|
How hot is too hot?
So now you know that most aircraft engine's exhaust gas
temperatures are around 1200 to 1600F but did you know that the
melting point for aluminum is around 1300F? In order to save
weight aircraft engines make extensive use of aluminum - in fact
aircraft engine manufacturers have much more experience with aluminum
alloys than the automotive world which didn't start using aluminum
in engines extensively until the 1970s and as we all know some
of those early aluminum engines didn't work out very well. So in
at least one respect aviation engines were technologically ahead
of their day.
Aluminum has some great properties that make it ideal for use
in aircraft both in terms of airframes and engines. However, aluminum
also has a couple of significant drawbacks when compared to say,
steel alloys. These are, (a) a relatively low melting point and
(b) work hardening. The first drawback - a relatively low
melting point is self explanatory, however the 'work hardening'
problem needs to be better explained.
Work hardening is an effect that causes metallurgical
structure of aluminum to break down and fracture. As an example,
take a piece of aluminum sheet metal and bend it at a moderate
angle of 45 to 60 degrees back and forth and after a few times
it will harden up and then simply snap in half. A simpler more
everyday test is to use a soda can. Bend the tab on top back and
forth 20 to 30 degrees and in a matter of a few times - clink -
it will break right off. This fracturing is known as work
hardening. Aluminum doesn't like to be bent back and forth.
Steel alloys however can handle such stresses quite easily but
their weight penalty limits their use to structural areas that
are absolutely necessary.
An aluminum air-cooled cylinder head is subjected to an
incredible range of temperatures. Prior to startup a cold engine
may be anywhere between a balmy 80F to as low as -40F depending
on where the engine is being used. After startup in a matter of
a few minutes the cylinder head temperatures rise to around 200F
at idle. During runup head temperatures rise to as high as 350F
and at takeoff and climbout 400 to 450F is not unusual. In the
worst case scenario an air-cooled engine's cylinder head
temperature can go from -40F to 450F in a matter of 5 to 10
minutes! That's a change of 500F! Then there is the cool down
cycle which as it turns out is more destructive than one might
think since the rate at which aluminum cools down has a direct
effect on its hardness. A slow cooling from a high (>325F)
has the effect of weakening the metallurgical structure of an aluminum
casting.
The repetitive heating and slow cooling of an aluminum head
both weakens the metalurgical structure and serves to create a
form of work hardening in much the same way that bending an aluminum
metal strip does. The structure of the casting becomes brittle
over time which when combined with extreme temperature changes
or temperature variations across a cylinder head leads to stress
fractures.
In the last 20 years the majority of automotive engines have
been designed with aluminum cylinder heads yet when compared to
aircraft cylinder heads, automotive heads rarely suffer from fatigue
cracks. Why is this? The answer is simple - heat! Too much heat!
As it turns out most automotive cylinder heads are hardened
to what is known as a T6 hardness. This hardening process is
done shortly after the part is cast and serves to relieve
casting stresses and to create a more uniform metallurgical
structure. The T6 hardening process involves heating the casting
to 1000F for about 6 hours and then quenching the part in water
for a few seconds. Next the part is 'aged' in an oven at about
320F for around 5 hours and then allowed to cool to ambient
temperature. The result is a part that has a Rockwell hardness
on the 'B' scale of around 84-88 and a nice dense and uniform metallurgical
structure.
The key to note is the aging temperature of 320F. If the part
is kept at or below 320F it will retain its hardness and uniform
metallurgical structure however, if it is repeatedly heated
above 320F the uniform metallurgical structure starts to break
down and the parts starts to become brittle.
Air-cooled cylinder heads regularly see temperatures over
320F and it is these high temperatures that lead to cylinder
head problems which can run the gamut of cracks, loss of valve
seats, loosening of valve guides and so on. But there is more to
this story.
It turns out that an air-cooled cylinder head has a wide
temperature variation across the head during operation. The
intake side of the head is seeing relatively frigid temperatures
from the intake mixture while the exhaust side of the head is
exposed to blast furnace temperatures. The result is a huge
temperature differential between the intake and exhaust valve
seats and its no wonder that this is the area where the majority
of cylinder head cracks are found.
