The Destruction of Exergy During the Combustion Process for a Spark-Ignition Engine

Abstract

The second law of thermodynamics provides the mechanism for assessing the quality of energy. The non-conserved property used for this assessment is called exergy, availability or available energy. For the internal combustion engine, the exergy of the fuel is distributed among work, heat transfer, exhaust, and is destroyed by several processes. The major destruction of exergy for the internal combustion engine is during the combustion process. This paper documents this destruction for a wide range of engine operating parameters, design parameters, and fuels. A 5.7 liter, spark ignition, automotive engine was selected for this study. Operating parameters that were examined included equivalence ratio, speed, load and spark timing. Design parameters that were examined included compression ratio, expansion ratio and the use of turbocharging. Combustion parameters and oxidizer were examined as well. The fuels examined included isooctane (base), methane, propane, hexane, methanol, ethanol, hydrogen and carbon monoxide. For the part load base case (1400 rpm and a bmep of 325 kPa) using isooctane, the destruction of exergy was 21% of the fuel exergy. For many of the engine operating and design parameters, this destruction was relatively constant (between about 20 and 23%). The parameters that resulted in the greatest change of the exergy destruction were (1) exhaust gas recirculation, and (2) inlet oxygen concentration. In general, the amount of the destruction of exergy during the combustion processes was associated with the level of the combustion temperatures.