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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 use of pilot-ignited, direct-injected natural gas in a heavy-duty compression-ignition engine has been shown to reduce emissions while maintaining performance and efficiency. Adding recirculated exhaust gas (EGR) has been shown to further reduce emissions of nitric oxides (NOx), albeit at the cost of increased hydrocarbons (tHC), carbon monoxide (CO), and particulate matter (PM) emissions at high EGR fractions. Previous tests have suggested that reducing the delay between the diesel and natural gas injections, increasing the injection pressure, or adjusting the combustion timing have individually achieved substantial emissions benefits. To investigate the effectiveness of combining these techniques, and of using them over a wide range of operating conditions, a series of tests were carried out. The first set of tests investigated the interactions between these effects and the EGR fraction.