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Aviation History
1917
1917 - 1170.PDF
• •••••': • '• •* '. "•• x •-- NOVEMBER 8, 1917. A PETROL ENGINE RING VALVE./^r-r'^-^;-,^" A COMPARISON WITH OTHER TYPES. By A. E. THE internal combustion engine ovtes its origin to a French engineer in 1862—55 years ago—since which time the main features of the engine have practically remained unaltered. The great attention given to minor details has made it the lightest and most compact power producing plant extant to-day. Of the main features, the tappet type of valve has remained practically unaltered, simply because nothing better has been put forward, although engineers, with the smooth, silent-acting slide valve of the steam engine ever before their eyes, have felt that finality in valve design for internal com- bustion engines had not been reached. Many attempts have been made, especially in the last ten years, to supplement the poppet valve for something giving freer port area, quieter action, and greater durability. These attempts have consisted of disc valves, rotary valves, plug valves, cone valves, piston valves, long and short sleeve valves, and combinations of the same. Many of these attempts failed because the internal pressures tended to lift the valve member off its seating and leak. Others having the full pressures on them were kept moving all the time, some were too difficult to properly oil or cool, others on account of their great reciprocating weight could not attain very high speeds, whilst the majority were slow in open- ing their ports. Many suffered from heat distortion and seizure, but whatever their faults they showed the desire to obtain something better than the tappet valve, which, like most of these valve systems, possesses faults of its own due to natural causes which cannot be eradicated. To deal with the faults of the poppet or tappet valve first; we have a disc of metal mounted on a stem, and in the case of the exhaust valve, lifted directly into the escaping exhaust flame, it is closed by being suddenly snapped into its seating, hammering loose pieces of carbon into its hot surface. This tappet or hammering action makes it a noisy contrivance unless the necessary gap between the value stem and its push-rod is very nicely adjusted. Prolonged running de- teriorates this valve, calling for frequent grinding to its seating—one of the dirtiest and most troublesome jobs imaginable, what with strong springs to contend with and the more often than not inaccessible and awkward position of the valve. Being unlubricated, wear takes place between the stem and its guide, often causing trouble due to leakage with both exhaust and inlet valves. The continual heating deteriorates the valve head, increasing its liability to snap off and fall into the cylinder with dire results. Other and natural faults are that, even with the best design, a sufficiently free and clear passage cannot be provided for the in or outgoing gases, whilst, in the case of the inlet valve, on shutting, gas is puffed back into the carburettor or inlet pipe. Its good points are:—Light weight, enabling it to be run at high speeds ; it can be actuated by cams, the most simple HAMMOND. would open the ports too slowly, therefore another cylinder or sleeve, operating in the reverse direction, and outside the first, is employed to give the required rapidity. As these two sleeves weigh about 20 lbs., it would seem that speeds above 1,400 r.p.m. are fraught with the serious risk of breaking a sleeve, entailing a long and costly repair. However, it appears to the author a very roundabout way for opening two norts. A far more simple system is to be found in the short. eve or ring type of valve, in which the ring itself forms the valve and is the only part moved. This valve may be likened to a broad piston ring, having a short reciprocating motion across two rings of circumferential ports, which in its central position it covers and seals, movement up and down uncovering one or other of the ports. The working surface for this valve conveniently consists of the cylinder bore itself, above the piston. The operation is by means of a light fork connected by means of rods to two pins projecting outside the cylinder at opposite points from the valve ring. The fork receives its motion, in one direction, from a cam and roller, a spring being used for the return. This keeps all the parts of the actuation in close contact, preventing noise—an impossible combination with the poppet valve, which must have a gap in its con- nection to allow it to seat firmly. The cam in this case is arranged to give the pause which is such a valuable asset to this valve, enabling it to remain at rest during the pressure periods. It is in this respect that it is an advantage on the steam engine slide valve, which has to move continuously under pressure. The ring valve, by being at rest, is thus able to take full advantage of the pressures, which help to seal it against leakage, also the actuating mechanism is relieved of undue strain thereby. Large and free ports, rapidly opened and closed, are a first essential for high speed and efficiency, and a little comparison will show the enormous advantage in this respect possessed by this slide valve over the usual poppet valve. Take a 4-ins. bore engine for example. This should have a valve of if in. diameter, having a f in. lift, the throat area available being ij sq. in. The slide valve (of same bore), with its circumferential ring of ports, -yvould, with the same valve lift, including its seal, give 2.98 sq. ins., or twice as much as the poppet. Even if the slide valve were only opened to give the same port area, it would still have the advantage of^_its direct gas flow, a thing a poppet valve can never have. '¥•? A fair idea of the durability of tins system may be obtained by comparing this ring with a piston ring, the conditions being almost identical, except that the valve has the best of it, moving only one-twentieth of the distance travelled by the piston ring, and having eight to ten times its wearing surface. As to heating, true, its inside surface, is exposed to the explo- sion, but as this explosion presses the valve tightly to the water- cooled cylinder wall, it stands little chance of ever getting Induction •Fig. I means of giving any desired timing, and the internal pressures help to hold it gas-tight on its seating; also it is cheap to make and replace. Of the other types of valve the double long sleeve has been the most exploited, backed by unlimited capital and scientific research made necessary by certain inherent faults, some of which, owing to natural forces, cannot be overcome. This type provides an ideal shaped combustion chamber, gives a fair port area, with a smooth, lubricated, silent operation. In this type the working cylinder is moved up and down, BO that ports cut therein may be opened and closed by passing over a packing ring held stationary in a fixed cylinder head. This reciprocation of the working cylinder, being accomplished by an eccentric or crank mechanism at half engine speed. much hotter than the latter. It is also subject to the cooling effect of the inlet gas, which passes directly over its inner surface. As this valve works at a fairly moderate tempera- ture, perfect lubrication is rendered possible, and it is in- teresting to observe how natural conditions within the engine are taken advantage of to bring this about, so we will follow this ring valve through its harmonious cycle of operations, and note what happens at each step.. (Figs. 1, 2, 3 and 4.) A charge of explosive gas is drawn in on the induction stroke. Fig. 1, when the ring uncovers the inlet port, and when the piston ascends on the compression stroke (Fig. 2), the valve will then be in its central position, covering and sealing both rings of ports, and held stationary by its cam. The compressed gas, by slightly expanding the ring, tends to further seal the^ports II7O
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