A Thermodynamic Evaluation of the Use of Alcohol Fuels in a Spark-Ignition Engine
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Although the use of alcohol fuels in spark-ignition engines has been investigated for over 100 years, consistent and thorough thermodynamic evaluations are few. The current work examines the detail thermodynamics of the use of methanol and ethanol by an automotive, spark-ignition engine. Overall engine performance parameters, detail instantaneous quantities, and second law parameters are determined as functions of engine design and operating conditions. In addition, the results for the alcohol fuels are compared to results for isooctane. Results include indicated and brake efficiencies, heat transfer, and exhaust gas temperatures as functions of engine speed and load. Operating conditions include constant equivalence ratio (stoichiometric), MBT spark timing, and constant burn duration. In general, the thermodynamic results are similar for the alcohol fuels and isooctane. Results for constant load (bmep = 325 kPa) resulted in a slightly more open throttle for the alcohol fuels, and hence, slightly higher thermal efficiencies for the conditions examined. In addition, the alcohol fuels resulted in slightly lower temperatures, lower nitric oxides, and less exergy destruction during the combustion process. The results are explained by detail knowledge of the relative heat transfer, pumping losses and cylinder gas temperatures. © 2009 SAE International.
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