Lycoming exhaust valve breakage -- Intro.txt - Bloc-notes

For over 30 years, Lycoming's parallel valve cylinders have experienced exhaust ... closely related to the volume of oil that flows to the rocker box of each.
13KB taille 14 téléchargements 321 vues
Lycoming exhaust valve breakage -- Intro.txt Lycoming exhaust valve breakage -- IntroLycoming exhaust valve breakage -- Intro For over 30 years, Lycoming's parallel valve cylinders have experienced exhaust valve and guide problems that resulted in either valve sticking and/or advanced valve guide wear that led to compression loss or valve disintegration. Numerous Lycoming publications have addressed the problem, but no single solution (or cause) has ever been found. The burden has been shifted to aircraft owners via the infamous "wobble test" (Service Bulletin 388B) that is considered mandatory every 400 hours on every engine the company makes. This may soon change. California based Grumman Tiger owner Bill Marvel and engine overhaul specialists Bill and Carol Scott of Precision Engine in Owensboro, Kentucky have made a discovery that is sure to turn heads. During the past two and a half years, they have been investigating the Lycoming valve guide wear problem and have made several very interesting observations. They are convinced that the problem is closely related to the volume of oil that flows to the rocker box of each cylinder and thus to the amount of valve and guide heat that oil can remove. Specifically, they have found an inverse correlation between exhaust valve guide wear and the volume of oil delivered to the rocker boxes (the more the oil, the less the wear). Additionally, they have discovered that the number 1 and 3 cylinders (those on the copilot side) invariably receive less oil than do the even numbered cylinders, and have correspondingly higher incidents of valve guide distress. These findings led them to the most interesting investigation of all -- Lycoming's oil system design in general and their mushroom style hydraulic lifter design in particular. Marvel and the Scotts researched the history of hydraulic lifters and learned that the Lycoming design is a variation of an automotive component first used in 1929. This automotive unit was initially developed for an in-line engine, which did not use either pushrods or rocker arms, components that are employed in present day aircraft powerplants. Lubrication was supplied to the valves in this engine by splash oil thrown off the crankshaft. When this automotive lifter design was later modified by Lycoming for use in their overhead valve engines, they did not provide any unrestricted oil flow path through or around the lifter to the rocker boxes, even though splash oil to the valves from the crankshaft was no longer available in their engine configuration. As a result, the only existing oil flow paths are past very tight clearances that allow only a tiny volume of oil to be provided for lubrication and cooling of the valves. This is the fundamental cause of the low oil flows to the rocker boxes that the Scotts and Marvel have measured. Interestingly, their efforts have revealed that Continental, in using this same type of automotive lifter for their aircraft engine, did incorporate an unrestricted flow path through the lifter that provides considerable oil to the rocker boxes. Very significantly, Lycoming's own Service Instruction 1479, issued in January 1996, appears to substantially corroborate, however inadvertently, Marvel and the Scotts' findings. This S.I. modifies the cylinder heads of Mooney TLS engines to incorporate oil cooled exhaust guides to address a problem of prematurely worn exhaust valves and guides. Marvel and the Scotts believe this modification, or a similar one, should be incorporated on most Lycoming parallel valve engines.

