Timeline: Emission Changes Since 2000

2002: Most diesel engine makers introduced exhaust gas recirculation, or EGR, as part of a consent decree with EPA to meet 2004 NOx standards early. (Cummins)

2007: Engine manufacturers added diesel particulate filters, or DPFs, to meet EPA's more stringent particulate matter emission standards. (Bloomberg News)

2010: Most diesel engine manufacturers introduced selective catalytic reduction, or SCR, in 2010 to meet tighter NOx emission limits. (John Sommers II for Transport Topics)

2014: Greenhouse gas emissions and fuel economy standards for heavy-duty trucks and engines go into effect under EPA and NHTSA's Phase 1 GHG rule. (Allika/Getty Images)

2021: Increasingly stringent greenhouse gas emissions and fuel economy standards for heavy-duty trucks and engines go into effect under EPA and NHTSA's Phase 2 GHG rule. (David Paul Morris/Bloomberg News)

2024: CARB's Advanced Clean Trucks rule is scheduled to begin requiring truck makers to transition an increasing percentage of their trucks sold in California to zero-emission vehicles. CARB also is developing its Advanced Clean Fleets rule, which would require motor carriers operating in the state to begin shifting toward zero-emission trucks. (Qilai Shen/Bloomberg News)

2027: Tighter NOx emission standards for heavy-duty trucks and engines go into effect under EPA's Clean Trucks Plan. CARB, meanwhile, is seeking a waiver from EPA to finalize its own regulation to further reduce NOx emissions for trucks operating in the state. EPA also plans to introduce Phase 3 greenhouse gas standards. (Midnight Studios/Getty Images)