EPA-420-P-98-020
 

Median Life, Annual Activity, and Load Factor Values

 for Nonroad Engine Emissions Modeling 

Report No. NR-005A 

December 9, 1997
 

revised June 15, 1998
 

Christian E. Lindhjem
 

and
 

Megan Beardsley
 

Nonroad Engine Emissions Modeling Team
 

Assessment and Modeling Division
 

EPA, Office of Mobile Sources
 

Purpose 

EPA’s NONROAD model computes emission inventories for nonroad engines. This 

report documents the default input values of median life, annual activity, and load factor for 

various types of nonroad engines. These values are used to calculate the fleet age distributions 

for each engine type included in this model. Annual activity and load factor values are also used 

to calculate the yearly emissions for each engine type included in the model. 

Introduction 

This report documents the default annual activity and load factor values. These estimates 

are used to determine emissions from engines by using the following equation. The discussion of 

the other default values of population, power, and emission factors are the subject of other 

reports. 

Emissions = (Pop) (Power) (LF) (A) (EF) 

where Pop = Engine Population 

Power = Average Power (hp) 

LF = Load Factor (fraction of available power) 

A = Activity (hrs/yr) 

EF = Emission Factor (g/hp-hr) 

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This report also documents the default values for the median life of engines. The first 

step in developing the age distribution is the determination of how long an engine is used in 

years. The median life, annual activity, and load factor are all variables used in estimating the 

median lifetime of nonroad engine. The median lifetime in years is determined from the median 

life expressed in units of hours at full load, the annual activity expressed in units of hours per 

year, and the load factor expressed in units of the fraction of available power as shown in the 

equation below. 

Median Life (hrs) Median Lifetime (years) = Activity (hrs/yr) * Load Factor 

The fleet of engines’ age distribution and scrappage rates are estimated as functions of the 

engine age expressed in the fraction of median lifetime. For instance, when engines of a given 

model year are used to their median lifetime, 50% of the engines sold in that model year have 

been scrapped and are not in use. In order to determine the engine population at any given in-use 

year, the number of engines in every model year remaining in service needs to be calculated 

based on the estimated median lifetime in years calculated with the equation above. A further 

discussion of how the age distributions and scrappage rates are used to incorporate fleet turnover 

is the subject of another report. (1) 

For most applications, equipment is used until the engine no longer works; that is, median 

engine life is considered synonymous with median equipment life. Marine vessels and other 

large equipment such as mining and construction applications are exceptions to this rule, because 

they typically use replacement engines when the original engine cannot be rebuilt instead of 

purchasing entirely new equipment. Installing a new engine is assumed to have the same effect 

on emissions as replacing the equipment, so engine life remains the relevant value for age 

determination for these applications. 

Values Used in NONROAD for Median Life, Annual Activity, and Load Factor 

The estimates for median life, activity, and load factor used as default data files in 

NONROAD are described in this work. The NONROAD model classifies engine applications 

and fuel types with the use of a source categorization code (SCC). The descriptions shown in the 

tables below were translated to SCC’s by the fuel type and application description. For diesel 

and gasoline engines, this translation is straightforward because the structure of the SCC’s 

differentiates engines by fuel type. For application-specific engines, the description of the 

application is used to determine the appropriate SCC. The translation procedure of using 

application description and fuel type to denote SCC’s is also described in the report describing 

default equipment populations. (2) 

2
 
Median Life in Hours at Full Load 

Engine life varies with engine type and power level. The NONROAD model uses the 

median life estimates that the California Air Resources Board in its OFFROAD model uses. (3) 

These estimates are listed by horsepower class and engine type in Table 1. The horsepower 

classes are defined in Table 2. 

Liquid petroleum gas (LPG) and compressed natural gas (CNG) engines are primarily 

four-stroke spark-ignition engines and hence have many similarities with four-stroke gasoline 

engines. Because LPG and CNG engines are similar in design to gasoline engines, their median 

life is likely to be similar to the median life of gasoline engines. For this reason and the lack of 

relevant data to the contrary, LPG and CNG engines are assumed to have the same median life as 

gasoline engines in the NONROAD model. 

The values described represent the only comprehensive review of the engine life 

associated with the engine type and power level available. They represent median values of 

engine life, not distinguishing the engine by make or model. Individual engine models may have 

longer or shorter median lives. Surveys of engine life by application or engine type are 

encouraged to improve the estimates shown below. 

