Dispelling myths on engine operation

Jaybird180

Jaybird180

Final Approach
Joined
Dec 28, 2010
Messages
9,034
Location
Near DC
Display Name
Display name:
Jaybird180
If all you know about an engine is that it has burned exhaust valves, what conjectures and speculation can be made about the operation of the engine and causes of the burned valves?

I'm not seeking to stir any pots here, but I am trying to plug holes in my own knowledge.

Can we eliminate LOP operations as a cause? How about excessive idling at full-rich? Running takeoff power on a cold (lukewarm) engine?

Looking for causes and also things that it cannot be (but common OWTs say it is).
 
Jaybird180 said:
If all you know about an engine is that it has burned exhaust valves, what conjectures and speculation can be made about the operation of the engine and causes of the burned valves?

I'm not seeking to stir any pots here, but I am trying to plug holes in my own knowledge.

Can we eliminate LOP operations as a cause? How about excessive idling at full-rich? Running takeoff power on a cold (lukewarm) engine?

Looking for causes and also things that it cannot be (but common OWTs say it is).
Click to expand...

There is no effective way of eliminating valve guide wear, and that is the primary cause of burned valves. When the valve guides wear the valve will not set on the valve seat properly and transfer its heat. If it can't do that it will over heat and warp or simply melt.
 
Thanks Tom. Didn't even cross my train of thought.
 
yeah, before starting a religious debate on pilots and levers, the first thing to do is to follow the maintenance manual. If you have a lycoming the valve wobble test should be a regular part of your vocabulary.
 
JHW said:
yeah, before starting a religious debate on pilots and levers, the first thing to do is to follow the maintenance manual. If you have a lycoming the valve wobble test should be a regular part of your vocabulary.
Click to expand...

Most pilots/owners will wait until they have a problem before they will have an A&P do the wobble check. then it is simply a verify the reason to replace the cylinder.
 
It is my belief that valve guide wear can be reduced by running 2 cycle oil in the gas.

One part per thousand.
 
As I understand it the valve wobble test is something you have to request and not expect your A&P to suggest. Is that correct?

I have been told that if the engine habitually runs rough for a set time after start then settles down that can be an indication of worn valve guides, but I don't know how rough it needs to be to make that determination.

Good thread I know the IO-360 can be bad about valve guides.
 
kgruber said:
It is my belief that valve guide wear can be reduced by running 2 cycle oil in the gas.

One part per thousand.
Click to expand...
same here except more like 300:1. Helps keep bladders supple too

It's funny, people will tell me I'm crazy to do that but the same people use MMO which is a total unknown during combustion. OTOH 2-stroke oil is designed to avoid deposits and not adversely affect detonation margin. I guess there is not enough "mystery" about it for some people.
 
JHW said:
same here except more like 300:1. Helps keep bladders supple too

It's funny, people will tell me I'm crazy to do that but the same people use MMO which is a total unknown during combustion. OTOH 2-stroke oil is designed to avoid deposits and not adversely affect detonation margin. I guess there is not enough "mystery" about it for some people.
Click to expand...


Exactly!

MMO? Might just as well spray WD40 in there.
 
Any, I mean Any combustable substance that is entering the combustion chamber will be consumed during the combustion process, and will do nothing for the exhaust valve.

In excessive dosages it can raise the BTU content of the mixture and cause damage.
 
There is sometimes, maybe often a relationship between burning ex valves and ignition timing. Of course, there is still a great deal of heat present during the exhaust stroke, but if the timing is off, that heat propagation can have an affect on the valve, seat and guide. Not always, but it's something to check.
 
Tom-D said:
Any, I mean Any combustable substance that is entering the combustion chamber will be consumed during the combustion process, and will do nothing for the exhaust valve.

In excessive dosages it can raise the BTU content of the mixture and cause damage.
Click to expand...
i don't think anyone is claiming otherwise. It's the stem of the intake valve that *may* benefit
 
When you need the 2 cycle oil the most is on takeoff and climb. At full throttle and full rich mixture there are unburned gasses going right past the exhaust valves.

Also, have you ever run your finger on the exhaust residue on your outboard. It is slippery from oil.
 
Last edited:
kgruber said:
When you need the 2 cycle oil the most is on takeoff and climb. At full throttle and full rich mixture there are unburned gasses going right past the exhaust valves.

Also, have you ever run your finger on the exhaust residue on your outboard. It is slippery from oil.
Click to expand...

