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SAVING SPACE BY COMPACTING THE ENGINE.
Page 102
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Page 104
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Unorthodox Methods are Here Suggested to Reduce the Space Occupied by the Power Unit.
THE smoothness and general desirability of the engine with more than four cylinders is gradually forcing the hands of designers towards the adoption of this form of power unit for their products, at least for the better-class passenger chassis. This development, however, involves one great disadvantage—the available passenger space is eaten into very much more than with the four-cylindered type of power unit, and so for general purposes the total capacity of the vehicle, when fitted with a six-cylindered engine, is curtailed somewhat more than is the ease in similar chassis with a four-cylindered power unit.
At first sight it would appear that, capacity for capacity, the six-cylindered engine should be very little longer, if any, than one with foer cylinders, but a little consideration of the matter will soon show that this Is not so. In the first place, the area of, say, a piston top varies its the square of its diameter. Obviously, then, very little difference in diameter makes a big difference to area. Again, the cylinder walls and water spaces are arranged more to suit the exigencies of moulding than the size of the cylinders; these dimensions therefore must be approximately equal in both four and six-cylindered engines.
Now, short of a general overall increase in efficiency, It is difficult to see where any material difference can be made in the proportions of the cylinder block of the six-eylindered type, because the length is governed entirely by dimensions which are practically unalterable. The only solution therefore is to look farther afield and to see if the cylinders cannot be arranged in different Iormation, so that they will either go beneath the floorboards and extend rearwards from the front of the vehicle, or be placed in one plane at the front of the chassis. The former method, of course, indicates a horizontal layout for the cylinders, whilst the latter indicates the possibility of arranging them radially around a common crankcase, as in air-cooled .aero-engine practice.
There are, of course. many points to consider, and each form of layout, with its possible variations, may tend to solve One or more of the problems with which designers are at present faced. For instance, a low platform level is a very desirable feature for passenger work, but with the present type of layout difficulties are already being experienced in providing sufficient axle-to-road clearance and axle-to-body clearance when proper allowance has been made for the axle construction.
n2.8
The six-cylindered engine develops its power usually at a greater speed than its counterpart in the fourcylindered class, with the result that lower final-drive ratios are necessary, a feature that Is leading designers to believe that a double-reduction arrangement of the final drive would be of advantage. Efficiency, on the other hand, is usually slightly higher in the multi' cylindered engine, due to the fact that greater crankshaft-revolution speeds are permissible with smaller and, consequently, lighter reciprocating parts.
The argument, of course, may be carried still farther. If the six-cylindered unit be smoother and develops rather more power than the "four," would not the
"eight" be even better? It is not proposed in this article to settle that long-fought-out discussion. There are staunch adherents to both sides, and each side brings forward infallible arguments in favour of its particular choice. On the other hand, the engine with six cylinders in line definitely possesses disadvantages from the bus or coach owner's point of view, and as he is the person chiefly to be considered it is obviously up to desigpers and production engineers to see if, by using any form of engine, whether orthodox or not, the problem of providing additional passenger space cannot be effectively solved.
The obvious solution of the problem of compacting the engine is to place the cylinders yee-wise, thereby making a "six " actually shorter but wider (which does not matter) than the equivalent "four." To arrange six cylinders like this is, of course, quite possible, and the resulting balance would be reasonably good. There is, however, a rather serious " couple" involved, which might possibly cause excessive vibration at certain speeds, and would certainly cause the engine to vibrate badly at high crankshaft speeds. The eight-cylindered V-type is, of course, quite good, and if the banks of cylinders were placed at an angle of 00 degrees to each other almost perfect balance could be obtained. Going one step farther, the horizontally opposed engine is very desirable from many points of view. Balance should be perfect; high revolution speeds—with attendant efficiency—can be achieved without vibration and without in any way causing trouble due to overstressing the reciprocating components. In addition, the engine can be made shallow in its general layout and, comparatively speaking, short.
