Prediction of NO Formation in Spark-Ignited Engines—An Analysis of Methods of Control

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Abstract
Lavoie, Heywood and Keck (1970) have recently shown how one can calculate the amount of NO formed during the combustion and subsequent expansion stroke of an IC engine. In this paper we have applied the method of Lavoie el al. to a spark-ignited carburetted IC engine and have incorporated into the computer program the engine variables; fuel type, fuel air equivalence ratio, humidity content of inlet air, % exhaust gas recirculation, inlet manifold temperature, inlet manifold pressure, RPM, compression ratio, piston connecting rod to crank ratio, and duration and position of combustion in the cycle. In addition to NO, we calculate indicated mean effective pressure and indicated specific fuel consumption. Brake specific values are calculated from an empirical correlation. Calculated NO levels show good agreement with data in the literature for the effect of the variables of air-fuel ratio, manifold pressure, spark timing, and humidity. Control methods capable of reducing NO emissions to the 100 ppm range have been analyzed to determine the effects on fuel economy and power output. Both exhaust gas recirculation and lean operation arc effective and will produce little loss in fuel economy provided that combustion commences between 10° and 20° BTDC with a duration of between 40° and 60° crank angle. Small effects such as reducing inlet air temperatures, and increasing compression ratio with an accompanying retardation of spark timing from minimum spark advance for best torque (MBT) can, be combined to reduce NO without loss in fuel economy.

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