Experimental investigation of the relationship between thermal barrier coating structured porosity and homogeneous charge compression ignition engine combustion

Abstract

Heat transfer has a profound influence on homogeneous charge compression ignition combustion. When a thermal barrier coating is applied to the combustion chamber, the insulating effect magnifies the wall temperature swing, decreasing heat transfer during combustion. This enables improvements in both thermal and combustion efficiency without the detrimental impacts of intake charge heating. Increasing the temperature swing requires coatings with lower thermal conductivity and heat capacity. A promising avenue for simultaneously decreasing both thermal conductivity and capacity is to increase the porosity fraction. A proprietary solution precursor plasma spray process enables discrete organization of the porosity structure, called inter-pass boundaries, which in turn produces a step-reduction in thermal conductivity for a given porosity level. In this investigation, yttria-stabilized zirconia is used to create four different thermal barrier coatings to study the potential of structured porosity as means of improving the “temperature swing” behavior in a homogeneous charge compression ignition engine. The baseline coating is “dense YSZ,” applied using a standard air-plasma spray process. Next, significant reductions of the thermal conductivity are achieved by utilizing the solution precursor plasma spray process to create inter-pass boundaries with a moderate overall porosity. Performance, efficiency, and emissions are compared against both a baseline configuration with a metal piston and an air-plasma spray “dense YSZ” coating. Experiments are carried out in a single-cylinder gasoline homogeneous charge compression ignition engine with exhaust re-induction. Experiments indicate that incorporating structured porosity into thermal barrier coatings produces tangible gains in combustion and thermal efficiencies. However, there is an upper limit to porosity levels acceptable for homogeneous charge compression ignition engine application because an elevated porosity fraction leads to excessive surface roughness and undesirable fuel interactions. Comparison of the coatings showed the best results with coating thickness of up to 150 µm. Thicker coatings led to slower surface temperature response and attenuated swing temperature magnitude.

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

“…While increases in NOx emissions have been noted in prior experiments using a YSZ coating in the HCCI engine, the changes were within the range of uncertainty of the analyzer ( \ 8 ppm NO x ). 10,14 However, the increases in the NO x emissions index for the series of experiments reported here were more significant than the measurement uncertainty, as illustrated in Figure 8. The increase in NO x emissions was greatest at 1200 r/ min, with a more than a five-fold increase over the metal baseline.…”

“…This has been noted in earlier FMPM studies with TBCs. 10,14 Despite the use of cooled EGR, reductions in unburned hydrocarbons were still realized with the GdZr-SP coating. The magnitude of reductions is more modest relative to FM operation due to the impact of dilution on chemical kinetics (refer to Table 5), as well as different phasing at lower speeds and the resulting decrease of the bulk gas temperature.…”

“…The outstanding improvement observed with GdZr inspired a reflection on previous efforts using the YSZ coating. 10,14 Hence, misfire limits observed with the ''dense'' YSZ coating on the piston top, as well as the YSZ coating with structured porosity were added to the plot in Figure 20. The two YSZ coatings produced a significant positive impact on the misfire limit, and the effect was relatively larger at higher speeds.…”

“…Therefore, following the findings reported by Powell et al, 10 the authors investigated the potential for further improvements by introducing greater porosity in the YSZ coating. 14 In general, porosity creates a well understood impact on both thermal conductivity and volumetric heat capacity of a coating. [15][16][17] However, excessive porosity fraction combined with fuel impingement can lead to erosion in a direct injection (DI) engine, as well as undesirable fuel storage within the open pores.…”

A numerical investigation of isobaric combustion strategy in a compression ignition engine

“…As such, improvements in thermal efficiency require systematic optimization in order to minimize various losses associated with gas exchange, combustion, heat transfer, and mechanical friction [6,7], along with the reduction of pollutant emissions like nitric oxides (NOx) and soot [8,9]. Over the past decades, various advanced combustion strategies were proposed, including the homogeneous charge compression ignition (HCCI) [9], the premixed charge compression ignition (PCCI) [10], and partially premixed combustion (PPC) [11,12], all of which aim to achieve higher efficiency and lower emissions.…”