EPA-420-P-98-025
 

Emission Deterioration Factors For the NONROAD Emissions Model 

Report No. NR-011 

September 4, 1998 

Chris Lindhjem
 

Greg Janssen
 

Mike Sklar
 

Rich Wilcox
 

Nonroad Engine Emission Modeling Team
 

Assessment and Modeling Division
 

EPA, Office of Mobile Sources
 

I. Purpose 

This report addresses the emission deterioration rates currently being used in the draft 

version of NONROAD and discusses other sources of emission deterioration estimates. The 

Nonroad Engine and Emissions Modeling Team (NEEMT) welcomes comments and suggestions 

concerning our approach to modeling nonroad engine emissions deterioration. 

II. Background 

As used here, the term “deterioration” refers to the degradation of an engine’s exhaust 

emissions performance over its lifetime due to normal use or misuse (i.e., tampering or neglect). 

Engine deterioration increases exhaust emissions, usually leads to a loss of combustion 

efficiency, and can in some cases increase nonexhaust emissions. The amount of emissions 

increase depends on an engine’s design, production quality, and technology type (e.g., spark 

ignition two-stroke and four-stroke, compression ignition). Other factors, such as the various 

equipment applications in which an engine is used, usage patterns, and how it is stored and 

maintained, may also affect deterioration. 

The term “deterioration rate” refers to the degree to which an engine’s emissions increase 

per unit of activity. Nonroad engine activity is expressed in terms of hours of use or fraction of 

median life. The term “deterioration factor” refers to the ratio of an engine’s emissions at its 

median life divided by its emissions when new. 

The terms useful life and median life are used in the following manner in this report in 

order to avoid confusion. Useful life is a regulatory term used to indicate the amount of time 

during the life of a nonroad engine that a manufacturer must certify to the U.S. EPA that the 

engine meets a required emission standard as defined by a regulation. Median life, as used in this 

1
 
report, refers to the age at which 50 percent of the engines sold in a given year have ceased to 

function and have been scrapped.1 

III. Core Challenge 

The core challenge associated with estimating nonroad engine deterioration is the 

development of reasonably accurate deterioration rates for the enormous range of nonroad engine 

types and applications from the limited amount of nonroad emission deterioration data that exist 

at this time. To estimate deterioration, the emission performance of engines at various ages is 

required. Such information can be obtained from a longitudinal study that examines the same set 

of engines periodically as they age, or from a sampling study that tests engines of various ages 

but the same basic design. In either case, the engines studied should be selected randomly from 

the population of engines actually being used in the field. 

Given the limited available test data, the NEEMT is currently unable to develop unique 

deterioration rates based on actual engine test data for the myriad of applications and power 

levels covered by NONROAD. The Office of Mobile Sources has conducted emissions testing 

on several dozen small spark ignition lawn & garden engines and a few large compressionignition engines. The nonroad engine industry and a few States have also conducted some 

nonroad engine emissions testing. However, the nonroad engine emissions data currently 

available are still limited when compared to the large number of nonroad engine types and 

applications for which these engines are used, particularly for the purposes of evaluating 

emission deterioration as engines age. The EPA has obtained extensive data on the emissions 

deterioration of light-duty highway engines, but these engines are unlikely to deteriorate in a 

fashion typical of nonroad engines due to fundamental differences in engine and emission control 

technology design, maintenance, and operation. Deterioration in emissions from light-duty 

vehicles (LDVs) is thought to be due in large part to gross failures of emission control aftertreatment systems, which nonroad engines do not have at this time. 

A related challenge is that EPA has essentially no data on the incidence of tampering 

and/or neglect of proper maintenance and only limited data that distinguish the effect of such 

malmaintenance from the effects of normal usage. These data are based on emission tests of two 

lawnmower engines that had various components, including the sparkplug, air filter, and oil, 

manipulated to simulate bad maintenance practices (i.e., not changing the sparkplug, air filter and 

oil on a regular basis, as recommended by the manufacturer). The results of this effort were 

inconclusive, suggesting that intentional disablement of engine components does not adequately 

simulate emissions deterioration from normal usage. The NEEMT requests that state and 

industry stakeholders share any data regarding the incidence of tampering and neglect of proper 

maintenance they may have with EPA. The NEEMT also requests that stakeholders share any 

data they have regarding the relationship between emissions deterioration due to normal usage 

and emissions deterioration due to intentional disablement of engine components with EPA. 

2
 
This report explicitly addresses the compatibility of NONROAD’s estimates of 

deterioration with the estimates of deterioration produced by other nonroad emission models. 

These models include the nonroad compression ignition regulatory Nonroad Emissions Model 

(NEM), the Phase 1 Nonroad Small Engine Emission Model (NSEEM1), the Phase 2 Nonroad 

Small Engine Emission Model (NSEEM2), and the California Air Resources Board (ARB) 

OFFROAD model. EPA used the NEM to support the proposed compression ignition (diesel) 

engine rule, the NSEEM1 to support the Phase 1 Small Spark Ignition Engine Rule, and the 

NSEEM2 to support the proposed Phase 2 Small Spark Ignition Engine Rule. During the 

development of NONROAD, the NEEMT has used many of the same data sources and modeling 

assumptions used in these earlier models. Once the final version of NONROAD is released, the 

EPA expects to rely on NONROAD as its primary emission inventory model for future 

rulemaking and inventory modeling activities. 

A discussion of the calculation methods used in each of the earlier models can be found 

in Section IV, Methods of Applying Deterioration. The specific deterioration inputs used in 

NONROAD and their basis will be addressed for spark ignition engines at or below 25 

horsepower; over 25 horsepower; liquid petroleum gas (LPG) and compressed natural gas 

engines (CNG), and compression ignition (diesel) engines in subsequent sections. 

IV. Methods of Applying Deterioration 

A. NONROAD Method 

Generally, the NONROAD model addresses the effects of deterioration in the 

inventory calculation by multiplying a zero hour emission factor for each category of engine 

by a deterioration rate as the engine ages. The following formula describes the basic form of 

the calculation: 

EFaged = EFo * DF (1) 

where: EF (aged) is the emission factor for an aged engine 

EFo is the emission factor for a new engine 

DF is the deterioration factor. 

In order for the NONROAD model to be compatible with the EPA’s small nonroad 

spark ignition engine rulemaking process and also be able to calculate deterioration for other 

engines, the NEEMT has derived the following multi-purpose deterioration function: 

DF = 1 + A * (Age Factor)b for Age Factor ' 1 (2) 

DF = 1 + A for Age Factor > 1 

3
 
where Age Factor =	 [Cumulative Hours * Load Factor]
 

Median Life at Full Load, in Hours
 

A, b = constants for a given technology type; b ' 1. 