Pge p

Lycoming exhaust valve breakage___a differing point of view.txt Lycoming exhaust valve breakage...a differing point of viewLycoming exhaust valve breakage ...a differing point of view [Reprinted from the August 1996 Australian AOPA magazine] by JG (JOCK) MCLEAN, AOPA 26788 I have been following the correspondence about the Lycoming exhaust valve breakage and wish to add the following information to the debate. It is possible to calculate the rate of heat transfer from the cylinder head to the engine oil, if the following assumptions are made. An amount of 100ml of oil per cylinder is used for the minimum oil flow to the valve gear as suggested by Ian Findlay (AOPA May 1996). Lycoming data is used for the temperature of the oil reaching the cylinder head for cooling which is 82 deg. C (controlled by the oil temperature thermostat) and is assumed to reach the maximum and red line temperature of 118 deg. C as it flows over the head. It can then be calculated that this oil flow picks up head at the rate of 119 watts per cylinder. The method of calculation is available for readers who may be interested. In a 180 horsepower four cylinder O-360 engine, the oil flow over the head and valves will collect only 476 watts of heat. At 75 percent power this engine puts 100,000 watts (100kW) into the propeller. It also passes approximately 117,000 watts into the cooling air and a similar amount out in the exhaust. This is the amount of heat absorbed by the oil as it passes over the whole cylinder head would make no significant contribution to the cooling of the exhaust valves as it is negligible when compared with the total heat output of the engine. Even the argument for a localized cooling effect is not sustainable as the 476 watts is minuscule when compared with the 117,000 watts dissipated into the cooling air. A similar situation holds true for other Lycoming engines. It is interesting to note that the Lycoming 320/360/540-series of engines have been very successful when installed in Cessna or Piper airframes. In the case of the 320 engines have been very successful when installed in Cessna or Piper airframes [sic]. In the case of the 320 engines, some flying-schools operating on engine life extension 288 programs have engines running well beyond the normal 2000 hour TBOs without any problems with exhaust valves. Flying-school operations are not easy on engines and no top-overhaul is necessary on these programs. If is necessary to look further at the possible causes of premature failure of exhaust valves in the Lycoming engines referred to by Ian Findlay. The exhaust valves essentially run dry in the guides and lubrication is not an issue. Some aircraft engines have no oil pressure feed to the valves, but rely for lubrications on splash from oil accumulated in the rocker covers. When compared with the Cessna and Piper range, it is interesting to note how tightly cowled some other makes of aircraft appear. While these tightly cowled planes may maintain the Lycoming-designated air pressure differentials across the engine, this gives no information about the actual mass of cooling air flowing over critical parts of the engine, such as the cylinder head. One problem with Lycoming engines has been exhaust valve sticking. To counter this problem some engine overhaul shops have reamed out the exhaust valve guides to give additional clearance to the valve stem. However, unless this is done carefully and the correct guide-to-stem clearances are maintained, then problems arise. It is essential that good contact is maintained between the valve stem and valve guide so that maximum heat transfer can take place to the cylinder head and ultimately to the cooling air. If the valve guides are over-enlarged, then rapid and premature wear will follow. The information about the Mooney TLS (Lycoming Service Instruction 1479) and the associated oil cooling of the exhaust valve guides must be put into perspective and may not be relevant to the problems identified by Ian Findlay. Because the Mooney TLS cruises at altitudes where the air density is reduced, then the mass or weight of air flowing through the engine is significantly reduced, resulting in decreased cooling. This is of particular importance when combined with the long climb, at high power settings, to cruising altitude. Because the turbocharged engine is able to maintain its power output at altitude, this Pge p

Lycoming exhaust valve breakage___a differing point of view.txt reduction in mass air is an important factor which must be taken into account when designing the cooling system. One solution is to rely on oil cooling in critical areas, such as the cylinder head. These problems are not usually encountered in normally aspirated engines operating below 10,000ft. In relation to Lycoming Service Bulletin 388B, I believe that is was introduced to cover certain situations in engines installed in helicopters. These engines are more highly stressed, for example some of the turbocharged 360-series for helicopters have RPM rated as high as 3,200. These severe operating conditions result in significantly lower TBOs when compared with similar engines in fixed-wing aircraft. Again, it is unlikely that SB 388B is relevant to the problems described by Ian Findlay. The issue of what constitutes the acceptable cylinder head temperature is worthy of further consideration. For example, the Lycoming Operator's manual for the 360-series engines quotes a maximum cylinder head temperature of either 475 deg. F or 500 deg. F, depending on the engine type. However, it qualifies this in all cases by stating, "For maximum service life of the engine maintain cylinder head temperature between 150 deg. F and 400 deg. F during continuous operation." There is a message here. I suggest that this is not a general problem with exhaust valves in the Lycoming 320/360/540 range of engines. Careful reading of CASA's airworthiness Advisory Circulars - Summary of Defects does not support the existence of a particular problem with these exhaust valves. Finally, many operators of fleets of Cessnas or Pipers with these engines claim to have a satisfactory history. For example, one operator maintaining a fleet of about 25 Pipers could only recall one exhaust valve problem in 14 years, with his fleet often exceeding 3,000 hours per month. Another with a number of Cessna 172s claimed that they always reached the normal 2000 hour TBO without any problems with the exhaust valves.

