Implications of fuel selection for an SI engine: Results from the first and second laws of thermodynamics

The current work examines the detailed thermodynamics of the use of eight (8) fuels by an automotive, spark-ignition engine using a thermodynamic engine cycle simulation. The fuels examined were methane, propane, hexane, isooctane, methanol, ethanol, carbon monoxide, and hydrogen. Both overall engine performance parameters and detailed instantaneous quantities are determined for each of the fuels. Results include thermal efficiencies, heat transfer, and exhaust gas temperatures as functions of engine speed and load. In general, the overall engine results were similar for the various fuels. The second law results showed that, for the same operating conditions, the destruction of exergy during the combustion process ranged between about 8% (for carbon monoxide) and 21% (for isooctane) of the fuel exergy depending on the specific fuel. The differences of the exergy destruction during combustion appear to be related to the complexity of the fuel molecule and the presence (or lack) of oxygen atoms in the fuel molecule.