Enhancing the efficiency benefit of thermal barrier coatings for homogeneous charge compression ignition engines through application of a low-k oxide

Abstract

Prior experiments reported by the authors have proven the hypothesis that achieving a dynamic temperature swing on the combustion chamber surface will lead to improved thermal and combustion efficiencies of the homogeneous charge compression ignition engine. A thin layer of yttria-stabilized zirconia, roughly 150 μm, was plasma sprayed on the piston top. It led to markedly advanced ignition and heat release in the gasoline homogeneous charge compression ignition engine, accompanied with reduced unburnt hydrocarbon and carbon monoxide emissions, improved combustion efficiency, and a higher thermal efficiency. A related computational study highlighted the critical role of coating thermal conductivity in achieving a desired dynamic response; hence, the second phase of experimental investigations focused on introducing structured porosity in the yttria-stabilized zirconia coating, as a means of reducing effective conductivity. Indeed, additional incremental improvements were observed, as well as limitations related to adverse effects of the surface roughness and the fuel interactions with the surface roughness and open pores. Erosion can also be a problem in a direct injection engine. Therefore, the third round of investigations focused on a material with a natively low conductivity (low-k), sprayed on the top of an Al piston in a relatively dense form, and in a way that yields a smooth surface. The objective was to capitalize on the low conductivity, while avoiding the pitfalls accompanying high-porosity formulations. The heat-storage capacity was limited by keeping the thickness relatively low. The results verify the paramount importance of thermal conductivity in the context of high “temperature swing” behavior and indicate a potential to improve the homogeneous charge compression ignition engine’s combustion efficiency roughly 1.5%, with the overall indicated efficiency improvement on the order of 5%, on a relative basis. In addition, the low-k oxide thermal barrier applied to the piston extended significantly the low-load homogeneous charge compression ignition operability limit.

A numerical evaluation and guideline for thermal barrier coatings on gasoline compression ignition engines

“…Gadolinium Zirconate (GdZr) has shown very encouraging results due to its low thermal conductivity, and the performance has been experimentally demonstrated. 54 The other candidate, Gen.2 Candidate#2, was one of the recently identified materials with superb thermophysical properties for coating application. The name of the material is not provided due to a potentially confidential disclosure for commercialization.…”

Low thermal inertia thermal barrier coatings for spark ignition engines: An experimental study

“…GZO has been identified as a substitute for yttria stabilized zirconia due to its lower thermal conductivity and has been successfully tested in conventional diesel 24 and advanced combustion modes. 25 One of the NC-coated pistons had a catalytically active topcoat applied (referred to as CCC). The seal coat was custom made using a commercial alkali binder and CCC (Ce:Co:Cu = 5:5:1) composite power.…”

Optimization of thermal barrier coating performance and durability over a drive cycle

“…70 Unfortunately, the modern engine literature has limited thermomechanical property data. Nonetheless, a few extra materials were included based on experimentally measured data 27,31,63 via private communication with the authors.…”

“…Coated walls have shown reduced combustion chamber soot deposits on the piston periphery, 26 while homogeneous charge compression ignition (HCCI) operational range has been shifted to lower loads. 27 Increased surface roughness provides more effective area and can increase near-wall turbulence. Additionally, the air-fuel mixing process can be affected, which in turn, affects chemistry.…”