The impact of piston thermal barrier coating roughness on high-load diesel operation

Thermal barrier coatings of various thickness and surface roughness were applied to the piston crown of a single-cylinder research engine and tested over a range of high-output diesel operating conditions, some near 30 bar gross indicated mean effective pressure. Three yttria-stabilized zirconia coated pistons were compared to a baseline metal piston. At each operating condition, a start-of-injection sweep was conducted to generate efficiency trends and find the optimal combustion phasing. Three variations of pistons coated with a graded-layer thermal barrier coating were tested: (1) 0.185 mm coating thickness with a surface roughness of approximately Ra = 11.8 µm, (2) 0.325 mm thickness with Ra = 11.8 µm, and (3) 0.325 mm thickness with Ra = 6.0 µm. Both coated pistons with Ra = 11.8 µm did not show any statistically significant improvement to engine performance when compared to the metal baseline piston, but did produce higher filter smoke numbers. The coated piston with Ra = 6.0 µm and 0.325 mm showed an increase of gross indicated thermal efficiency of up to 3.5% (relative) compared to the metal baseline piston for operating conditions comparable to standard engine operation and a reduction of filter smoke number back to the metal baseline. The increase in efficiency was found to correlate with additional late-cycle apparent heat release and a reduction in in-cylinder heat transfer. The very high-output conditions showed statistically insignificant changes in performance or heat transfer, which may have been related to the long injection duration used for these cases targeting outside of the piston bowl.