Small Off-Road Engines (SORE): Definition, Classes, and Phase 3 Limits

Small off-road engines are spark-ignition engines rated at or below 19 kilowatts, roughly 25 horsepower, that power outdoor equipment like lawn mowers, leaf blowers, chainsaws, string trimmers, and portable generators. The EPA regulates them under 40 CFR Part 1054 and sorts them into five classes based on whether the equipment is handheld or nonhandheld and how large the engine’s displacement is. Despite their size, these engines produce meaningful amounts of hydrocarbons, nitrogen oxides, and carbon monoxide, which is why federal and state regulators treat them as a distinct category.¹

What Counts as a Small Off-Road Engine

Two conditions have to be met. The engine must use spark ignition, meaning a spark plug firing a fuel-air mixture rather than a diesel compression-ignition design. And its maximum power output must sit at or below 19 kW. The engine also has to be a “nonroad” engine, which rules out engines designed to propel highway vehicles or to run permanently installed stationary equipment. Auxiliary marine spark-ignition engines, such as onboard generators, can also be certified under Part 1054 even though most marine propulsion engines fall under separate rules.¹

There is a narrow overlap zone with larger engines. Engines up to 30 kW with displacement at or below 1,000 cc may optionally be certified under Part 1054 instead of the large-engine rules in Part 1048.¹²

Equipment Powered by These Engines

The category covers a lot of familiar gear. Walk-behind and riding lawn mowers, string trimmers, leaf blowers, and chainsaws are the everyday consumer examples. Portable generators used for backup power or at construction sites account for a large share of the market, along with pressure washers, small air compressors, snow blowers, and log splitters. Commercial equipment like wood chippers, stump grinders, and turf-care machines also runs on engines in this power range.

Handheld vs. Nonhandheld

The EPA draws a big regulatory line between handheld and nonhandheld equipment, because handheld tools have to stay light enough to carry. Equipment counts as handheld if the operator carries it throughout use, if it is designed to work in multiple positions such as upside down or sideways, or if the combined engine-and-equipment dry weight stays under 16 kilograms with operator support required during operation. Augers under 22 kilograms and hand-supported jackhammers also qualify.³

Everything else under Part 1054 is nonhandheld: walk-behind mowers, generators, pressure washers, and other ground-supported or wheeled equipment. Handheld engines face less stringent limits, reflecting how hard it is to fit emission controls onto a device that has to weigh under 35 pounds.

The Five EPA Classes

Within Part 1054, engines fall into five classes. The nonhandheld/handheld split is the first cut, and displacement in cubic centimeters sets the specific class.³

Nonhandheld engines:

Class I: displacement below 225 cc. Typical of smaller walk-behind mowers and small generators. Engines at or below 80 cc within this class are subject to special provisions.
Class II: displacement at or above 225 cc. These power riding mowers, larger generators, and wood chippers.

Handheld engines:

Class III: displacement below 20 cc. Light-duty trimmers and small blowers.
Class IV: displacement from 20 cc up to but not including 50 cc. Most standard string trimmers and leaf blowers.
Class V: displacement at or above 50 cc. The largest handheld engines, powering heavy chainsaws, backpack blowers, and cut-off saws.

Phase 3 Emission Limits

The current federal limits, known as Phase 3, cap hydrocarbon-plus-nitrogen-oxide (HC+NOx) and carbon monoxide (CO) in grams per kilowatt-hour. Larger engine classes face tighter HC+NOx numbers, because larger combustion chambers produce more pollution per unit of work and have more room to fit emission-control hardware.

Nonhandheld engines:⁴

Class I: HC+NOx no more than 10.0 g/kW-hr; CO no more than 610 g/kW-hr.
Class II: HC+NOx no more than 8.0 g/kW-hr; CO no more than 610 g/kW-hr.

Handheld engines:⁵

Class III and IV: HC+NOx no more than 50 g/kW-hr; CO no more than 805 g/kW-hr.
Class V: HC+NOx no more than 72 g/kW-hr; CO no more than 603 g/kW-hr.

The gap is wide. A Class IV trimmer engine is allowed five times the HC+NOx of a Class I mower engine, reflecting a practical trade-off: two-stroke handheld engines run dirtier, and adding catalytic converters or fuel injection to a 10-pound chainsaw is far harder than fitting them to a 60-pound mower.

Useful Life and Durability

Testing a new engine once isn’t enough. The EPA requires manufacturers to demonstrate compliance over the engine’s projected “useful life,” measured in hours of operation and varying by class and intended use. The engine has to stay within the limits through realistic wear, not just on day one.⁶

For nonhandheld engines, useful life depends on residential versus commercial duty:

Residential: 125 hours (Class I) or 250 hours (Class II).
Extended-life residential: 250 hours (Class I) or 500 hours (Class II).
Commercial: 500 hours (Class I) or 1,000 hours (Class II).

For handheld engines in Classes III through V:

Light use: 50 hours.
Medium use: 125 hours.
Heavy use: 300 hours.

