Diesel Engine Tier Chart: EPA Limits by Tier and Power Band

The EPA’s diesel engine tier chart sets maximum allowable emissions of nitrogen oxides (NOx) and particulate matter (PM) for new nonroad compression-ignition engines, moving from Tier 1 (first enforced in 1996) through Tier 4 Final (fully phased in by 2015). Every value on the chart is expressed in grams per kilowatt-hour and applies to a specific engine power band and model year. A Tier 4 Final engine in the 56–560 kW range, for example, must keep PM at or below 0.02 g/kWh and NOx at or below 0.40 g/kWh, roughly a 90 percent reduction from early tier levels.¹

The authority behind the chart comes from Section 213 of the Clean Air Act (42 U.S.C. § 7547), which directs the EPA to set standards achieving the greatest emissions reduction available with existing technology.²³¹

Power Bands Come First

You cannot read a single row of the tier chart without knowing your engine’s rated power. Every limit is keyed to a kilowatt band, and the phase-in dates differ by band as well. The EPA divides nonroad diesel engines this way:

Below 19 kW (under about 25 hp): the smallest engines, typical of compact utility equipment. Least stringent limits and latest phase-in.
19 to under 56 kW (about 25–74 hp): skid steers, small tractors, similar mid-small equipment.
56 to under 130 kW (about 75–174 hp): loader-backhoes and mid-range excavators sit here.
130 to 560 kW (about 175–750 hp): full-size excavators, dozers, large wheel loaders.
Above 560 kW (over about 750 hp): mining haul trucks, large generator sets.

Part 1039 assigns different model-year start dates to each band. Engines in the 130–560 kW range fell under Part 1039 starting in model year 2011; engines in the 19–56 kW range began in 2008.¹ Smaller, more space-constrained engines got more time to integrate after-treatment.

The Limits, Tier by Tier

Tier 1

Adopted in 1994 and phased in starting in 1996 for engines above 37 kW, Tier 1 focused primarily on NOx. Most power categories carried a ceiling of 9.2 g/kWh. There was no federal PM limit yet for many bands. Manufacturers met Tier 1 with combustion improvements alone: better injection timing, revised piston bowls, tuned air intake. No exhaust after-treatment was required.

Tier 2

Phased in roughly 2001 through 2006, Tier 2 tightened NOx and introduced PM limits across all power bands for the first time. In the 75–130 kW range, PM dropped to 0.3 g/kWh; above 130 kW, to 0.2 g/kWh. Higher-pressure common-rail injection and electronic controls did the work.

Tier 3

Phased in from 2006 to 2008, Tier 3 lowered a combined NOx-plus-hydrocarbon limit while generally holding Tier 2 PM levels. Cooled exhaust gas recirculation, turbocharger refinements, and more sophisticated electronics kept the gains coming from inside the engine. Tier 3 was the last generation where base engine design alone could satisfy the rules.

Tier 4 Interim and Tier 4 Final

The EPA signed the final Tier 4 rule in 2004, phased it in from 2008 through 2015, and required roughly 90 percent reductions in both PM and NOx compared to Tier 3.⁴ The gap was too large to close with combustion changes alone. Exhaust after-treatment became mandatory.

The phase-in split into two stages. Tier 4 Interim demanded full compliance with the new PM limits but allowed transitional NOx levels. Tier 4 Final then brought NOx down to the target. For engines between 56 kW and 560 kW, the final limits are 0.02 g/kWh PM and 0.40 g/kWh NOx.¹ A Tier 1 engine in the same power range could legally emit 9.2 g/kWh of NOx, more than twenty times the Tier 4 Final ceiling.

Engines above 560 kW follow different numbers. Generator sets over 900 kW must meet 0.67 g/kWh NOx. Other engines above 560 kW carry a 3.5 g/kWh NOx limit and a PM ceiling of 0.03 to 0.04 g/kWh depending on application.¹

Key Numbers for the 75–130 kW Band

These figures show how PM and NOx limits dropped tier by tier for one of the most common construction-equipment power bands. All values are in grams per kilowatt-hour.¹

Tier 1: NOx 9.2. No federal PM limit for this power category.
Tier 2: NOx + NMHC combined, 6.6. PM 0.3.
Tier 3: NOx + NMHC combined, 4.0. PM stays at 0.3.
Tier 4 Interim: PM 0.02. NOx at transitional levels, up to 3.4 depending on credit path.
Tier 4 Final: PM 0.02. NOx 0.40. NMHC 0.19.

NOx fell from 9.2 g/kWh to 0.40 g/kWh over roughly two decades. PM went from unregulated at Tier 1 to 0.02 g/kWh at Tier 4 Final. Other power bands follow the same trend with different numbers and different phase-in dates.