The constants A and b can be varied to approximate a wide range of deterioration 

patterns. “A” can be varied to reflect differences in maximum deterioration. For example, 

setting A equal to 2.0 would result in emissions at the engine’s median life being three times 

the emissions when new. The shape of the deterioration function is determined by the second 

constant, “b.” This constant can be set at any level between zero and 1.0; currently, the 

NONROAD model sets “b” equal to either 0.5 or 1.0. The first case results in a deterioration 

curve in which most of the deterioration occurs in the early part of an engine's life; the second 

case results in a linear deterioration pattern, in which the rate of deterioration is constant 

throughout the median life of an engine. In both cases, the NEEMT decided to cap 

deterioration at the end of an engine's median life, under the assumption that an engine can 

only deteriorate to a certain point beyond which it becomes inoperable. For spark ignition 

engines at or below 25 horsepower, NONROAD uses the regulatory useful life values in 

Appendix F of the Phase 1 regulatory support document for median life values. For other 

engines, NONROAD uses the median life values from the Power Systems Research (PSR) 

database.2 These functions can be used to provide a close approximation to the shape of the 

deterioration curves used in NSEEM1 and NSEEM2 for spark ignition engines less than 25 

horsepower. 

SI engines have a wide range of designs that affect their emissions deterioration. To 

model these different deterioration patterns, NONROAD categorizes SI engines into 

“technology types” by their design and emission control equipment. A given technology type 

can apply to one or more horsepower-application categories, and a given horsepowerapplication category can be divided into more than one technology type. NONROAD applies 

a given deterioration function (that is, a given A and b value) to all engines of a given 

technology type, regardless of their application or power range. As a result, a single 

technology type may be applied to engines with very different median lives, but this 

difference is handled by expressing engine age in terms of the “Age Factor” defined above. 

The NEEMT believes this approach is reasonable, since deterioration patterns should be 

more closely related to the design of the engine and its emission control technology than to 

the kind of application in which it is used. Furthermore, the available data on emissions 

deterioration of nonroad SI engines is insufficient to develop separate deterioration functions 

for the many combinations of application, horsepower range, and technology type. 

NONROAD’s technology type feature allows each horsepower-application category 

to be divided into as many as ten technology types, each with its own deterioration pattern. 

The technology type feature gives the model flexibility to handle the full range of engine 

designs used in nonroad equipment. For example, the technology type feature can handle the 

33 distinct engine types that are defined by EPA’s Phase 1 and proposed Phase 2 Small 

4
 
Engine Rules, as shown in Tables 1-5. However, deterioration data for each technology type 

across different applications are not available at the present time. Thus, the NONROAD 

model does not apply different deterioration patterns to engines of the same technology type 

used in different applications. Instead, the model applies different deterioration patterns to 

engines within each engine type (i.e., two-stroke, four-stroke spark ignition, compression 

ignition) based on the more detailed engine classes defined in the Phase 1 and proposed 

Phase 2 Small Engine Rules instead of by application. In other words, NONROAD models 

deterioration for a tiller and a lawn mower equipped with engines of the same technology 

type using the same deterioration pattern. 

B. Methods of Applying Deterioration in Other Models 

1.	 California ARB OFFROAD Model 

The California ARB OFFROAD model applies deterioration based on a linear 

relationship. Emissions are modeled to increase by a fixed amount for every hour that 

the engine is used, up to a fixed limit. The function used by OFFROAD is as follows: 

EFaged = EF + DR * CumHours new at in-use load 

where EFaged = deteriorated emission factor 

EFnew = zero hour emission factor 

DR = fraction of the maximum deterioration added to the zero 

hour emission factor (EFnew) for each hour of age the engine 

accumulates 

CumHours at in-use load = Engine Age * Hours of use per year 

Without the emission rate cap, the linear function presented above could show an 

engine deteriorating indefinitely beyond one median life. This projection may not be 

a realistic or accurate one, since an engine can only deteriorate to a certain point 

beyond which it ceases to function and is scrapped. Like the NONROAD model, the 

OFFROAD model reflects the limit on how much an engine’s emissions can 

deteriorate before the engine ceases to function by capping deterioration once an 

engine has reached one median life. 

2.	 Phase 1 Nonroad Small Engine Emission Model (NSEEM1) for Small Gasoline Four 

and Two-Stroke (<25 hp) Engines 

The Phase 1 Small Engine Rule calculates deterioration using the following 

exponential function for both two and four-stroke spark ignition engines: 

DF = 1 + [(EFf

 - EFo)/EFo] * [1 - exp(-3 * {Age Factor})] (3) 

5
 
where EFf = emission factor for a fully aged engine

 EFo = zero hour emission factor

 Age = Age in years/Median life in years. 

The deterioration function (3) in the NSEEM1 model results in 95% of the 

deterioration occurring at an age of one median life. As stated above, the equation (2) 

used in NONROAD closely approximates the Phase I deterioration curve, but caps 

deterioration after one median life. The relationship between the deterioration in 

NONROAD and NSEEM1 at one median life is given below. 

A = [(EFf - EFo)/EFo] 	 (4) 

for Age Factor = 1 

3.	 Proposed Phase 2 Nonroad Small Engine Emission Model (NSEEM2) for Small 

Gasoline Four and Two-Stroke (< 25 hp) Engines 

The proposed Phase 2 Small Engine Rule and its supporting model 

(NSEEM2) use the following equations to calculate deterioration for small two and 

four-stroke engine HC, CO, and NOx emissions. It should be noted that the following 

deterioration functions were used to recalculate the original Phase 1 deterioration 

rates as well as calculate the Phase 2 deterioration rates. Also, particulate matter (PM) 

emissions and deterioration were not considered in either the first or second phase of 

the Small Engine Rule. 

The deterioration function for spark ignition two stroke engines is linear and is 

given by equation (5): 

DF = 1 + C * (Hourstotal) 	 (4) 

where C = A constant derived from a best-fit regression based on EPA 

and manufacturer test data3 

Hourstotal = Engine age * Annual hours of use 

For four-stroke engines, the deterioration function is a square root function 

and is given by equation (6): 

DF = 1 + C * (Hourstotal)

0.5	 (6) 

where C = A constant derived from a best-fit regression based on 

manufacturer test data 

Hourstotal = Engine Age * Annual hours of use 

6
 
This function produces a curvilinear plot similar in shape to the NSEEM1/Phase 1 

curve, where most of the deterioration occurs toward the early part of an engine’s 

median life. This function is based on manufacturer’s emissions testing data from 

engines undergoing accelerated aging in the laboratory and in the field.4 

4.	 Compression-Ignition Nonroad Engine Model (NEM) 

The NEM does not contain a deterioration function, since EPA assumes that 

compression ignition engines do not significantly deteriorate. This assumption is 

discussed in more detail later in this report, under the section addressing compression 

ignition engines. 

C. Using the NONROAD Deterioration Function to Represent Other Deterioration 

Patterns 

The NONROAD deterioration function can be modified to give results consistent 

with the Phase 1 and proposed Phase 2 Small Engine Rules. The NONROAD 

deterioration function also has the ability to describe other curvilinear or linear 

deterioration patterns, such as the California ARB deterioration rates, by changing the 

inputs for the function in equation (2). 

