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A new field problem associated with flakes of combustion chamber deposit (CCD) getting trapped on the exhaust valve seat has been reported by several car manufacturers in Europe. This causes difficulties in start-up and poor driveability. A road test procedure that is reasonably quick and sensitive to fuel changes has been developed to study the deposit flaking problem. The flaking of the deposits is believed to be caused by water - either generated by combustion or existing in the ambient air as water vapour - condensing on the deposits. Water is much more effective than fuel in causing deposit flaking. A way of quantifying the deposit flaking tendency has been defined and its repeatability established based on twenty-nine tests using two different cars and different fuels and additives. There are large differences between base fuels in terms of CCD flaking.

A simple mathematical model is proposed for the formation of engine deposits and their removal with detergent additive packages. The model is phenomenological and is intended to provide a framework within which the often complex experimental observations can be understood; it does not aspire to be fully predictive or to provide insights into the physics and chemistry of deposit formation. It is applied to inlet-valve deposit formation and removal, and the results demonstrate that two fuels, which might look similar in terms of deposit formation tendency, in a fixed duration engine test, might respond quite differently to the same detergent package. Two additive packages that give identical results in a keep-clean experiment can show quite different clean-up capabilities. A package that has very good keep-clean capability might not show any ability to clean up dirty valves but will do so at higher dose rates.