Air Intake IFilter & Induction System Filter Diesel Engine - Because a diesel engine requires close tolerances to achieve its compression ratio, and because most diesel engines are either turbo charged or supercharged, the air entering the engine must be clean..

http://www.engineersedge.com/power_transmission/air_intake.htm

Air Filter Performance Levels: IEST-RP-CC001.319 classifies filter performance levels as: Type A Filter Performance. Sometimes referred to as industrial types,

http://www.engineersedge.com/filtration/filter_performance_levels.htm