NO2 from Lean-Burn Engines - On its Lower Sensitivity to Leaning than NO

Investigations on the NO_x reduction for natural-gas-fueled spark-ignition engines with lean-burn techniques revealed that NO₂ is less sensitive to reduction measures than NO. For NO concentrations over 1000 ppm, the NO₂/NO ratio is generally less than 0.1. For NO concentrations below 50 ppm, the NO₂/NO ratio begins to exceed 1.0. The literature on burner and flow-reactor tests explains this phenomenon via the oxidation of NO with hydroperoxyl radicals through reactions very sensitive to component concentrations. Since NO is the precursor of NO₂, measures to further lower the NO production will ultimately result in a lower NO₂ emission. This has been demonstrated experimentally with an engine.