More sticky unburned @*&^ on the valve stem has GOT to help. Help something. I guess the question is what...
 
The best book I've seen on aircraft engines for the layman owner/operator is the Sky Ranch Engineering Manual. Despite the very technical, text book title it's a book that you can sit down and read like a novel, starting from virtually any page or chapter. A book that you will find hard to put down once you get into it.

As for what's been said here, valve guide wear is something that is normal and unavoidable and if all conditions are ideal the guides should last the life of the cylinder. When the rocker arm opens the valve there is a tangential force along the top of the stem pulling towards the fulcrum point of the rocker. Nothing can be done about this. Wobble checks can check for current condition but don't prevent the inevitable.

Another cause of valves not seating properly is lead deposits accumulating at the base of the stem in the port chamber. The lead is put in the fuel to achieve the octane (100) which is needed for the higher performance engines. If you have a lower performance engine, or if you are babying your engine, the lead deposits form. The amount of lead in the fuel is a compromise.

Some additives claim to mitigate this lead deposit issue. Alcor TCP is the only one that is recognized and approved by the FAA and manufacturers as far as I know.

I think it is also important as an operator to use aggressive ground leaning techniques and to operate the engine as it was designed to be operated which means not to baby it or try to treat it with kid gloves. They prefer to be run as intended and will thank you for it in the long run.
 
JHW said:
yeah, before starting a religious debate on pilots and levers, the first thing to do is to follow the maintenance manual. If you have a lycoming the valve wobble test should be a regular part of your vocabulary.
Click to expand...

That wobble test is for older engines.

There are two documents that we need to refer to for this: SB388C...
http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB388C.pdf

...where we find this:

"TIME OF COMPLIANCE: Helicopter Engines – 300 hour intervals or earlier if valve sticking suspected.
All Other Engines – 400 hour intervals or earlier if valve sticking suspected until exhaust valve guides are replaced with guides made of improved material. (Refer to latest revision of Service Instruction No. 1485.)"

So now we need to take a look at SI 1485...
http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1485A.pdf

...where we see this:

"Improved exhaust valve guides were initially incorporated into some cylinder assemblies beginning in April 1996. Since March 1, 1998, all engines, cylinder kits and spare exhaust valve guides shipped from Lycoming contain the improved material.
Cylinder assemblies which incorporated the improved “Hi-Chrome” exhaust valve guides when the engine was shipped from Lycoming are identifiable by the letter “C” stamped inside a circle on the boss for the drain back fitting. The individual guides made with the improved material can be identified by a 5° chamfer at the top of the guide (See Figure 1).
Once guides made from the improved material are installed in all cylinders on the engine, it is no longer necessary to complete the mandatory 400 hour inspections specified in the latest revision of Service Bulletin No. 388. It is recommended that the inspection procedure from the latest revision of Service Bulletin No. 388 be completed at 1000 hours of operation or half way to the recommended TBO, whichever occurs first. (For recommended TBO, refer to the latest revision of Service Instruction No. 1009."

So they dropped the 400-hour requirement and suggest that you do the check at 1000 hours. We ran a lot of Lycomings, and once they changed that guide material, we didn't worry about any 1000 checks and we never had any valves burn, or guide issues at all. All of them made it to TBO without any valve issues.

Dan
 
Silvaire said:
Some additives claim to mitigate this lead deposit issue. Alcor TCP is the only one that is recognized and approved by the FAA and manufacturers as far as I know.
Click to expand...

There's a far better FAA approved additive for my bird...50% to 75% MoGas!
 
JHW said:
i don't think anyone is claiming otherwise. It's the stem of the intake valve that *may* benefit
Click to expand...

The intake valve does not run hot enough to collect lead which is in solution within the fuel, and won't stick to anything.

The lubrication comes from the other side of the guide.
 
Jaybird180 said:
If all you know about an engine is that it has burned exhaust valves, what conjectures and speculation can be made about the operation of the engine and causes of the burned valves?

I'm not seeking to stir any pots here, but I am trying to plug holes in my own knowledge.

Can we eliminate LOP operations as a cause? How about excessive idling at full-rich? Running takeoff power on a cold (lukewarm) engine?

Looking for causes and also things that it cannot be (but common OWTs say it is).
Click to expand...

Spends a lot of time rich and low power, tends to climb at low RPM.
 
You must log in or register to reply here.
Back
Top