Abstract
In order to further optimize the efficiency of today’s internal combustion engines, specific coatings are used on functional surfaces to reduce internal engine friction and wear. In the current research project, oxymethylene ether (OME) is discussed because it is CO2 neutral and has a strong soot-reducing effect as a fuel or fuel additive. In some operational regimes of the internal combustion engine a dilution of engine oil by fuel must be assumed. In this paper, the frictional contact between piston ring and cylinder raceway is modelled using a pin-on-disk tribometer and the friction and wear behavior between a diamond-like carbon coating (DLC) and a thermal spray coating is characterized. The wear of the spray layer could be continuously detected by radionuclide technology (RNT). With the aid of photoelectron spectroscopic measurements (XPS), the steel thermal spray coating was chemically analyzed before and after the tribometer tests and the oxidative influence of OME was investigated. In addition, confocal microscopy was used to assess the topographies of the specimens. The measurements showed that the addition of OME to the lubricant reduced the viscosity and load-bearing capacity of the lubricating film, which led to an increase in the coefficient of friction. While almost no wear on the pin could be detected at 10% OME, the first visible material removal occurs at an OME content of 20% and the layer delaminated at 30% OME. The evaluation of the RNT wear tests showed that both the tests with engine oil and with engine oil plus 20% OME achieved very low wear rates. No corrosion of the thermal spray coating could be detected by XPS. Only the proportion of engine oil additives in the friction track increased with increasing OME concentration.
Highlights
For more than a century, the combustion engine has been used as a drive unit without its basic structure having changed significantly
While almost no wear on the pin could be detected at 10% oxymethylene ether (OME), the first visible material removal occurs at an OME content of 20% and the layer delaminated at 30% OME
Friction increases with increasing OME concentration at the same temperature
Summary
For more than a century, the combustion engine has been used as a drive unit without its basic structure having changed significantly. The majority is converted into heat and is lost through friction [1]. The current climate targets for greenhouse gas emissions require further optimization of the system. A major advantage is the increased oxidative effect of biofuels, which results in a more complete combustion, reducing emissions [3]. This raises the question of the compatibility of the existing engine components with an increased oxidative effect. In addition to the fuel-carrying system, the combustion chamber in particular can be considered for a possible corrosive effect due to the increased temperature level and fuel contamination of the lubricating oil. The current state of the art consists of a thermal spray coating that covers the Lubricants 2018, 6, 60; doi:10.3390/lubricants6030060 www.mdpi.com/journal/lubricants
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