The following information is reprinted with permission from Leonard Warren. Leonard is president and founder of TECH LINE COATINGS, INC. and has been actively involved in racing since the mid 60’s.

We wish to express our appreciation to John Erb, Chief Engineer KEITH BLACK RACING PISTONS, for his invaluable advice and technical assistance.

Coatings have been looked to as an instrument to achieve several desirable goals in thermal management, particularly in regard to the heat generated in the combustion chamber of internal combustion engines. There are five major goals;

If all five of these goals can be achieved we can increase thermal efficiency, reduce part temperatures, reduce detonation and increase performance.

The first goal is one that has been given the most attention, even though, as it turns out, it may not be the most important. For several years a variety of companies have experimented with ceramic TBC’s with mixed results, due to the inability of traditional ceramics to met the other goals. When we examine the combustion chamber, we find the temperatures can exceed 3000F for very short periods of time and exhaust gas temperatures can exceed 1600F.. Yet combustion chamber surfaces rarely exceed 600F. This is due to the cyclical nature of combustion chamber activity, combustion generating heat followed by a cooling, incoming air/fuel charge. Consequently, only a fraction of the heat generated is absorbed by the surfaces. This is not to say that a barrier is not desirable. Any decrease in part temperature reduces the burden on the cooling system and extends part life. In addition, if heat is not as rapidly lost, then greater force is exerted against the piston for a longer period of time during the power stroke, creating more power. However, all of these benefits are lost if the coating increases detonation or delaminates.

What, in reality, is of greater importance is to MOVE heat within the combustion chamber. In the combustion chamber there are three areas of concern, besides simple barrier action. The first deals with the movement of heat over the combustion chamber surfaces. The second, with the movement of heat away from the point of combustion into the chamber. The third concerns the introduction or transfer of heat into the incoming air/fuel mix, after the exhaust stroke, this heat is being retained by combustion chamber surfaces.

Hot spots develop on combustion chamber surfaces. These hot spots can Leed to detonation as fuel is elevated to a temperature where it self ignites, before the combustion event initiated by the spark is completed. In diesel engines, of course, combustion is generated by compression induced heat, not by spark. If the surfaces could be treated in such a manner that heat would easily flow from hotter areas to cooler areas, then detonation created by hot spots would be eliminated. Traditional ceramics have not functioned well in this regard. In 1987, Tech Line Coatings Inc. (Then known as G & L Coatings) pioneered the use of non-ceramic thermal barrier/dispersants. These coatings achieved the second goal quite well and also functioned as TBC’s. This was demonstrated in repeated testing and use. 

When we are speaking of Reflectability, we are looking at the ability of a surface to reflect heat. Generally this characteristic has been achieved by polishing the combustion chamber surfaces. A polished "bright" surface will reflect heat into cooler areas. This can be illustrated by considering an irregularly shaped room, lighted by a single bulb. There will be areas of the room that do not receive as much light, either because of being shrouded or simply because of being further from the light source. If you were to line the walls with mirrors the light would be reflected into the darker areas providing near equal levels of light through out the room. A combustion chamber would react the same way if the light/heat generated by ignition could be more evenly dispersed within the combustion chamber. More even and complete oxidation of fuel would occur, thus increasing the efficiency of the engine, making more power and reducing emissions. This has been demonstrated by the resulting increase in engine horsepower with a corresponding reduction in carburetor jet sizing.

The fourth goal in radiation deals with the ability or inability of a surface to transfer heat absorbed by combustion of the incoming air/fuel mix. When heat is transferred to the mix, the mix begins to expand; this at a time when you are first trying to get the largest, coolest volume of air and fuel into the chamber, and then trying to compress it. The results is a loss of efficiency, as a lesser volume of the mix is drawn into the chamber and the mix is expanding while the piston is still rising, trying to compress it. This not only takes power, but also can very quickly lead to detonation. This has been one of the major problems with traditional ceramics. These coatings absorb heat while not transferring it to the substrate or base metal, very rapidly. While this protects the substrate from absorbing the heat, it unfortunately provides a very hot surface for the new air/fuel mix to contact, creating the problems discussed previously. This was also a problem that non-ceramic coatings aggravated, through not to a great degree, due to the thinner film thickness. Traditional ceramics have been applied at thickness ranging from .002" to .025" and non-ceramic coatings have been applied at .005" to .0015" The amount of heat absorbed is in direct proportion to the insulating abilities and the thickness of the coating.

Tech Line Inc. has developed a coating to help maximize the efficiency. Tech Line suspends thermally conductive materials in a "ceramic like binder". After curing, the surface of the coating is burnished/polished to expose a micro- thin layer of conductive material. The barrier function is actually enhanced by this action, as the part now has a coating of a ceramic like material topped by a polished metallic layer. The conductive material, aluminum, does not degrade the effectiveness of the coating due to the tendency of heat flow to be interrupted any time it must pass through dissimilar layers, so that heat transferred to the substrate is further reduced. In addition, the surface coating will allow rapid movement of heat to reduce hot spots. When the surface is highly polished, excellent reflectivity is also achieved, leading to increased thermal efficiency. Since the micro thin aluminum coating cannot absorb or retain much heat, there is minimal radiation of heat into the incoming air/fuel mix. Only a small amount of heat will actually transfer into the mix, and there is speculation that a small amount of heat transfer would be beneficial. This would lead to exciting some of the wet fuel flow allowing more complete combustion. The polished surface would also reduce carbon buildup and maintain as new performance.

The final goal of durability is also achieved. While traditional ceramics are prone to flaking due to the nature of these materials, which require micro cracking to maintain adhesion, this is not the case with Tech Line’s material. The ceramic like coating is not achieved through the bonding or fusing of zirconia ceramics, as has been traditionally done. Rather the resin when cured takes on some of the characteristics of a ceramic. This type of material has a degree of flexibility and can expand and contract with the surface. The melt point of the coating is far above the temperature that would be generated by combustion and in testing has maintained adhesion when a piston was exposed to sufficient heat to actually melt the piston. Neither thermal shock nor physical impacts have damaged the coating, even when sufficient impact was used that the substrate was dented. The coating maintained adhesion and followed the deformation. A potential side benefit to the use of this coating is its ability to "strengthen" the substrate. On aluminum parts this means that not only is the surface less subject to damage, but in the case of a piston, a thinner dome could be utilized in certain applications. Testing in this area is proceeding.

Additionally, the new coating is applied at temperatures low enough that damage to the coated part is not a concern.

These new products either meet or exceed all of the goals identified for combustion chamber coatings.