40 CFR § 1066.610 - Dilution air background correction.

(a) Correct the emissions in a gaseous sample for background using the following equation:

$\begin{array}{c}{x}_{\left[\mathrm{emission}\right]}=x_{\text{[emission]dexh}}-x_{\text{[emission]bkgnd}}·\left(1-\left(\frac{1}{\mathrm{DF}}\right)\right)\\ \text{Eq. 1066.610-1}\end{array}$

Where:

x[emission]dexh = measured emission concentration in dilute exhaust (after dry-to-wet correction, if applicable).

x[emission]bkgnd = measured emission concentration in the dilution air (after dry-to-wet correction, if applicable).

DF = dilution factor, as determined in paragraph (b) of this section.

$\begin{array}{c}\text{Example:}\\ x_{\mathrm{NOxdexh}}=1.08305\mathrm{ppm}\\ x_{\mathrm{NOxbkgnd}}=0.12456\mathrm{ppm}\\ DF=9.14506\\ x_{\mathrm{NOx}}=1.08305-0.12456·\left(1-\left(\frac{1}{9.14506}\right)\right)=0.97211\mathrm{ppm}\\ \\ \text{(b) Except as specified in paragraph (c) of this section, determine the dilution factor,}DF,\\ \text{over the test interval using the following equation:}\\ \\ DF=\frac{1}{\left(1+\frac{\alpha }{2}+3.76·\left(1+\frac{\alpha }{4}-\frac{\beta }{2}\right)\right)·\left(x_{\mathrm{CO}2}+x_{\mathrm{NMHC}}+x_{\mathrm{CH}4}+x_{\mathrm{CO}}\right)}\\ \\ \text{Eq. 1066.610-2}\end{array}$

Where:

xCO2 = amount of CO2 measured in the sample over the test interval.

xNMHC = amount of C1-equivalent NMHC measured in the sample over the test interval.

xCH4 = amount of CH4 measured in the sample over the test interval.

xCO = amount of CO measured in the sample over the test interval.

a = atomic hydrogen-to-carbon ratio of the test fuel. You may measure a or use default values from Table 1 of 40 CFR 1065.655.

b = atomic oxygen-to-carbon ratio of the test fuel. You may measure b or use default values from Table 1 of 40 CFR 1065.655.

$\begin{array}{c}\text{Example:}\\ x_{\mathrm{CO}2}=1.456\%=0.01456\\ x_{\mathrm{NMHC}}=0.84\mathrm{ppm}=0.00000084\\ x_{\mathrm{CH}4}=0.26\mathrm{ppm}=0.00000026\\ x_{\mathrm{CO}}=80.4\mathrm{ppm}=0.0000804\\ \alpha =1.92\\ \beta =0.03\\ DF=\frac{1}{\left(1+\frac{1.92}{2}+3.76·\left(1+\frac{1.92}{4}-\frac{0.03}{2}\right)\right)·\left(0.01456+0.00000084+0.00000026+0.0000804\right)}=9.14506\end{array}$

(c) Determine the dilution factor, DF, over the test interval for partial-flow dilution sample systems using the following equation:

$\begin{array}{c}DF=\frac{V_{\mathrm{dexhstd}}}{V_{\mathrm{exhstd}}}\\ \text{Eq. 1066.610-3}\end{array}$

Where:

Vdexhstd = total dilute exhaust volume sampled over the test interval, corrected to standard reference conditions.

Vexhstd = total exhaust volume sampled from the vehicle, corrected to standard reference conditions.

$\begin{array}{c}\text{Example:}\\ V_{\mathrm{dexhstd}}=170.9m^3\\ V_{\mathrm{exhstd}}=15.9m^3\\ \mathrm{DF}=\frac{170.9}{15.4}=11.1\end{array}$

(d) Determine the time-weighted dilution factor, DFw, over the duty cycle using the following equation:

Where:

N = number of test intervals

i = test interval number

t = duration of the test interval

DF = dilution factor over the test interval

Example:

N = 3

DF1 = 14.40

t1 = 505 s

DF2 = 24.48

t2 = 867 s

DF3 = 17.28

t3 = 505 s

[79 FR 23823, Apr. 28, 2014, as amended at 86 FR 34583, June 29, 2021]