# 40 CFR 91.426 - Dilute emission sampling calculations.

(a) The final reported emission test results must be computed by use of the following formula:

(b) The mass flow rate (Wi) of an emission for mode i is determined from the following equation:

(c) Densities for emissions that are to be measured for this test procedure are:

(1) The value of DHC above is calculated based on the assumption that the fuel used has a carbon to hydrogen ratio of 1:1.85. For other fuels, DHC can be calculated from the following formula:

(2) The idealized molecular weight of the exhaust hydrocarbons, ie., the molecular weight of the hydrocarbon molecule divided by the number of carbon atoms in the molecule, MHC can be calculated from the following formula:

(3) The value of DNOx above assumes that NOX in entirely in the form of NO2.

(d) The dilution factor (DF) is the ratio of the volumetric flow rate of the background air to that of the raw engine exhaust. The following formula is used to determine DF:

(e) The humidity correction factor KH is an adjustment made to the measured NOX. This corrects for the sensitivity that a spark-ignition engine has to the humidity of its combustion air. The following formula is used to determine KH for NOX calculations:

(f) The absolute humidity of the engine intake air H is calculated using the following formula:

(g) The fuel mass flow rate Fi can be either measured or calculated using the following formula:

(h) The mass of fuel consumed during the mode sampling period, MFUEL can be calculated from the following equation:

(i) The grams of carbon measured during the mode GS can be calculated from the following equation: