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).
Article by Chrysler Corp. engineers on the factors influencing tank mileage and engine efficiency.
| Identifier | ExFiles\Box 149\3\ scan0001 | |
| Date | 1st January 1939 | |
| File for RM{William Robotham - Chief Engineer} 1263. Tank Mileage By W. E.{Mr Elliott - Chief Engineer} Zierer and J.{Mr Johnson W.M.} B. Macauley, Jr.{Mr James Royce} Chrysler Corp. SYMPATHIZING with the engineer who finds that the high efficiency designed into his engine fails to produce the anticipated increase in average miles per gallon obtained under normal driving conditions, the authors touch upon some of the factors, many of them out of the engine designer's control, which may completely mask the expected improvement. In so doing, they start with an engine of known specific consumption and show the effect of air resistance, chassis friction, gear ratio, and car weight on constant-speed road economy, comparing calculated values with actual test values available. Also discussed are such factors as climatic variations, traffic operation, cross-country driving and the individual driver, which have a definite effect upon economy, but over which the designer has little or no control. The effect of these factors is illustrated by the spread in tank mileage shown by a number of cars of similar model in fleet operation. “THE best laid plans of mice and men gang aft a-gley, And lea's us nought but grief and pain . . .” The grief and pain become a particularly acute ache to the engineer when the high efficiency designed into his engine fails to produce the anticipated increase in tank mileage in the car. It is the province of this paper to touch upon some factors, many of them out of the engine designer's control, which may mask completely the expected improvement. In the discussion that follows, mean road-load economy refers to the average miles per gallon obtained on constant-speed tests run in opposite directions on a level concrete road at 10-mile intervals of speed from 20 to 60 m.p.h. Tank mileage refers to the average miles per gallon obtained under normal driving conditions. In Fig. 1, the mean road-load economy of 21 different 1938 cars is plotted against displacement per car mile. The average curve drawn through the points is an equilateral hyperbola which indicates the approximate inverse ratio of these factors. When it is considered that displacement per mile does not take into account either thermal or mechanical efficiency, or rolling and wind resistance, the close proximity of the points to the curve is rather startling. It not only establishes the value of this factor as an approximate measure of the road-load economy of present-day cars, but also is indicative of a fair degree of uniformity in their efficiency and general design characteristics. It should be mentioned in this connection that, in certain types of operation, a reduction in displacement per car mile without a corresponding reduction in the weight of the vehicle, will result in such poor performance that the average tank mileage may be reduced. The dotted curve in Fig. 1 is a similar average of 15 1933 cars. Although the points are somewhat more scattered, the 11 per cent difference in the average curves is believed to be representative of the improvement made during this period. A typical example of the specific fuel consumption of a modern engine, plotted against load and speed, is shown in Fig. 2. Road test conditions have been approximated by operating the engine with a fan and muffler. The average full-throttle consumption over the speed range from 400 to 4000 r.p.m. is 0.64 lb. per b.hp-hr. which is equivalent to 20.9 per cent brake thermal efficiency based on a lower heating value of gasoline of 19,000 B.t.u. per lb. The brake thermal efficiency is reduced to 19.6 per cent at 3/4 load, 18.8 per cent at half load, 12.5 per cent at 1/4 load and, at 15 per cent load, it is only 8.5 per cent. The maximum efficiency at 1200 r.p.m. ranges from 23 per cent at full load to 12.1 per cent at 15 per cent load. [This paper was presented at the Semi-Annual Meeting of the Society, White Sulphur Springs, West Va., June 15, 1938.] January, 1939 29 Graph: Y-Axis: MILES PER GALLON X-Axis: CU. FT. DISPLACEMENT PER CAR MILE Legend: o -- 1938 CARS (C=3700) x --- 1933 CARS (C=3300) Caption: Fig. 1 - Mean road-load economy of 21 different 1938 cars and 15 1933 cars plotted against displacement per car-mile | ||