40 CFR 1065.715: Natural gas.

(a) Except as specified in paragraph (b) of this section, natural gas for testing must meet the specifications in the following table:

Table 1 of § 1065.715—Test Fuel Specifications for Natural Gas

Property Value a

Methane, CH4 Minimum, 0.87 mol/mol.

Ethane, C2 H6 Maximum, 0.055 mol/mol.

Propane, C3 H8 Maximum, 0.012 mol/mol.

Butane, C4 H10 Maximum, 0.0035 mol/mol.

Pentane, C5 H12 Maximum, 0.0013 mol/mol.

C6 and higher Maximum, 0.001 mol/mol.

Oxygen Maximum, 0.001 mol/mol.

Inert gases (sum of CO2 and N2) Maximum, 0.051 mol/mol.

a Demonstrate compliance with fuel specifications based on the reference procedures in ASTM D1945 (incorporated by reference, see § 1065.1010), or on other measurement procedures using good engineering judgment.

(b) In certain cases you may use test fuel not meeting the specifications in paragraph (a) of this section, as follows:

(1) You may use fuel that your in-use engines normally use, such as pipeline natural gas.

(2) You may use fuel meeting alternate specifications if the standard-setting part allows it.

(3) You may ask for approval to use fuel that does not meet the specifications in paragraph (a) of this section, but only if using the fuel would not adversely affect your ability to demonstrate compliance with the applicable standards in this chapter.

(c) When we conduct testing using natural gas, we will use fuel that meets the specifications in paragraph (a) of this section.

(d) At ambient conditions, natural gas must have a distinctive odor detectable down to a concentration in air not more than one-fifth the lower flammable limit.