Tier 1-4 Standards is the United States federal exhaust emissions standards for diesel engines of heavy equipment which were adopted in 1994 for engines over 37 kW (50 hp). The standard is applicable to all the heavy equipment engine manufacturer namely who signed the Environmental Protection Agency (EPA) regulations for Emissions for Compression-Ignition (Diesel) Engines.
Tier 1-3 Emission Standards
The 1998 heavy equipment diesel engine regulations were put forward into a 3-tiered progression. Each tier level involved a phase-in (by horsepower rating) over several years. Tier 1 standards were phased-in from 1996 to 2000. The more stringent Tier 2 standards took effect from 2001 to 2006, and yet more stringent Tier 3 standards phased-in from 2006 to 2008, Tier 3 standards applied only for engines from 37-560 kW.
Tier 4 Emission Standards
The Tier 4 emission standards was phased-in from 2008 through 2015 and introduce substantial reductions of NOx (for engines above 56 kW) and PM (above 19 kW), as well as more stringent HC limits. CO emission limits remain unchanged from the Tier 2 and Tier 3 stage.