Table 1 Expected Engine Life in Hours at Full Load (3) 

Engine 

Type 

HP 1 HP 2 HP 3 HP 4 HP 5 HP 6 HP 7 HP 8 HP 9 HP 10 

Diesel 1250 2500 2500 4000 4000 4000 6000 6000 6000 --

2-stroke 

Gasoline 

150 200 750 -- -- -- -- -- -- --

4-stroke 

Gasoline 

200 400 750 1500 3000 3000 3000 3000 3000 3000 

CNG/ 

LPG 

200 400 750 1500 3000 3000 3000 3000 3000 3000 

3
 
Table 2 Horsepower Classes for Median Life (3) 

HP Class Diesel 2-stroke 4-stroke 

HP 1 �15 �2 �5 

HP 2 16-25 3-15 6-15 

HP 3 26-50 16-25 16-25 

HP 4 51-120 26-50 26-50 

HP 5 121-175 51-120 51-120 

HP 6 176-250 121-175 121-175 

HP 7 251-500 176-250 176-250 

HP 8 501-750 251-500 251-500 

HP 9 751+ 501-750 501-750 

HP 10 -- 751+ 751+ 

Small (<25 hp) Spark-Ignition Engine Median Life 

The median life (the point where 50% of engines have been scrapped) for several 

applications of nonroad engines have been estimated previously in terms of years of use. (4) For 

the purpose of the NONROAD model, the median life was converted from units of years to units 

of hours at full load using load factors and median hours of use which are shown in Table 5 

below. To do this, we used the second equation shown above. Median life in years from Table 3 

was multiplied by hours per year and load factor given in Table 5 to create the column in Table 3 

and used as an input to the NONROAD model. 

The final values for the median life used in NONROAD for these selected applications 

are shown in Table 3. Lower or higher power level engines used the default values from ARB to 

distinguish power level among these applications because these engines are expected to be 

significantly different than the engines estimated previously. 

Table 3: Median Life for the Bulk of Several Small Spark-Ignition Engine Applications 

Application Use Power 

(min.) 

Power 

(max.) 

Median Life 

(Hours at Full Load) 

Median Life 

(Years) (4) 

Lawn Mowers Residential 1 6 47.9 5.8 

Commercial 1 6 268 2 

4
 
Trimmer/Edger/Cutter Residential 0 3 19.4 4.3 

Commercial 0 3 157.6 2.3 

Chainsaws Residential 0 6 28 4.3 

Commercial 0 6 136.4 0.9 

Leaf Blower\Vacuum Residential 0 6 21.5 4.3 

Commercial 0 6 324.3 2.3 

Tillers Residential 0 11 39.4 5.8 

Commercial 0 11 830.7 4.4 

Snowblowers Residential 0 6 12.3 4.4 

Commercial 0 6 209.4 4.4 

Commercial Turf Commercial 3 25 988.9 2.9 

Rear Engine Rider Residential 3 16 79.3 5.8 

Commercial 6 16 627 2.9 

Lawn and Garden 

Tractor 

Residential 3 25 114.8 5.8 

Commercial 6 25 920 2.9 

Activity in Hours per Year 

The default input values for the activity estimates come from Power Systems Research 

(PSR), an independent research firm. This firm conducts several yearly surveys of equipment 

owners and determines a median usage rate for engines by application and fuel type. (5) These 

usage rates are shown in Table 4 for 2 and 4 stroke gasoline and diesel engines. The PSR 

methodology is described and evaluated in a report by Pechan. (6) 

The PSR data represents the most comprehensive review of application specific activity 

available to the NONROAD team. It may be possible to determine activity regionally for a few 

specific types of nonroad equipment, and the results compared with the PSR estimates used in 

NONROAD. If such information becomes available, the Nonroad Engine Emissions Modeling 

Team would consider using such information in future versions of the model. 

No data exist regarding activity levels for LPG and CNG engines. These engines are 

similar to gasoline engines because they employ spark-ignition. Therefore, LPG and CNG 

engines are more likely to be used in applications where gasoline engines are used, so they are 

assumed to have the same usage pattern as gasoline engines. 

5
 
 Load Factor 

PSR also estimates the load factor from surveys of equipment users. PSR calculates the 

fraction of load from the estimate of hours of usage per year, the fuel consumption per year, and 

the fuel consumption rate at rated power for each engine in the field. A median fraction of 

available power is determined for specific applications. The load factors are also shown in Table 

4. As was the case for activity levels, LPG and CNG engines are assigned the load factors for 

gasoline engines. 

