You just missed an excellent webinar on the subject 2 days ago:
http://www.eaavideo.org/video.aspx?v=2274677932001
Solid presentation but,
If one were crazy enough to drop a lit match in a bucket of 100LL gasoline chances are very high a trip to the burn unit will follow. Unless the ambient temperature is lower than -20 degrees F or the experiment is being conducted in a chamber at some multiple of standard pressure the gasoline will have begun to evaporate immediately upon being poured in to the bucket. If the bucket is not agitated the vapors will form a concentration gradient from almost pure hydrocarbons right above the surface of the liquid to the makeup of the surrounding atmosphere at some other height above the liquid. The function that describes this gradient will change depending upon the ambient pressure, temperature, time since the gas was poured, geometry of the bucket, etc. What is important is that somewhere between the extents of atmosphere and pure hydrocarbons a layer that is a highly combustible mixture of hydrocarbons and air will exist. The match will likely never reach the liquid gasoline to be extinguished as it will ignite this combustible mixture layer as it drops through it. I have seen this experimentally demonstrated with many hydrocarbons even No. 2 diesel, and Jet-A will behave this way at low enough pressures or high enough temperatures. Neither of them will behave this way at any normal atmospheric temperatures or pressures, but if the bucket of Jet-A happens to be sitting on an active flow of lava it will behave just like the 100LL does at 100 degrees F.
All the statements about mixture for startup could benefit from additional clarification and explanation. Both so operators understand why priming or choking is needed and consequently why excessive prime is detrimental. No matter the temperature an internal combustion engine needs only to have a combination of air and fuel within the range of combustible ratios present in the cylinder to run. So if I had my controls set for a mixture that ran perfect when I shut the engine off hot why will the engine not cold start at that same mixture? The carburetor or fuel injection system will be happily metering out the same amount of fuel they were when the engine last ran and while the air going in will likely be more dense as it is not now being preheated on its way through the induction system, that in and of itself, is not enough to create a mixture too lean to support combustion. In fact fuel is being removed from the intake charge by all the cold surfaces the mixture comes is contact with just like humidity from the air condensing on a cold can of soda. This evaporation temporarily steals this fuel by making it unavailable for combustion. However this fuel does not disappear and as soon as the mixture has been enrichened enough for the engine to start this fuel will go back in to the mixture both from increased velocity through the induction path and the rising temperatures. One of the places fuel heavily condenses at initial startup is the cylinder walls themselves thereby diluting the oil present and producing a fuel oil mix of dramatically lower lubricity leading to increased ring wear. Use only the very minimum amount of enrichment necessary to get the engine running if you are priming and choking so heavily that the engine is running smooth from dead cold through warmup that is a sure indication of too much fuel. The best indications of minimum enrichment for cold start is an engine that is initially barely running perhaps even missing with a decidely flat exhaust note and no smoke from the exhaust.
In principal I agree with the contention that high CHT readings do not represent a path to long cylinder life, but the presentation neglects to consider a large number of variables and never even proposes the mechanism linking high CHT and increased cylinder wear. The data I have seen leads me to belive the mechanism is two fold:
1)Neither Lycoming nor Continental nor anybody else has invented a material or method to ensure pistons and cylinders will maintain optimum size and geometry through out the wide range of temperatures an air cooled engine is exposed to, liquid cooled engines are designed to run in a very narrow temperature band specifically to avoid this as of yet insurmountable problem.
2)Many operators continue to use archaic mineral oils in their aircraft engines, eventhough much data exists showing mineral oils experience substantial reductions in lubricity at or below the temperatures known to be present on the cylinder walls at the top of the bore.
Given one and two above the poor piston ring pack has its work cut out for it trying to effectively seal a piston of varying size and roundness operating in a bore of varying size and roundness all while being lubricated by an oil operating in and out of a temperature range where it can not perform effectively.
Any time specific engine operating criteria are being recommended on a blanket basis it makes me skeptical. Even if the presentation were only speaking to a single specific engine model inumerable factors related to the sensors (type, brand, construction), and their installation(location, heat transfer compound used or not, torque, electrical resistance to ground, etc, etc ,etc), would totally preclude making a blanket statement that 400 degrees on your gauge in your airplane equates to excessively hot. IE JPI directly states that their sparkplug gasket sensors usually read 50-60 degrees higher than a probe style sensor on a Lycoming engine, they also state that both Continental and lycoming engines will vary 25-50 degrees between the top sprak plug and the bottom plug so again which one is the right one?. So if one were to install both variations on the same engine one readout may show all CHTs around the presentations ideal 380 degrees while the other gauge is indicating all are about 430 degrees, so which is it ideal or in the red zone? I have also seen in the lab many times where contact temperature sensor data consistency between a pair of seemingly identical installations is foiled by things as innocuous as the type and amount of heat transfer compound used and the installation torque.
