The Case for the Oil Engine

22nd September 1950, Page 101

22nd September 1950

Page 101

Page 101, 22nd September 1950 — The Case for the Oil Engine

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By H. Scott Hall, M.l.Mech.E., M.I.R.T.E.

"The Commercial Motor" Costs Expert

The Problem of Choice Between Oilers and Petrol-engined Machines Exists To-day Only in Relation to the Lighter Vehicles. Reasons for the Selection of the Compressionignition Unit Are Propounded in this Article

GREATER economy in running costs, as compared with petrol-engined vehicles, gave rise to a preference amongst many transport operators for

oil-engined machines. Primarily, this saving lies in fuel costs—the reason for the oiler's initial success. ' Experience also showed economy in maintenance expenses and. concomitantly, a diminution in loss of use because the oilengined machine is less frequently off the road for servicing. On the other hand, the oiler, is initially more expensive than the petrol-engined machine.

The problem of choice between the two types must be tackled by setting the higher initial cost of the oiler against the savings resulting from its fuel economy. Other savings in maintenance expenditure and diminished loss of use may, in the first instance, be left for subsequent consideration.

The greater the payload that a vehicle is designed to carry, the greater the economy to be expected from an oiler, and the higher the price of fuel, the greater the savings become.

Comparative Costs

Take the case of a maximum-load multi-wheeler which would do 6 m.p.g. with a petrol engine or 10f m.p.g. with an oil engine. With petrol at 2s. 9f4. per gallon and oil fuel at 2.s. 6fd., the mileage costs for fuel are 5.544. and 2.94. respectively. This shows a saving of 2.644. per mile in respect of the oiler, equivalent to £402 a year, assuming an annual mileage of 36,000.

Corresponding figures for a 5-6-tonner are 11 m.p.g. on petrol and 19 m.p.g. on oil fuel. The mileage costs are 3.024. and 1.64. respectively, representing a saving of 1.424. per mile, or £213 a year over 36,000 miles. The saving in the case of the heavier vehicle is thus nearly twice that of the lighter machine.

If, however, petrol were only Is, 91d. a gallon and oil fuel Is. 6f4., the respective costs per mile would be 1,93d. for a petrol-engined 5-6-tanner and 0.92d. for a 5-6-ton oiler, The saving is thus only 1.014. per mile.

An obvious point may be mentioned: the greater the annual mileage to be run, the more economic it is to operate an oiler. This accounts for the popularity of the oil engine for public service vehicles, just as the savings per mile account for its popularity among heavy goods vehicle operators.

The problem of choice therefore exists at present only in relation to lighter vehicles up to and including 6-toriners. To-day, partly because of purchase tax, an oil-engined vehicle costs some £410 more than a petrol-engined machine. I have already shown that at current prices, the oiler saves 1.42d. per mile in fuel costs. Dividing 98,4001 (£410) by 1.42d„ 69,300 is shown to be the mileage which must be run to wipe out the difference in price. If, therefore, a vehicle covers 35,000 miles a year, the extra cost is amortized in under two years; and running 24,000 miles a year, it is written off in two years 10 months, To consider the oiler's economy in greater detail, the 10 items of vehicle-operating costs have to be taken into account: Licence, wages, garage rent, insurance and interest (the standing charges); and fuel, lubricants, tyres, maintenance and depreciation ((he running costs). The annual mileage must also be considered.

Typical prices of a 5-6-tonncr are £1,145 for an oilengined chassis and £735. for a petrol-enginecl version. If I allow £80 for a simple type of body, painted and lettered, the amounts become £1,225 and £815 respectively.

A set of tyres costs £140 and deducting that sum from the initial costs of the two vehicles leaves £1,085 and £675 respectively. Allowing a residual value of 10 per cent., I am left with £975 and £608 as the bases for calculation of depreciation.

Reckoning upon a life of 200,000 miles, I obtain 1.17d. and 0.73d. per mile as respective figures for depreciation. These figures give an advantage to the petrol machine. The life of an oiler is, however, usually longer than that of a petrol-engined vehicle, and an oiler's residual value is greater in proportion to its initial cost than a petrol vehicle's.

Another item of operating cost which is directly affected by the price differential is that of interest on capital outlay. In the case of the oiler, the interest on £1,225 at 3 per cent. is £36 15s. per annum, and for the petrol-engined lorry it is £24 9s.

It is now possible to set out in detail the figures for operating costs of the two types of vehicle. The annual standing charges of the oiler comprise the following: Tax, £35; wages, including provision for employees' insurance and holidays with pay, £312; rent and rates, £26; insurance, £25; interest, £36 15s.; total, £434 15s. Running costs. per mile are: Fuel at 2s. 61d. per gallon and running 19 m.p.g, I.6d.; lubricants, 0.18d.; tyres (£140 per set, with a life of 24,000 miles per set), 1.44.; maintenance, 1.654.; depreciation, 1.17d.; total, 64.

The petrol-engined vehicle bears the following standing charges per annum: fax, £35; wages, etc., £312; rent and rates, £26; insurance, £25; interest, £24 9s.; total, £422 9s. Running costs in pence per mile are: Fuel, at 2s. 91d, per gallon and 11 m.p.g., 3,024.; lubricants, 0.18d.; tyres, as above, 1.4€1.; maintenance, 1.84.; depreciation, 0.734.; total, 7.13d.

It should be noted that the oiler costs £12 6s. per annum more in standing charges, but 1.13d. less per mile in running costs.

Running 12,000 miles a year, the cost of an oiler comprises standing charges of £434, plus 12,000 times 64. '(the running cost per mile), which is £300, giving a total of £734.

The cost of running a petrol machine is made up of £422 for standing charges, plus 12,000 times 7.13d. (£357) for running costs, totalling £779. Thus an oiler saves £45 per annum. For an annual mileage of 24,000 the figures are £1,034 for the oiler and £1,135 for the petrol-engined vehicle, representing a saving of £101 a year. For an annual mileage of 48,000 the oiler costs £1,634 and the other vehicle £1,848, showing a saving of £214.

Prospective Economy An interesting new development is the fitting of oil engines to 3-ton vehicles. Inquiring into the prospective economy of 3-ton oilers, I will take the savings in fuel costs. Reasonable consumption figures are 15 M.p.g. for a petrcil vehicle and 25 m.p.g. for an oiler, equivalent to 2.24. and 1.22d, per mite respectively, a saving of ld. a mile. I will take a new 3-ton oiler as costing £920 and a similar petrol machine at £620. Operating costs, assessed in the same way as for the 5-6-tonners, will approximate to the following:—

For the oiler: Standing charges per annum—tax, £30; wages, £290; garage rent, £20; insurance, £21; interest on capital outlay, £29; total, £390. Running casts per mile— fuel, 1.22d.; lubricants, 0.14d.; tyres, 0.754.; maintenance, I.174.; depreciation, 0.924.; total, 4.2d.

For the petrol vehicle: Standing charges----tax, £30; wages, £290; ,garage rent, £20; insurance, £21; interest, £22; total £383. Running costs per mile—fuel, 2.2121.; lubricants, 0.14d.; tyres, 0.754.; maintenance, 1.37d.; depreciation, 0.594; total, 5.07d.

The total costs per annum for each type and the savings from the use of the oil-engined vehicle are likely to be as follows: For 12,000 miles per annum with the petrolengined vehicle, 1637, and with the oil-engined vehicle, £600, a saving of £37; at 24,000 miles per annum the petrol machine will cost £890, and the oiler £810, an economy of £80; for 48,000 miles per annum the petrol machine £1.397, and the oiler £1,230, a saving of £167.