BENEFITS OF OPPOSED-CYLINDER ENGINES.
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Consideration, it is Argued by the Author of this Article, Should be Given to the Design of a Type of Power Unit that is Suitable for Commercial Vehicle Chassis.
By Bertram Joy, 1$1.1.A.E., A.111.1.Mech.E.
THE power unit of the commercial vehicle has practically always occupied a position in the front part of the chassis, and it has consisted of an engine with cylinders disposed vertically over the crankshaft. The commercial vehicle was, to a large extent, evolved from lighter and earlier types of road vehicle, which accounts, perhaps, to some extent for the above-mentioned facts. Now, If we admit—and we must—that finality of design has not yet been attained, we can regard the possibility of other engine types with some interest—especially if it can be shown that there are not inconsiderable advantages to be recorded by varying the design of the machine that is responsible for furnishing the propulsive force.
If we look back to the historical aspect of another form of transport—the railway—it is curious to note that in this case the cylinders of locomotives were first placed vertically (inverted), they were then rotated to a diagonal p o s Ition and finally became horizontal, or nearly so. In the lastnamed position they remain to this day, and are likely so to remain, so long as the steam locomotive is a familiar object to be seen on our railways.
It is remarkable that t h e evolution of cylinder inclination occupied, in the case of the railway engine, , something like 30 years—just the period of time that the commercial road vehicle has been in existence. History, proverbially, repeats itself, so that may not the road vehicle pass through a somewhat similar form of mutation?
Whether change takes place quickly or with slow and sure steps, indubitably there will be change, and, therefore, the possibility of future engine development and variation better to suit working and other conditions is, perhaps, worth a few moments' consideration.
Now, regarding the motor lorry for the moment purely from the point of view of a load-carrying appliance, that machine would be most generally serviceable which combined the greatest area of platform with the shortest wheelbase. Large platform area provides spacious load-carrying capacity and a short wheelbase provides good manceuvring qualities. Naturally both of these qualities display themselves to the greatest advantage in the streets of towns—where practically all vehicles commence and conclude their working day. Thus it appears that the ideal vehicle would embody a sided rectangular platform extending, probably, forward of the front wheels and well behind the rear and driving wheels. With the exception of the driver's seat, canopy and controls, no part would reach a higher level than that of the platform. Indeed, the vehicle would present an appearance not unlike that of the average trailerpwhich, of course, is hauled by a tractor or tractor-vehicle.
In order to effect this desirable state of affairs, it is clear that, whilst modern transmission principles might well be retained, something drastic would have to be done to the engine.
So long as certain cylinder dimensions are necessary to produce some definite power output, the height of the engine from crankshaft centre to cylinder top cannot be diminished to any material extent.
This natur ally suggests some modification of engine design, such as the placing of the cylinders on a horizontal plane and opposing them for the sake of balance and symmetry. Without going closely into matters of design, it is a little difficult to say what the overall length of a suitable engine— from cy linder head to cylinder head—would be ; but, probably, assuming four cylinders of average stroke and bore, across-chassis dimensions would not 5e prohibitive. The height of such an engine—from crankshaft centre to highest point—probably need not exceed, say, 10 ins., so that the body could extend right over the engine and thus could be made, perhaps, 30 per cent. more in length than is now the case—a distinct advantage in the case of some types of load.
As regards accessibility, the more important com ponents—valves, ignition plugs, etc.—would be disposed one each side of the forward part of the vehicle, and, seemingly, at a convenient height from the road level, for inspection or removal. It is to be admitted, of course, that parts adjacent to the crankshaft and camshaft would he rendered less "get-at-able" by adopting the suggested design, but these parts are now far more reliable than was the case when time general layout of the present heavy chassis was first adopted.
Fig. 1 shows, in bare outline, a chassis of existing design and of about average proportions, whilst imme eiy beneath it (Fig. 2) is shown, also in outline, chassis of equal overall length and wheelbase, but assumed to be furnished with an opposed-cylinder horizontal engine. The actual length of platform is seen to be increased by about 50 per cent.; the available area for goods storage would not, however, have the same increment, as, naturally, some of the gained space would be occupied by the driver's seat, footboard and controls, all of which are presumed to be located on the extreme right hand of the chassis. In any case, regarded from the point of view of loading capacity, there does seem to be an advantage sufficiently enticing to warrant a carefully considered weighing-up of the whole proposition by some of those builders who have the development and advancement of the motor lorry in their hands.
The Pros and Cons of the Opposed-cylinder
Engine.
