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Sleeve valve

Bristol Perseus


The sleeve valve is a type of valve for piston engines that has a number of advantages over the more common poppet valve, used in most engines, as well as disadvantages that have precluded its widespread adoption to date. Sleeve valves were used in some pre-World War II luxury, sports cars and the Willys-Knight car and light truck, and saw substantial use in 1940s aircraft engines, but subsequently fell from use due to advances in poppet-valve technology (sodium cooling) and to their tendency to burn considerable amounts of lubricating oil or to seize due to lack of it.

Disadvantages of poppet valves

In a normal engine using poppet valves, the valves are opened by the camshaft pushing down on the top of the valve, sometimes via a long pushrod and rocker taking the power from the crankshaft area to the top of the cylinders. A spring wrapped around the valve stem closes the valve when the cam stops pushing on it.

The problem with this system is that as the RPM of the engine increases, the speed at which the valve moves also increases, which increases the loads involved due to the inertia of the valve, which has to be opened quickly, brought to a stop, then reverse direction and close and brought to a stop again. Large valves that allow good air-flow have considerable mass and require a strong spring to overcome the opening inertia. At some point, the valve inertia overwhelms the spring and stops following the cam profile, closing well after the cam lobe has moved away. This "valve float" can eventually cause the valve to not close at all before the cam comes round to open it again.

In some engines, the piston may not be able to travel its full stroke without colliding with an open valve, which does the piston and the valve no good at all. Even in "non-interference" engines, at some point the valve head can simply part from the valve stem due to the inertia effects. Very strong springs increase friction loads caused by the rubbing of the cam lobe against the parts that open the valve. Some claim the spring loads also cause simple mechanical losses (the cam has to push against the spring to open the valve), but the cam "regains" much of this energy when the spring closes, the valve, as it helps to push the cam around, as well. Thus, these losses are minor.

However, most of the problems operating poppet valves occur at much higher engine revolutions than aircraft piston engines ever operate at. The main problem is that the exhaust valves run very hot and may easily cause detonation that can destroy an engine in less than a minute, if ignored.

Sleeve valve description

The sleeve valve avoids all this. As the name implies, the valve is constructed as one or more sleeves that, typically, fit around the piston inside the cylinder wall. Several ports (holes) in the side of the cylinder replace the more normal intake and exhaust ports on the head. Similar holes in the sleeve(s) open and close the ports like a poppet valve would, but do so by being rotated into position. In some engines each sleeve has a gear ring on the bottom that runs in a channel and a small cut in the cylinder wall exposes the gear so that the sleeve can be turned. Otherwise a single sleeve is operated by a crank driven from the crankshaft, the sleeve moves in a circular path opening the cylinder ports in upper part of the circle. The advantage is that very large port openings can be arranged that increase the volumetric efficiency of the cylinder and the combustion chamber formed with the sleeve at the top of its stroke is almost perfect for complete, and detonation-free, combustion of the charge.

Another design is more "traditional" in that the sleeve is placed under the cylinder head. This has the advantage of being easier to build, as construction of a sleeve strong enough to bear the loads of the piston riding on it is not all that easy. A similar design rotates the entire cylinder head instead. However the advantages of this design compared to traditional valve systems is somewhat limited, and the rotating head version of the sleeve valve system did not see widespread use.

Advantages

There is no need for a spring in the sleeve valve, and the power needed to operate the valve remains largely constant with RPM – so the system can be used at very high RPM, and with no penalty for doing so. Furthermore it does away entirely with the camshaft, pushrods and rockers, replacing them all with a single gear running directly off the driveshaft. For an aero engine this sort of simplification and weight savings is an engineer's dream.

Another advantage of the system is that the actual size of the ports can be easily controlled. This is important when the engine runs over a wide range of RPM, because the speed at which the air can move into and out of the cylinder is defined by the size of the duct leading to the cylinder and varies with cube of the RPM. In other words at high RPM the engine typically wants larger ports that remain open longer in terms of one cycle, something that is fairly easy to arrange with a sleeve at the cost of a more complex gearing system.

Less important advantages include leaving the cylinder head empty so the spark plug can be placed wherever is best, the valve is not being continually "hammered" into the port leading to rapid wear, and the exhaust's heat is spread evenly around the cylinder, rather than generating a hot spot on the exhaust valves. Hot spots in engines must be avoided, they can often lead to the destructive problem of knock. In the sleeve valve engine this is not an issue, so they can be run at higher compression.

Disadvantages

The sleeve has one major disadvantage though, and that is that it can't be sealed well. In a normal engine the piston is sealed into the cylinder with piston rings (often at least 3 and sometimes as many as 8), and during the "breaking in" period any imperfections in one are scraped into the other. The result is a tight fit. This sort of fit is not possible on the sleeve valve however, because the piston and sleeve are moving in different directions and, in some systems rotating in relation to each other. Sleeves need sealing rings to the cylinder wall at the top and bottom of the cylinder. In the 1940s this was not a major concern because the poppet valves typically leaked anyway.

History

The sleeve valve principle, was invented in 1903 by an American, Charles Yale Knight. Initially, he was not able to sell his Knight Engine in the US directly, a trip to Europe got several luxury car firms to sign up and pay his expensive premiums. He first patented the design in Britain in 1908. Gabriel Voisin built nearly all of his cars to this design, and contrary to public opinion, they were fast; many won races. Daimler used the principle in its V-12 which it brand-named the 'Double Six'); another top-level firm was the Minerva of Belgium. Upon his return to America he was able to get some firms to use his design; here his brand name was Silent Knight (1905-1907) — the selling point was that his engines were quieter than those with valves. The most well-known of these were the Stearns Company of Cleveland, which sold a car named the Stearns-Knight, and the Willys firm offered a car called the Willys-Knight.

A number of sleeve valve engines were developed starting with a seminal research paper by the RAE, published in 1927 by Harry Ricardo. This paper outlined the advantages of the sleeve valve, and seemed to suggest that poppet valve engines would not be able to evolve much beyond 1500 hp (1,100 kW). Napier and Bristol started developments of sleeve valve engines that would eventually result in two of the most powerful piston engines in the world, the Napier Sabre and Bristol Centaurus.

After the war the sleeve valve rapidly disappeared. As it turned out the problems with sealing and wear on poppet valves were remedied by better materials, and soon the poppets were sealing very well indeed. Oil leakage dropped almost to zero, and the power used by the springs and camshaft was a small price to pay for such a tight seal. The problem with oil leakage in the sleeve is much more "built into" the system. Also, the inertia problems of large valves were solved to some extent by using several smaller valves rather than one large valve. This increases flow area and reduces mass, while not significantly reducing strength. These multi-valve engines are now nearly ubiquitous.

Recently the sleeve valve has started to make something of a comeback, owing largely to the same type of improvement that led to its demise. Newer materials and, more notably, newer and dramatically better construction techniques can make a sleeve valve engine that is so "tight" that it leaks very little oil. However most advanced engine research continues to look at entirely different designs, like the rotary engine, as opposed to more conventional improvements like the sleeve.