Manufacturers can voluntarily select a longer useful life in 100-hour increments, up to 3,000 hours for Class I or 5,000 hours for Class II, earning a “heavy commercial” designation. Engines above 19 kW that are optionally certified under Part 1054 must meet a minimum useful life of 1,000 hours.⁶

Evaporative Emissions

Exhaust from the muffler is only part of the pollution. Gasoline SORE equipment also releases hydrocarbons through fuel evaporation from tanks, lines, and carburetor vents. The EPA addresses this under 40 CFR Part 1060, which sets permeation limits on fuel-system components.

Fuel tanks may not exceed a permeation rate of 1.5 grams per square meter per day at 28°C, or 2.5 g/m²/day at 40°C. Low-emission fuel lines must stay at or below 10 g/m²/day; standard nonroad fuel lines at or below 15 g/m²/day. Cold-weather fuel lines get a more relaxed 225 g/m²/day allowance.⁷

These permeation rules are why modern small-engine tanks often use multi-layer plastic construction or barrier coatings. If you replace a tank or fuel line, using OEM-spec parts avoids creating a compliance problem.

Tampering and Defeat Devices

Federal law prohibits removing, disabling, or bypassing any emission-control device or design element on a SORE-certified engine, both before and after the engine reaches the end user. The rules also ban manufacturing, selling, or installing components that function as defeat devices.⁸

Three narrow exceptions apply. You can temporarily disable emission controls to make a repair, as long as you restore them afterward. You can modify controls to respond to a genuine emergency, again restoring them once the emergency passes. And a second manufacturer can modify and recertify a previously certified engine under its own engine family.⁸

Penalties are steep. Defeat-device violations can reach $44,539 per component. Manufacturers or dealers who sell noncompliant engines face up to $44,539 per engine; individuals face up to $4,454 per engine.⁸ Under the Clean Air Act’s inflation-adjusted schedule, manufacturers and dealers can face civil penalties up to $59,114 per engine for violations assessed on or after January 8, 2025, with non-manufacturer tampering capped at $5,911 per engine.⁹

What Isn’t a Small Off-Road Engine

Not every small gasoline engine is a SORE. Several categories sit under separate frameworks:

Highway vehicle engines fall under motor vehicle emission standards, not nonroad rules.
Recreational vehicles, including snowmobiles, ATVs, dirt bikes, and personal watercraft, are regulated under 40 CFR Part 1051.¹⁰
Marine propulsion engines face separate standards under 40 CFR Parts 91 and 1045, though auxiliary marine engines can be certified under Part 1054.¹
Spark-ignition nonroad engines above 19 kW are covered by 40 CFR Part 1048.²
Stationary engines that never move from their installed location are excluded from the nonroad definition, though they may still need to comply with certain Part 1054 provisions under stationary-source rules in 40 CFR Part 60, Subpart JJJJ.¹
Nonroad engines used solely for competition are excluded from emission standards. Modifying an in-service engine for competition-only use lifts the tampering prohibitions for as long as the engine stays in competition service, but the original emission label must be destroyed and any future owner informed in writing of the restriction.¹¹

California’s Zero-Emission Phase-Out

California treats SORE differently from the rest of the country. The California Air Resources Board has moved past tightening limits and is phasing out gasoline-powered small engines for new equipment sold in the state. The phase-out runs in two stages.¹²

Phase 1, model year 2024 onward: emission standards are set to zero for new engines in most equipment categories sold in California, including lawn mowers and leaf blowers. Generators and large pressure washers are temporarily exempt but must meet interim standards 40 to 90 percent stricter than previous limits.
Phase 2, model year 2028: generators and large pressure washers must also meet zero-emission standards.

A zero-emission standard effectively means battery-electric, plug-in, or hydrogen-powered equipment. The rule applies to manufacturers selling new equipment in California and does not ban the use or resale of existing gasoline-powered equipment, so a working gas mower can stay in service indefinitely.¹²

Under the Clean Air Act, California is the only state that can independently seek EPA authorization to adopt stricter nonroad engine standards. Other states may adopt California’s authorized standards, but only if they copy them identically and adopt them at least two years before the standards take effect. The Clean Air Act blocks all states, including California, from setting their own standards for nonroad engines used in construction or farm equipment rated below 175 horsepower.¹³

1
eCFR. 40 CFR 1054.1
2
eCFR. 40 CFR Part 1048 – Control of Emissions from New, Large Nonroad Spark-Ignition Engines and Equipment
3
eCFR. 40 CFR Part 1054 Subpart I – Definitions and Other Reference Information
4
eCFR. 40 CFR 1054.105 – Phase 3 Emission Standards for Nonhandheld Engines
5
eCFR. 40 CFR 1054.103 – Phase 3 Emission Standards for Handheld Engines
6
eCFR. 40 CFR 1054.107 – What Is the Useful Life Period for Meeting Exhaust Emission Standards
7
eCFR. 40 CFR Part 1060 Subpart B – Emission Standards and Related Requirements
8
GovInfo. 40 CFR 1068.101
9
Federal Register. Civil Monetary Penalty Inflation Adjustment
10
U.S. Environmental Protection Agency. Regulations for Emissions from Recreational Vehicles
11
eCFR. 40 CFR Part 1068 Subpart C – Exemptions and Exclusions
12
California Air Resources Board. CARB Approves Updated Regulations Requiring Most New Small Off-Road Engines Be Zero Emission by 2024
13
Office of the Law Revision Counsel. 42 U.S. Code 7543 – State Standards