Date of Award
Master of Science in Engineering (MSE)
The application of thermal barrier coatings (TBCs) in spark ignition (SI) engines has historically been avoided due to the knock penalty associated with higher surface temperatures induced by the ceramic layer. However, advances in low thermal inertia coatings (i.e., temperature swing coatings) that combine low thermal conductivity with low volumetric heat capacity can prevent excessively high surface temperatures during the intake stroke and reduce or avoid knock while improving performance and efficiency. This thesis experimentally evaluates the effectiveness of these low thermal inertia coatings in a single-cylinder research engine representative of modern SI engines.
First, four pistons coated with a novel, low thermal inertia material (called NC) and one piston coated with commercially available gadolinium zirconate (GZO) were tested. An average 0.15 percentage point absolute thermal efficiency gain was observed with the thinnest NC coating. This efficiency increase was enabled through spark advance, indicating that the piston surface temperature was lower than the metal reference surface temperature early in the cycle (i.e., during the intake stroke). Thicker NC coatings experienced a degradation in performance and efficiency due to charge heating increasing the knock propensity. Simulated cold-start tests demonstrated that the charge heating behavior observed with thicker coatings was beneficial for reducing unburned hydrocarbons and particulate matter emissions.
Once the best performing piston was identified, intake and exhaust valves with GZO coated combustion faces, backsides, and stems were installed to evaluate the effect of additional coated surface area on performance. With both coated valves and a coated piston, the net thermal efficiency increased by 0.20 percentage points, thus the valves contributed an average of 0.05 percentage points over the coated piston. A staged valve removal was performed to determine the contribution of each coated valve. It was found that the coated exhaust valve promoted knock and higher exhaust temperature, whereas the coated intake valve encountered lower knock propensity as heat transfer between the valve and incoming air was reduced.
Finally, a pseudo-durability test was performed to analyze the effect of naturally grown thermal TBCs (i.e., combustion chamber deposits – CCDs) on low thermal inertia TBCs. A coated piston and coated heat flux probe underwent a low-load operating condition for 62.5-hours to promote CCD growth. Every 12.5-hours, the performance at a knock limited condition was assessed and thermophysical property measurements on the heat flux probe were performed. Net thermal efficiency increased by 0.4 percentage points after 12.5-hours, but further CCD growth caused a dithering of efficiency between 12.5-hours and the baseline condition. The knock limited spark advance was consistently retarded throughout this period. External property measurements with the coated heat flux probe showed an improvement in the thermophysical properties of the TBC/CCD layer.
Gandolfo, John, "Experimental Investigation of Low Thermal Inertia Thermal Barrier Coatings for Spark Ignition Engines" (2023). All Theses. 4202.
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