1 0 F. = £16,000
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Effects of Temperature and Other Causes on Fuel Consumption of London Buses
F, this year, the mean atmospheric temperature should be I° F. higher -tan it was last year, London Transort's fuel costs would be reduced by bout E16,000. On the other hand, a ecrease from 0.848 to 0.846 in the pecific gravity of the fuel would cost ver £15,000.
Mr. A. T. Wilford. B.Sc., director of :search, London Transport Executive, ave these facts, on Tuesday, in a paper n the analysis and interpretation of :rvice records. He was addressing the aitomobile Division of the Institute f Mechanical Engineers, in London. In 1953, increased traffic congestion, s compared with 1952, was estimated ) have raised fuel consumption at the ate of 0.10 gallon per 100 miles of le Central London bus fleet nd cost f48,000. The country
us fleet, more particularly ireen Line coaches, was also dversely affected through a imilar cause. Whilst there /as a direct relationship between tmospheric temptrature and fuel conumption, there were, said Mr. Wilford, ther factors affecting fuel consumption rhich were also related to temperature. During the summer of 1950, the :asonal improvement in fleet fuel conimption failed to materialize, It was pparent from information provided by ie Tilling Group that at a popular :aside resort on the south coast, the id l consumption of the local fleet f buses remained roughly constant troughout the year, there being no tarked seasonal trend,, as was usually )und in London Transport's fleet.
• Effect of De-rationing
The explanation of the first of these pparent anomalies was that the early immer of 1950 saw the end of fuel ttioning. There was an immediate 'crease in the number of vehicles on te streets in London, and as a conseuence of the greater traffic congestion, lel consumption of the fleet of red uses increased by about 2 per cent., ins swamping the expected seasonal aprovemene In the case of the seaside resort, issenger loadings were much heavier summer than in winter, and the icrease in fuel consumption arising :om the greater weight which had to : carried was about equal to the :duction in consumption arising from :duced friction losses.
It had been shown that fuelmsumption figures varied considerably ith similar types of vehicle operated ader precisely similar conditions. welve new buses of the same make id type were operated on the 'same mite for five weeks and the fuel contraption in gallons per 100 miles tried from 12.2 to 13.4.
It might be suggested, said Mr. Pittard, that these discrepancies fleeted differences between drivers, it London Transport drivers were not lotted to a particular bus. The ,crage vehicle, he said, was handled / four drivers each day, and over five :eks any effect caused by differences
in driving technique would balance out.
It could be concluded that these differences in fuel consumption, which varied between 8.2 m.p.g. and 7.5 m.p.g., resulted from inherent differences between the vehicles. The variations of both individual vehicle and fleet fuel-consumption figures had called attention to some of the factors by which the figures were influenced.
These were; (a) Type of vehicle, including differences in weight of otherwise identical units; (b) inherent differences between vehicles of the same type; (c) mechanical condition of vehicle and accessories; (d) weather conditions, in particular atmospheric temperature; (e) passenger loading; (f) speed; (g) number of stops per mile; (h) engine idling speed and the time spent idling; (i) traffic congestion, which affected f. g and h; (j) daily mileage; (k) driving practices; (I) fuel characteristics; (m) lubricating-oil characteristics.
Fuel consumption was markedly affected by changes in engine-oil viscosity. Mr. Wilford was. however, at present unable to say whether any significant effect was obtained from changes in the viscosity of rear-axle lubricants.
Practical Application
Dealing with the practical use which was made of information derived from fuel-consumption studies, he said that it was naturally a matter of concern if the fleet fuel consumption for a particular year was worse than that obtained in the previous year.
Fuel consumption of London's fleet of red buses during 1953 was at the rate of 9.47 m.p.g., compared with 9.60 m.p.g. in 1952. [The relevant factors affecting these figures and their estimated effect are given in the accompanying table.) Analysis of the data showed that the slight reduction in specific gravity of the fuel, the growth in traffic density and the replacement of pre-1940 vehicles by a more robust, although somewhat heavier, type had together led to an increase in fuel consumption of 0.19 gallon per 100 miles. Offsetting this were reductions in consumption arising from a higher mean temperature and extended use of S.A.E. lOW engine oil, together amounting to 0.08 gallon per 100 miles.
The net increase in consumption of 0.11 gallon per 100 miles left 0.03 to be accounted for. This residue was attributed to "other unexplained causes," although attention was called to the fact that during 1953 there had been a small increase in the mean schedule speed.
Despite the adverse affect on fuel consumption of higher speed, an increase in schedule speed led to a reduction in total costs, as it permitted a more efficient use of vehicles and staff. The adjustment factors for both atmospheric temperature and traffic density were derived from a statistical analysis which indicated that fuel consumption was reduced by 0.035 gallon per 100 miles for 1° F. rise in temperature and rose by 0.0031 gallon per 100 miles for each 100 vehicles per week increase in flow. as recorded by the Road Research Laboratory's traffic counter installed in Grosvenor Road, London, S.W.1.
Result of Traffic Congestion
It had been estimated that since 1948, the fuel consumption of the fleet of red limes had increased by 4-5 per cent. as a result of greater traffic congestion.
One of the measures of the efficiency of a fleet is the number of miles run per involuntary stop, this being defined by London Transport as any delay exceeding five minutes. The figure is obtained by dividing the total mileage operated during a period of, say, a week, by the total number of failures and delays experienced during that period.
At the present time a London Transport bus runs, on an average, 58,000 miles before encountering an involuntary stop attributable to engineering causes. Failures due to specific causes, said Mr. Wilford, were unlikely to vary widely from week to week.
Failures from all causes would, however, exhibit much less variation, it being very unlikely that the " ups" or " downs " would coincide within a par ticular week. " Miles-per-failure figures would not, therefore, exhibit any marked short-term variations, although long-term trends would become apparent.
The figure was thus appropriate for review by the management rather than the engineer responsible for day-to-day maintenance of the vehicles. There were over 70 engineering causes which might lead to a failure or delay.