Sleeve valve


The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve valve engines saw use in a number of pre–World War II luxury cars and in the United States in the Willys-Knight car and light truck. They subsequently fell from use due to advances in poppet-valve technology, including sodium cooling, and the Knight system double sleeve engine's tendency to burn a lot of lubricating oil or to seize due to lack of it. The Scottish Argyll company used its own, much simpler and more efficient, single sleeve system in its cars, a system which, after extensive development, saw substantial use in British aircraft engines of the 1940s, such as the Napier Sabre, Bristol Hercules, Centaurus, and the promising but never mass-produced Rolls-Royce Crecy, only to be supplanted by the jet engines.

Description

A sleeve valve takes the form of one machined cylinders which fit concentrically between the piston and the cylinder block bore of an internal combustion engine having cross-flow induction/exhaust. These sleeves have inlet and exhaust ports machined in the periphery, analogous to a two-stroke motor. Ports in the periphery of the sleeves come into alignment with the cylinder's inlet and exhaust ports at the appropriate stages in the engine's cycle.

Types of sleeve valve

The first successful sleeve valve was patented by Charles Yale Knight, and used twin reciprocating sleeves per cylinder. It was used in some luxury automobiles, notably Willys, Stearns, Daimler, Mercedes-Benz, Minerva, Panhard, Peugeot and Avions Voisin. Mors adopted double sleeve-valve engines made by Minerva. The higher oil consumption was heavily outweighed by the quietness of running and the very high mileages without servicing. Early poppet-valve systems required decarbonization at very low mileages and were prone to valve spring failure before the later advances in spring technology.
The Burt-McCollum sleeve valve was named for the two inventors who applied for similar patents within a few weeks of each other. The Burt system was an open sleeve type, driven from the crankshaft side, while the McCollum design had a sleeve in the head and upper part of the cylinder, and a more complex port arrangement. The design that entered production was more 'Burt' than 'McCollum.' It was used by the Scottish company Argyll for its cars, and was later adopted by Bristol for its radial aircraft engines and the Halford-designed Napier Sabre. It used a single sleeve driven by an eccentric from a timing axle set at 90 degrees to the cylinder axis. Mechanically simpler and more rugged, the Burt-McCollum valve had the additional advantage of reducing oil consumption, while retaining the combustion chambers and big, uncluttered, porting area established in the Knight system.
A small number of designs used a "cuff" sleeve in the cylinder head instead of the cylinder proper, providing a more "classic" layout compared with traditional poppet valve engines. This design also had the advantage of not having the piston within the sleeve, although in practice this appears to have had little practical value. On the downside, this arrangement limited the size of the ports to that of the cylinder head, whereas in-cylinder sleeves could have much larger ports.

Advantages/disadvantages

Advantages

The main advantages of the sleeve-valve engine are:
  • High volumetric efficiency due to very large port openings. Harry Ricardo also demonstrated better mechanical and thermal efficiency.
  • The size of the ports can be readily controlled. This is important when an engine operates over a wide RPM range, since the speed at which gas 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; this is fairly easy to achieve with sleeve valves, but difficult in a poppet valve system.
  • Good exhaust scavenging and controllable swirl of the inlet air/fuel mixture in single-sleeve designs. When the intake ports open, the air/fuel mixture can be made to enter tangentially to the cylinder. This helps scavenging when exhaust/inlet timing overlap is used and a wide speed range required, whereas poor poppet valve exhaust scavenging can dilute the fresh air/fuel mixture intake to a greater degree, being more speed dependent. Greater freedom of combustion chamber design means that fuel/air mixture swirl at top dead centre can also be more controlled, allowing improved ignition and flame travel which, as demonstrated by Ricardo, allows at least one extra unit of compression ratio before detonation, compared with the poppet valve engine.
  • The combustion chamber formed with the sleeve at the top of its stroke is ideal for complete, detonation-free combustion of the charge, as it does not have to contend with compromised chamber shape and hot exhaust valves.
  • 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. A problem with high-speed engines that use poppet valves is that as engine speed increases, the speed at which the valve moves also has to increase. This in turn 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 poppet valves that allow good air-flow have considerable mass and require a strong spring to overcome their inertia when closing. At higher engine speeds, the valve spring may be unable to close the valve effectively for the required amount of crankshaft degree rotation before the next opening event, resulting in a failure to completely and/or remain closed. Harmonic frequency vibration produced at certain RPM can also cause a resonance with the poppet valve spring greatly reducing its spring strength and ability to quickly and maintain the valve closed and be correctly in time with the reciprocating mass. These effects, called valve float and/or valve bounce could result in the valve being struck by the top of the rising piston. In addition, camshafts, push-rods, and valve rockers can be eliminated in a sleeve valve design, as the sleeve valves are generally driven by a single gear powered from the crankshaft. In an aircraft engine, this provided desirable reductions in weight and complexity.
  • Longevity, as demonstrated in early automotive applications of the Knight engine. Prior to the advent of leaded gasolines, poppet-valve engines typically required grinding of the valves and valve seats after 20,000 to 30,000 miles 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 were also subjected to less intense heat build-up than poppet valves, owing to their greater area of contact with other metal surfaces. In the Knight engine, carbon build-up actually helped to improve the sealing of the sleeves, the engines being said to "improve with use", in contrast to poppet valve engines, which lose compression and power as valves, valve stems, and guides wear. Due to the continuous motion of the sleeve, the high wear points linked to poor lubrication in the TDC/BDC of piston travel within the cylinder are suppressed, so rings and cylinders lasted much longer.
  • The cylinder head is not required to host valves, allowing the spark plug to be placed in the best possible location for efficient ignition of the combustion mixture. For very big engines, where flame propagation speed limits both size and speed, the swirl induced by ports, as described by Ricardo, can be an additional advantage. In his research with two-stroke single sleeve valve compression ignition engines, Ricardo proved that an open sleeve was feasible, acting as a second annular piston with 10% of the central piston area, that transmitted 3% of the power to the output shaft through the sleeve driving mechanism in a Diesel engine. This highly simplifies construction, as the 'junk head' is no longer needed.
  • Lower operating temperatures of all power-connected engine parts, cylinder and pistons. Ricardo showed that as long as the clearance between sleeve and cylinder is adequately settled, and the lubricating oil film is thin enough, sleeves are 'transparent to heat'.
  • Continental in the United States conducted extensive research in single sleeve valve engines, pointing out that they were eventually cheaper and easier to produce. However, their aircraft engines soon equaled the performance of single-sleeve-valve engines by introducing improvements such as sodium-cooled poppet valves, and it seems also that the costs of this research, along with the October 1929 crisis, led to the Continental single-sleeve-valve engines not entering mass production. A book on Continental engines reports that General Motors had conducted tests with single sleeve valve engines, rejecting this kind of arrangement, and, according to M. Hewland also Ford around 1959..
Most of these advantages were evaluated and established during the 1920s by Roy Fedden, Niven, and Ricardo, possibly the sleeve valve engine's greatest advocate. He conceded that some of these advantages were significantly eroded as fuels improved up to and during World War II and as sodium-cooled exhaust valves were introduced in high-output aircraft engines.

