Development of a Device for the Nondestructive Thermal Diffusivity Determination of Combustion Chamber Deposits and Thin Coatings

Abstract

An experimental radiation chamber has been developed to nondestructively measure the thermal diffusivity of a combustion chamber deposit (CCD) layer. The accumulation of CCD shifts the operability range of homogeneous charge compression ignition (HCCI) to lower loads where the fuel economy benefit of HCCI over a traditional spark ignition strategy is at a maximum. The formation and burn-off of CCD introduce operational variability, which increases the control system burden of a practical HCCI engine. To fully characterize the impact of CCD on HCCI combustion and develop strategies which limit the CCD imposed variability, the thermal and physical properties of HCCI CCD must be determined without destroying the morphology of the CCD layer. The radiation chamber device provides a controlled, inert atmosphere absent of cyclical pressure oscillations and fuel/air interactions found within an engine. The device exposes temperature probes coated with CCD to controlled heat flux pulses generated by a graphite emitter and a rotating disk. CCD layer thermal diffusivity is then calculated based on the phase delay of the sub-CCD temperature response relative to the response of the temperature probe when clean. This work validates the accuracy of the radiation chamber's diffusivity determination methodology by testing materials of known properties. Wafers of three different materials, whose thermal diffusivities span two orders of magnitude centered on predicted CCD diffusivity values, are installed over the temperature probes to act as CCD surrogates. Multiple thicknesses of each material are tested over a range of heat flux pulse durations. Diffusivity values determined from radiation chamber testing are independent of sample thickness with each of the three calibration materials. The radiation chamber diffusivity values exhibit a slight, but consistent underprediction for all wafers due to residual contact resistance at the wafer–probe interface. Overall, the validation studies establish the radiation chamber as an effective device for the nondestructive thermal diffusivity determination of thin insulative coatings. The similarity of expected CCD diffusivity values to those of the validation specimens instills confidence that the methodology and device presented herein can be successfully utilized in the characterization of HCCI CCD layers.