Exhaust emission characteristics correlated to a chemically equilibrating gaseous system in an SI engine

Abstract

The chemical reactions leading to the destruction of NO and CO in the engine combustion gases are temperature dependent and it has been observed that they change over from 'shifting' equilibrium to 'frozen' equilibrium as the temperature drops during the expansion phase. Application of reaction rate theories to the prediction of exhaust emissions is complex and can at best be an approximation owing to uncertainties of the engine combustion reaction mechanism, the assumed rate constants and the estimated combustion temperatures. An attempt was made to correlate the computed equilibrium concentrations during the expansion phase to the measured emission levels of NO and CO to estimate the temperatures at which nonequilibrium effects become apparent. Comparison of the measured emission levels from a single cylinder spark ignition engine with the computed equilibrium concentrations for several engine operating conditions has shown that the concentration of NO freezes near its peak equilibrium value, whereas the destruction of CO continues during the subsequent expansion phase until the temperature has dropped to an effective 'frozen equilibrium temperature' at which the computed equilibrium CO corresponds to the measured level in the exhaust. This temperature is dependent on the time rate of expansion and was determined to be 1250 K at 600 rpm and 1300 K at 900 rpm for the steady state operating conditions considered in the present series of tests.An attempt has been made to correlate the computed equilibrium concentrations during the expansion phase to the measured emission levels of NO and CO to estimate the temperatures at which nonequilibrium effects become apparent. Comparison of the measured emission levels from a single cylinder spark ignition engine with the computed equilibrium concentrations for several engine operating conditions has shown that the concentration of NO freezes near its peak equilibrium value, whereas the destruction of CO continues during the subsequent expansion phase until the temperature has fallen down to an effective 'frozen-equilibrium temperature' at which the computed equilibrium CO corresponds to the measured level in the exhaust.