See the standard-setting part to determine if you may correct NOX emissions for the effects of intake-air humidity or temperature. Use the NOX intake-air humidity and temperature corrections specified in the standard-setting part instead of the NOX intake-air humidity correction specified in this part 1065. If the standard-setting part does not prohibit correcting NOX emissions for intake-air humidity according to this part 1065, correct NOX concentrations for intake-air humidity as described in this section. See § 1065.650(c)(1) for the proper sequence for applying the NOX intake-air humidity and temperature corrections. You may use a time-weighted mean intake air humidity to calculate this correction if your intake air humidity remains within a tolerance of ±0.0025 mol/mol of the mean value over the test interval. For intake-air humidity correction, use one of the following approaches:

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(a) For compression-ignition engines, correct for intake-air humidity using the following equation:

$$Example: xNOxuncor = 700.5 µmol/molxH2O = 0.022 mol/molxNOxcor = 700.5 · (9.953 · 0.022 + 0.832)xNOxcor = 736.2 µmol/mol

(b) For spark-ignition engines, correct for intake-air humidity using the following equation:

$$Example: xNOxuncor = 154.7 µmol/molxH2O = 0.022 mol/molxNOxcor = 154.7 · (18.840 · 0.022 + 0.68094)xNOxcor = 169.5 µmol/mol

(c) Develop your own correction, based on good engineering judgment.

[75 FR 23056, Apr. 30, 2010, as amended at 76 FR 57466, Sept. 15, 2011; 88 FR 4686, Jan. 24, 2023]