So you want to know what a burned exhaust
valve looks like? One that will give you about 15/80 compression? Well here you
Take a look at the
BeechTalk Thread contributed by Mike F. on three of his IO-550 jugs that
were giving him 15/80, 15/80 and 55/80 compressions. The valve on the left and
middle were at 15/80 and the one on the right was 55/80.
"...the one on the right doesn't look like
it was leaking (if any, hardly) while the engine was running... only during the
static compression check. I'd love to see an engine monitor download on it. That
one looks very much like a normal heat pattern on a healthy LOP-run valve with
VERY minor edge changes. The other two look like they were leaking significantly
while the engine was running."
Note the uneven burn patterns of the first and
middle valves as opposed to the valve on the far right.
Here is a close up of the first valve, the one
on the left:
Hopefully this will give you and your mechanic
an idea of what you might see in a Borescope when exhaust valve problems are
Stuart S., IA and Baron owner,
contributes this "Good" Valve picture above.
Stuart S., IA and Baron owner,
contributes this "Bad" Valve picture above and the below aircraft
valves had 45 hours on it since annual and was fine at that time. Notice the
small micro cracks in the second picture below.
Be Kind to Your
Here is a valve narrative from
Beech Talker Matt S., along with technical citations for sources:
Burnt valves always
have been and remain one of the largest repair issues in an aircraft engine.
Some are due to valve guide issues, but most are due to overheated valves.
Exhaust gas temperature is the major contributor to heating a valve. The valve
seat to the cylinder head is 80% of the cooling of the valve. Therefore,
operating with high CHT’s and EGT’s can overheat an exhaust valve and cause
valve damage, as many pilots have experienced.
Here is the science:
1. Taylor, MIT Cambridge 1985, The Internal-Combustion Engine in Theory and
a. “Exhaust valves have presented a difficult cooling problem ever since the
advent of the internal-combustion engine.”
b. “The mean EGT and velocity relative to the (exhaust valve) surface are higher
than at any other point in the engine.”
c. “….even a small reduction in valve temperature, as from 1400’F to 1350’F,
results in a marked improvement in valve life and reliability.”
2. Lumley, Sibley School of Mechanical and Aerospace Engineering, Cornell
University 1999, Engines:
a. “The problem areas (due to heat) are the exhaust valve and piston crown. The
exhaust valve head loses most of its heat to the valve seat (the amount lost to
the valve guide is relatively small, because the path is long and the conduction
area is small). Unfortunately, when the valve opens, it is exposed to the
exhaust gases, which flow past it at high velocities (making for good heat
transfer), while this is happening the valve head is not in contact with its
b. “….keeping the valve temperature as low as possible and seat depends in large
part on keeping the valve temperature as low as possible, to keep oxidation low
and to prevent warping of the valve.”
c. “….the cooling of the exhaust valve is such a critical matter.”
3. Sanders, Wilsted and Mulcahy, NACA Report 754 12/1943, “Operating
Temperatures of a Sodium-Cooled Exhaust Valve as Measured by a Thermocouple”
a. “Fuel-air ratio was found to have a large influence on valve temperature, ….”
b. “valve temperature has a vital influence on valve life.”
c. “Both richer and leaner mixtures produced lower valve temperatures, though
lean mixture operation did not result in so low a valve temperature as did
d. “Reducing the fuel-air ratio (from ROP) increased the valve temperature…..
“…. relatively large increases in over-all cylinder-head cooling are required to
maintain constant valve temperature.” i.e.: reducing the CHT’s will compensate
for higher EGT’s.
e. “Variation in fuel-air ratio (leaning) had a relatively great effect on valve
4. Baniasad, Khalil and Shen, SAE research paper 4/2006, Exhaust valve Thermal
Management and Robust Design ……:
a. “almost all the heat input to the exhaust valve is from the burning and
exhaust gases ….”
b. “Hotter exhaust gas due to fuel air mixture can cause possible valve
5. Hamada and Ohtsubo, SAE research paper 3/2000, Engine Valve Temperature
a. “Valve temperature is most affected by fuel air mixture.”
b. “The temperature on top of the (valve) head and under the head become maximum
when the EGT’s are maximum.”
6. Roth, SAE research paper 3/2003, Fatigue Analysis Methodology for Predicting
Engine Valve Life:
a. “Exhaust valve indicated a maximum temperature existed mid way up the valve
stem. This was due to high velocity exhaust gas flow around the stem during the
b. “The increase in the exhaust valve operating temperature decreases the useful
life of the valve.”
c. “Valve life is very sensitive to operating temperature.”
4. Wisniewski, SAE research paper 2/1998, Experimental Study of Heat Transfer on
Exhaust Valves …..:
a. “from 91% to 92% of heat was transferred from the exhaust valve through the
seat and from 7% to 8% through the valve stem.”
I cannot find any research papers, published data or books that support the
concept that exhaust valves’ reliability are not affected by EGT and CHT. We
look at 400 for a do not exceed temp for CHT. All the while we keep EGT's down
below 1800 as a sum.
Engine health relies on a good and accurate understanding of how a pilot's
actions affect the engine. Both EGTs and CHTs have a significant affect on the
exhaust valve temperature and its life.
Visit the CSOBeech engine ops narrative
HERE to learn more about
Advanced Pilot Seminars
and my experiences with LOP/GOP operations on my IO-470Ls.