Exhaust Valve Sticking Main Image
Exhaust valve sticking can be an inconvenience or a serious problem, such as a bent pushrod and broken pushrod tube that allows all the oil to drain from the engine. The important thing is to determine early on that you have a sticky valve and do something about it before the worst happens.
Valve sticking may show up in multiple ways. It most often presents as morning sickness, the uncontrolled engine shake when the engine is first started. As engine heat is transferred to the non-operational cylinder, the valve often releases and the cylinder springs back to life and smoothness returns. Many pilots treats this as a fouled spark plug - it is not. A bad plug will show up during a mag check. Any sign of morning sickness should be addressed with your mechanic immediately.
It can also show up while making a power change while either descending or leveling off after a climb. It can be a subtle shudder due to intermittent sticking or it may stick solid and bring the cylinder off-line with the attention grabbing roughness. Engine analyzers will show wildly fluctuating EGTS and CHTs before the cylinder goes cold.
Over the years, much has been said and many articles have been written about valve sticking in piston aircraft engines. Many "solutions" have been tried - ranging from major hardware changes to miracles-in-a-can, but the problem persists.
Figure 1: Valve and valve guides. Courtesy CB Performance
What is valve sticking?
The exhaust valves rides in special tubes called valve guides (figure 1). They are shown installed in a cylinder in a cut away picture (figure 2). One end of the valve and guide is exposed to very hot combustion/exhaust gasses (Hot Side), while the other end is bathed in relatively cool motor oil (Cool Side). The valve stem fits snugly into the valve guide. If enough deposit form on the valve stem or in the guide, the valve stem sticks in the guide.
Figure 2: Cylinder assembly showing valves guides and piston. Courtesy Alamy
There is good news and bad when it comes to sticky/stuck valves. The bad news is that there are two separate reasons for exhaust valve sticking Hot Side & Cool Side deposits. They are very different. The good news is we understand them both and minimizing them just involves subtle changes in engine operation and routine maintenance.
Most everyone, including pilots, engine manufacturers and oil companies, widely (and wildly) misunderstand the causes of the problem. As a fuels and lubes chemist and longtime gearhead, let me try to shed some light on this issue.
The exhaust valve environment
To say that exhaust valves operate under severe conditions is a colossal understatement. The valves seal against controlled cylinder explosions with temperatures up to 4000°F & pressures up to 1200PSI. And then, they have to survive jets of 2000° +F exhaust gas blazing past the open valve, at up to 25 times per second!
The difference in temperature between the Hot Side and Cool Side of the exhaust valve can be as much as 1500° F. The tulip end of the valve and guide are hot while the other side is much cooler as it's bathed in engine oil.
It is the heat along with the distinctive chemical activities at each end of the valve guide, which can lead to deposit formation and valve sticking. These are distinct unique phenomena.
Hot Side Deposits
Lycoming states the maximum allowable CHT is 500°F while Continental says 460°F, but both prefer to see a max cruise temperature of 400°F. And I agree. As to a lower limit, they really don't specify.
But, what is "too cool" and why?
Just because you can do something, doesn't mean you should. It seems that human nature has taken over once again when it comes to Cylinder Head Temperatures (CHTs) on our airplane engines. If 380° is better than 400°, 350° must be better than 380° and then 300° must be better than 350°. Unlike our water-cooled car engines, we have some say when it comes to CHTs in our airplanes. Throttle, mixture, cowl flaps and airspeed put some control in our hands and what we do with it has consequences in terms of engine reliability and cylinder life.
The race to the bottom, low CHTs, has evolved because of the confluence of several factors. One we have engine analyzers that inform us, literally to the degree, what the CHTs, EGTs and TITs are doing during every phase of the flight. Two, we have the GAMI gurus that have opened our eyes to the wonder of LOP operations, like the airlines used to run their big radials. You can run cooler, cleaner and save fuel. And three, you have human nature, if less is better, then a lot less is a lot better (the flipped side of "if some is good then more is better").
The convergence of these three effects has brought about a noticeable uptick in exhaust valve sticking reports. And while every engine is susceptible to this scourge, it is Lycoming engines that have most of the problems with this issue. Lately however, valve sticking has been reported in normally aspirated large bore Continental engines (470's, 520's and 550's).
This "Hot Side" example is the more common type of valve sticking, caused when lead compounds crystallize on the tulip end of valve stem, exposed to the hot exhaust stream. This deposit gets forced into the valve guide where it reduces the stem/guide clearance until the valve sticks.
The $64,000 question is what causes this kind of deposit to form in the first place and why is it seeming rearing its ugly head more often now.
I will delve further into the explanation of the Hot Side deposits, introduce the Cool Side deposits as well as how best to mitigate these problems in Part 2.
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