TFIE CASE FOR SINGLE-SLEEVE-VALVES Sir, I think that the Burt-McCollum single-sleeve-valve system deserves to be dissociated from the condemnation, however cautious, of the Knight twin-sleeve arrangement which these columns recently carried. The single-:sleeve layout is vastly
better than the Knight and, for many purposes, rather better than the poppet valve. It is this last fact that may not be very widely realised. I can claim only to be an amateur student of engineering matters, but I have formed this conclusion over a period of some years and it will, I hope, withstand critical examination.
The poppet valve is cheap, simple to make, economical of material quantity, and, in mass-produced motors, it need not be made of especially high quality material. It is less of a one-engine valve than the sleeve—small dimensional differences are possible between engines using the tame valves so that 1962’s model can be bored-out for ’63 without necessarily retooling for valves. The .actuating mechanism is fairly simple. Maintenance and replacement is easy. It is possible to have any or all of the following faster opening and closing, longer dwell, greater overlap, longer valve periods—that is, a wider range of timing and a wider choice of timing characteristics are available. Except with desmodromic drive, gas sealing is a little better than with sleeve valves, but falls off more quickly. (But, ease of starting apart, under running conditions slight leakage is not so important as is generally imagined.) In its more sophisticated forms the poppet valve can have a slightly better orifice coefficient, and so the gases will flow a little more freely than with sleeve valves at a given manifold pressure.
Against this its life is shorter. The exhaust valve is far lets thermally robust. Pre-ignition is a much larger risk. The vital cylinder-head temperature is always higher, and thermal stresses in general are far more likely to cause distortion with its consequent train of troubles. Clearance adjustments are required. Maintenance and replacement are more frequently required than with sleeves. In all high-output forms, even desmodromic, mechanical failure can result in piston/valve collisions and great consequential damage. In all reasonably efficient forms the valves and their operating gear result in a taller engine, and in o.h.c. form the camshaft drive is longer and more prone to wear and failure. They arc noisier.
The sleeve valve will give a much longer life with greater reliability and a more consistent performance. The operating mechanism is inherently desmodromic. Short valve drives are not only possible but unavoidable. Motors are, in general, more compact and there is a considerable reduction in their height. An uncluttered and ideally shaped combustion space is readily available. Ricardo maintains that with fuel of a given octane rating, one-higher compression ratio can be used than in a poppetvalve motor, which means, conversely, that with two engines of the same c.r. the sleeve-valve one will accept a lower octane fuel. Valve ” clearances ” are more-or-less self-adjusting. They are quieter and, not to repeat myself, all the poppet-valve .disad vantages stated above are absent. The valves are never starved of oil and the oscillatory elliptical movement between sleeve and cylinder wall has been described as the ideal motion between two metal surfaces for lubrication purposes.
Against this they are costlier, a little more exacting to make, and, in general, demand a larger quantity of better materials. The drive is a little more complex. With two similar engines, the sleeve-valve one would be slightly heavier. Maintenance and replacement, when needed, is more difficult and entails greater dismantling of the engine. Carboning-up of the inlet ports is not a troublesome factor with the single-sleeve system, but it must be admitted that the average oil consumption of a sleeve-valve engine is a little greater. This feature does tend to be exaggerated, however; for it must be remembered that by the time such motors develop a real thirst for oil, cylinder wear in their poppet-valve counterparts is such that they are in not much better case. In car use there would perhaps be two ” user” disadvantages : They should be allowed to warm-up at least briefly, after a coldstart, before being taken to high revs. The designer would no longer be able, by judiciously allowing valve-bounce to occur, to protect the engine from the insensitive types who want to scream her up in 3rd gear.
The different applications of the two types are thus fairly obvious : Low cost motors will use poppet valves. High-output motors restricted to atmospheric induction will use a refined poppet-valve system, preferably with positive valve-drive. Longlife motor will use sleeves valves. High-output motors, where the designer is limited by no laws but natural ones, %yin be given sleeve valves and superchargers. The Rolls-Royce Merlin’s 48 poppet valves practically won the War. But its ancestry was in the Kestrel and the R-type and dated back to about 1932. Asked to produce as much power as possible for use in the fairly _serious business then afoot, and with little room for error, and with no time for long development programmes—in other words, when the chips were down—the
designers of R.-R. gave us the sleeve-valve Eagle, while those at NaPiers produced the somewhat similar Sabre. Added to this, Bristol continued their tradition of big sleeve-valve radials with the Hercules and Centaurus and others. These latter had a matchless reputation for absolute reliability and, I think I am right in saying, was officially allowed to perform a greater number of flying-hours between overhauls than any other aero-motor then in service.
The poppet valve, then, has not got everything its own way. But we may as well learn to love it : it will be with us for as long as the piston engine is—and right now that may be anything from ten years to fifty.
Highbridp. K. W. P. WILLIAMS.
So farewell then, SWC...
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