|05-18-2013 08:13 PM|
why would there be no EGT guage? without an EGT there can only be a guess as to exhaust temp(s)
I ask a gain what type of A/C? cowls can cause cooling issues.If your comments were directed at me?
(For those not familiar with "lean of peak" operations.)I do understand and thats why I asked where the EGT sensor was located.
Is this your A/C? lots of the A/C engines still use sodium cooled exhaust valves.also if run at higher altitudes,the engine does not run very hot at all,for obvious reasons: sub zero temps and 40% loss of power or more
|05-18-2013 07:33 PM|
This is a normally aspirated Lyc, o-360. installed in a certified aircraft. Leaned IAW Manufacturer specs.
No EGT gauge. Due to the rising cost of fuel, leaning has become a "must do" kind of thing.
Had it been an IO-360, it could have been run "lean of peak" which would have done a better job of scavenging the lead, and not caused the fouling which led to the sticky/burned valve.
For those not familiar with "lean of peak" operations. The object of the game, is to lean the mixture untill the EGT peaks, (highest temp) then continue to lean untill it drops, by some amount (between 10 and 20 degrees). Problem is that carburetted engines, can't do that. There will be one cylinder running way rich, and one cylinder running way lean, and the others falling somewhere between, and one of those will be right at peak. With fuel injection, (port, not TB) you can get an even fuel flow to all cyls. Then you can successfully run LOP. Cars do it with a computer, and myriad sensors, piston single aircraft do it with a knob on the insrument panel, and an EGT gauge. Generally, once you reach your cruising altitude, you lean the fuel mixture untill the EGT gauge reaches it's highest temp, then continue to lean, untill the temp falls back down by some number of degrees. As long as all cylinders are running within a few degrees of each other, all is well.
Problem is with a carburetted engine, you cannot get all cylinders running at the same temp, so you must run them all rich. (some richer than others) Generally the ones that present the worst lead fouling problems are the ones running the coolest. Not running hot enough to burn off (scavenge) the lead, and the lead gets deposited in places that it does not need to be, such as valve stems.
|05-18-2013 02:15 PM|
If its carbed(o-360) then how is the mixture set? is the E.G.T. guage accurate and in what cylinder?Has there been any mods to the cowl? The rear cylinders can run hotter.If the aircraft is a homebuilt like a lancair/glassair then over heating of the rear cylinders can be attributed to many things under the cowl.
not enough information about the plane and how the EGT is monitored. Long x country trips may need multiple adjustments to make up for air temp and angle of attack as fuel burns off. also humidity,of carb heat was left turned on,,,,,,,,
|05-18-2013 01:40 PM|
|T-bucket23||Loose spark plug can also cause burnt valves. allows cold outside are to be sucked into the cylinder which causes some thermal issues and leans the mixture way out.|
|05-17-2013 08:25 PM|
The point that I was trying to make is to not fall into the OWT of lead in your fuel being beneficial to your engine. It is not, and never will be.
They quit makin' 80 octane avgas back in the very early '70s. An I haven't seen th' green stuff in as many years. so 100LL is all ya get, Some places offer mogas, but it's not legal to run it without the STc, and not all engines can get it, and in some cases, while you can get the STC, compliance is cost prohibitive. In some cases the STC is a simple matter of buying the paperwork, and a logbook entry. In other cases, it's all that plus extra fuel pumps, and different hoses, etc. And in other cases, it's not even available.
Nowadays, it's a moot point due to the alcohol in car gas. Which is a huge no-no in aircraft.
|05-17-2013 12:36 AM|
One of the "everyone knows that" areas of "common knowledge' (which means "usually wrong") is that the lead was the lubricant for the exhaust valve. (oil would be burned off due to the temps of the valve) so only lead would do the job to provide a "babbit bearing" sorta lube for the valve stem and for the actual valve as it rises out of and sets back into the seat.
Tetraethyl lead was a main component in raising the octane of gas from the abysmal levels of pre-WWII to the stuff that hi-po engines could actually use.
