In Part 1 of this article, I introduced exhaust valve sticking, the exhaust valve environment and the idea of "Hot Side" valve deposits. To explain how and why these "Hot Side" deposits form, we have to turn to the research on the chemistry of leaded fuel combustion, much of which was done 50-60 years ago.
Lead Fuel and Lead Scavengers
100LL avgas contains approximately 2 grams or tetraethyl lead (TEL) per gallon. TEL increases the octane of the fuel. And it does this very well. During compression, TEL is rapidly converted to lead oxide, which is actually the active octane booster. However, it was quickly observed that lead oxide coated the spark plugs and they stopped functioning, something not welcomed in an airplane.
Through trial and error, it was discovered that when ethylene dibromide is added to the leaded fuel, the spark plugs remain much cleaner. An exact amount of ethylene dibromide is added to the fuel to convert all the lead oxide to lead bromide. Not enough, and you get lead fouling of the plugs. Too much, and you form hydrobromic acid. which dissolves aluminium. Above 1100°F, lead bromide is a gas, while lead oxide just coats everything as a solid. This difference is where the term "lead scavenger" comes from. The "scavenged" lead can exit the engine as part of the exhaust gas.
Solving the puzzle
The ideal process proceeds thusly. The tetraethyl lead forms lead oxide, the active octane booster, during the comptression stroke and previous combustion cycles. During combustion, the lead oxide reacts with ethylene dibromide to form lead bromide that exits out the tail pipe with the exhaust gas. Unfortunately, the process is not that simple.
In actuality, tetraethyl lead quickly forms lead oxide, which then more "slowly" reacts with ethylene dibromide to form lead oxy-bromide in a series of 8 steps. The lead oxy-bromide eventually reacts fully to form lead bromide, which. as a gas at these temperatures, goes out the exhaust pipe. Chemical reactions are much faster at higher temperatures. It is the time it takes for this "lead oxy-bromide" to form lead bromide that creates the problem. And this time is intensely dependent on the peak combustion temperature. And guess what? You as pilot in command, can strongly influence the peak combustion temperature.
Explained more fully, if we look at the condensation temperatures of the lead compounds involved, a picture starts to emerge. The condensation temperature below which a compound precipitates from the hot exhaust gas and collects on a surface, which in this case is the valve stem. Condensation is the opposite of evaporation and can be considered the "dew point" of a chemical compound.
The approximate condensation/evaporation temperatures for these materials of interest are as follows:
Lead oxide - 1630°F (888°C)
Lead oxy-bromide - Initially 1470°F (800°C) decreasing to 1300°F (705°C) adter 8 temperature dependent steps.
Lead bromide - 1100-1175°F (593-635°C)
As you can see, there is a big difference in the condensation/evaporation temperatures of these compounds - and as it turns out, the substantial time element involved in the process is very important. The time necessary to complete these reactions is VERY dependent on combustion temperatures, of which mixture setting is a primary factor. Combustion temperature is reflected in both the Cylinder Head Temperature (CHT) and directionally, the exhaust valve temperature, which we do not measure directly. (Exhaust valve temperature follows cylinder head temperature because the valve guide is located in the cylinder head, to which it transfers heat from the valve.) Valve temperatures do not follow Exhaust Gas Temperature (EGT).
It is the mixture and, to a lesser extent, throttle, rpm, ambient temperature and cooling airflow that affords us some control of the actual combustion temperatures and thereby the CHTs and valve temperatures. If combustion temperatures are low enough that lead bromide cannot form in the allotted time, we end up with lead oxy-bromide condensing on the exhaust valve stems. This combined with reduced exhaust valve temperatures is a perfect recipe for valve sticking.
Exhaust Valve Temperatures
FIgure 3 shows the relative operating temperatures for sodium filled ezhaust valves (Lycoming) versus the solid exhaust valves (Continental). Exhaust valves run very hot and have to dissipate a lot of heat quickly or they will fail. The solid valve in Continentals dissipates most (81%) of its excess heat from the valve edge to the valve seat when the valve is closed. The remaining excess heat (19%) is transferred from the stem through the guide into the cylinder head.
In Lycoming valves, the sodium sealed in the stem, liquefies and transfers the heat a lot better than a solid valve stem. This leads to 50% of the heat being transferred from the valve edge to the valve seat when the valve is closed. The other 50% of the heat energy is transferred up the valve stem to the guide. This means there is more heat throughout the valve stem and guide.
Figure 4 shows lead oxy-bromide build-up in a Lycoming engine. The valve was stuck. This normally aspirated engine was run deeply LOP with CHTs well below 300°F. The deposit crept up to within a quarter of an inch of the other (cool) side of the guide.
Figures 5 and 6 show a valve that stuck in a Continental IO-550. Notice the large build-up of deposits on the lower part of the stem before the tulip flare. Above this shoulder is the part of the valve stem that enters the guide. Note the staining on this part of the stem because the deposits are scraped off the stem by the tight fit of the guide. At some point, enough deposit is forced into the guide and the reduced clearance sticks the valve.
Chemical analysis shows this deposit primarily consists of lead, carbon, bromine, and oxygen. We exposed a sample to high temperature in a thermogravimetric analyzer (TGA). The sample was heated to 1470°F where, surprisingly, only 25% of the mass evaporated after 25 minutes. This means that one of the hottest parts of the valve had lead oxy-bromide deposits condensing/crystalizing on it. In summary, lower combustion temperatures don't permit this conversion of lead oxy-bromide to lead bromide and the valves run cool enough to allow this lead oxy-bromide to form on the stem and this leads to (Hot Side) valve sticking.
Running either too LOP or ROP lowers peak combustion temperatures enough to cause this deposit problem. The low combustion temperatures are reflected in lower cylinder head and valve temperatures. Low combustion temperatures reduce both the conversion of lead oxy-bromide to lead bromide and the exhaust valve temperature, which allows the lead oxybromide to crystalize on it.
I will complete this story in part 3.
By Edward Kollin, Technical Director, Aircraft Specialties Lubricants
Aircraft Specialties Lubricants
2860 N Sheridan Rd., Tulsa, OK 74115
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