HINTS FOR HAULIERS.
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An Analysis of the Possible Errors in Fuel Consumption, Showing that it is Possible, from This Cause Alone, to Lose Money on a Haulage Contract.
DEALING with the 12 possible errors of observation, which I recently enumerated, as being likely to occur in connection with almost any amateur experimenter's determination of the fuel consumption of a commercial motor, I shall now be able to show how the consumption realized may vary within wide limits, although such result in itself may appear to the experimenter to be unassailable in its accuracy.
The following calculations' are naturally quite rough and approximate ; they would be no more satisfactory as a means of arriving at scientifically correct conclusions than the methods of experimenting with which we are actually dealing. They are sufficiently near the truth, however, to demonstrate my meaning, and no attempt has been made to exaggerate in either direction. For the sake of simplifying these calculations, I shall assume that the average speed of this non-stop run was 16 m.p.h. I shall also.take it that the gross weight of the lorry, including load and driver, was 10,000 lb. ; that is to say, slightly less than 4.i tons.
Errors of Observation in Ascertaining Fuel Consumption, •
Fault No. 1 was the negotiation, unknown to the driver of the lorry, of a substantial drop in the 32 mile's. In my last notes I said 400 ft.', but let us take a drop Of, say, 1,000 ft. ; this is equivalent to a gain, on the part of the lorry, of 10,000,000 f t.pounds of work in 2 hrs., which is equivalent to VI h.p. acting throughout the whole period. Now, VI h.p. for 2 hrs. is, in terms of petrol, equal to about 1 of a gallon ; that is to say, had the begin.Ling and end of this journey been on the same level instead of the end being 1,000 ft. below the beginning, it is probable that 2i gallons would have been used instead of only 2, in which case the m.p.g. would have been 13.i instead of 10, which is quite
a substantial difference, due to this one error alone.
The ,second fault referred to road surface conditions.' NoW, the best class of macadam road offers a resistance to traction of about 45 lb. per ton 01 the gross weight of the vehicle ; ordinary macadam has a resistance of 55 lb., while asphalt may present so little as 15 lb. per ton. It will, therefore, be very easy indeed,' hy, choosing a good' road, to reduce the resistance by 10 lb. per ton. In other words, since the journey with which we are concerned took..place over a road the surface of which was good, it is more than likely that in the•ordinary way-the resistance „generally encountered would be greater to the extent of 10 lb. per ton. On this assumption it is easy to calculate that a further of a h.p. would, generally, be necessary to propel the lorry. This is equivalent to another of a gallon of petrol, bringing the consumption up to 2 gallons for the journey, and reducing the m.p.g. to 12.8.
As possible fault No. 3, we assumed that a wind of 12 m.p.h. was following the vehicle, and therefore helping it throughout the journey. Assuming an ordinary high-sided body and• cab, it can easily be calculated that this is approximately equal to helping the lorry to .the extent of .6 h.p., so that, had there been no wind, a further consumption of 1-10th of a gallon would have been experienced, reducing the m.p.g. to 12.3.
The advantage gained by carrying out the experi. i merit on a fine dry day s one which it is diffi— ft to assess ; it depends largely upon the nature of the road surfaces. For example, a little wet on the surface of an asphalt road would hardly affect the consumption at all, whereas in the case of a poor macadam road it would very easily double the road resistance and bring about an increase of a of a gallon or so in the consumption. It cannot be said, then, that I am being ungenerous in assuming that the advantage gained as against average conditions. is equivalent to a reduced road resistance of 5 lb. per ton, equivalent to an extra consumption of a -Atli of a gallon, making the total 2.66 gallons and reducing the mileage to 12 per .gallon.
The Effect of Cutting Out Road Delays.
It is quite impossible to estimate the additional economy of petrol occasioned by the absence of any delay upon the road. That the consumption would have been increased by there being no necessity for oecasional stops, none will deny. In a future article i I propose to explain how it s that these frequent stops and starts have such an important effect upon petrol consumption. Meantime, we will agree that the fact that no stops or delays occur is in favour of the vehicle reducing its consumption and increasing its observed m.p.g.
Equally indeterminate is the amount of fuel saved by extra care of the throttle and ignition lever during the journey. Suppose we take it that, had there been a few stops, and had only ordinary attenticrn been paid to the control, an additional pint of petrol would . have been used. This would have brought the.„tots l consumption for the journey to 2.78 gallons, and would have reduced the mileage to 111 per gallon.
