Ceramic In-Cylinder Coatings

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Abstract: Zirconia based ceramic combustion chamber coatings originally developed for adiabatic or low heat rejection engines have been shown to reduce diesel emissions. Reported results indicate that in-cylinder zirconia coatings are capable of reducing the carbonaceous fraction of diesel particulates without increasing NOx or other regulated emissions. Reductions in total PM emissions may be achieved by combining zirconia coatings with diesel oxidation catalysts. In-cylinder coatings are most effective in reducing emissions from older technology engines of relatively low thermal efficiency.

Ceramic Coatings and Engine Performance

Ceramic thermal barrier coatings were originally developed and commercialized for gas turbine and jet engine applications. Many investigations have been conducted on various aspects of applying such coatings to the walls of combustion chamber in internal combustion engines. The prime objective which has been sought is to achieve higher thermal efficiencies by reduction of heat rejection from the combustion chamber. Experiments with diesel and gasoline engines suggest that thin coatings produce higher engine efficiency than thick coatings, in spite of being less effective as heat insulators [700]. This behavior of ceramic coatings has not been satisfactorily explained. It is believed that some detailed heat transfer characteristics must have a more profound effect on thermodynamic efficiency than the overall heat rejection rate from the engine.

Several ceramic materials such as zirconium oxide, chromium oxide, aluminum oxide, and mullite have been investigated as in-cylinder engine coatings. Zirconia, thanks to its low thermal conductivity and its thermal expansion coefficient which is compatible with that of metals, has become the preferred and most studied material. Ceramic coatings can be deposited by plasma spraying or from a ceramic slurry. The thermal spraying technique using a plasma torch has been used most extensively for this purpose. In the plasma spray process zirconia is fed as a powder into the plasma stream of the torch where it is melted at temperatures as high as 16,000°C. The high pressure plasma gas stream propels the molten particles onto the coated surface where they solidify. Powder and process parameters are used to control the structure and properties of the coating. The thickness of coatings can range from 0.05 to 2 mm. The optimal thickness of realistic materials is usually below 0.5 mm. Thin coatings were reported to exhibit both better performance and durability.

Besides improved thermal efficiency, advantages of ceramic coatings which have been proposed include improved engine durability, reduction in erosion and corrosion, less internal friction, lowered noise and reductions in exhaust emissions. A lot of work has been done on evaluating the effects of in-cylinder coatings on diesel engine performance and emissions. The results have been inconclusive and often contradictory. While most of published studies [146] [144] report potential emission benefits, some [143] [145] claim that the coatings have detrimental effects on fuel mixing and combustion, thus, deteriorating the performance and emissions. There is a significant variability in the coating effect between different engine types. The emission benefit of coatings appears to be related to their enhancing effect on the thermal efficiency of the engine. Therefore, higher emission effectiveness of coatings may have been possible in older technology engines which were characterized by relatively low thermal efficiency.

Effects of ceramic coatings on particular diesel emissions compiled from published experimental data are listed in Table 1.

Table 1. Effect of Ceramic Coatings on Emissions
Emission Effect of Coating
Total Particulate Matter No significant change
 - solid particulate fraction (carbon) Significant decrease*
 - organic particulate fraction (SOF) Increase
Visible Smoke Decrease
Nitrogen Oxides No change or slight decrease
Hydrocarbons Slight increase
Carbon Monoxide Decrease
* - decreases of up to 50% demonstrated in heavy-duty, 2-stroke urban bus engines