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THE VALVES AND THEIR LOCATION.

22nd April 1924, Page 9
22nd April 1924
Page 9
Page 10
Page 9, 22nd April 1924 — THE VALVES AND THEIR LOCATION.
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The Claims of Various Systems for Commercial Vehicle Usage.

I-.N all but a very few instances the poppet valve remains supreme for petrol engines of the motorvehicle type. The position of the valves has, however, undergone considerable modification in recent years with a view to increasing the efficiency obtainable from an engine of given capacity. In the domain of the pleasure car we find that, in over 36 per cent. of the vehicles now manufactured, the valves are placed in the head, instead of the

side-by-side position which

has held the field for so

long. Even in the com mercial-vehicle world, over head valves have their adherents, so that it is of interest to compare the merits and demerits of the two most-used positions.

There remains one vital factor -which influences designersetin both _spheres, namely, f u economy. When the valves are placed in the cylinder head a, very efficient „engine usually results, which emoans that more power is obtained and a greater distance covered per gallon of

petrol. However, a " but " creeps in here—as in most other cases. The efficiency of the overhead-valve engine depends upon the maintenance of correct tappet clearance, etc., to a marked degree, and such power units notoriously require more attention than the side-by-side valve types. In a commercial vehicle an engine which will run for long periods without attention, and without serious loss of efficiency, is very desirable, so that it can well be said that the side-valve unit possesses advantages to combat those of its rival. .

It must also be recognized that improvements in the design of the side-valve engine during recent years have greatly increased the efficiency, reducing the margin in favour of the overhead-valve variety. At one time the margin was considerable, and largely depended upon the fact that, owing to the excellent form of the combustion chamber obtained with overhead valves, a high compression pressure could be used without knocking• troubles. A high-compression pressure means greater power and efficiency. At this period, most side-valve engines were provided with heads of large area and small depth. The sparking plug was usually located over the inlet valve, and was therefore at a considerable distance from the centre of the head.

Consequently, the phenomenon known as " knocking ' was very liable to occur, as this consists of partial ignition of the charge near the plug, followed by detonation of the remainder. This fact has now been fully realized, and we see that in modern side-valve engines the head is so shaped that, at the top dead centre, the compressed gases surround the plug in a more or less uniform manner.

Indeed, in many cases, the compression volume is Very nearly as well arranged as in the overheadvalve engine.

This (question of improving the shape of the combus‘tion chamber has been tackled in a very ingenious mannerby the designer of the well-known Guy engines. Here, the side valves are inclined outwards and are operated through the medium of rockers from a camshaft placed in the crankcase. The combustion chamber is very compact, and the gear is most accessible.

Reverting to the overhead operated either by an overhead camshaft or by push rods and rockers operated by a camshaft placed in the crankcase. For commercial work, where speeds are not excessive, the latter method is preferable, as an overhead camshaft and the means for driving it are bound to complicate the mechanism. The only trouble met with when pushrods are used is that the change in length of the cylinder block, when heated, may differ from the change in the push-rods. In this .way tappet clearances will be altered materially when the engine is hot. This difficulty has, however, been combated in several ingenious ways, and in these designs the alteration in length is automatically compensated by the expansion of other parts of the gear.

Overhead Valves for High Speeds.

It has already been stated that the advantages offered by the overhead type of valve are most marked in high-speed engines. While the power unit of the commercial vehicle is usually limited to a low speed-1,000 to 1,500 r.p.m. or less—the case of the rnodern char-a-bance is an exception to this rule. Thus a 20-seater or 16-seater passenger vehicle may be called upon to give 25 m.p.h. or more, and at such road speteds the engine will have to revolve at over 2,000 r. p.m.

Hence, it is not surprising to find that several makers favour the overhead-valve engine for chassis primarily intended for fast passenger touring. It will be interesting to see whether a special type of highspeed engine will be evolved for motor coaches, and if this should occur the possibilities of the sleeve valve will merit the close attention of manufacturers.

This type of valve is, of course, exemplified by the Daimler engines, which are built on the .Knight system and embody two concentric sleeves, fitted between each cylinder and its piston. The sleeves are caused to reciprocate by connecting rods, driven from a layshaft, and admit and exhaust the gases through rectangular ports. In addition to working extremely )318

silently, this type of valve provides a very ample area for the incoming and anteing gases, and a very admirable shape of combustion head is obtained. Similar advantages are possessed by the sleevo valves used in Peugeot and Panhard vehicles, and the well-known Burt-McCollum single-sleeve system.

Undue weight and friction have often been advanced as disadvantages of the sleeve valve. As regards the former, it must be remembered that the sleeve moves comparatively slowly, and the weight is, therefore, not of such vital importance as one might at first suppose. Friction is certainly a disadvantage, but this has been reduced to a minimum by careful design in the well-known makes enumerated above. In this connection, it is of considerable interest to notice that the Panhard concern has recently adopted steel sleeves, which are much lighter than the cast-iron variety ; furthermore, in order to reduce the friction and minimize the possibilities of seizure, the surface of the valve is coated with a thin layer of white metal.

Some Comparisons.

Amongst so many pros and cons, what, then, are the conclusions to be deduced l The writer believes that these may be summed up as follows :—For the orthodox fouiacylinder power unit of a slow-speed lorry the side-by-side-valve system may be expected to maintain its popularity. Overhead aalves will probably become more widely used in the case of 1-ton chassis employed for rapid delivery work and high-speed passenger service. As regards the sleeve valve it is not safe at present to prophesy. Much depends upon the results of the extensive experimental work which is being conducted at the present time, and upon the fuels, other than petrol, which may come into general use in the future. Briefly, the sleeve valve is a serious competitor to the overhead-poppet valve for high speeds and high-compression ratios, and it is impossible to foretell which of these rivals will eventually become the most widely used.

The question of alternative fuels will come in for more serious eonsideration when home-produced alcohol or benzole are marketed in sufficiently large quantitiesAlcohol,' for example, enables the designer to use a. compression pressure of nearly double that which ean be employed for petrol, without fear of knocking. With such a fuel there is considerable difficulty in providing sufficient clearance for poppet valves, which, if given an effective lift, will either foul the piston or the cylinder head., owing to the cramped size of the combustion chamber.

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