V.	 Deterioration Inputs For Engines At or Below 25 Horsepower 

A.	 Deterioration Inputs Used In NONROAD For Spark Ignition Engines Less than 25 

Horsepower 

The NONROAD deterioration function is given by equation (2): 

DF = 1 + A * (Age Factor)b for Age Factor ' 1 

DF = 1 + A	 for Age Factor > 1 

where Age Factor =	 [Cumulative Hours * Load Factor]
 

Median Life at Full Load, in Hours
 

A, b = constants for a given technology type; b ' 1. 

In NONROAD, the constant ‘b’ is set at 0.5 for four-stroke engines, resulting in a 

square root relationship between age and deterioration. The constant ‘b’ is set at 1.0 for 

two-stroke engines, which produces a linear relationship between age and deterioration. 

This use of a curvilinear deterioration pattern for four-stroke engines and a linear 

deterioration pattern for two-stroke engines is similar to the approach used in the 

NSEEM2 model. 

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The inputs for the variable ‘A’ of the NONROAD deterioration function are 

shown in Tables 1-5 for the small engine classes defined in the Phase 1 and proposed 

Phase 2 Small Engine Rules. These values were derived from the values provided in the 

Phase I Regulatory Support Document for maximum life emission factors and new engine 

emission factors. For each pollutant and each engine type, variable ‘A’ represents the 

maximum deterioration rate reached at one median life. It should be noted that 

particulate matter (PM) standards were not considered or included in the Phase 1 and 

proposed Phase 2 Small Engine Rules. For the present time, PM deterioration in fourstroke engines will be equated to that of HC in NONROAD, and PM deterioration for 

two-stroke engines will assumed to be zero. The NEEMT believes this assumption is 

reasonable, since two-stroke engines are unlikely to experience the high PM deterioration 

rates seen in four-stroke engines given the high PM emission rates of new two-stroke 

engines. The NEEMT requests stakeholders with information about the PM emissions 

deterioration of two-stroke engines to submit such data to EPA. 

There are some small engine applications that are not covered by the Phase 1 or 

proposed Phase 2 Small Engine Rules. These include marine engines (SCC 

2282xxxxxx) and certain recreational equipment such as snowmobiles (226x001020), 

off-road motorcycles and all-terrain vehicles (226x001030), and specialty vehicle carts 

(226x001060). In NONROAD the two-stroke versions of the recreational equipment 

engines are assigned deterioration values equal to the G2N2 tech type shown in Table 2, 

but they use a tech type name of R12S since the emission factors differ from the other 

engine applications. Four-stroke versions of these recreational equipment engines are 

simply included in the G4N2O tech type since there are no different emission factors. 

Recreational marine engines are handled differently from the recreational 

equipment engines. Based on information gathered for the recreational marine engine 

rulemaking (61 FR 52087, October 4, 1996), two-stroke marine engines are modeled as 

having no deterioration for the same reasons mentioned above regarding particulate 

matter (PM) emissions. We request comment on whether this should be changed to 

model two-stroke marine engine deterioration similarly to other two-stroke engines or 

possibly use some other deterioration rate. NONROAD assigns four-stroke marine 

engines deterioration values equal to the G4N2O tech type shown in Table 2, but they use 

different tech type names such as M3, M10, M11, M12, and M16 depending on the 

specific control technology and application (outboard, personal water craft or inboard). 

8
 
Table 1
 

Class 1 (Displacement < 225 cc) Nonhandheld Deterioration Factor A
 

Engine Tech Type HC CO NOx PM BSFC 

G2N1 (gas 2-stroke nonhandheld Class 1, baseline) 0.201 0.199 0 0 0 

G4N1S (gas, side-valve, 4-stroke nonhandheld Class 1, 

baseline) 

1.1 0.9 -0.6 1.1 0 

G4N1O (gas, overhead valve, 4-stroke nonhandheld Class 1, 

baseline) 

1.1 0.9 -0.6 1.1 0 

G2N1 (2-stroke, Phase 1) 0.266 0.231 0 0 0 

G4N1S1 (Phase 1 side-valve, 4-stroke) 5.103 1.109 -0.33 5.103 0 

G4N1O1 (Phase 1 overhead valve, 4-stroke) 1.753 1.051 -0.30 1.753 0 

G4N1SC1 (Phase 1 side-valve, 4-stroke with catalyst) 5.103 1.109 -0.33 5.103 0 

G4N1S2 (Phase 2 side-valve, 4-stroke) 5.103 1.109 -0.33 5.103 0 

G4N1O2 (Phase 2 overhead valve, 4-stroke) 1.753 1.051 -0.30 1.753 0 

Table 2 

Class 2 (Displacement :225 cc; Power Rating < 25 hp) Nonhandheld Deterioration Factor A
 

Engine Tech Type HC CO NOx PM BSFC 

G2N2 (gas 2-stroke nonhandheld Class 2, baseline) 0.201 0.199 0 0 0 

G4N2S (gas, side-valve, 4-stroke nonhandheld Class 2, 

baseline) 

1.1 0.9 -0.6 1.1 0 

G4N2O (gas, overhead valve, 4-stroke nonhandheld Class 2, 

baseline) 

1.1 0.9 -0.6 1.1 0 

G4N2S1 (Phase 1 side-valve, 4-stroke with catalyst) 1.935 0.887 -0.274 1.935 0 

G4N2O1 (Phase 1 overhead valve 4-stroke) 1.095 1.307 -0.599 1.095 0 

G4N2S2 (Phase 2 side-valve) 1.935 0.887 -0.274 1.935 0 

G4N1O2 (Phase 2 overhead valve) 1.095 1.307 -0.599 1.095 0 

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Table 3 

Class 3 (Displacement < 20cc) Handheld Deterioration Factor A
 

Engine Tech Type HC CO NOx PM BSFC 

G2H3 (gas 2-stroke handheld Class 3, baseline) 0.2 0.2 0 0 0 

G2H31 (Phase 1) 0.24 0.24 0 0 0 

G2H3C1 (Phase 1 with catalyst) 0.24 0.24 0 0 0 

G2H32 (Phase 2) 0.24 0.24 0 0 0 

G2H3C2 (Phase 2 with catalysts) 0.24 0.24 0 0 0 

Table 4
 

Class 4 (20cc < Displacement < 50 cc) Handheld Deterioration Factor A
 

Engine Tech Type HC CO NOx PM BSFC 

G2H4 (gas 2-stroke handheld Class 4, baseline) 0.2 0.2 0 0 0 

G2H41 (Phase 1) 0.29 0.24 0 0 0 

G2H4C1 (Phase 1 with catalyst) 0.29 0.24 0 0 0 

G4H41 (Phase 1 4-stroke) 1.1 0.9 -0.6 1.1 0 

G2H42 (Phase 2) 0.29 0.24 0 0 0 

G2H4C2 (Phase 2 with catalyst) 0.29 0.24 0 0 0 

G4H42 (Phase 2 4-stroke) 1.1 0.9 -0.6 1.1 0 

Table 5
 

Class 5 (Displacement : 50cc; Power Rating <25 HP) Handheld Deterioration Factor A
 

