A Tier 4 generator is a diesel generator set whose engine is certified to EPA's Tier 4 nonroad diesel emission standards. Tier 4 Final is the full set of limits on nitrogen oxides (NOx), particulate matter (PM) and other pollutants that applies after the 2014 model year.
Whether a US site needs one depends mostly on how the set will run. A generator kept for outages and limited testing is an emergency engine, held to an earlier tier at most sizes. One used for peak shaving or as the site's main power source is a non-emergency engine. For engines under 10 liters per cylinder of model year 2015 and later, that means Tier 4 Final.
This article describes the federal rule in 40 CFR, read on eCFR on 29 October 2026. A site's air permit can add limits of its own.
Key Takeaways
- A stationary genset's standards come through 40 CFR 60 Subpart IIII. Displacement picks Part 1039 or 1042; model year, engine power and emergency or non-emergency use pick the limits.
- Emergency status has an hour budget. Maintenance and readiness testing get 100 hours a year, including up to 50 non-emergency hours. Running outside those limits makes the engine non-emergency.
- Above 560 kWm, generator-set engines carry a Tier 4 Final NOx limit of 0.67 g/kW-hr. Engines of the same power in other machines carry 3.5 g/kW-hr.
- Certified and compliant are different statuses. Non-emergency use calls for an engine certified to its model-year standards; third-party aftertreatment on an emergency engine makes a "compliant" package that stays emergency-class.
- Dated Tier 4 Final pricing at data-center sizes is thin. A March 2023 Kohler GSA list prices the 1,250 kWe-ESP KD1250-4 at $499,305, with no emergency set beside it.
What a Tier 4 generator is#
EPA finalized Tier 4 in 2004, with lower sulfur limits for nonroad diesel fuel, according to its final rule page. The final limits sit in 40 CFR 1039.101.
A stationary engine is one that does not meet the nonroad definition in 40 CFR 1068.30 (§ 60.4219). For these engines, 40 CFR 60.4201 in Subpart IIII ties certification to the Part 1039 standards "for the same model year and maximum engine power". Here kWm means maximum engine power, kept apart from a set's electrical kWe.
Under 40 CFR 60.4200, Subpart IIII covers owners of engines ordered after 11 July 2005 and manufactured after 1 April 2006, or after 1 July 2006 for certified NFPA fire pump engines. It also covers engines modified or reconstructed after 11 July 2005.
Engines of 10 to 30 liters per cylinder follow the marine standards in Part 1042, so this article covers the Tier 4 diesel generator below 10 liters per cylinder. Wider buying checks sit in the diesel generator procurement guide, and the generator catalog lists sets by size.
Emergency or non-emergency decides it#
Under the definition in 40 CFR 60.4219, an emergency engine supplies power "when electric power from the local utility (or the normal power source…) is interrupted", and otherwise runs only as § 60.4211(f) allows. Use decides the class; a standby or prime rating on the data sheet does not, as Kohler's April 2024 white paper explains. The prime power rating guide explains those ratings.
Under 40 CFR 60.4211(f), emergency running has no time limit. Maintenance checks and readiness testing get up to 100 hours per calendar year, with up to 50 non-emergency hours counted inside them. Those 50 hours cannot be used for peak shaving or non-emergency demand response.
