(596a) Development of a Functional Coating to Mitigate Coking in a Turbocharger Compressor

A turbocharger is used to recover thermal energy that would otherwise be rejected to the atmosphere through the exhaust system in a vehicle. The compressor housing of a turbocharger faces considerable fouling due to deposits (coking) arising from lubricant oil aerosols and particulate matter from the crankcase ventilation unit. Prior research has shown that this deposit formation can cause degradation of the compressor efficiency by as much as 10% which can subsequently lead to decreased fuel economy and increased emissions. This reduction in efficiency is correlated with the deposit surface roughness which in turn is dependent on the formulation of the lubricant. However, since oil choices by customers are beyond a car company’s control, efforts are needed to mitigate deposit formation irrespective of the type of lubricant. The carbonaceous deposits formed in the compressor are formed in a low temperature regime of 180 - 220°C. Limited literature exists on the coking mechanism and remediation approaches at these temperatures. In this study, we demonstrate a functional metal-metal oxide thin film (UM1) that offers low adhesion to the formed deposits and may enable the deposits to flake off the surface in the highly turbulent flow conditions of the compressor. The film was developed on an Al 6061-T6 coupon by sputter deposition. Coking is simulated using the 10W-30 CK-4 factory fill lubricant in an oil aerosol coking rig test with coated coupons placed inside the rig test tube. The deposits are analyzed using 2D imaging, 3D optical profilometry and optical laser interferometry for surface roughness values. Deposits formed on UM1 from the coking tests demonstrated protruded flaking in the temperature range of 210 - 225°C and maintained a smooth texture below 200°C. No deposit flaking or lack of adhesion is observed for the uncoated coupons. To demonstrate low adhesion of the deposits on UM1 and its applicability to remediate deposits in a real compressor, a novel laboratory adhesion test chamber was designed and setup that simulates the actual temperature and air flow conditions (Re > 60000). The adhesion test showed that the flaked deposits blow off the surface at air flow velocities as low as 5.6 m/s and near 100% deposit removal was achieved at 50 m/s (Re >60000). Due to low deposit adhesion, a slower rate of deposition and ease of deposit removal under the operating conditions, UM1 is a promising surface treatment approach to mitigate the low temperature coking observed in an automotive compressor.