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The auto-ignition or anti-knock quality of a practical fuel is defined by the Octane Index, OI = (1-K)RON + KMON where RON and MON are the Research and Motor Octane numbers and K is a constant depending only on the pressure and temperature variation in the engine. K decreases as the compression temperature in the unburnt gas at a given pressure in the engine decreases and can be negative if this temperature is lower than in the RON test. As spark ignition (SI) engine designers seek higher efficiency knock becomes more likely. Moreover such initiatives - direct injection, higher compression ratios, downsizing and turbocharging - will reduce the unburnt gas temperature for a given pressure and push the value of K downwards. In Europe there is evidence of a monotonic decrease in the average K value from 1987 to 1992. In 37 different Japanese and European cars (34 models) equipped with knock sensors that have been tested K has been found to be negative in most cases.

In previous work it has been shown that the autoignition quality of a fuel at a given operating condition can be described by its Octane Index, OI = (1-K)RON - KMON; the larger the OI, the more the resistance to autoignition. Here RON and MON are, respectively, the Research and Motor Octane numbers of the fuel and K is a constant depending only on the pressure and temperature history of the fuel / air mixture in the engine prior to autoignition. The value of K is empirically established at a given operating condition by ranking fuels of different RON and MON and of different chemical composition for their ease of autoignition. Another important parameter at a given operating condition is OI0, the Octane Index of the fuel for which heat release is centred at TDC. In previous work K and OI0 were measured at different operating conditions and were related empirically to pressure and temperature of the mixture before autoignition and to engine speed and mixture strength.

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.

This paper reviews the literature on deposits that form on all internal surfaces coming into contact with the fuel in a gasoline engine. It does not consider deposits that can form in areas primarily affected by the engine lubricant. The physical, and especially the chemical, mechanisms that lead to deposit formation are very complex and poorly understood, and are different in different parts of the engine. Deposits can impair engine operation in terms of fuel economy, power emissions, octane requirement and driveability. In the worst cases they could lead to engine damage. Deposits in most parts of the engine can be controlled through the use of fuel additives. Test procedures used in deposit-related studies are characterised by poor repeatability and reproducibility because of the complex nature of the processes involved. Deposit formation and, consequently, the effectiveness of deposit control additives also depend on base fuel properties and engine design and operation.