Sleeve valve
From Wikipedia, the free encyclopedia
The sleeve valve is a type of valve mechanism for piston engines, distinct from the more common poppet valve.
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[edit] Introduction
A sleeve valve consists of one or more machined sleeves. It fits between the piston and the cylinder wall in the cylinder of an internal combustion engine where it rotates and/or slides, thus providing openings which 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, the Willys-Knight car and light truck, and saw substantial use in aircraft engines of the 1940s, such as the Napier Sabre and Bristol Hercules and Centaurus. 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.
At war's end, the gas turbine replaced the single sleeve valve engine for military aircraft. Up to that point, the single sleeve valve won every contest against the poppet valve hands down in comparison of power to displacement. The difficulty of nitride hardening, then finish grinding the sleeve valve for truing the circularity, may be a factor of its lack of commercial applications.
[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 the engines pictured above each sleeve has teeth on its bottom which mesh with a gear turned (through other gears) by the crankshaft. These sleeves run in a pure circular fashion around the piston. In another variation, the sleeve is driven by a crank which is driven (indirectly) by the crankshaft. In this version the sleeve runs circular as the in the Perseus above but only a few degrees. Then up and down as well causing a circular movement opening the cylinder ports in the upper part of the movement. One motorcycle engine employed this method. Still 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] Different kinds of sleeve valve
The first successful kind of sleeve valve was patented by Willys-Knight, and used two sleeves in order to open and close passages for inlet and exhaust in four stroke engines. This was the system used by Daimler and Voisin luxury cars. This system promotes high oil consumption.
Another system was developed by the Scottish company Argyll for its cars, and adopted by Bristol for its radial aircraft engines. Instead of Willys-Knight two reciprocating sleeves, the Argyll uses a single sleeve, rotating around a timing axle set at 90 degrees to the cylinder axle, enabling a single sleeve to clear both the inlet and exhaust windows. Mechanically simpler and rugged, the Argyll system has the additional advantages of a reducing oil consumption (compared to other sleeve valve designs), while retaining the rational combustion chambers possible in the Willys-Knight system.
A small number of designs used a sleeve in the cylinder head instead of cylinder proper, providing a more "classic" layout compared to traditional poppet valve engines. This design also had the advantage of not having the piston ride on the sleeve, although in practice this appears to have had little practical value. On the downside, this arrangement also limited the size of the ports to that of the cylinder head, whereas in-cylinder sleeves could have ports of much larger size.
[edit] Case for sleeve valve
In a standard internal combustion engine, the poppet valves are opened by a shaped cam pressing on the top of the valve, while the valves are closed by a spring wrapped around the valve stem.
The main problem with most valve systems is that as engine speed increases, the speed at which the valve moves also has to increase. This increases 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 spring reaches its resonance frequency, causing a compression wave to oscillate within the spring, which in turn causes it to become effectively shorter and therefore unable to properly close the valve. 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 rising piston may even collide with the valve. The desmodromic system as used by Ducati in all of its current 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] Advantages
The main advantages of the sleeve valve engine are:
- an increase in volumetric efficiency due to very large port openings
- the combustion chamber formed with the sleeve at the top of its stroke is ideal for complete, and detonation-free, combustion of the charge, not having to contend with compromised chamber shape and hot exhaust (poppet) valve(s)
Most of the evaluation of these advantages was established during the 1920's by Sir Harry Ricardo, possibly the sleeve-valve engine's greatest advocate. He conceded however, as fuels improved up to and during World War II, as well as with the advent of the sodium cooled exhaust valve in high output aeroengines, these advantages were significantly eroded.
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.
Another principal advantage of the sleeve valve in early automotive applications (Knight engine) was longevity. Prior to the advent of leaded gasolines, poppet-valve engines typically required grinding the valves and valve seats after 20,000 to 30,000 miles (32,000 to 48,000 km) of service. Sleeve valves did not suffer from the wear and recession caused by the repetitive impact of the poppet valve against its seat. Sleeve valves are also subjected to less intense heat buildup than poppet valves, owing to their greater contact with other large metal surfaces. As a Knight engine is used, carbon build-up helps to improve the sealing of the sleeves, thus the engines were said to "improve with use", in contrast to poppet valve engines, which lose compression and power as valves and valve stems/guides wear.
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 major advantage is the single sleeve valve's laminar exhaust scavenging and vortex fuel mixture ignition. When the intake ports open, the fuel air mixture enters tangentially to the cylinder. This creates laminar exhaust scavenging, as opposed to turbulent poppet valve scavenging which mixes the exhaust - fresh air fuel mixture intake to a greater degree. A spinning fuel air mixture vortex is also created at TDC which greatly improves ignition.
A minor advantage includes the fact 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: 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. During the "breaking in" period (known as "running-in" in the UK) any imperfections in one are scraped into the other, resulting in a good fit. This type of "breaking in" 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. Unlike a traditional design, the imperfections in the piston do not always line up with the same point on the sleeve. In the 1940s this was not a major concern because the poppet valves of the time typically leaked appreciably more than they do today, so the oil consumption was poor in either case.
The oil consumption problem came from the Knight double sleeve valve. This problem was fixed with the single sleeve valve perfected by Bristol. At TDC, the single sleeve valve rotates in relation to the piston. This prevents boundary lubrication problems, as piston ring ridge wear at TDC and BDC does not occur. The Hercules top end was rated at 50,000 hr[citation needed] at wide open throttle.
[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 improving entirely different designs of internal combustion engine such as the Wankel.
[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', from 1909-1930), Panhard (1911-39), Mercedes (1909-24), Willys (as the Willys-Knight, plus the associated Falcon-Knight and Stearns-Knight), Mors, Peugeot, Argyll (Scotland), and Belgium's Minerva company, some thirty companies in all.[1] Itala also experimented with sleeve valves.
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 F.B. Stearns Company of Cleveland, which sold a car named the Stearns-Knight, and the Willys firm which offered a car called the Willys-Knight, which was produced in far greater numbers than any other sleeve-valve car.
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 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.
Potentially the most powerful of all sleeve-valve engines (that never reached production) was the Rolls-Royce Crecy, a 60 degree, V-12 two-stroke direct injected force-scavenged (turbocharged) aero-engine of 26.1 litres capacity. It achieved a very high specific output, and surprisingly good SFC - in 1945 the single cylinder test-engine (E65) produced the equivalent of 5,000 HP (192 BHP/Litre) when water injected, although the full V12 would probably have been initially type rated circa 2,500 HP.
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.
It is rumoured[citation needed] Keith Duckworth (of Cosworth F1 racing engine fame) experimented with a single-cylinder sleeve-valve test engine when looking at Cosworth DFV replacements.
[edit] Steam engine
Sleeve valves have occasionally been used on steam engines, for example the SR Leader Class.
[edit] See also
- D slide valve
- Piston valve
- Piston valve 2
- Corliss valve
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[edit] References
- ^ Georgano, G.N. Cars: Early and Vintage, 1886-1930. (London: Grange-Universal, 1985).