Engine Tech Type HC CO NOx PM BSFC 

G2H5 (gas 2-stroke handheld Class 5, baseline) 0.2 0.2 -0.031 0 0 

G2H51 (Phase 1) 0.266 0.231 0 0 0 

G2H5C1 (Phase 1 with catalyst) 0.266 0.231 0 0 0 

G2H52 (Phase 2) 0.266 0.231 0 0 0 

G2H5C2 (Phase 2 with catalyst) 0.266 0.231 0 0 0 

B.	 Other Sources of Deterioration Rates for Spark Ignition Engines At or Below 25 

Horsepower 

10 
The NEEMT considered several sources of information regarding deterioration 

factors. These sources included the Nonroad Engine and Vehicle Emissions Study5 

(NEVES), the technical documentation for the California ARB OFFROAD model, and 

the small engine regulatory impact analysis and support documents for the Phase 1 and 

proposed Phase 2 Small Spark Ignited Engine Rules. 

1. California ARB 

To estimate deterioration rates for pre-control, Tier 1, and Tier 2 two and fourstroke engines, the California ARB uses deterioration factors for HC, CO, and NOx 

derived from analysis of manufacturer emissions data submitted to EPA for 

consideration during the regulatory negotiation process for small engine standards, as 

well as additional manufacturer data submitted to the California ARB during its small 

engine rulemaking process.6,7 These data include the testing of engines that have 

undergone accelerated aging under both field and laboratory conditions. The 

deterioration rates for two-stroke engines appear in Table 6a and the rates for fourstroke engines appear in Table 6b. It should be noted that the four-stroke 

deterioration rates are weighted averages of the deterioration rates for side and 

overhead valve engines. At this time, the NEEMT has not received sufficient 

information from the California ARB to show the separate deterioration rates for side 

and overhead valve engines. However, all other things being equal, side-valve 

engines usually deteriorate significantly more than overhead valve engines, based on 

tests conducted by both EPA and the Equipment Manufacturers Association (EMA). 

Table 6a
 

Small Engine (<25hp) Spark Ignition Two-Stroke OFFROAD Deterioration Rates
 

Standards Model Yr. HP HC DR CO DR NOx DR PM DR 

Uncontrolled pre-95 0-2 0 0 0 0 

Tier 1 1995-99 0 0 0 0 

Tier 2 2000+ 0.2513 0 0 0.01117 

Uncontrolled pre-95 2-15 0 0 0 0 

Tier 1 1995-99 0 0 0 0 

2000+ 0.2513 0 0 0.00808 

Uncontrolled pre-95 15-25 0 0 0 0 

Tier 1 1995-99 0 0 0 0 

Tier 2 2000+ 0.2513 0 0 0.00808 

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Table 6b
 

Small Engine (<25hp) Spark Ignition Four-Stroke OFFROAD Deterioration Rates
 

Standards Model Yr. HP HC DR CO DR NOx DR PM DR 

Uncontrolled pre-95 0-5 0.0948 0.5196 0.0002 0.0026 

Tier 1 1995-96 0.0565 -0.067 0.0031 0.0026 

1997-99 0.0565 -0.067 0.0031 0.0026 

Tier 2 2000-03 0.0144 -0.3849 0.0065 0.0026 

Uncontrolled pre-95 5-15 0.0178 0.0337 0.0013 0.0002 

Tier 1 1995-96 0.0207 0.0895 0 0.0002 

1997-99 0.0207 0.0895 0 0.0002 

Tier 2 2000-03 0.0047 0 0.0035 0.0002 

Uncontrolled pre-95 15-25 0.0141 0.0276 0.0011 0.0002 

Tier 1 1995-96 0.0166 0.0345 0 0.0002 

1997-99 0.0166 0.0345 0 0.0002 

Tier 2 2000-03 0.0049 0 0.0032 0.0002 

2. EPA Phase 1 Small Engine Model (NSEEM1) 

For both uncontrolled and controlled four-stroke spark ignition engines, the 

maximum deterioration derived from the NSEEM1 model occurs at the end of one 

median life for HC, CO, and NOx. The deterioration rates in the two models, 

expressed in terms of the absolute increase in g/bhp-hr emission rates over the course 

of the engine’s life, are the same and can be found in the regulatory impact analysis 

and support document for EPA’s Phase 1 Small Engine Rule for engines at or below 

25 horsepower (19 kw).8 The NSEEM1 model uses the same deterioration rates 

found in the NEVES report for small four-stroke engines less than 20 horsepower 

(discussed in section 4 below).9

 These deterioration rates were originally based on 

tests done by Southwest Research Institute (SwRI) on three different four-stroke 

engines.

 The two-stroke spark-ignition deterioration factors for HC, CO, and NOx 

used by the Phase 1 Small Engine Rule Team used can be found in the regulatory 

impact analysis and support document for EPA’s Phase 1 Small Engine Rule for 

engines at or below 25 horsepower (19 kw). The two-stroke engine deterioration 

factors represent the ratio between the emission rates at one median life (when 

emissions reach their maximum level) and the emission rates at zero hours of age. 

The two-stroke maximum deterioration rates are 1.2, 1.9, and 1.0 for HC, CO, and 

NOx, respectively. This translates into a 20 percent HC increase, a 90 percent CO 

increase, and no change in NOx emissions over the engine’s life. The deterioration 

rates used in the Phase 1 Small Engine Rule were based on information from SAE 

technical papers and industry-supplied data. The OMS analysis of this information 

during the development of the Phase 1 Small Engine Rule supported changing the 

12
 
deterioration rates for two-stroke spark ignition engines from the ones found in the 

NEVES report. 

3. EPA Phase 2 Small Engine Model (NSEEM2) 

As discussed above, the Phase 2 Small Engine Model uses a linear function 

for two-stroke engines and a curvilinear square root function for four-stroke engines 

to define deterioration as the engine ages. The Small Engine Rule Team used these 

functions to derive deterioration rates for Phase 2 small engines, and they also revised 

the Phase 1 deterioration rates originally calculated for the Phase 1 Small Engine 

Rule. However, the uncontrolled deterioration rates remain unchanged from the 

original ones used in the Phase 1 Small Engine Rule. 

As shown above in the “Methods of Applying Deterioration” section (Section 

IV.A), the two and four-stroke Phase 2 deterioration functions employ a unique 

constant for HC, CO, and NOx for each class of small engines, as shown in Table 7. 

