Petrol Engines at Olympia.—By a Builder.
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It was with considerable satisfaction that I noted the complete absence of freak designs amongst .the engines staged at Olympia, or. fitted to the various vehicle and chassis exhibits. 1)esign has now become more ov less settled upon -regular lines, and, although there is yet much divergence of ideas on many important details, particularly with regard to the cubic capacity of the cylinders in relation to the gross weight of the vehicle and the gear ratio, the general lines of design are now settled down to a more or less uniform standard. At the Show in 1908, good engines were comparatively few, and in the best of them the design was more or less crude ; whereas, at last week's display there was not a single engine which could really be called a bad one. There were many on which important details were most inaccessibly placed2 but, on the whole, they were and are capable of capital service.
A tew makers, particularly on the Continent, still appear to like to tuck the engine away below the driver's footboards, the idea being to obtain a longer platform area for given overall length, but I do not think that, except for very special requirements, the advantage due to the saving in overall length is anything like so great as the undoubted disadvan tages of having an inaccessibly placed engine. In some cases, the arrangement is quite good, but, at the best, I consider the type is a bad one. I would like to see the wider adoption of the Renault. principle, as is exemplified on the Austin two-ton chassis, and the Bayard, in both of which cases every part of the engine is most accessibly placed, neither wings nor radiator impeding the workman in the slightest degree. The Austin design is a most interesting one, and the engine may very easily be removed bodily from the chassis, by the simple uncoupling of the fixing bolts, and then pulling it out from the front like a drawer. This engine, however, strikes me as being unduly long, in proportion to its bore and stroke, each of its four cylinders looking quite lonely as it stands singly on a long expanse of crankcase. I realize that this has been brought about by Mr. Austin's liberal allowance of crankshaft bearing, there being five long bearings to the shaft.
The Maudslay engine design is very well known, and, in the matter of accessibility of parts, it is probably unequalled by any other engine in the Show. Its overhead valves undoubtedly tend towards very high thermal efficiency, and, provided there is no backlash in. the skew gearing by which the overhead camshaft is driven, it is a most beautiful engine to run. I cannot, however, say that I am wholly in favour of this form of drive ; backlash very soon develops, and unsatisfactory running results. It would be better, in my opinion, to drive the camshaft by enclosed silent chains, but I fear that if this form of drive were adopted it would be no longer possible to swing the camshaft clear of the valves, which is one of the distinctive features of the Mandslay engine. On the whole, I prefer to have the valves mounted on one side of the piston, instead of directly over it, and this is the position which is gradually becoming the accepted ono.
Not nearly so many makers have adopted the chaindrive for eamshaft as I had been led to believe. was the case ; there appear to be certain stumbling Hocks attending the use of the chain-drive camshaft, one of which is undoubtedly that of adjustment to correct inaccuracies of machining, or to take up slack in the chain as stretch or wear develops. 1 notice, however, that on the smaller Bayard models, which have chain-drive camshaft, an adjustment is provided by housing the magneto pinion bearings in an eccentric casing, which easing also has an extension which carries the magneto, in order that, as adjustment is made, the alignment of the magneto and its driving pinion and coupling is not disturbed. Another engine in which the chain-drive has been adopted is found on one of the Aildays chassis, there being a chain-drive from crankshaft to camshaft in the usual position, whilst the pump is mounted at the flywheel end of the engine and is driven by a separate chain from a pinion on the crankshaft. This has the advantage of admitting a very large diameter centrifugal pump, but it has the disadvantage of necessitating a. very long suction pipe from the radiators
The Milnes-Daimler engines strike me as being particularly practical in the arrangement of their camshaft drive. It will be remembered, in their case, the timing pinion is mounted right in the middle of the csankshafts SO that for a four-cylinder engine they have afour-bearing crankshaft, with the timing pinion between the middle pair, and the drive is transmitted to the middle of the camshaft. This gives a clean and symmetrical appearance to the engine, and as the drive is transmitted from the middle of the camshaft, there is less tendency for lag on the further cams, due to torsion of the shaft, but this latter point is one, I think, on which the Mines-Daimlsr people lay unnecessary stress. The engine that appealed to me more than any other in the Show was the 32 h.p. Albion; I consider this a very beautiful piece of design ; it has a remarkable absence of that pipiness which characterizes so ninny machines. In this engine, the impeller of the centrifugal circulating numn is mounted on the tail end of the fan shaft, -arid the water jacket of the cylinder turms the pump casing. It is a very nest fled
clever arrangement, and reflects great credit upon its designer. There is one point, however, that I do not. like about this engine, and I may say the same applies to most other engines in the Show ; that is that water spaces around the valves and other parts of the cylindeiti are unnecessarily These spaces conk' be made much larger, and thus eliminate an possibility of local overheating, without increasing the weight to a prohibitive extent, and there is evidence of this in the Pagetield subvention engine which is the one engine in the Show, to my mind, that has sufficiently large and clear water spaces in the jacket ; it is one of the largest engines in the Show, and yet by no means the heaviest.
The three-bearing shaft appears to have become the accepted standard for the majority of manufacturers. There are a few exceptions from this rule ; for instance, there is the Milnes-Daimler and. the Austin, which I have already mentioned, and there is the smallest four-cylinder Thornyeroft model, which
is very compactly arranged, and has four cylinders cast en bloc, with a two-bearing crankshaft. This savours of American practice, but it is none the less sound ; it is a machine which has proved itself under most severe conditions in the service of Piekfotds, Ltd. It has long since passed through its experimental stages, and is a. type .of engine that in the future will have more attention paid to it in this cou n try.
The use of ball bearings for crankshafts does not appear to be extending very greatly, and I am not at all surprised. The ball bearings on one well-known maker's esankshafts rarely give any trouble; in fact, I have known engines of this make run for several years without requiring attention, but when they do require attention it is a matter of very great difficulty to remove them, and there is always grave, danger of fracturing the crankshaft during the process.
In so short an article, I can only touch on general considerations, but at a later date I hope to be able to dive more deeply into the consideration of modern commercial-vehicle petrol-engine design.