Exergy destruction during the combustion process as functions of operating and design parameters for a spark‐ignition engine
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The second law of thermodynamics provides different perspectives compared with the first law, and provides the property exergy. Exergy is a measure of the work potential of energy from a given thermodynamic state. Unlike energy, exergy may be destroyed, and for reciprocating engines, the major source of this destruction is during the combustion process. This paper provides an overview of the quantitative levels of exergy destruction during the combustion process as function of engine operating and design parameters, and for eight fuels. The results of this study are based on a spark-ignition, automotive engine. The amount of exergy destroyed during the combustion process has been determined as functions of speed, load, equivalence ratio, start of combustion, combustion duration, combustion rate parameters, exhaust gas recirculation (EGR), inlet oxygen concentration, and compression ratio. In addition, design parameters that were examined included expansion ratio and the use of turbocharging. The fuels examined included isooctane (base), methane, propane, hexane, methanol, ethanol, hydrogen and carbon monoxide. For the part load base case (1400rpm and a bmep of 325kPa) using isooctane, the destruction of exergy was 20.8% of the fuel exergy. For many of the engine operating and design parameter changes, this destruction was relatively constant (between about 20 and 23%). The parameters that resulted in the greatest change of the exergy destruction were (1) equivalence ratio, (2) EGR, and (3) inlet oxygen concentration. For the base case conditions, the exergy destruction during the combustion process was different for the different fuels. The lowest destruction (8.1%) was for carbon monoxide and the highest destruction (20.8%) was for isooctane. The differences between the various fuels appear to relate to the complexity of the fuel molecule and the presence (or absence) of an oxygen atom. © 2011 John Wiley & Sons, Ltd.
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