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Overview of Tier III and Tier IV Diesel Emissions Standards

06 April, 18 4:23 pm · Leave a comment · reddarc

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Since 1996, the EPA’s regulations for non-road diesel engines emissions have been phased in through four progressively more stringent tiers. Different tiers went into effect at different dates for different engines, but since 2015, all new non-road diesel engines, including stationary engines, but comply with the EPA’s Tier IV (Final) standard.

Today’s Tier IV compliant diesel engines are far more complex, efficient, and expensive than their unregulated ancestors from the early 1990’s.

How does Tier 4 compare with the previous generation (Tier III)?

For Tier III, the standard focused on reducing NOx (nitrous oxides) and NMHC (all hydrocarbons except methane) emissions to levels on par with federal requirements for on-road diesel engines. Since these non-road emissions were unregulated prior to the Tier Emissions Standards, we have no basis for a hard comparison, but the EPA estimates that since 2010, Tier III has been reducing NOx emissions by about 1 million tons per year. This is the amount of NOx produced annually by about 35 million cars.

Conspicuously absent from Tier III was any limit on PM (particulate matter), although the Tier II limit still applied to Tier III motors.

To comply with Tier III, most engine manufacturers relied on the following technologies:


  • Improved airflow through the engine (more responsive turbochargers, improved combustion chamber design, and variable valve timing) – This reduces NMHC and PM.
  • Precise control of injection (electronically controlled, common-rail injection systems) – A series of small fuel injections during the combustion cycle, instead of on single injection “blast” reduces both PM and NOx.
  • EGR (Exhaust Gas Recirculation: a portion of the exhaust is routed back into the intake air) – This dilutes the intake air with inert gases and absorbs heat, reducing NOx.

Tier IV had a long phase-in period, from 2008 until 2015. For most engines, it was broken into an “Interim” and “Final” phase. Tier IV (Interim) addressed particulate matter, calling for a further 90% reduction over Tier II. In addition, it required a further 45% reduction in NOx over Tier III. Tier IV (Final) now requires yet another 80% reduction in NOx over Tier IV (Interim), as well as some more modest additional PM reductions for some engine classes.

To comply with Tier IV, engine manufacturers have employed these additional technologies, among others:

  • DPF (Diesel Particulate Filter) – This reduces PM.
  • CCV (Closed Crankcase Ventilation: Blow-by gases that get into the crankcase are recirculated back into the engine instead of vented to the atmosphere) – This reduces NMHC and PM.
  • SCR (Selective Catalytic Reduction: A reduction reaction converts NOx to nitrogen, water, and CO2.) – This drastically reduces NOx.

These past two decades of Tiered Emissions Standards have driven a rapid evolution in diesel engine technology. Today’s Tier IV compliant diesel engines are far more complex, efficient, and expensive than their unregulated ancestors from the early 1990’s. Even more drastic, though, has been the reduction in emissions during this time: a 99% drop in both NOx and PM as well as a 95% decrease in tailpipe emissions overall.

Red-D-Arc carries a number of Tier 3 and Tier 4 diesel generators and diesel air compressors for rent.

Read more articles on diesel electric generators

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