What to ask a supplier for, by intended use (federal rule: 40 CFR 60 Subpart IIII and 40 CFR 1068.30)#
| Intended use | Federal class | Certification to ask for | Records and limits |
|---|---|---|---|
| Outage backup, plus maintenance and readiness testing | Emergency (§ 60.4219) | § 60.4202 standards for its model year and power: Tier 2 or Tier 3 at 37–2,237 kWm | An hour meter if it misses the non-emergency standards (§ 60.4209(a)); a run log and a "stationary emergency use only" label from model year 2011 at 130 kWm and above, 2012 at 56–130 kWm and 2013 at 19–56 kWm (Table 5) |
| Dispatch by the local balancing authority or transmission operator to avert voltage collapse or line overloads | Emergency, inside the 50 non-emergency hours (§ 60.4211(f)(3)(i)) | As above | A record of who dispatched it; an annual CEDRI report above 100 HP (§ 60.4214(d)) |
| Peak shaving, non-emergency demand response, selling power or main site power | Non-emergency (§ 60.4204) | Part 1039 standards for its model year and power: Tier 4 Final after model year 2014 | The manufacturer's emission-related instructions, or owner testing above 500 HP (§ 60.4211(g)(3)) |
| Portable unit on wheels, skids or a trailer, at one location for 12 consecutive months or less | Nonroad (40 CFR 1068.30) | Nonroad certification for its model year | A replacement's time counts toward the 12 months |
| Site with local limits, such as South Coast AQMD or Northern Virginia | The federal class above, plus the air permit | Certified Tier 4 Final, or a "compliant" package on an emergency engine | The permit's conditions |
Standards Subpart IIII applies to diesel engines under 10 liters per cylinder from model year 2007, by use and engine power (40 CFR 60.4201 and 60.4202)#
| Use | Maximum engine power and model years | Standards the engine is certified to | Section |
|---|---|---|---|
| Non-emergency | Up to 2,237 kWm, 2007 on | Part 1039 standards for the same model year and power (Tier 4 Final after model year 2014) | 60.4201(a) |
| Either use | Above 2,237 kWm, 2007 to 2010 | Table 1 to Subpart IIII | 60.4201(b), 60.4202(b)(1) |
| Non-emergency | Above 2,237 kWm, 2011 on | Part 1039 standards | 60.4201(c) |
| Emergency (other than fire pump) | Below 37 kWm, 2008 on | §§ 1039.104, 1039.105, 1039.107 and 1039.115, plus Table 2 to Subpart IIII | 60.4202(a)(1)(ii) |
| Emergency (other than fire pump) | 37 to 2,237 kWm, 2007 on | Tier 2 or Tier 3 nonroad standards for the same rated power | 60.4202(a)(2) |
| Emergency (other than fire pump) | Above 2,237 kWm, 2011 on | Tier 2 | 60.4202(b)(2) |
| Fire pump | NFPA nameplate power, from the Table 3 model years | Table 4 to Subpart IIII | 60.4202(d) |
Tier 4 Final limits by engine power#
Tier 4 Final emission standards for stationary non-emergency CI engines, by engine power, g/kW-hr (40 CFR 1039.101 table 1 as applied by 40 CFR 60.4201)#
| Maximum engine power | Application | PM | NOx | NMHC | NOx + NMHC | CO |
|---|---|---|---|---|---|---|
| Below 19 kWm | All | 0.40 | – | – | 7.5 | 6.6 |
| 19 to below 56 kWm | All | 0.03 | – | – | 4.7 | 5.0 |
| 56 to below 130 kWm | All | 0.02 | 0.40 | 0.19 | – | 5.0 |
| 130 to 560 kWm | All | 0.02 | 0.40 | 0.19 | – | 3.5 |
| Above 560 kWm | Generator sets | 0.03 | 0.67 | 0.19 | – | 3.5 |
| Above 560 kWm | All except generator sets | 0.04 | 3.5 | 0.19 | – | 3.5 |
CO is 8.0 g/kW-hr below 8 kWm and 5.5 g/kW-hr from 19 to below 37 kWm. Data center sets such as those in the 2 MW standby generator comparison sit in the top band.
In Table 7 of 40 CFR 1039.102, generator sets above 900 kWm already carried 0.67 g/kW-hr NOx with PM at 0.10 for model years 2011 to 2014. Engines of 560–900 kWm carried 3.5 NOx; Kohler's white paper equates them to roughly 500–800 kWe sets. After 31 December 2016, owners may not install non-emergency engines of 560 kWm or more that miss the 2015 model-year requirements (§ 60.4208(f)).
What Tier 4 Final hardware adds#
Selective catalytic reduction (SCR) injects diesel exhaust fluid (DEF), a urea solution, ahead of a catalyst that turns NOx into nitrogen and water. A diesel oxidation catalyst (DOC) cuts hydrocarbons and CO, and a diesel particulate filter (DPF) traps soot and burns it off. These descriptions follow a March 2014 Caterpillar white paper and the definitions in § 60.4219.
The Generac SD500, rated 500 kW standby at 60 Hz on a Perkins 2806F-E18TAG1, carries "EGR, DOC, SCR, and DPF", according to Generac's 2023 spec sheet, Rev. A. Kohler's KD Series Tier 4 Final sets use SCR alone, with no DPF or DOC, by Kohler's own description in its 2021 sell sheet. Where a DPF is used to meet the § 60.4204 standards, it needs a backpressure monitor (§ 60.4209(b)), and corrective actions after its alerts are recorded (§ 60.4214(c)).
3% to 5% — typical DEF use as a share of fuel burned on Cummins Tier 4 Final engines with SCR, in a September 2013 Cummins bulletin.
