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Proper mixture to prevent burnt valves in Continental and Lycoming aircraft engines

see also Exhaust valve leakage in Continental O200 cylinder

A. Exhaust valves erode "burn" when the seal between the seat and the valve face starts leaking. Leaking is usually caused by a worn valve guide. 

 Today's pilots lean their engines more than they did back in the 60's and 70's. As a result, exhaust temperatures have increased resulting in increased heat loading to the exhaust valve. Unfortunately, cylinder repair techniques haven't changed. This increased heat from the higher temperature exhaust gas is trapped in the valve and guide where it causes rapid guide wear.

Continental exhaust valve and seat

When the exhaust valve is open, it heats up as hot exhaust gas flows around the valve head. When the valve closes, it cools down as it conducts heat to the seat and finally into the atmosphere. When heat absorption isn't balanced by heat dissipation, exhaust valve temperature increases causing valve guide wear.

More contact area between the valve and seat allows more heat to flow out of the valve, thereby lowering the valve's  temperature. Contact area is determined by how much seat face the cylinder shop grinds into the valve seat. Thus, if it's late in the afternoon and the mechanic who grinds the valve seats is tired, you get narrow seats and hot valves.

The objective of cylinder shops when they grind a seat face is to achieve an airtight seal. This isn't sufficient. Foremost in controlling the valve and guide temperature is the cylinder shop grinding the proper width seat face.

If the engine is operated at low power settings, such as patrol aircraft, or with rich mixtures, heat loading is low and carbon deposit formation is high. The valve can be cooled with a narrow seat. The advantage of a narrow seat in this application is that the narrow seat increases contact pressure. This helps cut through carbon deposits that form on the seat face. Most aircraft are not operated at low power and with a rich mixture.

For example we will compare the Continental O-200 engine used in the Cessna 150 with the Lycoming O-235L2C engine used in the Cessna 152. The O-200 cylinder has minimal cooling fins on the exhaust side of the cylinder so the exhaust valve overheats resulting in rapid guide wear.

Almost no cooling fins on the exhaust port of Continental O-200 cylinder.  Cylinder fins are used to give the cylinder a large heat-transfer coefficient and of obtaining a reasonable uniformity of cylinder temperature

Lots of cooing fins on exhaust port of Lycoming O-235L2C cylinder. Notice cooling fins above spark plug pulling heat from the inside wall of the exhaust port. This is a high-crack area on all cylinders

If you grind a narrow (performance cut - high air-flow) seat face the valve will overheat and you will have worn exhaust guides and a burnt exhaust valve in as few as 50 hours. You need a wide seat to maximize the area in contact between the valve and seat to get the heat out of the valve. The O-235 has a sodium cooled valve and excellent cooling fins on the exhaust side of the cylinder so that valve cooling is not as critical as the O-200.

You can grind a narrow seat face without worry about valve overheat. In fact you should (must) grind a narrow seat face on this engine if using leaded fuel as this cylinder has poor combustion swirl and creates pockets of unburned fuel that allow build-up of carbon and lead. Many of these deposits occur between the exhaust valve seat and the cylinder wall where they can foul the exhaust seat. A narrow seat helps keep the seat face from building deposits. In fact combustion deposits in this cylinder are so bad that Lycoming ground pockets into the head next to the exhaust valve as a garbage collection area - this became the O-235N2C engine (rated 2 HP less).

If the engine is operated at high power settings and a lean mixture, then heat loading is high and carbon deposit formation is low. The valve needs the extra contact area that a wide seat provides to transfer heat out of the valve.

Cylinder repair shops that don't consider heat loading and heat transfer when they repair the cylinder can cause guide wear and valve burning.

Energy Flow Diagram Approximately 1/3 of the energy produced is sent past the exhaust valve. The same amount of work (1/3) that is propelling you thru the air. Two critical areas of cooling are the piston crown and the exhaust valve.

In a 100 HP engine approximately 75kW of heat is transferred to the cooling system and out the exhaust

What is the proper fuel mixture to prevent burning? I don't have a specific answer. Not only is seat width a factor but also some engines and installations are less efficient at removing valve heat than others. The Continental O200 in the Cessna 150 has small cooling fins in the exhaust port area when compared with engines such as the Lycoming O235. The O200 is less efficient at transferring exhaust valve heat to the atmosphere. It quickly wears out exhaust guides if the exhaust seat is narrow and the exhaust gas temperature is high.

Engine baffling directs the air flow through the cylinder's cooling fins. If air isn't flowing through the cooling fins, the exhaust side of the cylinder can overheat, thereby causing the valve to operate hotter. Because your CHT probe is located in the center of the cylinder you only see the average of the cold side of the cylinder (intake) and the hot side (exhaust). Baffling is like the radiator on your car. Keep it in good working order.

You can lean more aggressively at lower power settings because there is less mass flow past the exhaust valve. Fewer hot molecules are striking the valve so that the valve absorbs less heat even though the EGT temperature might be the same or higher.

Broken Continental exhaust valve caused by overheating from operating with valve leak. Good example of Hoop Stress. Hot portion of circle expands creating stress which creates the inward cracks. Eventually a chordal ( a half-moon shaped piece) breaks off and bounce around in the cylinder. At engine idle they suck back into the intake system and travel to another cylinder to do their damage.

Exhaust valve conditions - photo album

Bad valve seat job


Water Injection

Currently we use rich fuel mixture to cool the engine - especially on take-off where most aircraft are set for a fuel/air mixture that is richer than the best power mixture ratio. The engine requires a rich mixture, but the pilot wants the best power! By vaporizing excess fuel we extract heat from the combustion process. Our aircraft engines derive their power from heat so we trade off power for cooling during the most critical stage of flight --takeoff.

Wouldn't it be better if we used another less expensive liquid (such as water) to vaporize in the combustion chamber? 

CHT probe used in many Continental and Lycoming engines. Part number 177S34. This is a screw-in type probe. Some aircraft use a bayonet style probe as shown below.

CHT bayonet adapter part number 28202 used with bayonet style CHT probes

Your attachment to your plane is undoubtedly deep-routed. Parachute Sense US NAVY 1944

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