Oil-engine Influence on Costs
Page 24
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The Authors of this interesting Paper Base Oilengine Economy Mainly on Improved Thermal Efficiency, Particularly at Medium Speeds, Lower Fuel Cost and Good Pulling at Low Revolutions
LAST week a paper entitled "Diesel Influence on Road Transport Costs, Past Achievements, Present Position and Future Prospects," was read before the Diesel Engine Users Association by Messrs. P. M. Sanders, M.I.Mech.E M.Inst.T., and E. H. Wenlock. The authors begin by pointing out that if the short history of the development of the high-speed oil engine be any guide to its prospects there is little doubt that, as a means for reducing road-transport costs, it is likely to be a predominant factor.
The economy is mainly due to the lower cost of fuel and the greater mileage per gallon. The brake thermal efficiency is about 32 per cent, compared with 22 per cent, for the petrol unit, and the economy is particularly marked when running at 50-60 per cent. load, at which most vehicle running is accomplished. A further reason is greater pulling capacity at low speed. and it is often possible to ascend gradients on a higher gear than with a petrol vehicle. This also results in an indirect saving due to reduction in engine wear. Fuel is saved by the reduction of the warming-up period, and the fuel does not evaporate like petrol.
Rate Difficulties with Mixed Fleets.
The oil-fuel tax of 8d. per gallon had less effect upon development than was anticipated. It came too late to be more than a discouraging factor, but the increase did have a devastating effect upon one phase of development. This was the practice of converting petrol vehicles to run on heavy oil, but this'is somewhat inexplicable, for by the 1935 Act the Road Fund duties on oilers were reduced to the same as those imposed on petrol vehicles.
Undoubtedly the duty retarded the use of oil engines for light vehicles. Weight had something to do with this matter, because of the importance attached to keeping the tinladen weight of maximum-capacity vehicles down to 21 tons.
In addition to the direct influence of the oil tax on costs, these have been affected by changes in chassis for which the tax is largely responsible. Operators were relieved of differential taxation on weight, but makers realized that more importance than ever was attached to economy in fuel, oil and maintenance.
Results given by the direct-injection engine led to the wider adoption of this unit in preference to that employing pre-chamber combustion.
Comparative Costs in 1935 and To•day.
Graphs are given showing comparative costs in 1935 and dealing with petrol, steam and oil. At that time, considering a 4-ton four-wheeler, if it covered only 10,000 miles per annum the petrol vehicle was cheaper by 1.68d. per mile. Not until the mileage increased to 19,600 did the lower fuel cost cancel out the higher first cost, etc. At 50,000 miles per year, the economy shown was 0.72d. per mile. With a 6-tonner, the oiler was cheaper for all mileages over 10,000, whilst the costs of running on petrol and steam were equalized at about 29,000 miles. On a 15-ton four-wheeler with trailer, at 10,000 miles, steam was cheaper than oil or petrol by 0.042d. and 1.02d. per mile, respectively. At 17,500 miles oil and steam costs were equal.
Further graphs show the economies now given by oil, only those items affected by running on oil being taken-into consideration ; steamers are omitted.
The higher first cost of oilers increases depreciation and interest items, whilst reductions in fuel savings are due to the heavier tax. The costs of lubrication and repairs are now taken to be the same.
B24 So far as items affected by the oil engine are concerned, the operating costs of the smaller types show important reductions, attributable to several causes. Vehicle prices have fallen, the price of petrol has risen slightly, but the fuel casts of petrol vehicles are a fraction, lower, due to a higher operating efficiency and reduction in unladen weight.
An oil-engined 4-tonner covering 10,000 miles costs less to-day than a petrol 4-lonner of two years ago, the actual difference being 0.95d. per mile. Oil and petrol costs equalize at 19,500 miles, as before.
Although the licence duties on the petrol and oil types remain at £50 and £70 respectively for 6-ton four-wheelers, owing to lighter construction the former can now carry up to 8 tons and the latter to 7 tons, according to the weight of the bodywork, whilst the first cost of both four-wheelers and larger machines has been reduced. Comparing a 12-15ton four-wheeler and trailer of to-day and two years ago, although with the oiler greater economy is now obtainable, the difference is not quite so marked as formerly.
Rate Difficulties with Mixed Fleets.
One difficulty in the haulage industry has been that the man with, say, half a dozen petrol vehicles and one oilet, who would have been able to quote lower rates for the latter, would find that his customers did not understand why the goods should be subject to higher charges when they were carried on the petrol machines ; he thus found himself in the position of working the major portion of his fleet at unrentunerative rates or buying oilers to replace the vehicles using petrol.
A rate of 30s. per ton for a 100-mile haul about 1923-24 fell sharply to 25s. from the extensive introduction of the articulated vehicle. Then came the oiler, and the downward-rate tendency continued to a figure. in the neighbourhood of 14s. Although there are instances of rate-cutting to-day, there are, happily, other cases indicating a tendency for rates to harden.
Oiler Progress in Passenger Carrying.
The rapid progress of the oiler in passenger carrying was due to the importance attached to operating costs, and was little affected by comparison with the petrol unit from the point of view of the travelling public. From the first it was recognized that noisy running and an objectionable exhaust were more important defects in buses and coaches, and the success attending the efforts to effect improvements are evidenced by the greater demand for oil-engined vehicles by passenger-transport operators.
Two years ago the 26-32-seater was more economical when running on petrol at an annual mileage under about 24,000, and in the case of the largest bus, about 15,000. To-day a, 20-seater .oiler begins to be economical if the annual mileage exceeds about 16,000, and in the case of the 26-32-seater and the largest buses and coaches when approximately the same annual mileages are covered as formerly, although the economy obtainable with increasing mileage is not so great. It is now considered a creditable achievement to reduce oil consumption -to anything below 0.4 lb. per b.h.p.-hourthere are many that improve on that figure. Further progress in this regard may be expected, whilst enginemaintenance costs will, no doubt, be progressively reduced.
The realization of hither economies in first cost, fuel consumption and maintenance is also dependent upon the development of the high-speed two-stroke engine which, it is widely considered, will eventually supersede the fourstroke unit for road service.