"Initial Stages of Deposit Formation in Internal Combustion Engines – A Surface Analytical Study"

Christopher Baker-Brian

Shell Global Solutions UK, Cheshire Innovation Park, P.O. Box 1, Chester CH1 3SH, UK

Christopher.BB@Shell.com

Fuel-related deposits on surfaces within internal combustion engines are known to increase harmful environmental emissions and reduce engine performance. Deposit control is therefore a major aim in the development of new and cleaner fuels. The initial stages of deposit formation involve interactions between metal surfaces and active species in the fuels. These interactions have been studied by XPS, using fuels specifically formulated to promote or inhibit deposit formation. Results are correlated with screening tests and with deposits from engine tests.

After a survey of surface preparation techniques, air plasma treated iron foils with stable oxide surfaces were chosen as substrates for immersion tests. XPS was used to determine the relative amounts and chemical states of chemisorbed species as a function of immersion time, conditions and fuel types. Significant differences were seen between the behaviour of pro- or anti-deposit forming fuels, particularly in the amount and nature of minority S- and N-containing species. The XPS measurements were able to predict the results of an established screening test, and indicated how the accuracy of the test could be improved.

The results form the basis for a model for deposit formation. This will allow Shell Global Solutions to gain a better understanding of deposit formation in internal combustion engines, contributing to the production of future fuels that may prevent or reduce the formation of fuel deposits. There will be wider environmental benefits, as control and prevention of fuel deposit formation has been shown to decrease the emission of certain environmental pollutants, most notably oxides of nitrogen.