Effects of the compression ratio on nitric oxide emissions for a spark ignition engine: Results from a thermodynamic cycle simulation

Abstract

Past experimental work has not provided consistent results for the effects of the compression ratio on nitric oxide emissions. A thermodynamic cycle simulation for spark ignition engines, which included a formulation using multiple zones for the combustion process, was used to determine the effects of the compression ratio on nitric oxide emissions. An important feature of this simulation is the use of an adiabatic zone to capture the high temperatures in the core region of the cylinder better during combustion. This study was completed for a commercial, 5.71 spark ignition V-8 engine operating at a part load condition at 1400r/min with an equivalence ratio of 1.0 and ‘MBT’ (maximum brake torque) spark timing. The effects of the compression ratio on nitric oxide emissions are shown to be the result of a complex set of interactions involving the combustion gas temperature, pressure, species concentrations, chemical kinetics and other thermodynamic quantities. For the base case condition, the nitric oxide concentration first increases and then decreases with increasing compression ratio. This is shown to be a result of the relative values of the nitric oxide moles formed, the combustion gas volume and the moles of the exhaust gas. The computed results are shown to agree qualitatively with experimental values from the literature.