---------------------------------------------------------- Engines-List Digest Archive --- Total Messages Posted Sun 06/25/06: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 09:01 AM - Re: Lycoming Thunderbolt Engines (Gary Casey) 2. 09:47 PM - Re: Lycoming Thunderbolt Engines (Red Hamilton) ________________________________ Message 1 _____________________________________ Time: 09:01:24 AM PST US From: Gary Casey Subject: Engines-List: Re: Lycoming Thunderbolt Engines --> Engines-List message posted by: Gary Casey From the sound of some of the posts our friends at Lycoming and Continental haven't been reading the technical literature or have been asleep for the last 50 years or so. While they could perhaps be more progressive than they have I'm not so sure that some of the "easy" fixes make as much sense in aircraft engines as it sounds: 1. Roller-tipped rockers will reduce wear on the valve tip and will reduce side loading on the valve. that is especially important for engines with small-diameter stems, high spring loading and short- radius rockers (such as when increasing the valve lift of an automotive engine). Aircraft engines have relatively long rockers and low spring loads, so I don't see any critical need. It would reduce the wear and increase the life of the rocker arm (which now usually last over 4,000 hours), but it would introduce other failure modes. Aircraft engines are designed, as much as possible to have soft failure modes, such as the wear-out of a component. A roller tip, because of the higher contact forces, must be hardened and it and its ancillary components could break, putting hardened steel shrapnel into the engine. I don't think the increased horsepower (which would be miniscule) would be worth the risk. The same is true for roller follower, but these seem to be endorsed by Lycoming so perhaps the potential failure modes are addressed; I don't know. I would be reluctant to use needle-bearing rocker pivots for the same reason. 2. One big advantage of roller followers is that they allow a concave cam profile to be ground, increasing acceleration rates. If the spring loads were increased this would have a positive impact on the design of the cam profile, allowing a shorter overall duration for a given breathing capability. This would provide measurable improvements in BSFC, BMEP or both. Does anyone use concave profiles with the roller followers? I doubt it - it requires much more expensive cam grinding equipment. Is the power increase worth the trouble of going to roller followers? One clue would be for someone that changed to rollers to compare oil temperatures before and after. Oil temperatures are a function of lots of variables, but any friction savings that would result in a measurable power increase would have to also result in substantially less heat going into the oil. I'd be surprised if the gain were anywhere near 1 horsepower per cylinder. 3. Possibly the reason Lycoming endorses roller followers is the reduced probability of galling after extended periods of non-use. This has been a chronic problem with the Lycoming high-mounted camshaft and roller followers might be the only answer. 4. Aluminum rockers? Aluminum has a finite fatigue life, unlike steel, so aluminum can be effectively used in drag engines and other finite-life engines. I wouldn't use aluminum for this application in an aircraft engine. This is especially true since the likely failure mode is a fractured rocker, disabling one cylinder without warning. 4. One limitation to high valve accelerations is the length of the pushrods and these apparently are small enough in diameter to be prone to buckling. Very little that can be done without increasing their diameter. 5. Broken crankshafts? Don't know the root cause, so I can't comment intelligently. Certainly there isn't enough money in the aircraft engine business to pay for exotic engineering tools that are used in the automotive business, and weight sensitivity precludes burying uncertainties in cast iron (the old saying is that doctors bury their mistakes in the ground while engineers bury their mistakes in cast iron). More careful control of process parameters is probably the crutch that has to be used. 6. I can't comment on "machining tolerance", but aircraft engines can swallow a lot of garbage. Often the first sign of cam wear is a rough engine and inspection will show that the cam lobe is gone. For some time the engine tolerated that much iron and didn't fail. Fairly large clearances coupled with high-viscosity oil will allow such things. Do we want to give up damage tolerance for being able to use smaller crank journals or to increase the loading? Maybe not. I really like the idea of using a high-tech automotive engine in an aircraft and and did a lot of analysis. In the end I concluded that I could get equivalent power and durability and almost equivalent BSFC, but I couldn't avoid a 50-pound weight penalty. A big-bore air- cooled direct drive engine is a pretty good match for most aircraft. Gary Casey > > .....has been using a variety of these for about 50 years. > They are available in extruded aluminum, forged aluminum, and > stainless, > fully rollerized, or bushed and roller tipped, and in any ratio > desired. > (After correcting the ratio on a number of aircraft rockers, it is > obvious that > the factory does not pay close attention to this) > As far as strength, no contest here: Where an aircraft may have open > pressure > of approx. 300lbs, we run open pressures of around 1000 lbs. > Another significant advantage to the roller rockers is reduced valve > guide wear. > Not sure of HP gains on a slow turning ac engine, but might be > interesting to dyno each. > Archie ________________________________ Message 2 _____________________________________ Time: 09:47:56 PM PST US From: "Red Hamilton" Subject: Re: Engines-List: Lycoming Thunderbolt Engines Archie, If you ever do that please let us know. Thanks, Red Hamilton ----- Original Message ----- From: Archie To: engines-list@matronics.com Sent: Saturday, June 24, 2006 6:45 AM Subject: Re: Engines-List: Lycoming Thunderbolt Engines Two companies tried that in aviation, and failed. There are a series of parameters that do not directly apply to aircraft use, but for the most part, why re invent the wheel, when the racing industry has been using a variety of these for about 50 years. They are available in extruded aluminum, forged aluminum, and stainless, fully rollerized, or bushed and roller tipped, and in any ratio desired. (After correcting the ratio on a number of aircraft rockers, it is obvious that the factory does not pay close attention to this) As far as strength, no contest here: Where an aircraft may have open pressure of approx. 300lbs, we run open pressures of around 1000 lbs. Another significant advantage to the roller rockers is reduced valve guide wear. Not sure of HP gains on a slow turning ac engine, but might be interesting to dyno each. Archie ----- Original Message ----- From: n801bh@netzero.com To: engines-list@matronics.com Sent: Saturday, June 24, 2006 2:12 AM Subject: Re: Engines-List: Lycoming Thunderbolt Engines Geez,,, Roller rockers are pretty easy to fabricate, test and furnish.. Cheap HP gains from them too.. You would figure a company like Lycoming, with 60+ years of supposable R&D would have addressed that simple thing years ago. Now,,, if they can get their crankshafts from breaking they might go somewhere. You can bet if one does break on this new line of experimental engines the their response will be "_uckoff" ,,,,Their is something wrong with in installation.. They will not stand behind their certified engines.!!!!!!!!!!!!!!!!