What Made the Tier 4 Numbers Possible

The steep drop at Tier 4 exists because two pieces of after-treatment hardware showed up on the engine. A diesel particulate filter (DPF) traps soot in a ceramic or silicon carbide honeycomb and periodically burns it off, either passively during high-load operation or actively when the engine’s computer injects extra fuel to raise exhaust temperature. Selective catalytic reduction (SCR) handles NOx: the system injects a urea-based diesel exhaust fluid (DEF) upstream of a catalyst, where the DEF becomes ammonia and reacts with NOx to produce nitrogen and water.

DEF is a 32.5 percent urea solution and consumption runs about 3 to 5 percent of fuel use. The tank sits separately on the machine and needs regular refilling. Federal rules require on-board diagnostics to monitor DEF level and quality, and the SCR system must resist tampering. If the tank runs dry or the fluid is contaminated, the engine progressively derates, potentially limiting speed to as low as five miles per hour until the problem is corrected.⁵

Tier 4 after-treatment is also sensitive to fuel sulfur. Since June 2010, nonroad diesel has been required to meet the ultra-low sulfur standard of 15 parts per million maximum, a reduction of more than 99 percent from earlier levels.⁴ Fuel from an old bulk tank is worth verifying before it goes into a Tier 4 machine.

Reading the Tier off a Specific Engine

Every certified nonroad diesel engine carries an emission control information label, usually on the valve cover, rocker cover, or another visible spot on the block. The label lists the certified emission standard, power rating, and displacement, and by rule it carries the heading “EMISSION CONTROL INFORMATION” along with manufacturer name and maximum power.¹ If the label is gone or unreadable, cross-reference the model year and power rating against the phase-in dates in Part 1039 to find the minimum tier the engine had to meet when new.

Trust the label, not the machine’s model year. Under the EPA’s Transition Program for Equipment Manufacturers (TPEM), equipment makers can delay installing engines meeting the newest tier standard for up to seven years while they redesign their machines.⁶ That is why a brand-new machine at a dealer sometimes carries a previous-tier engine.

What the Chart Doesn’t Do to Older Equipment

The federal tiers apply at the point of manufacture. They govern what can be sold new, not what can stay in service. A Tier 2 dozer you already own can keep working. California and some other jurisdictions impose in-use fleet rules that restrict operation of older, higher-emitting equipment, so where the machine works matters.

Rebuilds and repowers are a different question. Under 40 CFR 1068.120, a rebuilt engine must be restored to its original certified configuration or to a same-or-later configuration that is equally clean or cleaner. A replacement engine dropped into existing equipment must meet standards at least as stringent as the engine it replaces.⁷ The certification stays with the machine for life, and the direction is one-way.

Stationary Engines Are on a Parallel Track

Backup generators, irrigation pumps, and compressor-station engines are regulated under 40 CFR Part 60, Subpart IIII, not Part 1039. In practice the numbers line up: stationary engine manufacturers certify to the same emission levels as the corresponding nonroad mobile standards for the same model year and power rating.⁸ Emergency stationary engines face less restrictive requirements than non-emergency ones because they run infrequently. An engine labeled as exempt from nonroad certification because it is stationary cannot legally be installed in mobile equipment, and the label itself carries that warning.¹

What Comes After Tier 4 Final

Tier 4 Final is still the current federal standard. The California Air Resources Board is developing what it calls Tier 5, aimed at reducing NOx and PM below current Tier 4 Final levels for new off-road diesel engines.⁹ Specific numbers are not final, but the direction points to tighter NOx and PM, likely a particle number limit similar to the European Union’s Stage V (which imposes a PN limit on engines between 19 kW and 560 kW), and possibly CO2 requirements. CARB standards have historically been picked up federally later, so the rulemaking matters even outside California.

1
eCFR. 40 CFR Part 1039 – Control of Emissions from New and In-Use Nonroad Compression-Ignition Engines
2
Office of the Law Revision Counsel. 42 USC 7547 – Nonroad Engines and Vehicles
3
eCFR. 40 CFR Part 89 – Control of Emissions from New and In-Use Nonroad Compression-Ignition Engines
4
US EPA. Regulations for Emissions from Heavy Equipment with Compression-Ignition Diesel Engines
5
US EPA. Diesel Exhaust Fluid
6
US EPA. Transition Program for Equipment Manufacturers (TPEM)
7
eCFR. 40 CFR 1068.120 – Requirements for Rebuilding Engines
8
eCFR. Standards of Performance for Stationary Compression Ignition Internal Combustion Engines
9
California Air Resources Board. Potential Amendments to the Off-Road New Diesel Engine Emission Standards – Tier 5 Criteria Pollutants and CO2 Standards