about one-third of its normal value. Traffic operation has the effect of reducing the economy to from one-half to two-thirds the road-load economy at the same speed. This ratio, of course, depends largely on traffic density and the individual driver, but it is not uncommon for a car which averages 24 miles per gal. road-load economy at 25 m.p.h. to run from 12 to 16 miles per gal. in city traffic. It is important, in this respect, to give consideration to off-idle mixture ratios and spark advance, and also to the selection of transmission ratios. Cars of the same weight and having equal high-gear acceleration and road-load economy at 25 m.p.h. may vary as much as 2 to 3 miles per gal. when driven in city traffic over the same route and at the same rate of speed. Graph Text: Fig. 11 – Mean road-load economy vs.{J. Vickers} average acceleration on two weight groups of recent-model cars and calculated results for Model "X" - Y-axis: MILES PER GALLON - MEAN ROADLOAD ECONOMY - X-axis: AVERAGE ACCELERATION (20 to 60) - FT/SEC^2 - Labels: 3300-3500 LBS., 4100-4700 LBS., ● CALCULATED - MODEL "X" Fig. 12 – Average monthly economy of a group of 45 cars in fleet operation compared with average monthly air temperature - Y-axis Left: AVERAGE TANK MILEAGE - MPG - Y-axis Right: AVERAGE MONTHLY AIR TEMP. - DEG. F.{Mr Friese} - X-axis: MONTH - Legend: o----o MPG, x----x TEMP Fig. 13 – Miles per gallon vs.{J. Vickers} length of trip when cars have stood over night at an air temperature of 20 deg. fahr. - Y-axis Left: MILES PER GALLON - Y-axis Right: TEMPERATURE °F.{Mr Friese} - X-axis: MILES - Labels: AVERAGE AIR TEMP - 20°F, AVERAGE SPEED - 25 MPH, MILES PER GALLON, WATER TEMP - CYL. HEAD, MANIFOLD TEMP - FRONT BRANCH | ||