Nitrogen Oxide Production in a Diesel Engine Fueled by Natural Gas

The effect of large exhaust gas re-circulation (EGR) quantities on NOx production in a natural-gas-fueled direct-injection heavy-duty diesel engine has been tested over a range of speed, load, and timing in controlled experiments with a single-cylinder engine. At the highest EGR ratio, as much as 50% of the cylinder- out NOx was NO₂. NOx results correlated well with oxygen mole fraction in the unburned gas because of the direct dependence of flame temperature on this quantity. Within the range of measurements, speed and load had little or no effect on the relationship between oxygen mole fraction and NOx production.

A multi-zone model for estimating combustion rate, flame temperature, wall heat transfer, and NOx production from engine operating conditions and the record of cylinder pressure development with crank angle, was used to interpret experimental measurements. The model showed the incompatibility of test data with the normal form of the extended Zeldovich model on NO production. However a modified form of it served to correlate experimental data with oxygen mole fraction - which was a nearly linear function of flame temperature. The model also served to represent the effects of engine timing (defined here as the crank angle corresponding to 50% cumulative indicated heat release) on NOx production. At highest EGR (lowest oxygen mole fraction) the NOx emissions were of the order of 1 g/kg of fuel. At this condition CO and unburned hydrocarbon emissions were high, indicating the need for enhanced burning rate.