“Soft” Engine Certification?

I came across an interesting little tidbit recently during an engine permitting exercise here in Pennsylvania.  A facility was seeking air quality permitting for a number of existing emergency generators each driven by a ~3,000 brake horsepower (bhp), diesel fuel-fired, U.S. EPA Tier II certified engine.  According to the certification documentation provided by the manufacturer of said engines, the engines are indeed certified to meet the U.S. EPA Tier II emission standards codified at 40 CFR §89.112 (i.e., PM/PM10, CO, and NO_X+HC).  However, at their intended operating load (i.e., full standby), the emission profile of the engines is actually higher than the U.S. EPA Tier II emission standard for certain pollutants.  Such engines are certified to meet the U.S. EPA Tier II standards via the emission testing procedures of ISO 8178 D2.  Without getting into the technical weeds of this specific testing procedure, it appears that the testing procedure prescribes testing engines at various operating loads (i.e., ¼ Standby, ½ Standby, ¾ Standby, Full Standby, and Full Prime).  The test data from multiple operating loads is used to determine a weighted emissions average at these various loads for comparison to the U.S. EPA Tier II emission standard.   As a result, it would seem that since the tested emissions resulting from ISO 8178 D2 represent a weighted emission average over multiple operating loads, engines certified to meet the U.S. EPA Tier II emission standards can still exceed said standards when operated at specific loads, where such loads result in emissions above the weighted average.  ALL4 reminds facilities to keep this in mind when developing air quality permit applications for engine operations so as to navigate this potentially confusing issue carefully.