Heat - The top end killer

About the Benefits
  • Overview
  • Getting rid of the heat
  • Heat - the top end killer
  • More power per cubic inch
  • Better performance and fuel economy
  • Radiator technology - past and present
  • Shock cooling - Problem solved!
  • The secret to achieving longer TBOs
  • Flying faster on the same power - reducing cooling drag
  • Side benefits - Safer cabin heat
  • Dispelling the Myths

  • 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.

    Liquid Cooled Air Power © 2001 Liquid Cooled Air Power. All Rights Reserved.
    1413 Prospect Ave., Hermosa Beach, CA 90254
    Proudly MADE in the U.S.A.
    Proudly MADE in the U.S.A.