Tier 4 emissions standards for nonroad diesel engines began taking effect with the 2008 model year and were fully phased in across all engine sizes by 2015. The EPA finalized the rule on May 11, 2004, aiming to cut particulate matter and nitrogen oxide emissions by roughly 90 percent compared with earlier standards.1U.S. Environmental Protection Agency. Final Rule for Control of Emissions of Air Pollution From Nonroad Diesel Engines and Fuel There isn’t one single start date, though. The specific year Tier 4 kicked in depends on the horsepower category of the engine you’re asking about.
Phase-In Dates by Engine Size
The EPA structured Tier 4 as a rolling phase-in rather than a single compliance date, giving manufacturers time to develop and integrate the aftertreatment systems the new limits required. Compliance was tied to model year and engine power rating, measured in kilowatts.
Engines Under 56 kW (Under 75 Horsepower)
The smallest nonroad diesel engines were first in line. Engines below 19 kW (about 25 horsepower) had to meet Tier 4 limits starting with the 2008 model year. Engines from 19 to 56 kW (roughly 25 to 75 horsepower) also began their Tier 4 phase-in in 2008, but final Tier 4 limits for this group didn’t take full effect until the 2013 model year.2eCFR. 40 CFR Part 1039 – Control of Emissions From New and In-Use Nonroad Compression-Ignition Engines This category includes compact excavators, small skid steers, and portable generators under 75 horsepower.
Engines From 56 to 560 kW (75 to 750 Horsepower)
This is the range that covers most full-size construction and agricultural equipment: excavators, wheel loaders, motor graders, and large tractors. These engines entered Tier 4 in two stages. Engines between 130 and 560 kW had to meet interim Tier 4 particulate matter and carbon monoxide standards beginning in 2011. Engines between 56 and 130 kW followed with interim standards in 2012. Final Tier 4 nitrogen oxide and hydrocarbon limits were phased in between 2012 and 2014, depending on the specific power rating and whether the manufacturer had banked emission credits from earlier compliance.2eCFR. 40 CFR Part 1039 – Control of Emissions From New and In-Use Nonroad Compression-Ignition Engines
Engines Over 560 kW (Over 750 Horsepower)
The largest nonroad engines, found in mining trucks, large cranes, and high-output generator sets, had transitional Tier 4 limits starting in 2011. Final Tier 4 standards for engines above 560 kW took effect in 2015, making this the last category to reach full compliance.2eCFR. 40 CFR Part 1039 – Control of Emissions From New and In-Use Nonroad Compression-Ignition Engines
Tier 4 Interim vs. Tier 4 Final
Equipment is often labeled “Tier 4 Interim” (sometimes Tier 4i) or “Tier 4 Final.” These aren’t marketing terms. They refer to two distinct compliance stages built into the regulation.
Tier 4 Interim required engines in the 56–560 kW range to meet the full final standard for particulate matter, but allowed a less strict nitrogen oxide limit while manufacturers developed selective catalytic reduction systems. Tier 4 Final tightened the nitrogen oxide and hydrocarbon limits to their lowest levels. For engines above 560 kW, the 2011 standards are called “transitional Tier 4,” and the 2015 limits represent Tier 4 Final.
The distinction matters when you’re looking at used equipment. A machine labeled Tier 4 Interim has significantly lower particulate matter emissions than a Tier 3 engine, but its nitrogen oxide output is still higher than a Tier 4 Final model. Some job sites and government contracts now require Tier 4 Final equipment specifically.
What Counts as a Nonroad Engine
Federal regulations define a nonroad engine broadly as any internal-combustion engine that is not a motor vehicle engine, a stationary engine, an engine used solely for competition, or an aircraft engine.3eCFR. 40 CFR 1039.801 – What Definitions Apply to This Part In practice, that covers excavators, bulldozers, skid steers, agricultural tractors, portable generators, forklifts, aerial lifts, irrigation pumps, and similar machines. If it burns diesel, sits on a job site or farm instead of driving on public roads, and isn’t bolted to a building foundation, it almost certainly falls under the nonroad rules and the Tier 4 timeline above.
