Manufacturers who signed the 1998 Consent Decrees with the EPA may be required to meet the Tier 3 standards one year ahead of schedule (i.e., beginning in 2005).

Voluntary, more stringent emissions standards that manufacturers could use to earn a designation of “ - Blue Sky Series” engines (applicable to Tier 1–- 3 certifications) are listed in Table 2.

Engines of all sizes must also meet smoke standards of 20%/15%/50% opacity at acceleration/lug/peak modes, respectively.

The regulations include several other provisions, such as averaging, banking, and trading of emissions credits and maximum “- "family emission limits" (FEL) for emissions averaging.

Tier 4 Standards
The Tier 4 emissions standards—- to be phased in from 2008 to 2015—- are listed in Table 3 for engines below 560 kW and in Table 4 for engines above 560 kW. These standards 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–- 3 stage.

In engines of 56- to 560-kW rated power, the NOx and HC standards are phased in over a few-year period, as indicated in the notes to Table 3. As an alternative to introducing of the required percentage of Tier 4–-compliant engines, manufacturers may certify all their engines to an alternative NOx limit in each model year during the phase-in period. These alternative NOx standards are:

• Engines 56–-130 kW:
• Option 1: NOx = 2.3 g/kWh = 1.7 g/bhp-hr (Tier 2 credits used to comply, MY 2012–-2013)
• Option 2: NOx = 3.4 g/kWh = 2.5 g/bhp-hr (no Tier 2 credits claimed, MY 2012–-2014)
• Engines 130–560 kW: NOx = 2.0 g/kWh = 1.5 g/bhp-hr (MY 2011–-2013)

Existing Tier 2–- 3 smoke opacity standards and procedures continue to apply in some engines. Exempted from smoke emissions standards are engines certified to PM emissions standards at or below 0.07 g/kWh (because an engine of such low PM level has inherently low smoke emissions).

The Tier 4 regulation does not require closed crankcase ventilation in nonroad engines. However, in engines with open crankcases, crankcase emissions must be measured and added to exhaust emissions in assessing compliance.

Similarly to earlier standards, the Tier 4 regulation includes such provisions as averaging, banking, and trading of emissions credits and FEL limits for emissions averaging.