Aggreko's "400 kW" Tier 4 Final rental set is rated 366 kW prime on its product sheet, with no frequency given for the rating. At 75% prime load it lists 0.88 gallons per hour of DEF against 16.83 of fuel: 5.2%, calculated.
SCR engines must warn operators before the DEF tank is empty (§ 1039.110). Limiting engine performance to force a DEF refill counts as an emission control under 40 CFR 1039.665. Inducement can shut a certified generator down, by the account in Kohler's white paper.
An EPA-approved emergency auxiliary emission control device (AECD) can suspend that control where a fault risks human life; EPA's own example is a data center routing 911 calls. It switches off within 120 hours of cumulative run time (§ 1039.665(b)(7)), and the engine must meet Tier 1 limits while it is active (§ 60.4204(f)).
Cummins Tier 4 Final engines must run on ultra-low sulfur diesel of 15 ppm or less, according to Cummins' fluids page. Subpart IIII has required nonroad diesel fuel since 1 October 2010 for engines under 30 liters per cylinder (§ 60.4207(b)).
Certified or compliant#
Owners of 2007-and-later engines comply "by purchasing an engine certified" to the standards for the engine's model year and power (§ 60.4211(c)). For a non-emergency engine those are the § 60.4204(b) standards, and § 60.4208 bars installing engines that miss the model-year requirements after set dates.
Kohler's white paper defines a "compliant" package as an EPA-certified stationary emergency engine with third-party aftertreatment. The engine underneath keeps its emergency certification.
Compliant packages answer local limits beyond the federal tier. The same Kohler white paper says South Coast AQMD PM limits often call for a DPF. Northern Virginia's ozone-driven generator limits, it says, are met by cutting NOx with SCR or a low-NOx calibration. The Clean Air Act's Title V operating-permit program is a separate driver: sites with many large standby sets, such as data centers, can exceed its major-source thresholds.
Owners who depart from the manufacturer's emission-related instructions for installing, operating and maintaining an engine take on compliance duties of their own. Above 500 HP (373 kWm, calculated), those are a maintenance plan, an initial performance test within a year, and repeat tests every 8,760 hours or 3 years (§ 60.4211(g)(3)).
What Tier 4 Final costs#
Dated Tier 4 Final list prices sit on a Kohler GSA price list effective 20 March 2023. Only two small sizes carry tier labels on both units of a pair.
Tier 4i and Tier 4 Final list prices for two small sets; no emergency Tier 2 or 3 comparator and no data-center size (Kohler GSA price list effective 20 March 2023; March 2023 dollars)#
| Size on list | Tier 4i part, list price | Tier 4 Final part, list price | Difference (calculated) | $/kW, 4i to Final (calculated) |
|---|---|---|---|---|
| 30 kW, rating basis not stated | 30REOZK, $20,113 (2023) | 30REOZK4 with 24-hour fuel tank, $41,946 (2023) | +$21,833 (+108.6%) | $670 to $1,398 (2023) |
| 40 kW, rating basis not stated | 40REOZK, $27,368 (2023) | 40REOZK4 with 24-hour fuel tank, $45,104 (2023) | +$17,736 (+64.8%) | $684 to $1,128 (2023) |
The Tier 4 Final parts on the Kohler GSA price list include a 24-hour fuel tank, so each gap covers more than emission hardware. Both pairs compare interim and final Tier 4 on small sets, so they say nothing about SCR cost at 560 kWm and above.
The same list prices the KD1250-4 at $499,305 in March 2023 dollars. Kohler's 2021 sell sheet rates it 1,250 kWe ESP and 1,120 kWe PRP, certified Tier 4 Final, with no frequency stated. That is about $399 per ESP kWe (2023), calculated. The list's two other 1,250 kW sets, rating basis not stated, carry no tier label, so the list offers no clean comparison at that size.
At data-center sizes, the gap between emergency Tier 2 and Tier 4 Final sets has to come from dated quotes, read alongside the pricing tool. In a December 2020 article, Volvo Penta argues that a Tier 4 Final certified system costs less to buy and own than "a Tier 2 system with aftertreatment". That baseline is the compliant route, not a plain emergency Tier 2 set.
DEF use scales with fuel burned, so every engine hour a battery takes over also saves DEF. On a Kansas City data center build, a 500 kW battery let a 275 kW generator (rating type not stated) average 3.5 hours a day for seven months. Those figures are United Rentals' own, from its case study. The data center battery storage guide covers permanent sites.
