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DOES THE OIL ENGINE Sc E FOR 30 m.p.h. VEHICLES?

28th July 1939, Page 32
28th July 1939
Page 32
Page 33
Page 32, 28th July 1939 — DOES THE OIL ENGINE Sc E FOR 30 m.p.h. VEHICLES?
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Some Comparative Figures for Opera/it Costs, Which Show that the Oil Engh Might, With Advantage, Be Used for

Number of 30 m.p.h. Chassis

By S.T.R.

IHAVE often been charged with being antagonistic towards the oil engine, especially in respect of its use on vehicles coming within the 30 m.p.h. category. The figures published in The Commercial Motor Tables of Operating Costs give the lie to that charge, as is shown by the following typical example.

In Tables II and III of the current issue of The Commercial Motor Tables of Operating Costs are figures relating to petrol and oil-engined chassis for loads of 3 tons to 8 tons, and, of those, vehicles carrying up to 5 tons are assumed to come within the 30 m.p.h. class. If the figures for cost of operation of 5-ton petrol and oil-engined vehicles be compared, the oil-engined type shows to advantage for all mileages from 200 to 1,000 per week.

At 200 miles per week the figures are 11.5.3d. per mile for petrol and 11.53d. for oil—a saving of 0.03d. per mile, or 6d. per week. At 1,000 miles per week, the corresponding figures are 5.29d. for petrol and 4.98d. for oil—a saving of 0.33d. per mile, or 27s. Od. per week, in favour of the oil-engined vehicle.

It is true that when a small operator asks my advice I usually recommend him to go slow if he be thinking of changing from petrol to oil. In particular, I warn him that he may find his maintenance costs likely to offset *30 his savings in fuel cost, especially if his mileage he low_ That, I think, is what is disliked by the protagonists of the oil engine.

There arc two reasons for my attitude, as thus disclosed—a fundamental one and a practical one.

The fundamental one is that I do not consider it the business of small operators to pioneer new developments. That is the concern of big operators, and it has to be admitted that the responsibility involved is one which they have always shown themselves ready to shoulder.

Maintenance Cost Considerations.

The other reason, the practical one, is that maintenance costs are apt to be higher, in the case of the oiler, when the vehicle falls into inexperienced hands and into the ownership of operators without facilities for routine maintenance and without the opportunity to carry out the work and the experience of how to do it. That is a general rule, to which, of course, plenty of exceptions can be found.

It is, I think, widely appreciated now that, in comparing the merits of petrol and oil-engined chassis, consideration has to be given to the following matters :— First, on the credit side, there is the saving in expenditure on fuel. Secondly, on the debit side, there is the extra expenditure arising out of the heavier initial outlay and (a controversial matter into which I am not at present going to enter) the extra cost of maintenance, if and when there be any such extra cost.

For weekly mileages in excess of 300, the fuel economy may be taken as approximately half the average expenditure on petrol. That is not an estimate which can be universally applied. It is, for example, well known that the direct-injection type of engine is more economical on fuel than the ante-chamber type.

The latter, on the other hand, has advantages as compared with the former, which, in many conditions of use, make the slight extra fuel cost worth while. That, incidentally, is typical of the difficulties which arise in comparisons such as this. There are intangible assets which cannot be assessed in terms of Z. s. d.

Now, in the case of fair weekly mileages, the saving of approximately half the petrol cost has to be set against those items of operating cost which art greater in the case of the oil-engined vehicle than in that of the petrol-engined type. The items are : Depreciation, interest on first cost (both arising out of the greater capital outlay), and any increase in maintenance cost.

If depreciation be reckoned, as it should be, and as it always has been in The Commercial Motor Tables of Operating Costs, on a mileage basis, the comparison is simplified. Taking again the example of a 5-h:inner, the fuel costs, according to the Tables, are : Petrol, 1.70d. per mile; fuel oil, 0.83d. The saving is 0.87d. per mile.

Depreciation, assuming a £200 difference in initial cost and a vehiclelife of 110,000 miles, is 0.44d., and the saving, thus far, is 0.43d. per mile. If an allowance of 0.07d. per mile be made on account of extra cost of maintenance, the net saving is reduced to 0.36d. per mile. There is still the item of 3s. per week interest amongst the standing charges, which reduces the net saving still further to the amount named earlier.

A weakness in this method of reckoning depreciation on a mileage basis is, as I have so often explained, that the provision for renewals, which is, after all, the practical interpretation of " depreciation," is insufficient when the mileage is low; the vehicle does not wear out before it becomes due for replacement on account of obsolescence.

It will be noted that provision is

made ior that circumstance in the current issue of The Commercial Motor tables of Operating Costs. This actually involves an increase in the allowance per mile for depreciation, and it applies, of course, in respect of the more-expensive oilengined vehicle, as well as with the lower-priced petrol chassis.

With the former, however, the extra fuel .cost at low speeds is sufficient to offset the increased allowance for depreciation. It is that fact that accounts for the continued economy of the oil-engined vehicle, in comparison with the petrolengined machine, all down the scale of annual mileages.

The Perkins Perpetuity Scheme.

An alternative method of assessing the comparative costs of operation of the two types of vehicle is that for which F. Perkins, Ltd., is responsible. It is called the Perkins Perpetuity Plan, and is, in reality, a special application of a method of replacing the engine as and when such replacement becomes desirable.

The replacement is made at a predetermined charge and can be effected an number of times. At each replacement the engine is guaranteed.

Assuming that the engine originally cost £250, or, alternatively, that the difference between the price of a petrol-engined chassis and one equipped with the Perkins P6 engine be £250, replacement is made at a charge of E62 10s. for the first and second time, and £72 10s., on subsequent occasions.

The time when replacement becomes due is a matter for the owner of the vehicle to decide. It is variously estimated as being from 60,000 to 200,000 miles.

Actually, 80,000 miles is probably a figure at which it is advisable for this work to be done, except in unusual circumstances, when the engine has been carefully used, and that period may be lengthened. I advocate 80,000 miles myself.

On the basis of replacement at 80,000 miles, when the vehicle has done 400,000 miles, the engine will have cost the owner £520, made up as follows : First cost, £250; two replacements at 262 10s. each and two replacements at 272 10s. each. That is equivalent to 0.312d. per mile, covering depreciation and a considerable proportion of maintenance cost. That shows a considerable saving on the figures quoted.

The application of the terra "Perpetuity" is as wide as can reasonably be expected, since the foregoing conditions as to replacement apply, although the engine be transferred from chassis to chassis and from owner to owner. When a new chassis is bought, the operator is saved the high initial cost of the oil engine. He buys a chassis without an engine and has the unit transferred at a nominal cost.

There is no delay involved in engine overhaul, all that is necessary being the replacement of the unit, which can be done in a few hours.

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