Stationary emergency engines, such as backup generators at hospitals or water treatment plants, run under a separate set of rules and are not covered by the nonroad phase-in dates.
How Far the Limits Dropped
The actual numbers show why the phase-in was staged over several years. Tier 4 Final limits, expressed in grams per kilowatt-hour, vary by engine size:4eCFR. 40 CFR 1039.101 – Tier 4 Exhaust Emission Standards
- 19 to 56 kW: 0.05 g/kW-hr for particulate matter, with combined nitrogen oxide and hydrocarbon limits capped at 7.5 g/kW-hr.
- 56 to 560 kW: 0.04 g/kW-hr for particulate matter and 0.40 g/kW-hr for nitrogen oxides. These are the strictest categories.
- Over 560 kW (non-generator sets): 0.07 g/kW-hr for particulate matter, with combined nitrogen oxide and hydrocarbon limits at 6.2 g/kW-hr.
- Over 560 kW (generator sets): 0.05 g/kW-hr for particulate matter and 1.07 g/kW-hr for nitrogen oxides.
A typical Tier 1 engine in the 130–560 kW range was allowed 0.54 g/kW-hr of particulate matter. Tier 4 Final cut that to 0.04 g/kW-hr, a reduction of more than 90 percent. Numbers that low cannot be reached through combustion improvements alone, which is why Tier 4 forced widespread adoption of diesel particulate filters and selective catalytic reduction.
Why Some Post-2015 Equipment Still Isn’t Tier 4 Final
Even though the last Tier 4 Final phase-in date was 2015, you can still find newer machines that were built with older-tier engines. That’s because the EPA built flexibility provisions into the rule. Under 40 CFR 1039.625, equipment manufacturers could continue producing a limited number of machines with engines certified to earlier standards during the transition period.5eCFR. 40 CFR 1039.625 – What Requirements Apply Under the Program for Equipment-Manufacturer Flexibility
The program offered two approaches. Under the percent-of-production method, a manufacturer could use exempted engines for a portion of its total U.S. production, as long as cumulative exemptions within a power category didn’t exceed 80 percent of production over a seven-year window. Under the small-volume method, a manufacturer could produce up to 700 total units with exempted engines over the seven-year period, with no more than 200 in any single year.5eCFR. 40 CFR 1039.625 – What Requirements Apply Under the Program for Equipment-Manufacturer Flexibility The small-volume caps were lower for engines above 130 kW (350 units over seven years) and set at 525 units with no more than 150 per year for engines below 130 kW.
These flexibility windows have now expired for every power category. The last one covered 2015 through 2021 for the largest engines. Any new nonroad diesel engine manufactured today has to meet full Tier 4 Final standards unless a specific exemption applies.
Earlier Tiers That Led Up to Tier 4
Tier 4 didn’t appear out of nowhere. The EPA rolled out progressively stricter standards for nonroad diesel engines over more than a decade beforehand:
- Tier 1 (1996–2000): The first federal limits on nonroad diesel emissions, phased in by engine size, addressing nitrogen oxides, hydrocarbons, carbon monoxide, and particulate matter at relatively generous levels.
- Tier 2 (2001–2006): Tightened the same pollutants further, pushing manufacturers toward improved combustion and fuel-injection technology.
- Tier 3 (2006–2008): Reduced nitrogen oxide and hydrocarbon limits again for most engine categories, though particulate matter standards stayed at Tier 2 levels for many sizes.
Each round pushed engine technology forward, but particulate matter and nitrogen oxide emissions remained high enough to cause serious air quality problems near construction sites and in urban areas. Tier 4 was a much larger leap than any previous tier.
Is There a Tier 5?
As of early 2026, no federal Tier 5 standard exists. The California Air Resources Board released a draft proposal in February 2026 for state-level Tier 5 standards that would cut nitrogen oxide emissions by up to 90 percent and particulate matter by up to 75 percent below Tier 4 Final levels. If adopted, those California standards would phase in between 2031 and 2036, though the proposal would not cover engines below 175 horsepower used in construction or farm equipment. Without a matching federal rule, the California standards would apply only within states that adopt California’s emission framework. Whether the EPA will pursue a national Tier 5 remains an open question.