The load factor is determined from actual and maximum fuel consumption rates for 

various pieces of equipment. If alternative surveys become available, they would be considered 

for future versions. For reference, the PSR methodology is explained in detail in the Pechan 

report. (6) 

Table 4: PSR Estimates of Activity and Load Factor for Gasoline and Diesel Engines* 

PSR Application Description SCC Activity 

Diesel 

(Hrs/year) 

Load Factor 

Diesel 

(% of Max. Power) 

Activity 

Gasoline 

(Hrs/year) 

Load Factor 

Gasoline 

(% of Max. Power) 

Generator Sets 22xx006005 338 74 115 68 

Air Compressors 22xx006015 815 48 484 56 

Pumps 22xx006010 403 74 221 69 

0il Field Equipment 22xx010010 1231 92 1104 90 

Underground Mine Equipment 22xx009010 1533 68 260 80 

Refrigeration/AC 22xx003060 1341 28 605 46 

Tactical Military Equipment not modeled 260 62 225 68 

Terminal Tractors 22xx003070 1257 82 827 78 

Welders 22xx006025 643 45 208 51 

Forklifts 22xx003020 1700 30 1800 30 

Other Material Handling 22xx003050 421 59 386 53 

Locomotive not modeled 848 63 15 91 

Scrubbers/Sweepers 22xx003030 1220 68 516 71 

Surfacing Equipment 22xx002024 561 45 488 49 

Forest Equipment 22xx007015 1276 71 35 70 

Marine Auxiliary not modeled 2608 67 175 61 

Chippers/Grinders 22xx004066 465 73 488 78 

Cranes 22xx002045 806 43 415 47 

Excavators 22xx002036 859 57 378 53 

Scrapers 22xx002018 914 72 540 70 

Graders 22xx002048 821 61 504 64 

Crawler Dozers 22xx002069 936 64 700 80 

Rubber Tire Dozer 22xx002063 899 59 900 75 

Rubber Tire Loader 22xx002060 761 68 512 71 

Crush/Processing Equipment 22xx002054 955 78 241 85 

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Paving Equipment 22xx002021 622 53 175 59 

Other Construction 22xx002081 606 62 371 48 

Bore/Drill Rigs 22xx002033 466 75 107 79 

Skid Steer Loader 22xx002072 818 55 310 58 

Rollers 22xx002015 745 56 621 62 

Off-Highway Truck 22xx002051 1641 57 450 80 

Pavers 22xx002003 821 62 392 66 

Trenchers 22xx002030 593 75 402 66 

Tractor\Loader\Backhoe 22xx002066 1135 55 870 48 

Irrigation Sets 22xx005060 749 65 716 60 

Agricultural Tractor 22xx005015 475 70 550 62 

Other Agricultural Equipment 22xx005055 381 51 124 55 

Combines 22xx005020 150 70 125 74 

Swathers 22xx005045 110 55 95 52 

Balers 22xx005025 95 58 68 62 

Powerboats (Inboard Marine) 