Table 7
 

Constants Used in the Phase 2 Small Engine Rule Deterioration Function
 

G2H3 G2H4 G2H5 G2N1 G2N2 G4N10 G4N1S G4N20 G4N2S G4H4 

Phase 1 

Res. 

HC 0.002 0.002 0.002 0.000 0.000 0.050 0.1300 0.0800 0.0400 0.1840 

CO 0.002 0.002 0.002 0.002 0.000 0.022 0.0220 0.0110 0.0110 0.0020 

NOx 0.000 0.000 0.000 0.000 0.000 0.030 0.0200 0.0200 0.0200 0.0230 

Phase 1 

Com. 

HC 0.0003 0.0003 0.0003 0.000 0.000 0.0245 0.0637 0.0566 0.0283 0.0751 

CO 0.0003 0.0003 0.0003 0.002 0.000 0.0110 0.0110 0.0080 0.0080 0.0008 

NOx 0.0000 0.0000 0.0000 0.000 0.000 0.0147 0.0098 0.0141 0.0141 0.0094 

Phase 2 

Res. 

HC 0.002 0.002 0.002 0.000 0.000 0.050 0.1300 0.0280 0.0400 0.1840 

CO 0.002 0.002 0.002 0.000 0.000 0.022 0.0220 0.0110 0.0110 0.0020 

NOx 0.000 0.000 0.000 0.000 0.000 0.030 0.0200 0.0060 0.0200 0.0230 

Phase 2 

Com. 

HC 0.0003 0.0003 0.0003 0.000 0.000 0.0245 0.0637 0.0198 0.0283 0.0751 

CO 0.0003 0.0003 0.0003 0.000 0.000 0.0110 0.0110 0.0080 0.0080 0.0008 

NOx 0.0000 0.0000 0.0000 0.000 0.000 0.0147 0.0098 0.0042 0.0141 0.0094 

13
 
The other input used in the deterioration functions is the median life in hours 

for each equipment type. This value is derived by multiplying the annual hours of 

activity by the half-life (B50 value) in years of each equipment type. The half-life in 

years is the point on the NSEEM2 scrappage curve when 50 percent of the equipment 

from a given model year are no longer functioning. These values are included in 

Tables F-05 by F-06 in the Phase 2 Small Engine Rule Regulatory Support Document 

and are reprinted here in Table 8.10 

14
 
Table 8
 

Phase 2 Small Engine Rule Average Annual Use and Load Factor
 

Equipment Use Hr/Year*Load Factor Hr/Yea 

r 

B50 

LN MOWERS res 8.38 25.4 5.8 

LN MOWERS prof 134.12 406.42 2 

TRIM/EDGE CUTTER res 4.55 9.1 4.3 

TRIM/EDGE CUTTER prof 68.64 137.29 2.3 

CHAINSAWS res 6.25 12.5 4.3 

CHAINSAWS prof 151.25 302.5 0.9 

LEAF BLOW/VAC res 4.8 9.59 4.3 

LEAF BLOW/VAC prof 141.14 282.29 2.3 

GENERATOR SETS res 6.08 8.95 5.8 

GENERATOR SETS prof 97.36 143.18 2.3 

TILLERS res 6.69 16.73 5.8 

TILLERS prof 188.64 471.6 4.4 

SNOWBLOWERS res 2.97 8.48 4.4 

SNOWBLOWERS prof 47.49 135.68 4.4 

COMM TURF res NA NA NA 

COMM TURF prof 340.85 681.69 2.9 

REAR ENG RIDER res 13.5 35.54 5.8 

REAR ENG RIDER prof 216.07 568.6 2.9 

LN/GROUND 

TRACTOR 

res 19.84 45.08 5.8 

LN/GROUND 

TRACTOR 

prof 317.37 721.31 2.9 

PUMPS res 9.34 13.54 5.8 

PUMPS prof 149.44 216.58 2.3 

ALL OTHER 

EQUIPMENT 

prof 7.3 14.61 5.8 

ALL OTHER 

EQUIPMENT 

res 116.84 233.68 2.3 

4. Nonroad Engine and Vehicle Emissions Study (NEVES) 

15
 
Four-Stroke SI Engines 

The deterioration rates used in the NEVES for four-stroke engines at or below 

20 horsepower were applied to zero-hour emission factors to generate an in-use 

emission factor. These deterioration rates were derived from tests done by Southwest 

Research Institute (SwRI) on three four-stroke engines.11 A table of the SwRI testing 

results can be found in Appendix 1 of this report. For four-stroke engines, the 

maximum deterioration factor at the engine’s average useful life based on the SwRI 

tests are multiplicative factors of 2.1, 1.9, and 0.4 for exhaust hydrocarbons (HC), 

carbon monoxide (CO), and oxides of nitrogen (NOx), respectively (i.e., 110 percent 

increase for HC, 90 percent increase for CO, and 60 percent decrease for NOx at the 

median life of this type of engine). The particulate matter (PM) deterioration factor 

taken from the NEVES report is 3.6, which translates into a 260 percent increase in 

emissions at the median life for this engine category.12 The decrease in NOx 

emissions was not seen as being unreasonable, given that wear and tear on small 

engines tends to cause the fuel to air ratio to become more fuel-rich, thereby 

increasing products of incomplete combustion such as hydrocarbons, particulate 

matter, and carbon monoxide emissions while suppressing NOx formation.13 The 

authors of NEVES assumed that four-stroke LPG and CNG engines deteriorated the 

same amount as their gasoline counterparts, since design differences between the 

gasoline and LPG/CNG engines were negligible. 

Two-Stroke SI Engines 

With the exception of recreational marine engines, the NEVES report equated 

all two-stroke SI engine deterioration rates for HC and CO to the four-stroke SI 

engine deterioration rates in NEVES for below-25 horsepower (19 kw) engines. The 

authors of NEVES used this approach because the available two-stroke SI emissions 

testing data from Southwest Research Institute (SwRI) provided inconclusive results. 

As discussed in the NEVES report, SwRI tested two small (at or below 20 hp), aged 

two-stroke SI engines , but these results differed widely for HC and CO. The SwRI 

deterioration data for small four-stroke SI engines fell between the results for the two 

two-stroke engines, and the authors of the NEVES report decided to use this fourstroke deterioration rate for all small SI engines (both two- and four-stroke). This 

approach was considered to be preferable to basing the small two-stroke engine 

deterioration estimates on two data points that differed so greatly. For two-stroke 

recreational marine engines the NEVES report used a multiplicative deterioration 

factor of 1.2 (20 percent) that was based on data submitted by the National Marine 

Manufacturers Association (NMMA). 

For NOx, both of the two-stroke SI engines tested by SwRI showed almost 

equal or slightly higher emissions compared to new engine emission factors for twostrokes. These consistent test results prompted NEVES’ authors to use a deterioration 

16
 
factor of 1.0 (zero percent deterioration) for NOx emissions from all small two-stroke 

SI engines (including recreational marine engines), instead of using the four-stroke SI 

engine NOx deterioration factor. 

