Detonation/pre-ignition is a different matter again, caused by the fuel/air charge to reach ritical self ingnition prematurely & exploding causing supersonic shock waves to occure and blasting metal fragments off.
Listed below is a number of references to “information” explaining the advantages of protecting an engines internals from the ravages of combustion heat. I have given other forum members ideas towards boosting performance (& reliability). In reading the correspondence it is obvious there is some difficulty in understanding some of the physics involved so I will briefly explain how the concept can work in the context of, using in my case, VHT paint as a heat shield/protectant.
As with specialist ceramic spray on coatings used by NASA and high performance engine manufacturers i.e. porsche, Wiesco etc, VHT paint as you get from auto accessory shops contains ceramic compounds that are very poor in transferring heat i.e to the piston, from combustion heat, yet itself is impervious to the effects of heat. Ceramics generally are clay based compounds. Exhaust gum (exhaust joint sealing goo) contains clay, suspended in an epoxy carrier. When applied to a joint the epoxy carrier burns and chemically changes to act as a rigid binder. VHT paint behaves in a similar manner except the epoxy carrier (the paint) retains its original state to a higher temperature.
How is this paint rated to 1500f (815cel) able to stand up to combustion temperatures up to 2500c. In the same context working limits for Aluminium alloys is about 500c, cylinder wall oil film about 450c. Ever done the lighter flame test across the palm of your hand. As the article from Warwick explains, “In a typical internal combustion engine, combustion is over in less than 4 ms”. Combustion temperature,peaks of 2500c measure than 1ms with total burn periods less than 4. Additional to this there is the effect of what’s called boundry layer, which is a microscopic gas layer in this case over the surface of the metal acts as a sort of temp regulator that fluctuates significantly less, comparatively speaking than the combustion temp peaks.
In the application as a heat shield e.g. on a piston crown, the surface layer of the paint will burn (oxidise) to an degree, but this oxdising process does not degrade from the paints ability to shield heat. It will over a period of time, depending on extremes, disperse or in a sense evaporate and effectively wear off the coating, which will need reapplying at some stage dependent on conditions. In the “performance” applications that I experimented with in coating piston crown, combustion chamber & exh port it in a small 50cc 2 stroke air cooled engine, setup as a blower as an experiment, I experienced consistant 40-50celcius drop in working load head temperatures. If the head experiences this temporature drop, then the piston will too, my conclusion is that the hot underside of the piston, being exposed to the crank case charge will also heat the charge which is less desirable. This was all the proof I needed for myself, to justify such an easy and cheap modification. The advantages in confining the combustion temperatures to the gases themselves means the temperature can do what it is meant to do..expand the air charge more..creating more force on the piston..translating to more power..its logical that the paint don't burn off the outside of exhaust pipes (that glow read hot) why should it be different on th inside of the pipe, it works, don't knock it till you tried it !!
The same discussion has taken place in another forum several years ago that raised some of the same issues..
http://forums.corner-carvers.com/archive/index.php/t-2253.html what more can I say
http://www.customclassictrucks.com/techarticles/137_0312_cerm/
Spray-On horsepower Ceramic Coatings By Bob Ryder
Who would have thought that horsepower could be gained from coating internal engine components? Internal engine components are made from dissimilar metals. Due to the lack of metallurgical similarities of these components, they absorb and dissipate heat at different cycling periods. The ability to protect and cool engine internal and external components actually contributes to noticeable horsepower and performance gains.
http://www.swaintech.com/twostroke.html
Why Coat A Two Stroke Piston?
Pistons are the highest stressed and the most critical part of an internal combustion engine. This is especially true for a two stroke motor.
http://users.tkk.fi/~tpyrjovu/ceramics/
BENEFITS ON APPLYING CERAMICS TO COMBUSTION ENGINES
The purpose of this document is to be a brief study on applying ceramic materials for usage in combustion engines. The document was written as a literal part of the seminar presentation lectured for the course “Kon-67.104 Introduction to materials science and selection of materials” at Helsinki University of Technology in Laboratory of Engineering Materials. Because of the lack of sources and the limitations due the course there is not any cutting edge information nor break through results.
http://www.rpw.com.au/Products/HPC Coatings/HPC Coatings.htm
Performance Engine Coatings
HPC offers two coatings for pistons and valves that can be used together separately of each other based on needs and some class regulations in racing. HPC's thermal barrier coating (TBC) is applied to the combustion face of the piston and a wettable solid dry film (SDF) applied to the skirt.
http://www.speedoptions.com/articles/1513/
Thermal barriers as the name implies are barriers or shields to thermal events or heat. An internal combustion engine is basically a thermally controlled air pump. So various engine events will depend on heat to function properly while others depend on the elimination of heat to live. Remember, an engine's working fluid is air, introduce an air charge into the combustion chamber, provide an ignition source with a combustible compound to heat it at the correct time, and you produce work. Sounds easy, however heat is the key to making a little power, a lot of power, or seizing the engine.
http://speedoptions.com/articles/1213/
what are thermal coatings and what do they do? Thermal coatings come in two basic categories; thermal dispersants to help get rid of heat and thermal barriers to block heat. Thermal dispersants help the coated part shed heat faster than that part normally would dispense with were it bare or un-coated.
In early 1998, the INDY LITES organization decided to conduct a test of their then current oil cooler and thermal dispersants. They had the oil cooler thermal dispersant coated, it was then tested on and off the track. They experienced a 15% reduction in oil temperatures; the smaller older coated oil cooler out performed its newer and larger replacement. All INDY LITES teams now run coated oil coolers.
http://www.eng.warwick.ac.uk/oel/papers/2002/2001-11-05.pdf
2 Introduction
Heat transfer measurements within internal combustion engines has become increasingly important with the drive towards higher efficiencies and cleaner exhaust emissions as well as increased energy levels at the exhaust for turbo-charging. Although internal combustion engines have been studied for many years the combustion chamber temperature and heat transfer rates have been investigated to a lesser extent. During a combustion cycle the peak gas temperature can reach levels around 2500 K. The metal components of the combustion chamber can withstand approximately 600 K for cast iron and 500 K for aluminium alloys (Lim, 1998). Hence, cooling of the cylinder head, block and piston is required.
http://www.eng.warwick.ac.uk/oel/papers/2002/2002-01-0747.pdf
INTRODUCTION
Difficulty in understanding and measuring the combustion process is partly due to the short duration of the flame. In a typical internal combustion engine, combustion is over in less than 4 ms. The flame-front is multi-spectral, in terms of emitted radiation, threedimensional and highly turbulent. In order to make a worthwhile measurement of the burning process, each of
these parameters must be taken into account. Add into this variation in pressure, mixing of fuel and air, combustion chamber geometry, engine speed and loading; the problem becomes highly complex
...then knock it