Hour creep, rentals and the 12-month rule#
Running past the § 60.4211(f) limits makes the engine non-emergency, and it "must meet all requirements for non-emergency engines". Grid-reliability dispatch under § 60.4211(f)(3)(i) is the one paid use inside the 50 hours. Duke Energy's PowerShare Mandatory 50 program is the example in EPA's May 2025 backup generator fact sheet.
Portable or transportable units, with signs such as wheels, skids or a trailer, are nonroad engines under 40 CFR 1068.30.
12 consecutive months — the stay at one location after which a portable engine stops being nonroad, with a replacement doing the same job adding its time to the count under 40 CFR 1068.30.
A converted engine keeps the model year in which it was manufactured (§ 60.4219). Temporary replacement units on site for less than a year, certified to nonroad standards, need meet no other Subpart IIII provision (§ 60.4200(e)). Log each unit's arrival and swap dates and its hours by reason.
Checking a used or rental generator's status#
The emission label comes first. Under 40 CFR 1039.135, it carries "EMISSION CONTROL INFORMATION", EPA's engine family designation, the power category, the date of manufacture and the emission-control system. The date can sit elsewhere on the engine, and the control system in the owner's manual. Above 560 kWm, generator-set engines carry "FOR GENERATOR SETS AND OTHER APPLICATIONS" and other engines "NOT FOR USE IN A GENERATOR SET".
Engines certified only as stationary carry "stationary" in place of "nonroad" (§ 60.4210(c)(3)(ii)). EPA's nonroad compression-ignition certification data covers model year 2011 onward, with an archive back to 1996, updated quarterly. Search the label's engine family there, and for a stationary-only family ask for the manufacturer's certification documentation as well.
Reconstruction can give an engine a new date of manufacture, once new and refurbished components exceed 75% of the fixed capital cost of a comparable new one (§ 60.4219). Ask for the records § 60.4214 names for the engine's class: certification documentation, maintenance, the hour log and any DPF corrective actions.
The installation deadlines do not apply to engines moved between locations (§ 60.4208(i)), while the model-year standards apply for the engine's life (§ 60.4206). The used 2 MW standby checklist covers the mechanical side.
FAQ: Tier 4 generators#
Do emergency standby generators have to be Tier 4 Final?#
Not under the federal rule while they meet the emergency definition (§ 60.4202; Table 4 for fire pumps). Two federal carve-outs extend emergency standards to other engines. In Guam, American Samoa and the Northern Mariana Islands, engines under 30 liters per cylinder meet the emergency standards whatever their use (§ 60.4215(a)). In remote Alaska, owners may choose the emergency standards, though model year 2014 and later non-emergency engines must still be certified to at least Tier 3 PM (§ 60.4216(c)).
Do all Tier 4 engines need DEF?#
Only engines built with SCR use DEF. Cummins Tier 4 Final engines rated above 75 hp (56 kW) use SCR, per the same 2013 bulletin, and the Tier 4 Final table sets no separate NOx limit below 56 kWm. The label or owner's manual names the emission-control system (§ 1039.135(c)(8)).
Are natural gas generators Tier 4?#
Subpart IIII and its Tier 4 link cover compression-ignition engines. Under 40 CFR 60.4230, spark-ignition gas engines fall under Subpart JJJJ. A dual-fuel engine averaging less than 2 parts diesel per 100 parts fuel over a year, on an energy basis, counts as spark ignition (§ 60.4219). Gas units are covered in the used natural gas generator checks.
Can a Tier 2 generator be upgraded to Tier 4?#
Adding aftertreatment to a Tier 2 emergency engine makes the "compliant" package described above, and the engine stays emergency-certified. A modified or reconstructed non-emergency engine takes on the standards for its own model year, power and displacement (§ 60.4204(e)). Unless certified, it needs a performance test within 60 days of resuming operation (§ 60.4211(e)).
Did EPA change the Tier 4 generator standard recently?#
Not the limits. EPA's 30 August 2024 final rule amendments added electronic reporting to Subpart IIII and related engine rules. EPA stated it "is not making changes to the emission limits". The Tier 4 delay EPA proposed on 14 May 2026 covers light- and medium-duty vehicles, to model year 2029, according to an EPA fact sheet.
Before the next order#
SecondWatt publishes equipment research and helps buyers source power equipment; it earns a fee when a sale it arranges closes. A request through the generator request form that states the intended use, expected annual hours and site lets suppliers quote the right certification. Owners releasing standby sets can start from the seller page.