Attachment E-4 

Definition of Similar Engines 

As stated in the Audit Protocol at Paragraphs I.B.2.a.v, emissions measured from an engine that has 

been stack-tested (a “Stack-Tested” engine) may be used to determine the emissions from an engine 

that has not been stack-tested (a “Non-Stack-Tested “ engine) if the Non-Stack-Tested engine is 

determined to be similar to the Stack-Tested engine. A Stack-Tested engine is one that has been (1) 

tested using any of the methods set out in the Audit Protocol at Paragraphs I.B.2.a.v, or (2) subject to 

emission tests by its manufacturer or for which the manufacturer has provided reliable emissions data.

Definition of “similar”

A Non-Stack-Tested engine may be considered similar to a Stack-Tested engine if its emissions of a 

particular pollutant are expected to be no greater than such emissions from the Stack-Tested engine, 

based on the criteria in the table below. 

EPA developed the concept of similarity for use in Clean Air Act programs that require manufacturers to 

certify that their engines comply with federal emissions standards. EPA described the concept in the 

nonroad engine certification program. That program specifies how similar engines are combined into 

groups known as engine families (see 40 CFR Parts 89.116, 90.116, and 1048.230). An engine 

manufacturer uses a single engine model and rating to certify emissions for an entire family of nonroad 

engines, even though individual engine ratings and applications may vary significantly. Because nonroad 

generator engines and stationary generator engines share basic technology, the methods used to 

identify groups of engines in nonroad applications also serve as a suitable foundation for establishing 

groups of engines in stationary applications. The methods that EPA established for nonroad engine 

certification are now specified in the new source performance standards for stationary spark-ignition 

and compression-ignition engines (see 40 CFR 60, Subparts IIII and JJJJ). 

For purposes of the Audit Protocol, the criteria for determining engine similarity are included in the 

following table. These criteria, which include fuel selection, method of aspiration, cooling mechanism, 

fuel delivery system, and cylinder displacement volume, among others, are key characteristics that are 

specified in the nonroad engine certification program. Accordingly, a Non-Stack-Tested engine would be 

deemed similar to a Stack-Tested engine if all of the statements in the right-hand column are true and 

there are no additional engine characteristics, such as ignition timing, operating cycle, engine controls, 

or emissions devices that indicate that the comparison would not be technically justified. Additionally, a 

Non-Stack-Tested engine can be determined to be similar to a Stack-Tested engine when one or more of 

the statements in the right-hand column are false, if it can be shown that the difference in 

characteristics can be expected to have little or no impact on emissions, with any justification being 

documented in the final audit report.
Criterion Stack-Tested Engine Non-Stack-Tested Engine 

Type of fuel consumed Diesel Same (i.e., diesel)

Gasoline Same (i.e., gasoline)

Natural gas Same (i.e., natural gas)

Kerosene Same (i.e., kerosene)

Other Same

Method of air aspiration Natural aspiration Same (i.e., natural aspiration)

Turbocharged Same (i.e., turbocharged)

Cooling mechanism Water-cooled Same (i.e., water-cooled)

Air-cooled Same (i.e., air-cooled)

Cylinder displacement 

volume

Any Same or lower displacement volume than tested 

engine (within -10% of tested engine volume)

Combustion chamber 

configuration (e.g., round v. 

“hemi”)

Any Same (i.e, both engines have a round, “hemi,” or 

other shaped combustion chamber)

Combustion chamber / fuel 

delivery system design

Direct injection Either direct injection or common rail injection

Common rail 

injection

Either direct injection or common rail injection 

Indirect injection Same (i.e., indirect injection)

Carbureted fuel Same (i.e., carbureted fuel)

Cycle Two-stroke Same (i.e., two stroke)

Four stroke Same (i.e., four stroke)

Combustion timing Any Same (i.e., either set by manufacturer or 

similarly adjusted)

Other relevant criteria (e.g., 

ignition timing, operating 

cycle, engine controls, 

aftertreatment devices, 

exhaust gas recirculation)

Any Same (e.g., both engines are electronically 

controlled; both engines lack aftertreatment

devices or the Non-Stack-Tested Engine has a 

device at least as effective as the Stack-Tested 

Engine)