**** 

2282010005 

2282020005 

200 35 100 38 

Trimmer/Edger/Cutter** 22xx004025 

22xx004026 

60 43 34 68 

Agricultural Mowers 22xx005030 363 43 175 48 

Snowblower** 22xx004035 

22xx004036 

400 65 45 78 

Cement/Material Mixers 22xx002042 275 56 84 59 

Pressure Washer 22xx006030 145 30 115 85 

Tillers** 22xx005040 172 78 43 71 

Dumpers/Tenders 22xx002078 566 38 127 41 

Plate Compactors 22xx002009 484 43 166 55 

Specialty Vehicle/Carts 22xx001060 435 65 65 58 

Lawn and Garden Tractor** 22xx004055 

22xx004056 

544 57 104 62 

Aerial Lifts 22xx003010 384 46 361 46 

Lawn Mowers** 22xx004010 

22xx004011 

320 55 76 70 

Leaf Blower/Vacuum** 22xx004030 

22xx004031 

120 40 56 75 

Commercial Turf Equipment** 22xx004071 1068 55 733 60 

Off-Highway Tractors 22xx003070 855 65 155 70 

Sprayers 22xx005035 90 58 80 65 

Chainsaws** 2260004020 

2260004021 

2260007005 

70 60 26 92 

Snowmobiles** 22xx001020 50 40 121 81 

Light Plants\Signal Boards 22xx002027 535 78 318 72 

Other General Industrial 22xx003040 878 51 713 54 

7
 
Wood Splitter 22xx004060 

22xx004061 

265 55 76 69 

Other Lawn Garden** 22xx004075 

22xx004076 

433 65 61 58 

Concrete/lnd. Saws 22xx002039 580 73 610 78 

Sailboat Auxiliary*** 22820xx020 

22820xx025 

68 32 62 35 

Railway Maintenance 228500x015 943 47 184 62 

Aircraft Support 22xx008005 732 51 681 56 

Rear Engine Rider** 22xx004040 480 64 86 67 

Rough Terrain Forklifts 22xx002057 662 60 413 63 

Hydro Power Unit 22xx005050 790 48 450 56 

Front Mowers 22xx004055 

22xx004056 

480 56 86 65 

Gas Compressors 22xx006020 8500 60 8500 60 

All-Terrain Vehicles\Off-road 

Motorcycles 

22xx001030 0 0 135 72 

Personal Water Craft  2282005015 0 0 100 42 

Mini-Bikes 22xx001040 0 0 55 62 

Golf Carts 22xx001050 1150 49 1080 46 

Tampers/Rammers 22xx002006 460 43 160 55 

Shredders 22xx004050 

22xx004051 

22xx007010 

120 40 50 80 

Crawler Tractor (Agricultural 

Tractor with Crawler Tracks) 

No population 

not used 

936 64 700 80 

2-Wheel Tractors 22xx005010 544 62 286 62 

Outboard Engines** ** 2282005010 

2282020010 

150 29 100 32 

* Gasoline values are used for LPG and CNG engines. 

** See discussion below of Small Spark-Ignition Engines for alternate use and load factors.
 

*** Gasoline sailboat auxiliary is included in outboard and powerboats (inboard in the SCC
 

nomenclature)
 

**** New values for gasoline recreational marine shown below
 

Small (<25 hp) Spark-Ignition Engines Activity and Load Factor 

Regulations to be proposed for small spark-ignition engines are described in greater detail 

in another report (2), and the supporting documentation for those regulations include estimates of 

activity and load for a few important applications. (4) The activity and load factors and power 

levels for these applications, mainly lawn and garden equipment, were derived from data 

supplied during discussions regarding the proposed regulations. These alternate values are 

shown in Table 5 and represent a departure from the PSR values shown above. For applications 

not shown in Table 5, the PSR values were used because the PSR survey includes such categories 

8
 
as construction, agricultural, commercial, and many other engines applications already 

considered commercial and not included in the data submitted for the rulemaking. 

Many of the small spark-ignited engines are used in lawn and garden equipment 

applications, and lawn and garden equipment is used both by commercial and residential users. 

The commercial users are grounds keepers for nonresidential, large apartment complexes, and 

some single-family homes. Commercial equipment therefore has significantly different usage in 

terms of hours per year and weekday and weekend day than residential equipment. To account 

for these differences in equipment usage, separate SCC’s were defined to distinguish commercial 

from residential lawn and garden equipment. 

9
 
Table 5: Usage and Load Factors for Several Small Spark-Ignition Engine Applications 

Application Use Annual Hours Load Factor 

(fraction of power) 

Lawn Mowers Residential 25 0.33 

Commercial 406 0.33 

Trimmer/Edger/Cutter Residential 9 0.50 

Commercial 137 0.50 

Chainsaws Residential 13 0.50 

Commercial 303 0.50 

Leaf Blower\Vacuum Residential 10 0.50 

Commercial 282 0.50 

Tillers Residential 17 0.40 

Commercial 472 0.40 

Snowblowers Residential 8 0.35 

Commercial 136 0.35 

Commercial Turf Commercial 682 0.50 

Rear Engine Rider Residential 36 0.38 

Commercial 569 0.38 

Lawn and Garden Tractor Residential 45 0.44 

Commercial 721 0.44 

Recreational Marine Engines 

As described in another report (2), recreational marine engines are divided into three 

applications; inboard, outboard, and personal watercraft. The outboard category includes the 

inboard\outboard sterndrive engines formerly considered as a separate application in the SCC 

system because the engine types are similar. The inboard engines are most closely associated 

with the PSR category ‘powerboats’ in Table 3. Personal watercraft were included in the PSR 

category formerly reserved for Off-Road Motorcycles which are now included with All Terrain 

Vehicles. 

10
 
 

A study of recreational boating activity indicates that boating activity may be 

considerably different than PSR estimates. (5, 7) In order to determine and estimate for activity 

in hours per year, it was necessary to estimate the number of engines (E), yearly fuel 

consumption of the engines (FC), brake-specific fuel consumption (BSFC), average horsepower 

(Hp), and a load factor (LF) by type of engine as described in the equation below. 

