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Addition of nitroalkanes to gasoline is shown to reduce the octane quality. The reduction in the Motor Octane Number (MON) is greater than the reduction in the Research Octane Number (RON). In other words addition of nitroalkanes causes an increase in octane sensitivity. The temperature of the compressed air/fuel mixture in the MON test is higher then in the RON test. Through chemical kinetic modelling, we are able to show how the temperature dependence of the reactions responsible for break-up of the nitroalkane molecule can lead to an increase in octane sensitivity. Results are presented from an Homogenous Charge Compression Ignition (HCCI) engine with a homogeneous charge in which the air intake temperature was varied. When the engine was operated on gasoline-like fuels containing nitroalkanes, it was observed that the combustion phasing was much more sensitive to the air intake temperature. This suggests a possible means of controlling combustion phasing for HCCI.

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.