Engine load affects the engine's fuel-air ratio, which affects exhaust emissions. When a diesel engine is operated at a low load, a significant quantity of the exhaust is excess air that does not contribute to the combustion process. It has been demonstrated that engine faults may go undetected until an engine is operated at a sufficiently high fuel-air ratio. Furthermore, a maladjusted maximum fuel setting is impossible to detect unless the engine is tested at, or very near, full load. This will maximize the likelihood of detecting engine faults.
Diesels require excess air for complete combustion.
Without enough excess air, unburned and partially oxidized products of combustion are formed and exhausted into the atmosphere. When the timing is retarded, the opposite is true. A stain tube is a vacuum-sealed glass tube about the size of a pen, which contains indicators that change color when exposed to concentrations of specific gases. The engine emission test procedure is performed by breaking the tip of a stain tube 7 to 8 meters downwind of piece of operating diesel equipment. An emission concentration is determined by observing the stain against a scale printed on the tube.