For PM, the NEVES report set the PM deterioration factors for all two-stroke 

spark-ignition engines to a multiplicative factor of 1.0 (zero percent) due to the 

perceived inappropriateness of applying the four-stroke spark-ignition deterioration 

rate (3.6 or 260 percent over the median life of the engine) to two-stroke engines. PM 

emission rates from new two-stroke SI engines are already so high that the NEVES 

authors believed that they could not experience such a large increase in emissions and 

remain operational. In the absence of other information, they chose to assume no 

deterioration in PM emissions. A copy of the table containing these deterioration 

rates can be found in Appendix 2. 

C. Discussion of Deterioration Factor Data for Spark-Ignition Engines At or Below 25 

Horsepower 

The deterioration rates used in NONROAD approximate the levels of 

deterioration found in testing, including the testing summarized in NEVES and the testing 

done to support EPA’s Phase 1 and proposed Phase 2 Small Engine Rules. Where these 

test results differ, the NEEMT has chosen to give greater weight to data taken from 

engines which have experienced usage patterns that reflect expected field conditions. The 

test data submitted to EPA for the proposed Phase 2 Small Engine Rule, for example, 

reflects testing of engines that have undergone accelerated aging which the NEEMT does 

not believe to be representative of the aging experienced by engines in use. After 

evaluating all available data, the NEEMT has determined that the level of deterioration 

used in NSEEM1 and Phase 1 Small Engine Rule provides a reasonable basis for the 

deterioration rates used in NONROAD. These deterioration rates are generally higher 

than the deterioration rates used for regulatory purposes in NSEEM2 and the proposed 

Phase 2 Small Engine Rule, but are generally smaller than those used in NEVES. The 

NEEMT believes that the deterioration rates used in NONROAD are more reflective of 

the deterioration rates that one would expect to find out in the field when equipment 

powered by small spark ignition engines is used by the average person than are the 

deterioration rates found in NSEEM2 and the proposed Phase 2 Small Engine Rule. 

VI. Deterioration Inputs for Spark Ignition Engines Greater than 25 Horsepower 

A. Deterioration Factors Used in NONROAD for Spark-Ignition Engines Greater Than 

25 Horsepower (19 kilowatts) 

Currently, there are few emission testing data concerning dedicated nonroad fourstroke spark ignition engines over 25 horsepower. Both the NEVES and the California 

ARB used deterioration rates for these types of engines that originated from on-highway 

17
 
spark-ignition engine emissions data. These approaches will be discussed later in this 

document. 

For the purpose of the draft release of NONROAD, the NEEMT has decided to 

equate deterioration of four-stroke SI engines over 25 horsepower to that of the Class 2, 

non-handheld four-stroke engines, which are the largest class of engines found in the 

Phase 1 and proposed Phase 2 rules. Since four-stroke engines over 25 horsepower are 

not currently regulated, have overhead valve technology, and have no catalytic technology 

applied to them, the NEEMT will use the baseline deterioration factors found in Table 2 

listed next to the label “G4N2O” (i.e., gasoline, four-stroke, non-handheld, Class 2, 

overhead valve engines). The deterioration rate for the large ones will be curvilinear, 

using 0.5 for the ‘b’ constant, as is the case for small four-stroke engines. 

Two-stroke SI engines over 25 horsepower are also unregulated at this time. For 

these engines, the NEEMT has decided to use the pre-control deterioration inputs being 

used in NONROAD for the largest class of small engines in the Phase 1 and proposed 

Phase 2 Small Engine Rules. These can be found in table 2 next to the label “G2N2" 

(i.e., gasoline, two-stroke, non-handheld, Class 2 engines). The deterioration rate for the 

large ones will be linear, using 1.0 for the ‘b’ constant in the NONROAD deterioration 

function, as is the case for small two-stroke engines. 

B.	 Other Sources of Deterioration Factors for SI Engines Greater than 25 Horsepower 

1.	 NEVES Deterioration Factors for Gasoline Spark Ignition Engines Greater than 20 

Horsepower 

The data for the large four-stroke SI deterioration rates in the NEVES report 

were developed by a 1983 joint testing program by EPA and the Equipment 

Manufacturers Association (EMA) to test heavy-duty on-highway engines, including 

both spark-ignition and compression-ignition engines. For heavy-duty, four-stroke, 

spark-ignition engines, the program used an unknown number of 1979 to 1982 precontrolled highway engines. EMA’s calculation of the deterioration rates used 

regression equations based upon a number of assumptions, most notably that the 

engines had logged 55,000 miles. Based on this assumption, the engines had reached 

the halfway point of their median life in relation to the existing heavy-duty onhighway gasoline engine regulations, which defined the median life of such engines to 

be 110,000 miles. Deterioration rates for HC and CO were calculated by dividing 

in-use engine emissions by those of new heavy duty gas engines. The deterioration 

factor for HC was calculated to be 1.5 (a 50 percent increase), while the deterioration 

factor for CO was calculated to be 1.3 (a 30 percent increase). NOx emissions did not 

show a significant increase in emissions, so a deterioration factor of 1.0 (zero percent 

increase) was used. No testing was done to measure PM deterioration. The NEVES 

report assumed that large nonroad four-stroke SI engines experienced similar 

18
 
 

deterioration patterns, in part because of the similarities in engine design between 

such nonroad engines and the uncontrolled highway engines tested by EMA and EPA. 

The NEVES report equated the large two-stroke spark ignition deterioration 

rates to the deterioration rates for the greater than 20 horsepower (19 kW) four-stroke 

spark-ignition engines in NEVES for HC, CO, and NOx. As stated in section V.B.4, 

the NEVES report assumed no deterioration in PM emissions for all two-stroke spark 

ignition engines. 

2.	 California ARB's Deterioration Factors for Spark Ignition Nonroad Engines Greater 

than 25 Horsepower 

The California ARB nonroad model, OFFROAD, uses deterioration rates for 

gasoline four-stroke spark-ignition engines over 25 horsepower that are based on precontrol on-highway engine deterioration rates found in EMFAC7E, the California 

ARB on-highway emissions model. These deterioration rates, in the form of percent 

deterioration per percent of median life, are shown in Table 9.14 The California ARB 

matched nonroad and on-highway engines as closely as possible by horsepower and 

used a ratio to convert the on-highway deterioration rates based on gram/10,000 miles 

traveled to rates based on number of hours an engine has been used. This ratio is 

shown below, with the median life for off-road engines expressed in terms of hours 

and the median life for on-highway engines expressed in terms of miles traveled. The 

deterioration factors are expressed as the percent increase in emissions per percent of 

median life. Since there were no equivalent on-highway engines for the 25 to 50 

horsepower category of nonroad engines, the California ARB set the deterioration rate 

for these engines equal to that used for the 50 to 120 horsepower engines. No data 

were available for PM. 

