Combustion Chamber Thermal Dynamics

[MichaelPGriffith]

I'm in the process of doing a home build Honda J series V6. The motor is going into an MK1 MR2.
My initial plan was to do a thermal barrier on the piston domes, chamber face, valve faces, and full exhaust port, but I'm concerned about excess heat build-up on the cyl-wall. Also, I'm entertaining the idea of not coating the intake valves to help vaporise the fuel sprayed onto them. How would you guys weight in on these two issues?

 

[BrianPetersen]

If you're going to do thermal barrier coating, coat everything. Don't worry about fuel vaporisation, you'll never norice the difference. In fact, I'd rather have the fuel go into the cylinder atomized (not vaporized) because it takes up less space that way, and let heat of compression take care of the rest. I'd rather have the fuel cool the intake charge by evaporative cooling and let the cooling system carry away the heat, rather than boil the fuel and take up more space.

 

[WhizzMan]

Would there be any significant difference if you would coat the inlet valves? I doubt they would heat up from the cold air/fuel mixture, so it would only be blow-back during overlap that would heat them. I don't see a benefit of coating anything else than the "inside" of the dish of the inlet valves. If there is, I'd love to learn.

 

[SMOKEY44211]

Do not coat the intake valves. There is an SAE paper written by an engineer who was involved with the R+D of the Caddilac northstar engine. Great detail as to why the temperature of the intake is critical for fuel vaporization. Also do not be overly concerned with temperature build up in the cylinder walls. Even with coatings most of the heat goes out the head and exits with the exhaust gas.-------Phil

 

[naturalextraction]

Thermal barriers are designed to keep more heat energy to do work in the chamber against the piston. Yes, intake temperatures are critical. Looking back at SAE papers and in John Heywoods books, developing and maintaining "ideal" intake and or charge temperatures are part of the efficiency of work performed by the pressure and internal chemical energy. I would assume the build up of excess heat would be due to constant high rpm due to the intended use of this vehicle rather than inappropriate mixtures? Invariably it is about atomization and not vaporization of the intake charge and getting it to it's optimum "state". Out of curiosity what is your intended use for the vehicle?
Some find polishing for deflection works as well (especially if you can DIY) without the expense of coating. However the costs for that seemed to have come down considerably over the years.

 

[HRHtex]

Thermal barrier coatings to keep heat in the combustion chamber never quite make sense to me. I have always thought that one of the main objects of an aluminium head was to conduct the heat away more quickly thus allowing a higher CR without detonation - having a thermal barrier would seem to work against this aim.

 

[gruntguru]

In thermodynamic terms it is better for power and efficiency to keep the heat in the gas. Once any heat gets into the metal it is preferrable to conduct it away.

Engineering is the art of creating things you need, from things you can get.

 

[MichaelPGriffith]

naturalextraction; The intention is to have a weekend warrior/ NASA time attack vehicle.

Thanks everyone for your input.

I went ahead and started another thread to ask my elementary questions and there is a link to a web-album of the buildup there.

http://eng-tips.com/viewthread.cfm?qid=283754&page=1

 

[L4189]

The reason for cooling cylinder heads for instance, is to keep them from cracking. Same thing with cylinders and pistons. If a piston overheats it expands and seizes in the cylinder with spectacular consequences. The latest figures on the "heat balance" is that about 25% of the energy evolved from the fuel is absorbed by the cooling system. Ceramic thermal barrier coatings have been successfully used in spark ignited engines for racing to further increase output through increases thermodynamic efficiency. The thin heat reflective coating is best for this. It has to have high reflectance and low heat capacity. Thick thermal barriers were tried in diesels but because the surfaces stayed hot at the start of the intake of cool charge air. This reduced engine power and resulted in increased NOx. On spark ignited engine they would cause detonation as in the bad old days with lead or carbon deposits on the cooler surfaces.

http://www.usceramiccoatings.com/