Disadvantages

A number of disadvantages plagued the single sleeve valve:
  • Perfect, even very good, sealing is difficult to achieve. In a poppet valve engine, the piston possesses piston rings which form a seal with the cylinder bore. During the "breaking in" period 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 valve stems of the time typically leaked appreciably more than they do today, so that oil consumption was significant in either case. To one of the 1922–1928 Argyll single sleeve valve engines, the 12, a four-cylinder 91 cu. in. unit, was attributed an oil consumption of one gallon for 1,945 miles, and 1,000 miles per gallon of oil in the 15/30 four-cylinder 159 cu. in.. Some proposed adding a ring in the base of the sleeve, between sleeve and cylinder wall, or a Dykes ring on the 'Junk Head'. Single-sleeve-valve engines had a reputation of being much less smoky than the Daimler with engines of Knight double-sleeve engines counterparts.
  • The high oil consumption problem associated with the Knight double sleeve valve was fixed with the Burt-McCollum single sleeve valve, as perfected by Bristol. The models that had the complex 'junk head' installed a non-return purging valve on it; as liquids cannot be compressed, the presence of oil in the head space would result in problems. At top dead centre, the single-sleeve valve rotates in relation to the piston. This prevents boundary lubrication problems, as piston ring ridge wear at TDC and bottom dead centre does not occur. The Bristol Hercules time between overhauls life was rated at 3,000 hours, very good for an aircraft engine, but not so for automotive engines. Sleeve wear was located primarily in the upper part, inside the 'junk head'. Hewland and Logan claimed solving the Oil consumption in their single sleeve valve, single cylinder, 500 cc prototype engine, by adding a ring in the sleeve base and a Dykes ring on Junk Head.
  • An inherent disadvantage is that the piston in its course partially obscures the ports, thus making it difficult for gases to flow during the crucial overlap between the intake and exhaust valve timing usual in modern engines. The 1954 printing of the book by Harry Ricardo The High-Speed Internal Combustion Engine, and also some patents on sleeve valve production, point out that the available zone for ports in the sleeve depends on the type of sleeve drive and bore/stroke ratio; Ricardo tested successfully the 'open sleeve' concept in some two-stroke, compression ignition engines. It not only eliminated the head rings, but also allowed a reduction in height of the engine and head, thus reducing frontal area in an aircraft engine, the whole circumference of the sleeve being available for exhaust port area, and the sleeve acting in phase with the piston forming an annular piston with an area around 10% of that of the piston, that contributed to some 3% of power output through the sleeve driving mechanism to the crankshaft. The German-born engineer Max Bentele, after studying a British sleeve valve aero engine, complained that the arrangement required more than 100 gearwheels for the engine, too many for his taste.
  • A serious issue with large single-sleeve aero-engines is that their maximum reliable rotational speed is limited to about 3,000 RPM, but the Mike Hewland car engine was raced above 10,000 rpm without toil.
  • Improved fuel octane, above about 87 RON, have assisted poppet-valve engines’ power output more than to the single-sleeve engines’.
  • The increased difficulty with oil consumption and cylinder-assembly lubrication was reported as never having been solved in series-produced engines. Railroad and other large single sleeve-valve engines emit more smoke when starting; as the engine reaches operating temperature and clearances enter the adequate range, smoke is greatly reduced. For two-stroke engines, a three-way catalyst with air injection in the middle was proposed as best solution in a SAE Journal article around the year 2000.
  • Some feared the build-up of heat inside the cylinder, however Ricardo proved that if only a thin oil film is retained and working clearance between the sleeve and the cylinder was kept small, moving sleeves are almost transparent to heat, actually transporting heat from upper to lower parts of the system.
  • If stored horizontally, sleeves tend to become oval, producing several types of mechanical problems. To avoid this, special cabinets were developed to store sleeves vertically.
  • Equivalent implementations of modern variable valve timing and variable lift are impossible due to the fixed sizes of the port holes and essentially fixed rotational speed of the sleeves. It may be theoretically possible to alter the rotational speed through gearing that is not linearly related to the engine speed, however it seems this would be impractically complex even compared to the complexities of modern valve control systems.