Impact of Thermal Barrier Coatings on Intake and Exhaust Valves in a Spark Ignition Engine
Spark ignition knock is highly sensitive to changes in intake air temperature. Hot surface temperatures due to ceramic thermal barrier coatings increase knock propensity by elevating the incoming air temperature, thus mitigating the positive impacts of low heat transfer losses by requiring spark retard to avoid knock. Low thermal inertia coatings (i.e. Temperature swing coatings) have been proposed as a means of reducing or eliminating the open cycle charge heating penalty of traditional TBCs through a combination of low thermal conductivity and low volumetric heat capacity materials. However, in order to achieve a meaningful gain in efficiency, a significant fraction of the combustion chamber must be coated. In this study, a coated piston and intake and exhaust valves with coated combustion faces, backsides, and stems are installed in a single-cylinder research engine to evaluate the effect of high coated fractions of the combustion chamber in a knock-sensitive architecture. Spark timing sweeps demonstrated a small, but notable increase in net thermal efficiency compared to the coated piston by itself. Here, a staged valve removal demonstrated that the coated intake valve prevents heat transfer to the incoming air, reducing knock propensity compared to the coated exhaust valve individually.