An examination of the combustion characteristics of compression ignition engines fuelled with gaseous fuels
LccTJ 805 L58 1995
LcshDiesel motor - Alternate fuels
Fuel - Combustion
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AbstractThe development of gas fuelled dual fuel compression ignition engines provides an alternative approach for internal combustion engines to improve economy, reduce exhaust emissions and achieve greater power output. The present work investigates the problems encountered in dual fuel engine operation by developing and using a detailed chemical kinetic scheme, a quasi-two zone and a multi-zone analytical models. The chemical kinetic scheme developed in the present work consisted of 137 elementary reaction steps with 32 species. It has been examined through carrying out sensitivity and species consumption path analyses over the wide operating conditions for the oxidation of common gaseous fuels and their mixtures employed usually in dual fuel engines. The quasi-two zone analytical model developed is based on the effective energy releases of the pilot fuel while using the detailed chemical reaction kinetic scheme for the oxidation of gaseous fuels. This model is employed mainly to predict the onset of knock under dual fuel engine operation. In the multi-zone analytical model developed, the complex interaction of the gaseous and pilot liquid fuels during the combustion process is considered while applying detailed chemical kinetics to the individual zones. This multi-zone model is then employed to examine comprehensively and predict the performance of dual fuel engines. The effects of the admission of the gaseous fuel in dual fuel engines on the length of ignition delay period and cyclic variations are examined analytically over a number of consecutive cycles while accounting for the presence of residual gases and employing an experimentally based formula for the ignition of the pilot fuel. It is shown that the changes in the temperature and pressure levels due to the variation in the physical properties of the mixture, preignition energy release, external heat transfer and the contribution of residual gases are the main factors controlling the extent of the delay period and cyclic variations. It has been shown that the onset of knock in a dual fuel engine is strongly dependent on the type of gaseous fuel used and operating conditions. The knock observed with propane and methane admissions results from the autoignition of the gaseous fuel-air mixture in the neighbourhood of ignition centres. However, with hydrogen admission, mainly at low intake temperatures, the observed knock is more likely to be associated with high rates of energy release resulting from the very rapid flame propagation following ignition. The features of combustion processes and engine performance could be predicted relatively well when using the multi-zone model developed. It is shown that the low efficiency and poor emissions at light load are due primarily to the incomplete combustion of the gaseous fuel-air mixture outside the pilot fuel region. Measures such as increasing concentration of the gaseous fuel, enhancing the intake temperature and employing a larger pilot fuel can increase the combustion zones and improve significantly the performance and exhaust emissions.
Bibliography: p. 268-287.