Pge p

More Lycoming Exhaust Valve Breakage Observations.txt More Lycoming Exhaust Valve Breakage ObservationsMore Lycoming Exhaust Valve Breakage Observations by Bill Marvel and Bill Scott, USA Jock McLean's article in the August issue contained a number of interesting aspects that we would like to address. Although it was intended as a response to reader Ian Findlay's comments in the May issue, the article involves an area that we have studied extensively; hence our desire to reply. First and foremost, we must emphasize that the significant question here is how much heat is carried away from the exhaust valve and valve guide by air and oil, not from the cylinder head itself. It is, after all, the valve and guide that are experiencing failure due to excess heat, not the cylinder head. Whether cylinder head heat rejection by airflow is 117,000 watts or one million watts, it matters little if that does not include sufficient exhaust valve and guide heat to keep the temperature of these components in check. Our investigations have shown, as has Lycoming's S.I. 1479 (oil cooled exhaust valve guides for the Mooney TLS), that with normal cylinder head temperature, premature valve and guide failures are occurring due to excess heat. As an example, the Mooney TLS encounters CHT levels in cruise of 380 to 420 (F), indicating normal cylinder head cooling. And yet its engine routinely experiences compression loss due to exhaust guide wear at about 400 hours or less. Why? Because of the inability of the cylinder head to transfer sufficient valve and guide heat to the atmosphere while operating at a normal cylinder head temperature. Mr. McLean's view is that our recommended minimum oil flow to the rocker boxes has a minuscule effect in cooling the cylinder head, and we fully agree. However, when this oil flows directly onto the exposed exhaust valve and guide it has a very significant cooling effect on them. It is through the use of this concept that Lycoming solved the TLS problem. Consider this simple, real-life analogy. If one held a safety pin with his fingers and heated its point red hot, it would be a few moments before enough heat transferred out of the pin for it to be safe to touch. On the other hand, if one merely dunked the red hot pin into a single drop of water, the pin would cool almost immediately. It would do this because it has a low thermal mass and water rapidly transfers heat from the pin into itself. In like manner, the thermal mass of the exhaust valve is very small compared to that of the cylinder head. While air in tremendous volume is required to cool the cylinder head, only a relatively small amount of oil making direct contact with the very hot valve stem and valve guide will effectively cool those components. Based upon the above analogy, it is important to note that this limited oil flow most certainly is not intended to cool the entire cylinder head. Overall heat input to, and rejection from the head via air flow is massive, as Mr. McLean correctly states. Oil cooling of the guide and valve is minimal by comparison. However, please note that inlet oil makes its very first contact with a hot object at the guide and valve stem and immediately transfers heat away from these components. Since the valve stem is not very well thermally coupled to the cylinder head (the Mooney TLS problem in a nutshell), this additional oil becomes highly significant from the standpoint of cooling the valve itself. Think about it for a moment. If the problem had been caused by insufficient air flow through the cowling, as the article suggested, wouldn't Lycoming simply have told Mooney to increase air volume? Wouldn't the design of the air cooling system be faulted and then altered to function properly? Instead, Lycoming redesigned their own oil system to provide additional oil cooling of the valve and valve guide and did so at their own expense, not Mooney's. This alone is quite telling. Second, as we have already written in your magazine, the mission of the aircraft is of paramount importance in determining whether or not a given engine will encounter valve and guide distress. Prolonged exposure to cruise power settings with leaned mixtures, even with normal CHT levels, contributes substantially to this problem. It was noted in the article that many flight school aircraft in Oz Pge p

More Lycoming Exhaust Valve Breakage Observations.txt easily reach TBO and beyond, as is the case in this country also. However, from a thermal standpoint as seen by the exhaust valve and guide, the mission profile of a training aircraft is quite benign. High power is used only occasionally for takeoff and power-on stall practice and is immediately followed by a cooling cycle. Prolonged flight at cruise power and mixture settings for these aircraft is much less common. And yet it is this latter operation that bears directly on the valve and guide distress we have investigated. (Incidentally, we have discovered that in the O-320H engines used in many Cessna 172 aircraft, that the barrel type of lifter in that engine flows approximately ten times as much oil to the rocker boxes as do the mushroom style lifters used in most Lycoming engines. The O-320H, although earlier plagued with cam and lifter spalling problems, has a very good history of upper end longevity.) A third area raised by Mr. McLean's article involves S.B. 388B, which is the infamous "wobble check." Although he believes that it originally came about due to guide wear in turbocharged helicopter engines, the current situation is much different. This S.B. now applies to every engine Lycoming manufactures regardless of horsepower, number of cylinders, turbocharged or normally aspirated, or airframe in which it is to be installed. Regarding helicopters, take the Robinson R-22 for instance. This normally aspirated O-320 engine operates at about a 350 (F) cylinder head temperature due to excellent shrouded, forced air cooling. Despite the low CHT and the normally aspirated induction system, this engine installation frequently requires valve and guide replacement at about 400 hours. It experiences this problem because of the prolonged heat soaking phenomenon mentioned earlier. As a helicopter engine, it of necessity operates continually at about 75% power, which takes its toll on guides and valves despite its comfortable CHT. Again, the problem is that the parallel valve cylinders, by their very design, are unable to adequately transfer exhaust valve and guide heat to the cylinder head in some operational situations without the added benefit of oil cooling augmentation. And this leads to our final comments. As was stated in the article, Lycoming publishes that, "For maximum service life of the engine, maintain cylinder head temperature between 150 F and 400 F during continuous operation." Mr. McLean goes on to say that, "There is a message here." We agree, but would like to go one step further and decode it -"The Lycoming parallel valve cylinders are marginal in their ability to shed heat from the exhaust valve and guide during some operational conditions despite normal CHT levels. If you operate in these areas, the boundaries of which we cannot define, you will experience premature valve guide wear and subsequent compression loss. If you own a Mooney TLS we will correct this problem by installing oil cooled exhaust valve guides to augment the heat transfer ability of the cylinder head. But if you own another aircraft with an engine with the exact same cylinders as the Mooney TLS and have the exact same problem, we have no solution. Our recommendation is to operate at a sufficiently low power and corresponding cylinder head temperature that the cylinder head can be expected to transfer sufficient heat from the valve and valve guide to prevent their early failure. CHT levels between 150 and 400 F ought to do it, but we cannot be certain of this fact."

Pge p