In effect an air-cooled aluminum cylinder head is destined to
fail after a relatively short lifespan of service. It is
considered acceptable practice not to run cylinder heads more
than twice the TBO of the engine before being replaced. There
are even those that recommend replacing the cylinder heads at
each overhaul and based on service data it can be shown that the
second time around cylinder heads are more likely to encounter
cracking or other fatigue failures.
Sticky exhaust valves?
Now that we have addressed the cylinder head lets look at the
exhaust valves. As mentioned earlier, exhaust valves are subject
to incredible temperatures. The heat that is absorbed by the
exhaust valve must be dissipated into the cylinder head. In an
air-cooled head the process of heat transfer from the exhaust
valve face to the valve seat isn't sufficient so the excess heat
travels up the valve stem to the next best place - the exhaust
guide. This is the fundamental cause for sticking valves in most
air-cooled engines. The problem is that the temperatures of the
valve stem can reach as high as 600 to 800F. Oil vaporizes at
those temperatures but not without leaving carbon deposits in a
process known as coking. Yet oil is necessary to lubricate the
valve stem and guide. Here is one of the very real drawbacks of
any high powered air-cooled engine - one that cannot be easily
solved. How do you lubricate an area that is running at such a
high temperature? Answer - you don't or at least you keep the lubrication
to a minimal amount so as to reduce the rate at which carbon
deposits build up. But if you don't lubricate the valve
stem/guide sufficiently the result is excessive wear on both the
valve stem and the guide. In short the result is an unacceptably
short service life fraught with problems.
The solution - keep it cool!
Sounds simple but that's the truth. Again the only reasonable
method to achieve cooler cylinder head temperatures is to use
some form of water cooling. Water cooling solves all of
the problems described above. A water cooled engine's cylinder
head is usually kept to around 200F, well below the critical
temperature of 320F above which a T6 heat treatment is
destroyed. Furthermore a T6 heat treated cylinder head will
retain pressed in valve seats substantially better reducing the
possibility of one coming loose and the resulting catastrophic
results. A water cooled cylinder head also has far less temperature
differential across the cylinder head during operation further
reducing the thermal stress.
By far the greatest benefit of water cooling is the lifespan
of the exhaust valve and guide. With the majority of heat being
removed from the valve face through the seat area the valve stem
temperatures are kept well below the point at which oil becomes
carbonized and the valve stem and guide area won't suffer from
carbon buildup and the resulting stickiness and excessive wear.
Finally the use of exotic exhaust valves is eliminated. Certain
air-cooled aircraft engines incorporate sodium filled exhaust
valves that are ridiculously expensive, typically ranging in
the $225 to $275 price range each. The only reason such valves are
used is to allow better transmission of heat from the valve face
up the stem - the very heat that carbonizes the oil, causes
excessive stem wear and sticky valves. This heat transfer is
necessary in order to prevent the outright failure of the valve
at the point where the stem joins to the valve end and it is
the very area that is fully exposed to the blast furnace
temperatures of the exhaust gases.
Reduced Oil Consumption
Last and certainly not least is that water cooled cylinder
heads allow the use of valve seals that prevent excess oil
from being sucked through the stem/guide area into the intake
and exhaust streams. Oil mixed with the fuel-air intake lowers
the effective octane level of the mixture, reduces combustion
efficiency and lowers detonation resitance. Because of the
very high operating temperatures of air-cooled
aircraft engines valve stem seals cannot be used. A substantial
amount of oil gets sucked past the valve guides and is
where the majority of oil consumption loss occurs.
All of the benefits of water cooling point to better engine
reliability and longer component life which in the end
translates to longer engine life and longer times between
overhaul with fewer if any top end related service problems
during an engine's lifespan. Furthermore Liquid Cooled Air Power
is researching alternative exhaust valves to replace the exotic
sodium filled exhaust valves that will offer better service life
and improved flow which further translate into improved engine
performance.
|