Activity (hrs/year) =(FC) / [(E) * (Hp) * (LF) * (BSFC)] 

According to the report (7), the fuel consumption and number of boats by boat type were 

determined and given in Tables II-3 (page II-6) and II-8 for gasoline-fueled engines. Cabin 

cruisers and other cabin boats were assumed to be inboard gasoline. Personal watercraft were 

determined uniquely, and all other boats were assumed to be driven by outboard-style engines. 

The total number of inboard, outboard, and personal watercraft boats were estimated to be 

998,000, 9,065,000, and 200,000 respectively. Table II-9 shows the fraction of boats that have 

one and two engines where 26.8% of inboard boats have two engines; 2.7% of outboard boats 

have two engines; and no personal watercraft engines. So it is estimated that there were 

1,265,000 inboard, 9,309,755 outboard, and 200,000 personal watercraft engines nationally in 

1990-1991, the time period for the survey. 

Total fuel consumption was determined from Table II-3 based on the criteria described 

above to be 257,173,000 gallons for inboard, 672,009,000 gallons for outboard, and 25,063,000 

for personal watercraft for the survey year. 

To determine the fuel consumption of the engines, data was submitted to EPA which 

indicated fuel consumption as a function of power level and is shown in Table 6. (8) The 

fraction of engines by power level was determined from PSR data for the 1990 in-use year and 

associated with fuel consumption estimates to determine an average fuel consumption of 0.216 

for outboard, 0.100 for inboard, and 0.160 for personal watercraft in units of gallons/hp-hr. 

Table 6 Fuel Consumption by Power Level 

Engine Type Power Level Fuel Consumption

 (gallons/hp-hr) 

2-stroke 0-16 hp 0.27 

>16 hp 0.16 

4-stroke 0-25 hp 0.14 

25-40 hp 0.12 

>40 hp 0.10 

11
 
The average power for the three types of engines was determined from the PSR estimates 

of the 1990 in-use populations. Price Waterhouse estimate average power level by boat length 

and type of boat rather than a direct measure of average power level. The average power level of 

outboard engines was determined to be 37 horsepower from PSR compared with roughly 66 for 

the Price Waterhouse estimate. The average power level for inboard engines was estimated to be 

166 from PSR compared with 179 for the Price Waterhouse. The average power level for 

personal watercraft was estimated to be 52 from PSR compared with 50 for Price Waterhouse. 

By using the PSR data for average power level, we are consistent with the use of PSR in 

determining the population distributions for this version of NONROAD. 

Other supplied comments indicates that EPA should revise the load factor to accurately 

reflect the duty cycle in the certification test of 20.7% of rated power. In-use data of load factor 

is not available, and the PSR estimates of load factor (32% for outboard, 38% for inboard, and 

42% for personal watercraft) are not currently documented. 

Using the combined information and estimates, the average activity is estimated to be 75 

hours per year for inboard, 45 hours per year for outboard, and 73 hours per year for personal 

watercraft engines compared with the PSR estimate of 100 hours for all types of recreational 

marine gasoline engines. It is helpful to remember that this implied activity is related to the 

number of hours that the engine actually operates, not necessarily how many hours the boat is inuse on the water. 

References 

(1)	 EPA, “Growth and Scrappage Methodology Report for NONROAD,” NR-008, Jan. 1998. 

(2)	 EPA, “Nonroad Engine Population Estimates,” NONROAD Documentation, NR-006, 

Dec., 1997. 

(3)	 ARB, “Documentation of Input Factors for the New Off-Road Mobile Source Emissions 

Inventory Model,” Energy and Environmental Analysis, Inc., February, 1997. Table 3-1. 

(4)	 EPA, “Statement of Principles for Nonroad Phase 2 Small Spark-Ignited Engines; 

Proposed Rule,” 62 FR 14740, March, 27, 1997. 

(5)	 Power Systems Research, “Reference Guide, US Parts Link Edition 6.2,” St.Paul, MN. 

(6)	 E.H. Pechan, “Evaluation of Power Systems Research (PSR) Nonroad Population Data 

Base,” prepared for Office of Mobile Sources, U.S. Environmental Protection Agency, 

September 1997, EPA Contract No. 68-D3-0035, Work Assignment No. III-107, Pechan 

Report No. 97.09.003/1807. 

(7)	 Price Waterhouse, “National Recreational Boating Survey,” Draft Final Report, Prepared 

for the U.S. Fish and Wildlife Service and the U.S. Coast Guard, March 9, 1992. 

(8)	 Ted Morgan, “Emission Data for Recreational Marine Engines,” communication to Ken 

Zerafa of the EPA, September 13, 1991. 

(9)	 Ted Morgan, communication to Rich Wilcox of the EPA, April 13, 1998. 

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