Oil Engine Influence on Road, Rail and Air Transport
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I N his paper, " The Oil Engine and Its Influence on Road, Rail and Air Transport," the author introduced the subject by stating that it is such a vast one that, in the time permitted, he might not be able to do more than touch its fringes, but he hoped that it would promote a discussion which would bring out many points of value and interest.
After a brief comparison of the respective principles of petrol and oil engines, he pointed out that the burning of oil fuel is in three stages, the process occupying probably
less than 1-500 of a second. The main problem is to arrange for intimate contact between the injected fuel and the air, to carry out the combustion from a liquid state in much the same time as that permitted for the mixture in a petrol engine, and to arrange these functions to proceed at the proper rate at all loads and speeds.
The Future of the Two-stroke Unit.
Many existing engines operate on the four-stroke cycle, but increasing attention is being given to the two-stroke. This type, however, requires some means for pre-compressing the air. The maximum pressure in a normal petrol engine is 400 lb. per sq. in., but the oil unit develops between 700 and 1,100 lb. This is the main reason for the additional strength of construction necessary. The higher pressures are also of longer duration, thus throwing more stresses upon the bearings, but by careful design, the weight of a commercial oil engine can be brought to within a little of that of the petrol unit.
There are difficulties in the detail design in respect of the combustion chamber and fuel injection, but the oil unit is actually much simpler than the petrol, and there are fewer auxiliaries to cause trouble, no fouling of sparking plugs, freezing of carburetters or shorting of electrical equipment; in fact, the only trouble—and that infrequent= is choking of individual -injector nozzles. The all-round efficiency is about 39 per cent. and that of a petrol engine from 10 to 26 per cent., according to the throttle opening.
In this country, last year, the petrol consumption was 1,320,000,000 gallons, whilst the oil-fuel consumption for road vehicles alone MSC from 150,000 gallons in 1930 to 38,000,000 gallons in 1935. For practically all this we are dependent upon overseas supplies, and economy, particularly in tithes of national emergency, is vitally necessary, whilst it is impossible to estimate how long the liquid-fuel supplies of the world will last. Thus any power unit which can use these supplies to the best advantage should receive favourable consideration.
In hot countries petrol loses by evaporation about one per cent. per week, whereas oil is unaffected. Oil depots and dumps are less vulnerable, and the fuel can be stowed in the double bottoms of ships, whilst there is no risk of lire at sea.
Successful experiments are being carried out with superchargers giving an increased output of 50 per cent., and a decrease of 5 to 8 per cent, in the consumption per b.h.p.
The oil engine has a big interest for Temple Press Ltd., the publisher of The Commercial Motor, for as early as 1903 a Diesel engine coupled to a 530-volt 140-amp. dynamo was installed and is still doing good work.
The Oil Engine on the Road.
Due credit must be given to Germany for pioneering the employment of the high-speed oil engine for road-transport vehicles. It was the arrival in this country, in 1928, of a Mercedes-Benz oiler that began the movement in Britain.
In Germany, the total number of lorries in 1935 was
about 220,000, of which 15,000 had oil engines. Of tractors, practically all the most powerful use oil, and of a total of 6,120, 3,270 are so equipped. New registrations show more striking results. In 1935, of 32,000 of the
goods type, 9,000 had oil engines, and of 1,830 new buses, 1,000 were oilers; but in Germany the tax on gas oil is about half that on petrol, the authorities being more fully alive to the value of the unit than are ours.
In France, practically all long-distance road transport for goods is by oilers, and their use continues to develop.
In England, in 1935, -the figures for oil-engined vehicles were 5,541 passenger type and 5,429 goods, a total of nearly 11,000, and the number is rising rapidly.
Although the oil engine is now more flexible than formerly, it is better to run it normally at speeds at which it will give its best characteristics, averaging, say, between 1,200 and 1,400 r.p.m. Recent tests with a British vehicle have shown a great saving when utilizing six speeds. It seems an anomaly that the commercial vehicle should use the same top-gear ratio when running fully laden or light, and there appears to be a strong case for the overdrive. The provision of closer steps giving reasonably constant engine speed is also desirable. Although the oil engine may give good torque at lower revolutions, it pays for this in extra stressing.
Considerable space in the paper was devoted to the use of the oil engine for rail vehicles and locomotives, but this phase is somewhat outside the scope of this journal.
It may be mentioned, however, that the author stated that, compared with the steam train, the oil engine cuts out an enormous bulk and weight of water and solid fuel, the rolling stock can be lighter, when the oil engine is at rest there is no fuel cost, steam does not have to be raised, and no stoking is required; the weekly boiler washout is abolished, and refuelling is, in many instances,
carried out only weekly. A good oil engine can ;run 150,000 miles between major overhauls—equal to twice the distance achieved by a steam locomotive.
The Oil Engine in the Air.
It was considered that the field of use for the oil engine in the air presents almost infinite potentialities, but the future may depend upon the attitude of the public.
Dealing with the fire risk, which is perhaps the most important point, oil fuel will frizzle and may catch fire if it falls on a red-hot pipe, but with the oil engine the risk is almost negligible, because of the low exhaust temperature and the elimination of exhaust flame. The penetration of an oil tank, even by a tracer bullet, would be unlikely to cause a conflagration, whilst the lessening of the carbon monoxide gas prevents slow poisoning.
Ignition systems are particularly liable to develop faults, and in a recent case in which an aeroplane was struck during an electric storm the insulation was almost burnt from the wires. Some nations claim to possess the secret of waves which will upset electrical ignition. In the oil engine these vulnerable auxiliaries are replaced by the fuel pump and injectors. The cooler running of the oil engine means less weight in the fins (if air-cooled), water and radiators. Under full load the oil engine is about 80 per cent. more economical than the petrol, the percentage increasing proportionately at lower loading, and as most aircraft cruise at 75 per cent. power the economy should be nearly 100 per cent.
Special petrols with octane numbers of from 88 to 100 are being developed, but their cost, which may be as high as 5s. per gallon, at present precludes them from competition on a commercial basis. Between 20 and 25 per cent, of the German fleet of air liners, considered numerically, is equipped with engines of Junkers make, and oil engines will be,employed exclusively for the North Atlantic service.
In high tying, the oil engine has already shown that it loses less power than a petrol .unit, and supercharging Will undoubtedly do still more to increase this superiority: