ENGINE COATINGS PISTONS, CYLINDERS & COMBUSTION CHAMBER COATINGS

Two primary benefits to pistons and cylinders are friction reduction and heat reduction.

Friction reduction is accomplished due to the Moly's natural lubricity and high load carrying abilities (some formulations can withstand loads in excess of 325,000 psi and still provide lubrication) while Teflon (PTFE) is very slippery and is an excellent friction reducer when used in right application. Compared to Molybdenum based lubricants Teflon has low load carrying capabilities (generally less than 10,000 psi) By reducing friction we also reduce heat generated by friction, further heat reduction is accomplished due to other properties of our coatings which we will discuss in detail later.

A combination of coated pistons and cylinder walls reduce damage from piston rock at the bottom of the stroke. Galling and scuffing is greatly reduced and piston life is increased. Unlike other coatings that can have an adverse effect on ring seal, our coatings improve ring seal. Leakdown tests have shown a 2 to 3 percent improvement.

The exterior of the piston is coated to a thickness of approximately 1/2mil (.0005) and burnishes to .0000 upon use. The functional material is bonded to the pores of the metal. After short use the coating will looked streaked on the skirts. This is normal and does not represent a loss of coating as it is still in the pores of the metal providing lubrication. In the ring land area this provides a lubricant that will allow the rings to move freely and inhibit sticking due to excessive heat and lubrication failure.

Heat reduction is accomplished in two ways, one of which we have already discussed (friction). The second is through the heat dissipating characteristics of our coatings. The pigments are thermally reactive and when bonded to a surface exposed to a source of heat react to that heat. The heat, generated either through friction or combustion, is rapidly transferred over the coated surface and radiated away from the part, in the combustion chamber this tends to increase thermal efficiency. On other parts it helps the oil absorb heat and reduces the operating temperature of the parts and the engine as a whole.

On piston domes a thermal dispersant coating (cermet) will improve the thermal efficiency of the combustion chamber by providing more even heat distribution and inhibiting the amount of heat that the piston will absorb. This is accomplished by the coatings ability to move heat approximately three times faster across the surface than through it.

Coatings have been looked to as an instrument to achieve several very 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. Insulating (thermal barrier), transfer heat, reflectability, radiation, and durability. If all five of these goals can be achieved we can increase thermal efficiency, increase part life, reduce part temperatures, reduce engine operating temperatures and reduce detonation.

The first goal has been the 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 meet the other goals. When we examine the combustion chamber, we find that 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 generated 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 thermal barrier action. The first deals with the movement of heat over combustion chamber surfaces.

The second with the movement of heat away from the point of combustion into the chamber. The third concerns introduction or transfer of heat into the incoming air/fuel mix, that remains after the exhaust stroke. This heat being retained by combustion chamber surfaces. Hot spots develop on combustion chamber surfaces, these hot spots can lead to detonation as fuel is elevated to a temperature where it self ignites, before the combustion event initiated by the spark plug 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 surface hot spots would be eliminated. Traditional ceramics have not functioned well in this regard.

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 light 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 nearly equal levels of light throughout the room. A combustion chamber would react the same way if the light/heat generated by ignition could be more evenly dispersed within the chamber. More even and complete oxidation of fuel would occur, thus increasing the efficiency of the engine, making more power and reducing emissions.

The fourth goal of radiation deals with the ability or inability of a surface to transfer heat absorbed by combustion to the incoming air/fuel mix. When heat is transferred to the mix, the mix begins to expand, this at a time when you are trying to get the greatest, coolest volume of air and fuel into the chamber, and then trying to compress it. The result is lost efficiency, as a lesser volume of the mix is drawn into the chamber and the mix is expanding while the piston is still trying to compress it. This has been one of the major problems with traditional ceramics. These coatings absorb heat, while not transferring it to the substrate 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. this was also a problem that non-ceramic coatings aggravated, though not to as great a degree, due to their thinner film thickness. Traditional ceramics have been applied at thickness ranging from .002" to .250" and non-ceramic coatings have been applied at .0005" to .0015". The amount of heat absorbed is in direct proportion to the insulating abilities and the thickness of the coating.

What is needed is a coating that has the insulating characteristics of ceramics, good heat transfer, can be polished for reflectability, and will not radiate absorbed heat into the incoming air/fuel mixture very readily. And of course, stay on the part.

Our coating meet these goals, we suspend a thermally conductive material, in a ceramic 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 ceramic 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 heat transferred to the substrate is further reduced. In addition, the surface coating will allow rapid movement of heat to reduce hot spots. Since the surface can be highly polished, excellent reflectability is also achieved, leading to increased thermal efficiency.

Since the thin aluminum coating cannot absorb nor 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 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 our material. The ceramic coating is not achieved through the bonding or fusing of zerconia ceramics, has been traditionally done, rather the resin when cured becomes 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 impact have damaged the coating, even 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.

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