Date of Award

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Automotive Engineering

Committee Member

Zoran Filipi, Committee Chair

Committee Member

Mark Hoffman

Committee Member

Robert Prucka

Committee Member

Fadi Abu-Farha

Abstract

The inherent thermodynamic benefits of Homogeneous Charge Compression Ignition (HCCI) make it a likely choice for meeting the increasing demands of fuel economy legislation. Unfortunately HCCI suffers from reduced combustion efficiency and operational variability due to the buildup of carbon deposits. However, the unique thermo-kinetic nature and thermal sensitivity of Low Temperature Combustion (LTC) provides an opportunity to improve combustion efficiency through manipulation of the in-cylinder thermal environment. This body of work sought to create a wall temperature swing using a thin Thermal Barrier Coating (TBC) to reduce combustion heat transfer and improve LTC combustion and thermal efficiencies. The first TBC used was a thin, dense YSZ coating, which provided modest gains in thermal and combustion efficiencies in addition to accelerating LTC burn rates and advancing combustion. This confirmed the original hypothesis, so coatings with higher porosity were pursued as a means of further reducing thermal conductivity and increasing the temperature swing magnitude. This direction of investigation yielded further incremental improvements in thermal and combustion efficiencies, however pitfalls experienced due to interactions of combustion gases with the surface roughness and open porosity of highly-porous TBCs discouraged this area of inquiry. An investigation into porosity and roughness interactions confirmed the impacts of surface roughness, however the open porosity effects were not representative of the impacts witnessed with the TBC, due to the porosity becoming blocked by carbon deposits. The next step focused on alternative low thermal conductivity materials, such as gadolinium zirconate, as a way to achieve durable, low conductivity TBCs. This area of investigation proved successful, providing a thin coating with a 0.65 W/m-K conductivity that created a large temperature swing, boosting thermal efficiency by up to a 5.9% and combustion efficiency by up to 1.5%. As a separate approach to improving combustion efficiency, catalytically active coatings were investigated. Experiments indicated catalytic activity with the use of a low temperature CuOx – CoOy – CeO2 (CCC) catalyst specifically developed for LTC aftertreatment over a YSZ thermal barrier provided a modest boost to combustion and thermal efficiencies. Combined, these investigations provide guidance on thermal barrier coating design for LTC to remove one of the last hurdles to mass-adoption for HCCI engines.

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