4.2.2. NO₂

Whenever formed, the effect of SO₂ addition was to increase the concentrations of NO₂. This can be seen in Figure 149.

Figure 149: Experimental readings of NO₂ from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S

At 0.833 equivalence ratio (fuel-lean) small amounts of NO₂ were formed at short distances from the atomiser nozzle (41 ppm-wet). However, it disappeared rapidly within the ensuing 100 mm. SO₂ increased NO₂ concentrations by 8 ppm-wet, but after 200 mm the amounts of NO₂ remaining were negligible and thus sulphur could no longer exercise any effect.

In stoichiometric conditions the formation and emission of NO₂ is significant. At the closest distance investigated (100 mm) 60 ppm-wet NO₂ were measured, but no S-N interaction was observed as sulphur was not able to modify that amount. The concentration of NO₂ decreased as the sampling probe was displaced away from the atomising nozzle, but the interaction with sulphur raised as sulphur addition increased the concentration of nitrogen dioxide at all other sampling locations. Eventually, emissions of NO₂ were raised by 7 ppm-wet, which represents 33 % of the emissions without SO₂ addition.

The effect of sulphur is most intense at fuel-rich equivalence ratio. At 100 mm there is no difference in the amounts of NO₂ found with or without addition of SO₂. However, within the following 100 mm an increment of 23 ppm-wet was established by the addition of SO₂. More nitrogen dioxide is formed subsequently, but the increase of NO₂ concentration is sustained steadily up to the point of exhaust. NO₂ concentrations were increased by 26 ppm-wet from 247 ppm-wet on exhaust at = 1.200.

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