We will assum.e, too, thatithis allowance of one pint suffices to cover the effects of fault No. 7, that of keeping the vehicle travelling at a moderate and even pace throughout the journey, sedulously avoiding any attempt to race. There is plenty of room for error in connection with the speedometer reading alone. The instrument itself, even, may not be giving correct readings. We will, however, assume that it is correct, or that a hub hodometer, which is less likely to err, is being used. The real and calculable mistake will most likely. occur in connection with the tyres. These, even if new, are rarely to size ; they may be either over, or under, according to the practice of the maker. The odds are, however, that they are considerably worn ; an inch of wear is easily possible, and this in a tyre the nominal diameter of which is 870 ram. would mean, supposing that the speedometer was correct with a new tyre, that it would register 6 per cent. over the actual mile, so that, although the distance recorded on the instrument is 32 miles, actually only 30 have been covered. Even without any other error this faulty reading would have reduced the real consumption to 15 miles per gallon instead of the apparent 16 miles. Added to all the other faults,. however, it has the effect of bringing our actual m.p.g. down to 10.8.
Now we come to the question of the actual quantity of petrol in a standard two-gallon can. I know of old that any doubt cast up en the veracity of these popular receptacles will bring such a hornets' nest about my head as will make it sing for weeks ; I've had some ; and besides, it so happens that I have to have frequent dealings with the leading officials of the petrol companies, and they are all men of rather hasty temper. Perhaps the best thing I can do is to say that they might he in error in either direction, although between me and my readers and the yardpost—well !!
Filling the Tank when the Vehicle is on Unlevel Ground.
The most prolific source of error is fault No. 10— the ehicle not being on an even keel when the observations of fuel level in the tank were made. The most absurd results can be reached as a consequence of this error alone. Assuming an extreme case, it is possible to make a difference of four gallons in the amount of petrol presumed to be in the tank. I have assumed such an extraordinary result and illustrated it by the accompanying sketches. An ordinary macadam road has a camber, near the kerb, of 4 ins. in 6 ft. ; the average chassis has a wheel track of approximately 5 ft. 6 ins. ; moreover, a driver, unsuspicious of the error which such action would involve, would naturally draw his vehicle up near the kerb to replenish his tank, and, incidentally, make his observation as to the level of the fuel in that tank while the vehicle was still in that position. Now the petrol tank which I have sketched has a capacity of 15i. gallons. It is a rectangular tank 15 ins. wide, 6 ins, deep, and 4 ft. long. There will be a difference in level as between one end and the other of 21 ins, when the vehicle happens to be !qanding near the kerb. If, at the start, the tank is filled with the vehicle standing at the near side of the road, the filler orifice being then as shown im the left-hand figure of the sketch, it will fill right up before the. level of the petrol approaches the opening. Suppose, however, that this non-thinking driver, not appreciating the significance of his action, refilled his .tank, on completion of his journey, with the vehicle standing near the off-side kerb, a circumstance which could very well be brought about as the natural result of his having to make deliveries at that side of the road, and supposing there, again, he simply poured in petrol until the level was apparently the same as before, the tank would still be nearly four gallons short of being full. Actually, in respect to the particular journey which we have in mind, it would appear that the 32 miles have been covered without any loss of fuel whatever, which, cf course, is absurd-, and would immediately have drawn the driver's attention to the mistake he was making. It should be pointed out, too, that the suggestion made by some, that making the observation by noting the level only when the petrol has risen in the neck of the.orifice would obviate such errors, would be incorrect in this case, as it would be impossible to fill the tank owing to the presence of air in the upper corner. Even however, if we assume quite a small difference in level as between the two ends of the tank. it is clear that considerable errors in the reading will result. Let us take in, as being this difference—this, with ninety-nine drivers out of a hundred, would pass unnoticed, yet it would give rise to a difference one way or another of as much as two-thirds of a. gallon. It would increase the real consumption by that extent and, apart from any other errors, would reduce the actual mileage from 16 to 12 for each gallon. If we add the effect of this mistake on to all the others which we have considered, then our resultant m.p.g. is down to 9.28.
Fault No. 11 would tell in the opposite direction to the others ; that is to say, it would., if it had any significance at all, improve the figure for consumption if it was accurately recorded. The difference, however, would only amount in the worst case to about tooth part of a gallon, so that its effect on the mileage is not, in a case of this kind, worth calculating. ;
Tuning-up for a Test Always Gives a Misleading Result.
As to fault No. 12, it is a fact that a vehicle in quite ordinary everyday condition will easily consume from 10 per cent. to 15 per cent. more petrol than one specially tuned up for a test ; machines in more or less dilapidated condition would consume 25 per cent, more fuel. It. is certainly true, at any rate, that the majority of commercial motor vehicles on the road are in such a state that they regularly get rid of 15 per cent. more petrol than would be needed if they were in good trim. We must, therefere, make a further reduction in Our figure of 9.28 miles per gallon if we are to arrive at the probable consumption of this vehicle when it has been in service for a lengthy period. This would bring our consumption down to 8 miles per gallon' which figure, as I know from correspondence which I am receiving, is very near. what is actually experienced in a good many cases.
I do not pretend that all these error's are likely to occur in any single case. They might do so, and some of them most certainly would. I have shown, however, that one or two of the most likely ones could easily affect results sufficiently to vitiate any -figures obtained as the result of an occasional test.
ma SIKOTC11.