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lapuwali |
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Not another one! ![]() ![]() ![]() ![]() Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743 ![]() ![]() |
In another thread, TimT posted a graph produced by Pratt & Whitney showing the relationship of CGT, EGT, mixture, and power in air-cooled aero engines. Despite the often held assertion here and elsewhere that "our" engines need to run rich to stay cool, the chart showed that CHT fell as the mixture got leaner from "best power" (about 13:1). It also fell as it got richer from best power. There was a question in my mind how well this generalized to the Type IV.
My 1.7 w/ Webers had been, I knew, running rich on the idle circuits, and I was waiting on a set of leaner idle jets. These showed up, and I installed them this morning (50s down from 55s). Crisper running, and I hope much better gas mileage (was below 20mpg). My CHT gauge held the real surprise, however. It's installed on the hottest cylinder (No. 3), and previously warmed up very quickly to 300dF after just 30 seconds or so after a cold start. It would climb to 350dF as soon as any load was put on the engine, and up to 400dF when pulling up a two-mile long 6% grade near my house. With the leaner idle jets and no other changes, it was much slower to warm up, and never exceeded 275dF idling and puttering around the neighborhood at 25mph. Up that long hill, it just barely touched 350dF. I don't have a wideband mixture meter (yet), so I have no idea if I'm still rich or somewhat lean. Drivability is still good, so it can't be all that lean. I haven't yet tried to do a plug check (used to be sooty black). In any case, it's one data point that seems to support the P&W chart. |
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TimT |
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retired ![]() ![]() ![]() ![]() Group: Members Posts: 4,033 Joined: 18-February 03 From: Wantagh, NY Member No.: 313 ![]() |
Well I knew the air frame mechanics would come out and say this and that..
Yes they have magneto ignitions with static timing... and they dont operate at over XXX rpm....I hope your car has full ignition advance at 55mph... However there are some things the .49cc engine you had as a kid (your model airplane engine), the engine in your 914, and the lycoming engine, or any water cooled engined all have in common... Laws of thermodynamics..... AVweb has a host of incredibly informative articles a portion is quoted here QUOTE ie #7: Aggressive leaning results in burned valves and detonation. Fear of the red knob is one of the most pernicious areas of misinformation among general aviation pilots. Most pilots operate way too rich most of the time, and do so because of the mistaken belief that leaning will harm their engine. The result is usually trouble: fouled spark plugs, accelerated exhaust valve guide wear, and stuck exhaust valves. Lycoming has long authorized leaning to peak EGT at any cruise setting up to 75% power. TCM authorizes leaning to peak EGT up to 65%, and its latest recommendations even endorse lean-of-peak operation for many big-bore engines, provided the engines will run smoothly when operated that lean. Contrary to popular belief, aggressive leaning doesn't cause burned valves. Most burned valves are the result of excessive valve guide wear or valve stem contamination. Aggressive leaning doesn't cause detonation, either. Most of our engines are incapable of detonation at cruise power settings, provided that we don't exceed CHT red-line or try to burn contaminated fuel. Furthermore, recent tests on Lycoming engines by ASTM revealed this fascinating result: detonation is most likely to occur at a mixture setting 11% richer than stoichiometric (i.e., substantially richer than peak EGT). Lean as aggressively as the book allows. For Lycomings, that means peak EGT at all cruise power settings to 75%. For Continentals, lean to peak EGT up to 65%, 50°F rich of peak at 75%. For turbocharged engines, also limit TIT to 1600°F. Lean during all ground operations except for engine start. It is particularly important to lean for taxi and runup. Since EGT is usually off-scale at idle power, the best method is to lean for peak RPM at idle. once the engine has climbed the hill... brought the boat onto a plane, or got the plane into the sky,, the engine can "relax" and doesnt need to produce max hp per unit of fuel...Savings of a few GPH cant be ignored... All this run lean stuff doesnt mean shit with carbs...with EFI I can optimize the mixture at any rpm, manifold pressure etc... BTW I would only use lean run stuff on a street car.. |
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