Air combustion engine

January 10, 2022


Stationary engines are internal combustion engines using reciprocating motion to convert heat energy into mechanical work and which is not mobile.

Mobile engines are used to propel a motor vehicle or non-road engine. Engines that are portable are excluded from permitting and RICE rule requirements.

Combustion turbines, although not reciprocating internal combustion engines, may require an air permit and be subject to other federal air pollution regulations (NSPS KKKK and NESHAP YYYY).

Stationary engines are typically found at manufacturing, agricultural, municipal utilities, hospitals, retail centers, schools, and other types of businesses. This page is a resource for helping the owners and operators of these engines to determine whether they require an air permit for their stationary engine(s) and whether or not they are compliant with the federal rules for reciprocating internal combustion engines (RICE), 40 CFR 63 Subpart ZZZZ. Additional information can be found on the United States Environmental Protection Agency’s (U.S. EPA) Stationary Internal Combustion Engines page.

Regarding the regulated entities that have been assisted, the Indiana Department of Environmental Management (IDEM) is finding most communities are satisfying the federal requirements of the RICE rule due to good maintenance practices. However, we are finding that some of the sources failed to obtain the required air permits.

Do I need a permit for my engine?

If you have stationary engines such as generators, pumps, blower motors, or other stationary combustion equipment such as boilers, flares, furnaces, or turbines, you should evaluate these to determine if an air permit is required.

Are my combustion activities combined or separate for permitting purposes?

To determine whether an air permit is necessary, one must identify all the various pieces of stationary air pollutant emitting equipment at that property. Air permits are issued holistically based on the combined emissions of all stationary air pollutant emitting equipment at a single property or source, not on each individual piece of equipment. It is also possible that equipment at multiple property locations may be considered as a single source for permitting purposes if the properties are adjacent (near one another), are dependent on one another, or share staff. Below you will find more information regarding source determinations for equipment on multiple properties.

Do I need an air permit?

The type of air permit that is required is based on the combined equipment’s potential to emit air pollution, not on the actual air pollution emitted from the equipment. The potential to emit (PTE) is essentially a worst-case calculation presuming the equipment will operate at its maximum capacity 24 hours per day, 365 days per year (8, 760 hours per year).

If the PTE of a single criteria pollutant, from all your combined equipment, meets or exceeds the permit threshold then a permit is required. The permit threshold varies by pollutant. The thresholds for the most common pollutants of concern are below.

Pollutants of Concern
Criteria Air Pollutant Criteria Air Pollutant Symbols Threshold Requiring an Air Permit (tons/year)
Nitrogen Oxides NOx 10
Sulfur Dioxide SO2
Volatile Organic Compounds VOC
Particulate Matter PM
Carbon Monoxide CO 25

To help determine the applicable air permit for your source, the emission thresholds for various levels of permits are discussed within the Threshold Table for Sources based on Potential to Emit [PDF] as well as the IDEM Permit Guide, specifically the Registration Thresholds Table and the Title V Thresholds for Major Sources.

Do I have a peak shaving/load shedding agreement?

If an engine is committed for a peak shaving or load shedding agreement with the electrical utility, the generator is considered “non-emergency use” for permitting purposes, and the PTE must be calculated based on 8, 760 hours per year of operation. The peak shaving/load shedding issue is being reviewed. Entities should familiarize themselves with the difference between a “non-emergency use” engine and an “emergency use only” engine as well as review the IDEM Nonrule Policy Documents (NPD) for any changes regarding this issue.

How do I determine my source’s potential to emit?

Emissions are calculated using the power rating or heat input rating of the engine, an emission factor for the pollutant of concern, and the potential hours of operation of the engine. The power rating or heat input rating for a diesel or gasoline-fired engine is measured in horsepower (HP), while a natural gas or digester gas-fired engine is measured in BTU/hour. Regarding the potential hours of operation for a particular engine, “emergency use only” engines, as defined within NPD-Air-037 (available on the IDEM Nonrule Policy Documents page), may utilize 500 hours per year while every other engine should utilize 8, 760 hours per year.

“Non-emergency use” equipment as small as 74 HP diesel-fired engines, 100 HP 4-stroke lean burn gasoline-fired engines, and 0.5 million BTU/hr (fuel input) natural gas-fired engines will require the lowest level of air permit. Please note, a source with three (3) “emergency use only, ” diesel-fired, 434 horsepower engines would also require the lowest level of air permit.


A source has three (3) “emergency use only” 450 HP diesel fired engines. The emission factor for NOx for diesel engines less than 600 horsepower is 0.31 lbs NOx/hp – hr. The emissions calculation is:

3 * 450 HP = 1350 HP

1350 HP * 0.031 lbs (NOx / HP HR) * 500 HR/YR * 1(ton/2000lbs) = 10.5 tons NOx/YR

How do I find the horsepower or heat input capacity of my equipment?

1 KW = 1.34 HP
1 KW = 9380 BTU (diesel)

Owners of stationary “emergency use only” engines are knowledgeable of the generator power rating measured in kilowatt hours (KW). However, they are typically unaware of the horsepower (HP) for a diesel or gasoline engine, or heat input capacity measured in BTU/hr for a natural gas engine. If a permit is required, the horsepower or heat input capacity will be needed for the permit application. (They can be obtained from the manufacturer or from the engine plate.) The actual horsepower or heat input capacities are preferred, but for an approximation, one could use a conversion factor.

Effect of Hot Air Intake on 4- stroke Engine
Effect of Hot Air Intake on 4- stroke Engine

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