Abstract:
The high frequency reciprocating rig (HFRR) is typically used to determine the lubricity of diesel fuels and has been accepted as the universal test apparatus in most countries of the world. A study was undertaken to analyse results obtained with the HFRR instrument by identifying crucial factors contributing to friction and wear mechanisms during a lubricity test under given conditions that may lead to a better understanding of test results obtained. The transient temperature distribution on a semiinfinite disk due to a circular continuous oscillatory heat source was also modelled. The model was used to calculate the contact temperature of a contact similar to what can be expected in the HFRR apparatus. For lubricated tests a mixture of n-hexadecane (cetane) as base fluid and palmitic acid as lubricity additive was used. Experimental results, including results obtained by running unlubricated (dry) tests and results obtained from literature were all considered to formulate the hypothesis that a fully functional additive film will not survive the full duration of the lubricity test, which is 75 minutes (ISO 12156-1, 2003). Friction values obtained in the experiments conducted were much higher than what is expected for boundary films formed by long chain carboxylic acids. Comparable friction values were obtained in the first few seconds of the tests, but the friction values increased indicating the destruction of the boundary film. Metal oxide lubrication dominates for the remainder of the test. Information of the exact mechanism prevailing in the crucial first few seconds of the test should be obtained by increasing the data acquisition rate of the apparatus. It is believed that junction growth through adhesion is the dominant factor in the initial stages of the test. GCxGC/TOF-MS showed the formation of hexane and heptane, which is a strong indication of the catalytic decomposition of the C16-hydrocarbons in the sample.