Accessibility, especially for the power unit, is often quoted as being an essential feature. Whilst it is agreed that some components must be easily get-atable, and some other components easily removable for general service attention, it might prove illuminating to examine the matter superficially and to see how the horizontally opposed eight-cylindered engine, for example, would compare with the more orthodox four and six-cylindered types. In the first place it is obvious that the cylinder blocks, or at any rate the cylinder heads, must be very easily removable, whilst such items as the carburetter, magneto, dynamo, etc., must be placed in convenient positiOns.
Now the chief value of the horizontally opposed type of engine is the fact that it can be made very shallow. Indeed, there does not seem to be any reason why, say, a 31-litre engine, having eight horizontally opposed cylinders, should exceed 6 ins. or 7 ins, in height from the crankshaft centre-line. Such a design, however, calls for all the auxiliaries such as camshaft, valves, distribution drive, manifohling, etc., to be carried on the under-side of the unit. This, of course, permits the engine to be hung comparatively low in the frame; say, at the usual height for the crankshaft centre, so that the floor of the vehicle would then be able to be carried right over the unit. It would, of course, provide vastly greater passenger accommodation for the chassis than with the normal type of six or eight-cylindered engine, with the cylinder block arranged in a vertical plane; but how would accessibility fare with an engine of this type?
It would appear that some form of tray to cover In the entire under-side of the power unit would be a desirable feature, but this could -be made quickly detachable, so that the whole of the unit could be reached from beneath the chassis. This, of course, may give rise to some criticism, but against such criticism it might be argued that the modern -type of power unit requires very little roadside adjustment. Sparkling plugs, for instance—components that in the past have, given an enormous amount of trouble—seldom require even cleaning nowadays, whilst valve-tappet ad-. justment should only require checking over about once every 1,500 miles, or at even less frequent intervals. Carburetters, too, have, by the addition of air stranglers and hot-spot arrangements, been improved out of all recognition, with the result that the float chambers need not be flooded to get a start from cold, whilst, by the institution of a really sensible filter in the petrol system, choked jets can be avoided.
There are many ways in which the auxiliaries—water pump, dynamo, magneto, etc.—can be driven and disposed, possibly the most useful arrangement being that In which the business ends of the instruments point upwards, Where they can be made accessible by providing removable floor boards. There is one-other aspect connected with the multi-cylindered engine, as against the larger bore four-cylindered type—the higher rate of revolution speeds permitted by the smaller cylindered engine enables a greater overall efficiency for the unit to be attained.
Among the illustrations will be found a curve denoting the horse-power obtained from a range of six-k cylindered engines of varying capacity. Each engine has approximately the same bore-stroke ratio, and the valve gear on each is identical. The curve is, of course, sell-explanatory, but the rapid fall in power as the capacity of the engine is increased is very noticeable. It is safe to assume, then, that, apart from considerations of balance and smooth running, the multi-cylindered engine, as outlined above, would prove to be more powerful and probably more economical to run.
Digressing for a moment, let us turn to a consideration of the ethics of the proposal to adopt a layout that is unconventional. Commercial vehicles, indeed motors generally, are often referred to in the femi*Sine gender, which is, In reality, not very surprising when it is considered how closely they follow the human species in certain respects. Mode and fashion are factors of very great importance—although it is generally admitted that they should not be—and any layout of an unconventional nature is always the subject of comment, mostly unfavourably disposed.
Many concerns, even those producing the best-class passenger chassis, have to keep a very tight rein on the designing staff, in order to maintain the vehicles produced by the concern as saleable propositions. It is obvious, then, that there is no lack of enterprise on the technical side of the industry, but any sales manager will-admit that he is literally terrified of anything unorthodox. Development and further increase in engine efficiency may alter things by degrees, but he would be a bold man who would state that convention can be thrown to the winds and an entirely new line of thought adopted without a great deal of pioneer work by the Press and possibly by an enthusiastic section of the trade.
