An Analytical Examination of the Chemical Kinetics of the Combustion of N-Heptane-Methane Air Mixtures

A number of practical applications where dual fuel systems are used, such as in gas fuelled diesel engines, involve the combustion in air of a gaseous fuel, such as methane in the presence of a higher hydrocarbon vapour. There is a need to examine and understand the nature and extent of any chemical interaction that may take place between the gaseous and higher hydrocarbon fuel components and how this interaction influences the combustion processes and hence the performance of dual fuel engines. Detailed chemical kinetics for the oxidation of the typical higher hydrocarbon fuel n-heptane, representing the behaviour of diesel vapours, in the presence of methane, are examined while using a comprehensive kinetic scheme (1966 reaction steps and 380 species) at different conditions relevant to engine applications, including constant temperature, constant pressure and constant volume processes. The extension of these calculations to consider the combustion reactions of complex mixtures of fuels, including the representation of the composition of typical natural gases will be described, with special emphasis on its application in the simulation of knock reactions in engines. On the basis of these comprehensive scheme calculations involving the use of supercomputers, reduced schemes of varying degrees of detail requiring very much less computing capacity are being formulated. Some of the main aspects of the procedure followed in this reduction and the extent of their success and limitations are to be presented and discussed with examples.