Although, for reasons not at all obvious, the engine with horizontally opposed cylinders has not met with favour, actually it has several really excellent features. Tn the first case, the task of balancing an engine of this description is not at all a difficult one, and, provided that each pair of cylinders on opposite sides of the crankshaft have axes which coincide—not at all difficult of attainment—the moving parts become almost automatically balanced. The crankshaft can be short arid stiff, with but two throws against the four throws of a standard shaft for a four-cylindered unit. The perfect balancing of this shaft is, actually, an easy matter. The crankcase is much less bulky and can have an axial length of not much more than half that of the more usual design. The camshaft is, equally with the crankshaft, of less length by about half.
On the other hand, there are, of course, objections, although it Is thought that these are really of quite a minor character. There may he a slight tendency for the cylinder bores to wear oval. (In the case of such small piston weights this criticism surely may be considered hardly worthy of mention, although,necessarily, in larger cylinders of, say, 20 ins, or more in diameter the question cannot be ignored.) The exhaust and carburetter pipes may become a little intricate, since the opposing cylinder heads are separated by several feet, and there are other points, but so minor as not to be worthy of mention.
Whatever the apparent result of weighing up the pros and cons, there really does not appear to the writer to be much room for adverse criticism of the opposed-cylinder horizontal engine as a type, and this opinion is apparently held also by no less an authority than Dr. F. W. Lanehester, who made the following statement in a paper read before the Institution of Automobile Engineers (Proc. I.A.E., Vol. viii, p. 205) so far back as 1.914. " The author attributes the fact that this type of engine has not come into prominence almost entirely to popular prejudice. The popular idea has been for many years that an engine cannot I ab right unless it is vertical. It would show quite as high a degree of popular intelligence if an eligine were not deemed the thing' unless painted blue."
Another Engine Possibility.
There is another type of engine that lends itself fairly well for instalment where "head room" must he cut down to a minimum—viz., the engine with cylinders arranged diagonally. The overall height of this type is clearly less than that of the vertical engine, whilst its width across from cylinder head to cylinder head is less than that of the horizontally opposed type.
This kind of engine seems, however, to occupy rather a large amount of transverse chassis space; moreover, it has not the balancing possibilities of the previous type nor has it the compactness of the standard type of vertical engine. Engines of this class were built very freely for boat propulsion during the latter stages of the war, and for this purpose they appear to have been very well suited. It, however, seems to the writer that if in the future the vertical engine is to give place to some other type, it will be the best for the designer to adopt a bold policy and to lay the cylinders down flat rather than to adopt the weaker policy of employing the diagonal engine.
Some Other Points Discussed.
One will not be very inaccurate in stating that, in the average commercial chassis about one-third of the total length of the vehicle is occupied by •the engine and the driver—the latter being usually furnished with sufficient space to invite several of his friends to accompany him if he so desires. The present design of L.G.O.C. bus, in order to secure more passenger space, pushes the driver forward and compels him to ride beside the engine instead of behind it. It would not appear to be an indignity, therefore, if the commercial driver were to be treated in like fashion, somewhat as is suggested in Pig. 2, accompanying these notes, in which, of course, the driver is intended to occupy a position on the extreme right of the body and in line with the engine.
It certainly seems uneconomical to place the driver behind the engine, since neither he nor his engine really needs a width much exceeding one-quarter of the wheel track.
[It must be remembered, however, that it is often necessary or desirable to carry on the vehicle a second man or boy for loading work, etc.—En.} Occasionally the driver has been placed immediately. over the engine, and this brings about economy of length ; the resulting elevated position, moreover, is, perhaps, not detrimental, to driving. Presumably, the main objection to the position is that fairly substantial superstructure has to be put up over the engine, to which to attach the control levers—foot and hand— and other important parts. '
Referring once again to Figs. 1 and 2, it will be noticed that the wheelbase is the same in each case, and the two diagrams are intended to show the possibility of increased body capacity. Clearly there will be a considerably greater proportion of the load than Is usual upon the front axle, but this can, of course, be righted by adjusting the position of the rear axle.
The saving of space upon our roads, particularly in large towns, is becoming of more and more importance each year, and if we be correct in saying that the average petrol lorry has but two-thirds of its length available for load carrying, it is easy to see where an attempt should be made to economize. By adopting some such principle of design as has been outlined, it would seem that not only might there be a. saving of length approximating to one-third of overall dimensions, but that, on the other hand, by a shortening of wheelbases, vehicles would become More manageable.
No one is more ready to admit than the writer that the present design of heavy road vehicle is the result of gradual and carefully considered development; but this process of development is to continue, and may it not eventually lead us to a more thorough study of the possibilities and advantages of the opposed-cylinder type of power unit?