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94 Alexandru Radu et al. Spark timing is important because of ignition delay issues. Ethanol actually has a slightly higher flame speed in smooth combustion than does gasoline. However, it has a much longer delay between application of the spark and a fully-formed flame (the “ignition delay”). This means that the spark timing may need to be advanced. Ethanol flammability range is much wider for air-ethanol mixtures comparative to gasoline (0, 3...1, 56 versus 0, 4...1, 4) provi<strong>din</strong>g engine run stability in the area of lean mixtures (Pană et al., 2007). The enthalpy of evaporation of ethanol is 842-930 kJ/kg that is higher than the value of 330-440 kJ/kg for gasoline. This property can contribute to the increase of engine power and efficiency because of the resulting higher density of the mixture, but this also causes ignition problems at low temperatures (Cowart et al., 1996). 3. Modelling the Engine Cylinder Process The main reason for the growth in engine modelling activities arises from the economic benefits and to reduce the time. Real engine testing cannot be replaced by models, but they are able to provide good estimates of performance changes resulting from possible engine modifications. There are now available comprehensive “commercial” models, which have a wide purpose of use with refined inputs and outputs to facilitate their use by engineers. Also, many universities have produced their own thermodynamic models, of varying degrees of complexity, scope and ease of use. Such software is AVL BOOST v.2011.6 with which we have developed a model for the engine running simulation with gasoline and ethanol-gasoline blends and with normal intake and supercharged admission. The engine numerical model was validated by comparing the obtained values with the experimental data when using gasoline and E20 as fuels, on a 1.5L DOHC engine (76.5 mm bore, 81.5 mm stroke, 9.2 compression ratio). The theoretical researches were carried at engine speed 2500 rpm, full load and different dosage values (λ=0.8; 0.85; 0.9 for gasoline and λ=1.0; 1.1; 1.2 for blend) for two fuels: gasoline and E20 (80% gasoline blend with 20% ethanol) and with different intake pressures for the normal admission engine and supercharged engine with 1.2 bar. For E20 the tests were carried out for dosages λ>1 in order to emphasize the main advantages of using ethanol in the lean mixtures field. The present investigation specifically concerns the engine performance and reduction of pollutant emissions from the exhaust of engines using ethanol fuel as a substitute for gasoline. 3.1. Results Fig. 1 shows that peak cylinder pressure (pmax) and indicated mean effective pressure (IMEP) increases for E20 gasoline-ethanol blend comparative
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 95 with gasoline in both cases: normal intake and supercharged. Values of the two parameters have decreased for leaner mixtures in the field of λ=1…1,2. Increased peak cylinder pressure is due to higher burning rate of ethanol determinates a reduced of the combustion duration. Fig. 1 – Effect of ethanol addition on peak cylinder pressure. In Fig. 2 can be observed that the peak cylinder temperature is higher in case of the E20 fuel comparative with gasoline especially to supercharged engine. In order to reduce the NOx emissions at the engine fuelled with E20 the dosage must be leaner (e.g. λ=1.2). Fig. 2 - Effect of ethanol addition on peak cylinder temperature.
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BULETINUL INSTITUTULUI POLITEHNIC D
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VICTOR COTOROS şi EMIL BUDESCU, As
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ALEXANDRU RADU, CONSTANTIN PANĂ an
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