When lead was abolished from gas in the 70's, many engines experienced valve recession and some also had greatly increased wear on valve stem guides and so the "common knowledge" came about.
I assume this is a Lycoming IO-360? (The one I owned for a while in a PA-28 was the 150 hp version and only needed 80/87 fuel) As I recall, the 100LL usually lead-fouled the plugs something fierce, but was a compromise fuel supposedly better than the (then common) 100/140
|05-16-2013 05:48 PM|
The only reason for running it in the 8.5:1 engines is that the STC to allow car gas, specifies that it must contain exactly 0% alcohol, and must be tested for it (simple to do), But that's not readilly available just anywhere any more.
Apparently lead is the only approved antiknock compound approved by th' manufacturer.
What prompted this thread, was the fact that a friend carried a set of 289 ford heads in to be worked. And the guy at the shop told him that If he didn't install hardened seats, or run lead additive in the fuel, he could not warrenty against burned valves within 10k-12k miles.
Airplane engines running mid grade unleaded, (alcohol free) have none of these problems, like sticky valves, fouled plugs, grey goo in the oil, sticky grey stuff on the underneath, etc.
Most all of them have hardened seats of one kind or other.
With a bore and stroke of 5.125"(bore) and 4.375" (stroke) and a ratio of 8.5:1 the actual compression using an automotive typ compression tester, is pretty high, (plugs pulled, turning with starter, 3 rounds) about 145-150.
I would expect it to be higher when idling at ~600 rpm.
Anyway, why would someone reccomend using lead additive, to prevent burned valves, when lead deposits, cause the sticky/burned valve trouble.
|05-15-2013 07:41 PM|
Lead in fuel was found to prevent seat erosion. But, it also contributes to sticking valves by both gumming up the lower part of the guide and stem as well as by contaminating the oil which gets into the upper guide. Add that to typical hotter valve temps of an air-cooled engine vis-a-vis a liquid cooled with a little tight guide clearance and you've got a recipe for a sticking valve. Then add to that an airplane motor, like a boat motor, works a lot harder than the typical car motor. Take off power, everything its got till you’re a up a ways, climb power, almost everything its got till you get to altitude which might be a while, cruise power what 60-70 percent of everything's its got till you throttle back for descent. If you operated the typical car engine that way it would be junk and a couple months.
Is this low lead av gas or something else? And I guess with such a low compression motor why lead at all?
|05-15-2013 05:21 PM|
70 is good using a differential pressure guage. 80 psi is about the max that a person can keep the crank from turning. so holding 70/80 is pretty good. It's a leakdown test, you can listen where the air is escaping, and determine what's wrong.
In this particular case it's a 360 cubic inch aircraft engine. Has 550 hours since rebuild. (should have gone over 2000 hours)
Found the trouble to have been caused by a combo of two things. #1 leaded fuel, and #2 valve guides on the tight side, but still within allowance.
Lead buildup on the valve stem, caused stickin' valve, thus causing burned valve. (note the discolored stuff on the intake valve That's lead)
That's ok because he can get a complete cylinder assembly New from the engine manufacturer, including new piston, and rings, for about $1100 +/- a few bucks.
Now just exactly how izzit that lead is supposed to prevent valve troubles?
|05-14-2013 08:12 PM|
but possibly A vacuum leak on that one cylinder . How many miles on this motor?
Is this a test?
|05-14-2013 07:48 PM|
Wazzit (type of engine, etc.)?
70 is still low if you're talking psi.
Valve adjusted too tight will overheat/burn/warp an exhaust valve due to insufficient seat time...
|05-14-2013 07:33 PM|
Not sure what Happened, but here ya go....
I tried to post this as new thread, but somehow it ended up as reply to one of my own threads, totally unrelated. ???
Anyway, Burned exhaust valve. Cast iron seat, Stainless valve, leaded fuel.
Low compression engine (~8.5-1). Compression on that cylinder was 41. Should have been at least 70, the others were 75+. Leakdown proved it to be exhaust valve.
I have an idea as to the cause. But I want to hear from someone else.
I'll post the findings as soon as I find them.