REQUIREMENTS OF THE C ERCIAL PETROL ENGINE.
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How Higher Speeds and Greater Pow Needs of Modern Conditions of Service and Commercial Power
e Been Rendered Possible to Meet the Difference Between the Work of Private Engines of the Future. 74XTRAORDINARY progress has been made during ' litte past two or three years in the development of comparatively high-speed buses and coaches, and even lorries have shared in this upward trend. Not so very long ago 20 m.p.h. was considered quite sufficient for the majority of large, passenger vehicles and, in many instances, even that speed was not encouraged by the manufacturers. Now, a maker will state quite calmly that a particular standard vehicle will reach 40 m.p.h., and even higher speeds, with its full load, and put up an average for long distances which would shame many private cars.
How has this improvement, which would have been thought well-nigh impossible of achievement until recently, been obtained, and wherein lies the chief difference between the old and new models?
The employment of pneumatic tyres has, of course, exercised a considerable influence—perhaps the greatest. Lowering the centre of gravity has also helped to a considerable degree, but neither of these causes would have been sufficient without marked modiCcations in that primary source of speed, the engine.
The modern commercial-vehicle power unit is not only different from the earlier types but it can be compared with many private-car engines, with great disadvantage to them, for on the score of working life and freedom from trouble it now wins the premier position. Very few engines on private cars will run more than, say, 20,000 miles without requiring atten
tion to the bearings, yet the majority of commercialvehicle engines are often confidently expected to run 50,000 miles or more between their periods of overhaul, rand, in fact, we know of cases where twice this distance has been covered without the engine having to be dismantled or otherwise attended to, with the exception of decarbonizing and valve grinding.
To no one point in design can these recent improvements be traced. It is a combination of many details and it must be remembered that the commercial vehicle Is practically alwaysrun on cheaper and, therefore, presumably inferior grades of fuel, and this renders even more difficult the problem of obtaining high power from a unit of moderate size.
• Perhaps the most important development has been in the lightening of the reciprocating parts. It used to be considered essential to build heavily with the idea that only weight could cope with heavy duty, and speed had to be sacrificed to strength and long life. However, experience has proved that this is not necessarily the case. By the use of suitable materials, sound designs and really adequate lubrication, power units have been considerably reduced in weight, whilst generating far greater power and still retaining the ability to run for very long periods with a minimum of wear.
It is interesting to compaae the performance of the c32 engine on the commercial vehicle vith that on the average private car. In many commercial vehicles the engines are very little larger in volumetric capacity than those on private cars, and yet the loads carried are many times heavier and the speed is by no means disproportionate.
Many people find it difficult to understand why this should be the case, but it may be summed up by saying that the commercial-vehicle engine is practically all out all the time, whereas in the private car the engine is hardly ever called upon to develop its full power, and then only for very short periods.
Some actual figures giving the average sizes of power units on commercial vehicles of different load-carrying capacities may be of interest. The small lorry, carrying from 20-25 cwt., has an engine with an average R.A.C. rating of 17 h.p.; the vehicle carrying from 1i tons to 2 tons has an engine of 21.7 h.p.; from 21, tons to 3 tons, 28.4 h.p.; from 34, tons to 4 tons, 82 h.p.; 5 tons and over, 38.2 h.p. In the case of . passenger vehicles, such as buses and coaches, the engines are somewhat larger ; for instance, the 15-20seater has an engine of 27.5 h.p.; the 21-25-seater, 29 h.p.; the 33-36-seater, 34.3 h.p.; the 54-seater, 38.8 h.p.; and the largest 68-seater six-wheeled bus, 48.6 h.p.
An Increasing number of engines is being equipped (with light-alloy pistons and, in some cases, Duralumin connecting rods. Certain engines now have double oil pumps, one pump supplying the main and big-end bear ings and the other such parts as the camshaft and timing gear. The reason for this development is that where one pump is used to lubricate all parts the oil leads have to be comparatively small, otherwise the pressure is lost. By the new arrangement the leads to the really important bearings can be greatly increased in size without robbing the remaining parts of their lubricant.
Higher engine speeds in themselves have rendered passible higher vehicle speeds, for by improved design the power curve of the engine can be kept practically straight to speeds of 2,000 r.p.m. and more. With the old engines the curve used to drop very quickly as the speed rose.
Great attention is now being paid to the designing of the combustion space so that this provides the greatest possible degree of turbulence, and many engines have what is known as the Ricardo head.
The valves and valve ports have also receivedconsiderabIe attention, with a view to the prevention of wire-drawing at high speeds.
The main requirements in the commercial vehicle engine are powerful, steady pulling at a comparatively low number of r.p.m. and a capacity for providing rapid acceleration (both of which features render unnecessary undue recourse to the change-speed gears), economy in consumption of both fuel and Oil, the ability to run for very long periods without breaiidown, ease of maintenance and accessibility.
The last-named point is not always a quality dependent upon the engine itself. A unit of perfectly good design may not be get-at-able owing to the design of the rest of the chassis and/or the body. For instance, in some vehicles it is impossible to remove or replace the engine as a unit without first taking off the flywheel. This is only one example of many which could be quoted.
There is another important essential in a commercial engine and that is ease of starting. Many engines are far too difficult in this respect. It should not be neces sary to employ three or more men or a special, and usually expensive, appliance for the purpose. Something might be done in the way of providing decompressors, but reliance must not be placed upon the ordinary electric starter, as this is almost useless with a large engine when starting from cold. Even if the power be available, the drain on the battery in such circumstances is excessive. A cheap fuel is often the cause of difficult starting, and it might pay to provide a small auxiliary tank of really good quality fuel which could be used for the first filling of the carburetter ; the amount used for this purpose would have a negligible influence upon the running costs.
What are we to look for as the engine for the commercial vehicle of the future? There are all sorts of possibilities. Perhaps the most extreme is the internalcombustion turbine, but we think it will be a long time before an engine of this type will become a commercial
possibility. We are somewhat inclined to base our hopes upon engines designed on the Diesel principle, for with them it is not only possible to employ much lower grades of fuel and thus economize in a commodity the supply of which at its present rate may not continue for so long as many think, but they will dispense with certain engine auxiliaries, such as the magneto or coil ignition and the carburetter, which are sources of possible derangement. In the Diesel the only important ) Ruxiliarieg are the oil pump and fuel distributor.
It is quite possible that alcohol or compound fuels composed chiefly of this will become popular in the future, and this will necessitate certain engine modifications. Suction gas is already gaining in favour and will undoubtedly be an invaluable fuel in those countries where liquid fuels are unobtainable except at excessively high prices. Gas producers can work on coal, coke, wood-charcoal and even wood itself. The chief disadvantage is that a supply of water is essential to form the gas, but the quantity used in this manner is not excessive; a small amount of liquid fuel is also required at starting. Engines designed to run on suction gas must have a much higher compression than that normally employed, otherwise there is a tconsiderable fall in the power output. With higher compression there is an increased difficulty in startling unless decompressors are employed, but these are not difficult to arrange.
We are inclined to the belief that the engines of the future will grow smaller and smaller, with a proportionate increase in the power developed per unit of volumetric capacity. It may well be that in order to take up less room in the chassis such engines will not have their cylinders in line as at present, and it is quite poSsible that we shall see a return to the original scheme „of carrying the engine with its crankshaft across the chassis instead of longitudinally.