Off-Road d.f. * Median Life = On-Highway d.f. * Median Life 

Off-Road Zero Hour Factor On-Highway Zero Mile Factor 

19
 
 

Table 9
 

California ARB Deterioration Rates For Four-Stroke 

Spark-Ignition Engine Over 25 Horsepower
 

(Percent increase per percent of median life)
 

HP 

Category 

Equivalent 

On-hwy. 

On-hwy. 

MYR 

On-hwy. 

Useful Life 

Off-hwy. 

HC d.f. 

Off-hwy. 

CO d.f. 

Off-hwy. 

NOx d.f. 

25 to 50 None N/A N/A 1.38% 0.83% 0.064% 

50 to 120 LDGT 1969 

NCAT 

120,000 1.38% 0.83% 0.064% 

120+ HDGV 1970 

NCAT 

120,000 0.37% 0.56% 0.140% 

The technical support document for the California ARB’s OFFROAD model 

stated that baseline deterioration rates for two-stroke SI engines had been set equal to the 

two-stroke deterioration rates contained in the NEVES report, with the exception of twostroke nonroad motorcycles, all terrain vehicles (ATVs) and snowmobiles. For nonroad 

motorcycles and ATVs, the California ARB calculated deterioration rates based on data 

from pre-control on-highway motorcycle engines. Snowmobile deterioration rates in 

OFFROAD were set to zero percent because the California ARB believed that it did not 

have sufficient data concerning their emissions. 

C. Discussion of Deterioration Factors for Spark Ignition Engines Greater Than 25 

Horsepower 

The NEEMT has several misgivings about using the on-highway engine deterioration 

rates derived from the EPA/EMA testing program and used in NEVES. Nonroad and 

highway engines are used in different applications and on different operating cycles, 

experience different maintenance patterns, and are operated at different power levels. 

Highway SI engines also may be tuned to run differently or be configured differently than 

those used in nonroad applications. These differences are likely to result in significantly 

different rates of deterioration for nonroad engines when compared to highway engines of 

similar rated power levels. The cycles used in the EPA/EMA test program were not 

documented in the NEVES report and supporting documentation, but it is likely that the 

program used highway testing cycles since it used highway engines and since EPA had not 

yet developed representative test cycles for nonroad applications at that time. The NEEMT 

also has reservations about using the California ARB deterioration factors, and its general 

approach for deriving nonroad deterioration rates from highway engine test data, for similar 

reasons. Furthermore, neither the EPA/EMA test program nor the California program 

measureed PM emissions, although PM deterioration could be assumed to be equal to HC 

deterioration, as the California ARB did for small four-stroke SI engines. 

20
 
Given the paucity of actual nonroad testing data for spark-ignition engines greater 

than 25 horsepower and the concerns with the deterioration factors used in NEVES report 

and the OFFROAD model, the NEEMT has chosen to equate spark-ignition engine 

deterioration for large (>25 hp) engines to the deterioration of Class 2 SI engines in its draft 

version of NONROAD. Class 2 engines are the largest class of under-25 horsepower sparkignition engines, and there is sufficient test data on such engines to permit estimation of their 

deterioration rates without having to use data taken from tests of highway engines. The 

NEEMT requests comments regarding the appropriateness of this approach and welcomes 

suggestions regarding more appropriate alternatives. The NEEMT also requests that 

commenters submit any additional data that could be used to assess deterioration for large 

spark-ignition engines (both two and four-stroke). 

VIII. Liquid Petroleum and Compressed Natural Gas Spark-Ignition Engines 

Because liquid petroleum gas (LPG) and compressed natural gas (CNG) engines are 

primarily four-stroke engines, the NEEMT decided to assume that they would deteriorate at 

the same rate as the corresponding gasoline-powered four-stroke SI engines for all pollutants. 

The NEEMT is not aware of any deterioration data available for LPG and CNG engines and 

requests that commenters submit any such data they may have to the NEEMT. If such data 

become available, the NEEMT will revise the deterioration rates for these engines in 

NONROAD accordingly. 

IX. Diesel Engines 

In the draft version of NONROAD, the NEEMT has chosen to assume that no 

deterioration takes place in compression ignition nonroad engines. This approach is consistent 

with the approach used by EPA in modeling emissions from highway compression ignition 

engines. However, recent preliminary test results suggest that nonroad diesel engines may 

experience significant rates of deterioration. In a 1997 test program conducted by Southwest 

Research Institute for OMS 15, nine late-model, in-use nonroad diesel engines were tested. Four 

of the engines had significant problems necessitating repairs. Manifold or turbocharger leaks 

were found in all four of the malmaintained engines; two of the engines also required other 

repairs to the fueling system in addition to the leaks. Leaks in the manifold or turbocharger 

would affect the functionality of the turbocharger and would likely increase emissions. The 

manifold leaks made it impracticable to perform emission measurements on these engines, so 

repairs were required before emissions testing. Therefore, it is likely that the measured emissions 

and fuel consumption of these engines underestimated their in-use emissions. EPA expects to 

have a testing program commencing soon that will investigate deterioration rates in nonroad 

compression ignition engines. The NEEMT hopes to incorporate the data from this testing 

program in future versions of NONROAD and, if time and resources allow, in the final version 

of NONROAD. 

21
 
The EPA/EMA testing program mentioned above in Section VI.B.1 tested on-highway 

heavy-duty diesel engines as well as heavy duty gasoline engines. The results of this testing 

showed no increase in HC or NOx and only a slight increase in PM as engines aged. These 

results are reflected in EPA’s MOBILE emissions models and the Nonroad Emissions Model 

(developed to support EPA’s 1997 proposed nonroad diesel engine standards), both of which 

assume that diesel engines experience no emissions deterioration. The California ARB 

incorporates diesel engine deterioration rates in its OFFROAD model that were derived from onhighway data in the California ARB’s EMFAC model. The same concerns regarding the 

relevance of highway engine emissions testing data to nonroad engine deterioration rates 

discussed in Section VI.C also apply to nonroad diesel engines. The NEEMT is not aware of any 

nonroad diesel emissions testing programs that were designed to isolate the effects of 

deterioration and that could be used to confirm or refute the EPA/EMA and California ARB test 

results. 

Endnotes 

1.	 Median life is defined as the midpoint of the scrappage curve at which half of the engines in a given 

population cease to function and are scrapped. For more information, please refer to the technical report on 

activity, load factors and median life ( NR-005) and the technical report about scrappage (NR-007). 

2.	 See endnote 1. 

3.	 Regulatory Impact Analysis and Regulatory Support Document, Control of Air Pollution; Emission 

Standards for New Nonroad Spark-Ignition Engines At or Below 19 Kilowatts, Office of Air and 

Radiation, Office of Mobile Sources, A-93-25, May 1995, section C.1.1, p. C-4. 

