The accumulation and burn-off of combustion chamber deposits create uncontrolled shifting of the homogeneous charge compression ignition operability range. This combustion chamber deposit–created operational variability places increased control burden on a multi-mode engine. However, the operational variability can be mitigated by manipulating combustion chamber deposit accumulation. A magnesium zirconate thermal barrier coating was applied to the piston of a homogeneous charge compression ignition engine in an effort to reduce combustion chamber deposit accumulation through elevated piston surface temperatures. While reduced combustion chamber deposit thicknesses were observed on the magnesium zirconate piston periphery, combustion chamber deposit accumulation in the bowl region increased relative to aluminum piston operation. Additionally, combustion chamber deposit thicknesses on the aluminum cylinder head were reduced during operation with the magnesium zirconate coated piston. Chamber-wide alterations to combustion chamber deposit accumulation taken together with the increased burn duration and hydrocarbon emissions measured during operation with the magnesium zirconate piston indicate significant interaction between the directly injected fuel spray and thermal barrier coating porosity. The porosity and surface roughness of the magnesium zirconate thermal barrier coating are speculated to create fuel pooling/absorption within the piston bowl, increasing combustion chamber deposit accumulation in the bowl and leaning the remaining fuel–air charge. The charge leaning lengthens the magnesium zirconate burn duration and reduces cylinder head combustion chamber deposit accumulation. Furthermore, hydrocarbon emissions were increased during magnesium zirconate operation due to late desorption and subsequent incomplete burning of fuel from piston bowl and the presence of incombustibly lean areas in the remaining cylinder charge.
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.
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