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 from a journal discussing vehicle fuel economy, comparing road-load economy to trip economy, and the effects of driver behaviour.
| Identifier | ExFiles\Box 149\3\ scan0006 | |
| Date | 5th January 1939 guessed | |
| 34 S. A.{Mr Adams} E.{Mr Elliott - Chief Engineer} JOURNAL (Transactions) Vol. 44, No. 1 [GRAPH] Y-Axis: ROAD-LOAD ECONOMY - MILES PER GALLON X-Axis: AVERAGE - TRANSCONTINENTAL TRIP - MILES PER GALLON Fig. 14 – Average economy of a group of cars on transcontinental trip vs.{J. Vickers} road-load economy In cross-country driving, the effect of speed and acceleration is greater than is realized generally. Taking the road-load economy at 20 m.p.h. as 100 per cent on a group of 20 1938 cars ranging in weight from 2330 to 4670 lb., the economy is reduced to 96 per cent at 30 m.p.h., 87 per cent at 40 m.p.h., 77 per cent at 50 m.p.h., 66 per cent at 60 m.p.h., and 54 per cent at 70 m.p.h. The percentage reduction is greater on the smaller cars and, at 70 m.p.h., the average of the four smallest cars in the group is 46 per cent and that of the four largest cars is 64 per cent. The characteristic shape of the wide-open-throttle economy curve is fairly flat and, from 20 to 60 m.p.h., averages about 38 per cent of the mean road-load economy. Fig. 14 is an example of the average economy of a group of cars driven on transcontinental test trips at average speeds, based on total driving time, of 44 to 49 m.p.h. Here, the average economy of each car for the trip is plotted against road-load economy at the same speed. Although there is a wide variation among individual cars, the results indicate, in a general way, the economy obtained in this type of operation. The “trip” economy of all the cars ranges from 69 to 87 per cent of the road-load economy and the average of the group is 77 per cent. Traffic Test Described Typifying the effect of individual-driver variation, we might describe a traffic test which recently was run in Detroit. A 10-mile course through city traffic was laid out, and the same car was driven over this course in two different ways. In the first test, maximum acceleration through all gears was maintained up to a maximum speed of 30 m.p.h. The car was driven in such manner as to cover the course as rapidly as possible while observing the 30 m.p.h. speed limit. In the second test, the car was started in second gear, shifted into high as soon as practicable, and accelerated slowly so as not to bring the step-up into action. The car was coasted up to traffic lights with the clutch disengaged and, in general, the precautions were observed which would be recommended to an owner who wished to obtain maximum economy. In the first test, the 10-mile course was completed in 39 min. The average fuel economy was 8.5 miles per gal. In the second test, the course was covered in 44 min. and the economy was 17 miles per gal. By sacrificing 12 per cent in time the driver was able to improve his fuel economy 100 per cent. Throttle Stop Improves Mileage We know of a case in fleet operation where an improvement in tank mileage of 20 per cent was achieved by restricting the performance of the cars by means of a throttle stop. This method, of course, did not change the mean road-load economy of the car at all. The result was accomplished by combining the performance restriction with a considerable amount of driver education. Where the same equipment was used minus the latter ingredient, the results were, in some cases, negative. It is such factors as these which cause the spread in mileage as shown in Fig. 15, which covers 45 cars of similar model. In this chart the cars are bracketed in groups depending on their average mileage over a period of a month. The squares represent the number of cars giving mileages within the bracket indicated. The observed spread is from 13.7 to 23.4 miles per gal., and it may be noted that the standard of maintenance of this group of cars is probably both higher and more uniform than a similar group of cars taken from the hands of individual owners. The car owner is concerned with tank mileage. He measures economy by the number of times he has to reach for his pocketbook. I know of one case where mileage complaints on a certain model were reduced greatly by fitting a larger gas tank. The gap between this point of view and that of thermal efficiency is prodigious. We cannot hope to bridge it in this paper. We hope that we have thrown a strand or two across the chasm. [CHART] MILES PER GALLON | UNIT 13.5 13.9 14 14.4 14.5 14.9 15 15.4 15.5 15.9 16 16.4 16.5 16.9 17 17.4 17.5 17.9 18 18.4 18.5 18.9 19 19.4 19.5 19.9 20 20.4 20.5 20.9 21 21.4 21.5 21.9 22 22.4 22.5 22.9 23 23.4 Fig. 15 – Spread in miles per gallon with 45 cars of similar model | ||