The U.S. Environmental Protection Agency requires large engines to meet strict pollution-emission requirements to be certified roadworthy. Engine manufacturers don’t need to test every engine, just a particular design. Thus having the test equipment to perform the tests would be costly, since they would only be used when a design changes. So they ship a new engine to a test facility where such testing occurs all the time.
This facility puts the engine into a test cell, and couples it to a dynamometer. By running the engine through an EPA-specified speed/torque cycle, the real world of uphill / downhill, fast / slow, continuous / irregular driving can be simulated. And by measuring the pollutant concentration in the engine’s intake air and the concentration in its exhaust during this cycle, one can judge the pollution emission contributed by the engine. Since the cycle is specified, the tests are repeatable.