# 7.87 Test to determine the maximum fuel-air ratio.
(a) *Test procedure.* (1) Couple the diesel engine to the dynamometer and connect the sampling and measurement devices specified in § 7.86.
(2) Prior to testing, zero and span the CO and NO<sub>X</sub> analyzers to the lowest analyzer range that will be used during this test.
(3) While running the engine, the following shall apply:
(i) The parameter for the laboratory atmospheric factor, f<sub>a</sub>, shall be: 0.98≤f<sub>a</sub>≤1.02;
(A) The equation is f<sub>a</sub> = ( 99/P<sub>s</sub>) * ((T<sub>a</sub> + 273)/298)
<sup>0.7</sup> for a naturally aspirated and mechanically supercharged engines; or
(B) The equation is f<sub>a</sub> = (99/P<sub>s</sub>)
<sup>0.7</sup>* ((T<sub>a</sub> + 273)/298)
<sup>1.5</sup> for a turbocharged engine with or without cooling of the intake air.
Where:
P<sub>s</sub> = dry atmospheric pressure (kPa)
T<sub>a</sub> = intake air temperature (°C)
(ii) The air inlet restriction shall be set within ±10 percent of the recommended maximum air inlet restriction as specified by the engine manufacturer at the engine operating condition giving maximum air flow to determine the concentration of CO as specified in paragraph (a)(6) of this section.
(iii) The exhaust backpressure restriction shall be set within ±10 percent of the maximum exhaust backpressure as specified by the engine manufacturer at the engine operating condition giving maximum rated horsepower to determine the concentrations of CO and NO<sub>X</sub> as specified in paragraph (a)(6)of this section.
(iv) The air inlet restriction shall be set within ±10 percent of a recommended clean air filter at the engine operating condition giving maximum air flow as specified by the engine manufacturer to determine the concentration of NO<sub>X</sub> as specified in paragraph (a)(6) of this section.
(4) The engine shall be at a steady-state condition when the exhaust gas samples are collected and other test data is measured.
(5) In a category A engine, 1.0 ±0.1 percent CH<sub>4</sub> shall be injected into the engine's intake air.
(6) Operate the engine at several speed/torque conditions to determine the concentrations of CO and NO<sub>X</sub>, dry basis, in the raw exhaust.
(b) *Acceptable performance.* The CO and NO<sub>X</sub> concentrations in the raw exhaust shall not exceed the limits specified in § 7.84(b) throughout the specified operational range of the engine.