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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.

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.

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.

The anti-knock or octane quality of a fuel depends on the fuel composition as well as on the engine design and operating conditions. The true octane quality of practical fuels is defined by the Octane Index, OI = (1-K)RON + KMON where K is a constant for a given operating condition and depends only on the pressure and temperature variation in the engine (it is not a property of the fuel). RON and MON are the Research and Motor Octane numbers respectively, of the fuel. OI is the octane number of the primary reference fuel (PRF) with the same knocking behaviour at the given condition. In this work a wide range of fuels of different RON and MON were tested in prototype direct injection spark ignition (DISI) engines with compression ratios of 11 and 12.5 at different speeds up to 6000 RPM. Knock Limited Spark Advance (KLSA) was used to characterize the anti-knock quality of the fuel. Experiments were also done using two cars with DISI engines equipped with knock sensor systems.