This preamble has been included to introduce the possibilities of the radial engine, a type that does not seem to have been explored very thoroughly, if at all. One or two attempts have been made in the past to introduce the radial engine into the privatecar market ; the writer has, in fact, assisted in producing such a car, and is therefore cognisant of the difficulties which have to be encountered. Cylinder spa6e is one of them, but as the car in question was a small one the difficulties in this direction were obviously very much greater than would be the case in a large commercial chassis. Again, as already explained, quite a small increase in the overall size of the cylinders enables a big increase in capacity to be attained. Indeed, the more one probes the question of the radial engine for heavy commercial passenger chassis, the more attractions does the scheme appear to offer.
So compact can an engine of this type be made that it should not encroach upon the passenger space at all. By mounting the unit on the forward face of a cross-plate connecting the main frame members an excellent fixing could be obtained. The plate would, of courSe, form part of the forward construction of the body, and there would be no necessity for any component to protrude into the passengers' compartment.
Among the illustrations will be seen a drawing depicting the manner in which a radial engine could be housed. ina coach chassis. The engine itself is entirely cowled in on its outer diameter by a circular type of .bonnet, whilst the radiator—also circular in shape— could be fitted at the front. It would be necessary,
n30 of course, to make the radiator quickly and easily detachable.
As a suggestion for facilitating matters in this direction it might he possible to run the water connections to, say, a circular flange on the timing case, which, with a thick rubber washer interposed between the two elements, would make a watertight joint and provide, a means for insulating the radiator from engine vibration. It should, therefore, prove to be a very easy matter to effect valve adjustments, decarbonize the cylinders, etc., whilst there does not appear to be any insuperable difficulty in housing the auxiliaries in convenient and readily get-at-able positions.
Attention has already been drawn to the fact that smaller and more efficient engines—the multi-cylindered engine comes into this category as well—usually develop their power at high revolution speeds. The engines are, however, lighter, more economical to run, and are incomparably smoother at both high and low speeds. A great trouble, however, with this overall increase in crankshaft speeds is the fact that the final drive becomes a problem, in order to make the pinions sufficiently robust to withstand the driving torque yet at the same time to provide an adequate reduction ratio.
The radial unit offers quite good opportunities for introducing a reduction gear at the engine end. Helical spur gears with herringbone teeth, are indicated, and, if disposed on the rear face of the engine mounting, should bring the clutch, gearbox and transmission line well below the floor of even an ultra-lowhung chassis. At the same time, the rear axle could be made much smaller in overall height—so that both road and body clearances wouldbe much better than with the single-reduction gear contained within the axle itself.
So far, all suggestions that have been made have called for an entirely new layout for the power unit, making it quite different from the type in use. There are, however, ways and means whereby a few inches of space could be saved which might be just sufficient to allow an extra line of seats to be included in the body. The obvious thing to do, of course, is' to place the engine as far forward as possible in the chassis, and there is no doubt that this has been done to a large extent. For instance, the radiator takes up an enormous amount of space. The general run of radiators are at least 6 ins. or 7 ins, deep, whilst interposed between the front face of the cylinder block and the radiator itself is a fan, which often accounts for another 6 ins. or 7 ins. Further, timing gears have become a little unwieldy of late, and if every part were pinched as much as possible there can be little doubt that a few inches might be taken off the overall length of the engine. For instance, double-row timing chains could, and should, be avoided.
Returning now to .the question of the radiator. By dividing this corn-' ponent and mounting the halves separately at the forward end of the chassis, but with the cylinder block. protruding between the two units thus formed, a saving in the overall length of the power unit could be effected to the extent of about 12 ins.
Have we said enough to arouse the interest of manufacturers and users alike? The present vehicle with a long bonnet, whilst being of good appearance, undoubtedly follows convention too slavishly. There are many more ways by which engine space might be saved and the pockets of the operators of the vehicles .enriched accordingly, but space forbids the exploration of these. In alater issue we hope to deal with another aspect of the engine problem—that of introducing the supercharger.