4.	 Regulatory Impact Analysis and Regulatory Support Document, Control of Air Pollution; Emission 

Standards for New Nonroad Spark-Ignition Engines At or Below 19 Kilowatts, Office of Air and 

Radiation, Office of Mobile Sources, A-93-25, May 1995, Section 5.4.2. 

5.	 Nonroad Engine and Vehicle Emission Study - Report and Appendices, USEPA, Office of Air and Radiation, Office of 

Mobile Sources, 21A-2001, November 1991. 

6.	 Documentation of Input Factors For The New Off-Road Mobile Source Emissions Inventory Model, 

Prepared for the California Air Resources Board by Energy and Environmental Analysis, Inc., August 

1995, p. 6-21. 

7.	 Fax sent to NEEMT on 4/3/98 from Archana Agrawal of the California Air Resources Board. The fax 

contains tables of the official adopted deterioration rates in the California OFFROAD model as of 3/26/98. 

8.	 Regulatory Impact Analysis and Regulatory Support Document, Control of Air Pollution; Emission 

Standards for New Nonroad Spark-Ignition Engines At or Below 19 Kilowatts, Office of Air and 

Radiation, Office of Mobile Sources, A-93-25, May 1995, Appendix C. 

9. A slight discrepancy exists between NEVES and the small engine nonroad regulation concerning the 

classification of small versus large spark ignition engines. The NEVES report divided smaller and larger 

22
 
four-stroke and two-stroke engines at 20 horsepower. The regulations that have been promulgated since 

the publishing of NEVES set the dividing point at 25 horsepower (19 kilowatts). 

10.	 EPA Regulatory Support Document, Phase 2: Emission Standards for New Nonroad Spark-Ignition 

Engines At or Below 19 Kilowatts, Appendix F: Nonroad Small Engine Emission Model Tables, Tables F05 and F-06, December 1997. 

11.	 Nonroad Engine and Vehicle Emissions Study, November 1991, Appendix I, Table I-14, p. I-61. 

12.	 The first and second phases of the small engine rule did not address PM emissions. 

13.	 Nonroad Engine and Vehicle Emission Study, USEPA, Office of Air and Radiation, Office of Mobile Sources, 21A2001, November 1991, Appendix I, p. I-12, Section 2.2.1. 

14.	 Recreated from Table 6-9, p. 6-22, in Documentation of Input Factors For The New Off-Road Mobile 

Source Emissions Inventory Model, Prepared for the California Air Resources Board by Energy and 

Environmental Analysis, Inc., August 1995 

15.	 Fritz, S. G., Emission Factors for Compression Ignition Nonroad Engines Operated on Number 2 Highway 

and Nonroad Diesel Fuel, Southwest Research Institute. EPA EPA 420-R-98-001, March 1998. 

23
 
Appendix 1 

SwRI In-Use Small Utility Engine Test Results 

Engine Test HC HC CO CO NOx NOx PM PM 

g/hp-hr test/EF g/hp-hr test/EF g/hp-hr test/EF g/hp-hr test/EF 

4-Stroke 

2 yr. WBM 1A 67.9 1.80 650 1.51 0.94 0.47 1.35 1.80 

4 yr. WBM 1A 83.9 2.23 928 2.16 0.37 0.18 1.11 1.48 

2A 112.6 2.99 1033 2.40 0.47 0.23 2.05 2.73 

8 yr. WBM 1A VOID VOID VOID VOID VOID VOID VOID VOID 

2A 77.3 2.05 835 1.94 0.90 0.45 6.27 8.36 

3A 74.9 1.99 829 1.93 0.71 0.35 4.08 5.44 

New Engine EFs 37.7 430 2.02 0.75 

In-use adjustment (avg. test/EF) 2.10 1.90 0.40 3.60 

2-Stroke 

11 yr. WBM 1 187 0.90 415 0.85 0.51 1.76 5.75 0.75 

2 177 0.85 418 0.86 0.52 1.79 6.61 0.86 

New Engine EFs 208 486 0.29 7.7 

4 yr. String Trimmer 1 1369 6.11 2244 3.11 0.77 0.86 61.3 15.36 

2 1205 5.38 1936 2.68 0.69 0.77 54.3 13.61 

New Engine EFs 224 722 0.90 3.99 

24
 
Appendix 2 

Baseline and In-Use Emissions Numbers from EPA’s 1991 Nonroad Study 

Equipment Category Baseline In-Use I n - Adjusted In-Use 

(g/kw-hr) (g/kw-hr) Use Factors 

Factor 

HC CO NOx HC CO NOx HC CO NOx HC C NOx 

O 

4-Stroke Engines 2.1 1.9 0.4 2.1 1.9 0.4 

Lawnmowers 50.5 576.4 2.71 106.13 1095.17 1.09 

Trimmers/Edgers/Brush 32.41 527.27 2.71 68.07 1001.81 1.09 

Cutters 

Chainsaws NA NA NA NA NA NA 

Leaf Blower/Vacuum 26.01 509.79 2.72 54.61 968.59 1.09 

Generator Sets 12.73 473.19 2.72 26.74 899.06 1.09 

Tillers 50.54 576.41 2.71 106.13 1095.17 1.09 

Snowblowers 50.54 576.41 2.71 106.13 1095.17 1.09 

Commercial Turf 12.6 474.53 2.83 26.46 901.61 1.13 

Rear Engine Riders 12.47 473.19 2.72 26.18 899.06 1.09 

Lawn and Garden Tractors 12.6 474.53 2.83 26.46 901.61 1.13 

Pumps 12.47 473.19 2.72 26.18 899.06 1.09 

All Other Equipment 2.1 1.9 1.0 1.2 1.9 1.0 

2-Stroke Engines 

Lawnmowers 278.82 486 0.39 585.52 1237.8 0.39 

Trimmers/Edgers/Brush 301.1 728.22 1.22 632.14 1854.72 1.22 

Cutters 

Chainsaws 399.46 699 1.29 842.9 1780.29 1.29 

Leaf Blower/Vacuum 288.59 716.81 1.29 606.05 1825.66 129 

Generator Sets 278.82 486 0.39 585.52 1237.8 0.39 

Tillers 278.82 486 0.39 585.52 1237.8 0.39 

Snowblowers 278.82 486 0.39 585.52 1237.8 0.39 

Commercial Turf 278.82 486 0.39 585.52 1237.8 0.39 

Rear Engine Riders NA NA NA NA NA NA 

Lawn and Garden Tractors NA NA NA NA NA NA 

Pumps 5.74 113 9.44 12.05 287.8 3.78 

All Other Equipment 

N/A=Not Applicable 

Note: All Other Equipment includes the following: Distributed Loose Engines, Commercial Turf Equipment, Other Lawn and Garden, Wood 

Splitters, Pressure Washers, Front Mowers, Welders, Specialty Vehicles and Carts, Shredders, Cement/Mtr Mixers, Golf Carts, Paving 

Equipment, Air Compressors, and Sprayers. 

25