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I read an interesting post here about how ceramic coatings would increase the surface area of a head and actually increase the heat transfer from the charge in the combustion chamber. Thermodynamically, this is looking in the oposite direction of the dominant effects. As a gas increases in temperature relative to a solid it is in contact with, radiative heat transfer goes up with the fourth power of the temperature. At the 3000 to 3500F peak temperature of a gasoline/air charge at TDC with a head temperature of only a few hundred degrees, radiative heat transfer exceeds convective and conductive heat transfer by two orders of magnitude. In english, this means that there is over 100 times more infrared energy transfer than conduction. If you want to stop some of this heat transfer and keep the pressure in the cylinder where it can do some good, you need a mirror; an infrared mirror like a refractory metal ceramic. Refractory metals are those with that are very hard, brittle, dense, lousey conductors (for metals) and are usually used as alloying additives in small percentages. Some of them are Molybdenum, Vanadium, Tungsten, Zirconium, Titainium, etc. In specific oxide form, they have awesome reflectivity in the infrared spectrum where all radiative heat transfer occurs. A smooth mirror is a marginally better reflector than a rough one, but both prevent absorbtion of heat to the mirror and whatever lies under it.
The effect has to work as advertised or even the rotary engine cited by the recent ceramic critic would not have gone faster. By his own admission, it did. The rotor face only occupies a fraction of the combustion chamber surface area compared to the coated surfaces of a ceramic equipped piston engine. It is the excessive surface area to volume ratio of the rotary combustion chamber that is in fact the primary thermodynamic limiter of these engines.
I appreciate people's opinions about how engines work, however I have a hard time when basic physics is ignored. There is nothing in engines that I have seen in 25 years of motorsports and over 30 years of aviation experience that cannot be explained with basic thermodynamics and mechanical engineering knowledge. Refractory metal ceramic coatings on the piston crown, head, and valves decrease specific fuel consumption by simply reflecting waste heat back into the combustion chamber where it results in more complete combustion of poorly atomised fuel that otherwise would burn in the exhaust tract too late to produce power. By doing this, less fuel can be injected resulting in lower EGT (less post burned fuel), lower engine temperatures (less heat transfer), and more power from less fuel with the same peak temperature and pressure as before the coatings were used.
A note on detonation. Combustion is a propagation of reaction in gasses that moves at about 30 to 70 meters per second for gasoline/air. The speed variance is mostly dependant on mixture ratio of the gaseous elements, liquid fuel droplet size, heat transfer to the surrounding environment (piston and head), and polutants (like residual exhaust gasses from the last cycle and ADI injection). Detonation is propagation of the reation at the speed of sound of the reaction gasses, about 350 to over 2000 meters per second for gasoline/air depending mostly on temperature (it's faster when it's hotter). Achieving detonation conditions in the entire combustion chamber of any rational piston engine is impossible; detonation is caused by localized lean and hot areas. The more uniform the combustion process and the more uniform the temperature is throughout the combustion chamber, the less likely it is to detonate. (Proper) Ceramic coatings reduce detonation potential through two mechanisms. First, they reduce the heat transfer to the surface of the combustion chamber and with it, the hot spots that can led to uncontrolled ignition away from the spark plug. A mirror in front of a fire has a colder surface than a bare piece of aluminum; try it. Second, by keeping heat in the combustion chamber that would otherwise be conducted away, large fuel droplets are evaporated and combusted more efficiently. Without this effect, large fuel droplets are free to remain in liquid form leaving the gas volume nearby too lean. It is interesting to note that in highly atomized mixtures, the benfits of ceramic coatings are reduced by about half. Sadly, it takes orders of magnitude better atomization to achieve this effect than Lycoming or Continental achieves with even their new "FADEC" systems. In the meantime, ceramic coatings are the best performance per dollar modification I've seen in 20 years.
BTW, Zehrbach is a foot shorter than Bachman, and I'm MUCH taller than either of them. I doubt if either Zehrbach or Bachman could ever pass for each other.
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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