Background
Since the mid-19th century, dynamometers (or "dyno" for short) has been used to measure torque and rotational speed (measured in rpm) from which power produced by an engine, motor or other rotating prime mover can then be calculated. A chassis dynamometer measures power from the engine through the wheels. The vehicle is parked on rollers which the car then turns and the output is measured. These dynos can be fixed or portable. Because of frictional and mechanical losses in the various drivetrain components, the measured horsepower is generally 15–20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer. Historically though, dynamometer emission tests are very expensive, and have usually involved removing fleet vehicles from service for a long period of time. Also, the data derived from such testing is not representative of “real world” driving conditions, and cannot be deemed as quantifiable, especially due to the relatively low amount of repeatable tests at such a facility.
Read more about this topic: Portable Emissions Measurement System
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