OCR Text |
Show Some features of engine operation The presence of carbon dioxide with the methane brings about various adverse changes to the characteristics of the combustion process in engines fuelled with such a mixture. Though the fuel admitted can be stepped up to ensure approximately the same thermal loading, the indicated power output, efficiencies and cyclic variations generally deteriorate with the increased presence of carbon dioxide with the methane, as shown typically in Fig. (10) for a spark ignition engine[13]. Regular spark ignition engine operation is possible when the volumetric concentration of carbon dioxide in the fuel mixture does not exceed around 7 0 % by volume. The very significant lowering of the flame propagation rates with the increased presence of carbon dioxide with the methane in the spark ignition gas fuelled engine can be reflected in the corresponding substantial increases in the average length of the combustion period , ( the time between establishing a propagating flame following the passage of the spark and end of combustion ) ignition lag times and the associated burning rates, as shown in Fig (11). The effects of these changes can be modified somewhat through appropriate changes to the spark timing as the concentration of carbon dioxide in the fuel is altered. The optimum spark timing for maximum indicated power output is a rninimum at around the stoichiometric mixture. At the same time, as the volumetric concentrations of carbon dioxide in the fuel were increased, the optimum spark timing , had to be advanced, Fig. (12). The optimum spark timing for maximum power was throughout the same as that needed for minimum combustion duration. The average exhaust gas temperature is a maximum at around the stoichiometric region but tended to increase as the volumetric percentage of carbon dioxide in the fuel was increased, when operating with very lean or relatively rich mixtures, a reflection of the slowing down of flame propagation. W h e n excessive concentrations of carbon dioxide were present in the fuel, the exhaust temperature dropped sharply due to the lowering of the overall temperature level and the onset of even occasional "misfire". The concentrations of carbon monoxide in the exhaust does not increase substantially as the concentration of carbon dioxide with the methane is increased indicating that much of the carbon dioxide acts largely as a diluent. Throughout, it is evident that the spark timing is a convenient controlling variable that has to be adjusted towards the optimum value in order to achieve maximum power, efficiency, low emissions and smooth burning [13]. The presence of carbon dioxide with the methane will not undermine for most engines and operating conditions , the excellent knock resisting qualities of methane. Moreover, oxides of nitrogen emissions are substantially reduced. Diesel engines may be converted to operate in the dual fuel mode, where some diesel fuel injection provides ignition of the cylinder charge, made up of a mixture of a gaseous fuel such as methane and air. The bulk of the energy released then comes usually from the combustion of the gaseous fuel. This c o m m o n approach can provide an efficient means for utilizing effectively various gaseous fuel resources for the production of power in conventional compression ignition engines [14]. With dual fuel engine operation involving a fiiel that is made up of methane containing carbon dioxide, the ignition delay period increases substantially as the concentrations of the diluent in the methane is increased, as shown typically in Fig. (13). This reflects the marked changes that will occur to both the physical as well as the chemical parts of the delay in the presence of gaseous fuel. Generally, for a given diluent concentration in the fuel and a pilot fuel quantity, lower power, efficiency and maximum cylinder pressure are produced when carbon dioxide presence in the fuel is increased [15]. This deterioration, however, is reduced significantly at high loads. Moreover, the 9 |