OPINIONS FROM OTHERS.
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The Efficiency of the Tractor-lorry.
The Editor, THE COMMERCIAL MOTOR.
121911 Sir,—We agree with your editorial remarks on the subject of basing haulage rates on fuel consumption figures which cannot be attained in regular tunmng.
As makers of a vehicle which has been conclusively proved to give, in the hands of ordinary users, much better ton-mile consumption figures than ordinary four-wheeled lorries, we have always been most careful not to exaggerate the undoubted advantages we can give.
We have never stated that such a figure as 110 tonmiles per gallon can be had-in ordinary work, nor are we aware that any of our customers have ever claimed results so good. as this. At the same time, we agree with you that preposterous claims•of fuel economy are often put forward, both.by users and, we are sorry to note, by manufacturers, who ought to know better.
Having agreed with you up to this point, we wish, however, to dispute the figures you give and the implication you appear to convey—that the tractorlorry, and, in particular, the 12-ton machine, is not definitely more economical than ordinary lorries operated under the same conditions.
As a special performance,-, over roads with a hard surface, with no traffic delays and no bad gradients, 110 ton-miles per gallon can be obtained on our machines, and we might mention that on a series of tests on the Watford-Aylesbury route we have got figures of 103, 105, and 108 ton-miles per gallon on
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thre.,_ runs of 50 miles with different sprocket ratios. Considering that all estimates of overall thermal efficiency mast be based on an assumed rolling resistance figure, we are surpised at your dogmatic statement that the overall thermal efficiency of a wheeler doing 110 ton-miles per gallon is "-over 21 per cent." In view of the great Lumber of variablesaffeeting the!rolling resistance of the vehicle, your statement seems rash, to say the least of
You give no figure for rolling resistance, but my experience of the Scammell six-wheeled tractor-lorry goes to show that, on a hard road at about 15 miles an hour, a Lilly loaded machine has a resistance of between 35 and 40 lb. per ton. This corresponds to thermal efficiencies (neglecting transmission losses) of 13 to 20 pericent.
As the thermal efficiency of the latest Scammell • engine, when "tuned up," is about 23 per cent., it will be seen-that there is a margin for transmission losses, which are, in any case, exceedingly small on the Scammell, owing to the use of bevel and chain drive, practically all the running in the tests referred to being done on the top gear, which is direct. Such results are, ofcourse, unobtainable in normal work, with its frequent stops, traffic delays, and energy losses,. due to braking on steep hills, but over 90 ton-miles is being obtained by users of our latest machines, and our experience is that it is easier to get 90 ton-miles per gallon on a 12-ton Scammell than to get 70 ton-miles on the ordinary 5-ton lorry.—Yours faithfully,
SCAMMELI. LORRIES, LTD.,
O. D. Nonni, Technical Manager.