Sleeve valve
From Wikipedia, the free encyclopedia
The sleeve valve is a type of valve mechanism for piston engines which have traditionally relied on the more common poppet valve.
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[edit] Introduction
A sleeve valve consists of one or more machined sleeves that fit within a piston engine's cylinders and are designed so as to move so that their openings align with the cylinder's inlet and exhaust ports at the appropriate stages in the engine's cycle.
Sleeve valves saw use in some pre-World War II luxury cars, sports cars, and the Willys-Knight car and light truck, and saw substantial use in 1940s aircraft engines, such as the Napier Sabre and Bristol Hercules and Centaurus, but they 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.
[edit] History
The sleeve valve principle was invented in 1903 by the American inventor Charles Yale Knight. Although he was initially unable to sell his Knight Engine in the US, a trip to Europe secured several luxury car firms as customers willing to pay his expensive premiums. He first patented the design in Britain in 1908.
Among the companies using Knight's technology were Gabriel Voisin (in his Avions Voisin cars), Daimler (in their V-12 'Double Six'), and Belgium's Minerva company.
Upon Knight's 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 standard poppet valves. The best known of these were the Stearns Company of Cleveland, which sold a car named the Stearns-Knight, and the Willys firm which offered a car called the Willys-Knight.
A number of sleeve valve aircraft engines were developed following a seminal 1927 research paper from the RAE by Harry Ricardo. This paper outlined the advantages of the sleeve valve, and suggested that poppet valve engines would not be able to offer power outputs much beyond 1500 hp (1,100 kW). Napier and Bristol began the development 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.
Following World War II the sleeve valve disappeared from use, as the previous problems with sealing and wear on poppet valves had been remedied by the use of better materials, and the inertia problems with the use of large valves were reduced by using several smaller valves instead, giving increased flow area and reduced mass.
[edit] Case for sleeve valve
In a standard internal combustion engine, the poppet valves are opened by the a shaped cam acting on the top of the valve, while the valves are closed by a spring wrapped around the valve stem.
The main problem with this system is that as the RPM of the engine increases, the speed at which the valve moves also increases, increasing the loads involved due to the inertia of the valve, which has to be opened quickly, brought to a stop, then reversed in direction and closed 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 around to open it again and in some engines the piston may even collide with the valve.
The desmodromic system as used by Ducati in some of its motorcycle engines uses mechanical methods to close the valve, but this system requires precision engineering and is markedly more expensive than spring-closed valves.
[edit] Sleeve valve description
As its name implies, the sleeve valve is constructed as one or more sleeves that fit around the piston inside the cylinder wall. Ports (holes) in the side of the cylinder replace the more normal intake and exhaust ports on the head, and similar apertures in the sleeve(s) open and close the ports 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, alternatively the sleeves are operated by a crank driven from the crankshaft, with the sleeve moving in a circular path opening the cylinder ports in the upper part of the circle.
The advantage of the sleeve valve 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 involves a reduced height sleeve placed beneath the cylinder head. This has the advantage of being easier to construct, as it does not need to be strong enough to withstand the forces generated by a piston moving within it.
[edit] Advantages
No springs are involved in the sleeve valve system, therefore the power needed to operate the valve remains largely constant with the engine's RPM meaning that the system can be used at very high speeds with no penalty for doing so. In addition, the camshaft, pushrods, or rockers can be dispensed with, as the sleeve valves are generally driven by a single gear running directly off the driveshaft. For an aircraft engine this produced desirable reductions in weight and complexity.
An additional advantage of the system is that the size of the ports can be readily controlled. This is of importance when an engine runs over a wide range of RPM, as the speed at which air can enter and exit the cylinder is defined by the size of the duct leading to the cylinder and varies according to the cube of the RPM. In other words, at higher RPM the engine typically requires larger ports that remain open for a greater proportion of the cycle, something that is fairly easy to arrange with sleeve valves, but difficult in a poppet valve system.
A minor advantage includes the fact that the cylinder head is not required to house valves, therefore allowing the sparkplug to be placed in the best possible location for efficient ignition of the combustion mixture.
[edit] Disadvantages
The sleeve valve has one major disadvantage, in that perfect sealing is difficult. In a poppet valve engine the piston possesses piston rings (often at least 3 and sometimes as many as 8) which form a seal with the cylinder bore, and during the "breaking in" period any imperfections in one are scraped into the other resulting in a good fit. This type of "breaking in" (known as "running-in" in the UK) is not possible on a sleeve valve engine however, because the piston and sleeve move in different directions and in some systems even rotate in relation to one another. In the 1940s this was not a major concern because the poppet valves of the time typically leaked appreciably more.
[edit] Modern usage
The sleeve valve has begun to make something of a comeback, due to modern materials and newer and dramatically better engineering tolerances and construction techniques which produce a sleeve valve that leaks very little oil. However, most advanced engine research is concentrated on entirely different designs of internal combustion engine such as the rotary engine, as opposed to improvements to existing engines such as the sleeve valve.
[edit] Steam engine
Sleeve valves have occasionally been used on steam engines, for example the SR Leader Class.
