The composition and structure of heavy-duty diesel engine piston deposits generated in a Caterpillar 1Y540 single-cylinder bench test engine according to the Caterpillar 1N test procedure have been investigated using infrared spectroscopy, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), XPS depth profiling and transmission electron microscopy (TEM). The test engine piston was modified to allow deposits to be examined without removal from their original surface, allowing the structure of the deposit/substrate interface to be studied for the first time. Detailed microcharacterization of specimens generated under typical conditions has shown that deposit formation is initiated by adsorption of a thin lubricant layer on the piston surface during start-up. Oil-soluble additive degradation products within this thin lubricant layer react with surface oxides to form a phosphate-rich layer. With increasing engine run time, degraded lubricant additive species react with fuel combustion acids to form inorganic sulphates and phosphates that become embedded in a gum-like matrix of non-volatile hydrocarbon residues and partial oxidation products. Extended running times result in the build-up of a concentration of crystalline species near the deposit/substrate interface as a result of agglomeration and annealing. The layer structure is completed by deposition of further lubricant additive material during over-run on completion of the engine test.

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