Demands for improved fuel efficiency and reduced CO 2 emissions of diesel engines have been the driving force for car industry in the past decades. One way to achieve this would be by using thermal spraying to apply a thermal insulation layer on parts of the engine's combustion chamber. A candidate thermal spray process to give coatings with appropriate properties is suspension plasma spray (SPS). SPS, which uses a liquid feedstock for the deposition of finely structured columnar ceramic coatings, was investigated in this work for application in light-duty diesel engines. In this work, different spray processes and materials were explored to achieve coatings with optimized microstructure on the head of aluminum pistons used in diesel engine cars. The functional properties of the coatings were evaluated in single-cylinder engine experiments. The influence of thermo-physical properties of the coatings on their functional properties has been discussed. The influence of different spray processes on coating formation on the complex piston head profiles has been also discussed. The results show that SPS can be a promising technique for producing coatings on parts of the combustion chamber, which can possibly lead to higher engine efficiency in light-duty diesel engines. Keywords diesel engines Á fuel efficiency Á microstructure Á suspension plasma spraying Á surface engineering Á thermal barrier coatings & Wellington Uczak de Goes wellington.uczak
Thermal barrier coatings (TBCs) are considered a promising solution for improving the efficiency of internal combustion engines. Among the thermal spray processes, the relatively newly developed suspension plasma spray (SPS) is an attractive candidate due to its unique microstructural features that have already demonstrated increased performance in gas turbine applications. To achieve these features, thermal spray conditions play an essential role. In specific uses, such as piston of diesel engines, parameters as spray angle and spray distance pose challenges to keep them constant during the whole spray process due to the complex geometry of the piston. To understand the effect of the spray distance and spray angle, a comprehensive investigation of the produced thermal spray microstructure on the piston geometry was conducted. Flat and complex geometry surfaces were coated using the same plasma parameters while the spray angle and distance were changed. Characterization was performed using scanning electron microscopy (SEM) combined with the image analysis technique to perceive the variation of the thickness and microstructures features such as pores, cracks, column density, and column orientation. The results showed that the changes in spray angles and spray distances due to the complex shape of the substrate have a significant influence on the microstructure and thermal properties (thermal conductivity and thermal effusivity) of the coatings. The thermal conductivity and thermal effusivity were calculated by modeling for the different regions of the piston and measured by laser flash analysis combined with modeling for the flat-surfaced coupon. It was shown that the modeling approach is an effective tool to predict the thermal properties and thus to understand the influence of the parameters on the coating properties. Connecting the observations of the work on the microstructural and thermal properties, the complex geometry’s influence on the produced coatings could be diminished by tailoring the process and generating the most desirable TBC for the internal combustion engines in future applications.
Internal combustion engines have a key role in the social and economic advancement of modern society but also a significant contribution to greenhouse gas emissions. For these engines, to preserve their role, a higher efficiency, that dramatically reduces the environmental impact, is necessary. To achieve increased engine efficiency, a technical solution is to lower the heat losses in the combustion chamber. Among them, the heat losses to the pistons are the preferential route, due to their extensive impact on fuel consumption. In this paper, porous thermal barrier coatings with large pores were applied to the pistons of diesel engines to improve engine efficiency. Atmospheric Plasma Spray (APS) process and porosity former TBC feedstock were employed to obtain high porosity coatings with large pores. Scanning Electron Microscopy (SEM) was utilized to investigate the microstructure of the coating in coupons and pistons. The optical properties of the coatings were explored with two methods: the spectral normal hemispherical reflectivity at room temperature (SNHRRT) and spectral normal emissivity at high temperature (SNEHT). The coatings’ behavior under thermal cyclic conditions was assessed by Flame Rig Test. Microstructure analysis was also performed before and after the test to identify the failure mechanisms. The engine efficiency was evaluated by measuring the Indicated Specific Fuel Consumption (ISFC) in a single-cylinder engine test. The results showed that porous coating with large pores combined with a higher emissivity can withstand the engine environment well and have the potential to provide enhancements in engine efficiency.
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