United States Air and Radiation EPA420-P-99-019 

Environmental Protection May 1999 

Agency 

Determining Repair 

Effects on IM240 Cold 

Start Emissions for 

1981 and later Light Duty 

Vehicles 

DRAFT

 Printed on Recycled Paper 
EPA420-P-99-019 

May 1999 

Determining Repair Effects on IM240 Cold Start Emissions for 

1981 and later Light Duty Vehicles 

Report Number M6.IM.002 

Janet C. Kremer
 

Assessment and Modeling Division
 

Office of Mobile Sources
 

U.S. Environmental Protection Agency
 

NOTICE
 

This technical report does not necessarily represent final EPA decisions or positions.
 

It is intended to present technical analysis of issues using data which are currently available.
 

The purpose in the release of such reports is to facilitate the exchange of
 

technical information and to inform the public of technical developments which
 

may form the basis for a final EPA decision, position, or regulatory action.
 
-DraftDetermining Repair Effects on IM240 Cold Start Emissions 

for 1981 and later Light Duty Vehicles
 

Report Number M6.IM.002
 

May 1999
 

Janet C. Kremer
 

U.S.EPA Assessment and Modeling Division
 

Introduction 

When the MOBILE model calculates credits from Inspection and Maintenance (I/M) programs, 

one important input is the difference between emissions before and after successful I/M repairs. 

These “repair effects” may vary with vehicle type and model year. They also may vary with test 

type and test cutpoints, and may be different for running and start emissions. 

This document examines the repair effects on start emissions. It describes how the database for 

this analysis was developed. It also describes how to calculate cold start emissions from the 

Federal Test Procedure (FTP) bag results. The analysis in this report covers Light Duty Vehicles 

(LDVs) and Light Duty Trucks (LDTs) with model years ranging from 1983 through 1991. 

Repair effects for running emissions are found in EPA report number M6.IM.001. 

Background 

The current version of the MOBILE model is MOBILE5. MOBILE5 calculates I/M credits using 

a module called TECH5. TECH5 is an external FORTRAN program which calculates and 

exports the I/M credit values to MOBILE5 as built-in code or as an external file. 

For MOBILE6, the new I/M credit methodology will be built into the model’s code. This new 

methodology will operate automatically every time an I/M program is called by the model. The 

MOBILE6 I/M credits will have several new features. They will incorporate new basic emission 

rates and I/M credits will be calculated for start and running emissions separately. Also 

MOBILE6 has new classifications for defining emitter categories. In MOBILE5, there were four 

emitter classifications: normal, high, very high, and super. For MOBILE6, the emitter categories 

are handled by two classifications, normal and high. Normal emitters are low emitting vehicles 

with emission control systems that generally function properly. High emitters have average 

emission levels which are considerably higher than the overall mean emission levels. 

MOBILE6 will apply separate benefits to start and running emissions to generate I/M credits for 

1981 and later model year vehicles. MOBILE6 will apply the same benefits to start and running 

emissions for pre-1981 model year vehicles. 

More information on the new methodology for I/M credits can be found in the report “MOBILE6 

Inspection / Maintenance Benefits Methodology for 1981 through 1993 Model Year Light 

Vehicles,” report number M6.IM.001. 

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Data 

A lack of “real world” data on repair effects was a constraint on this analysis. In order to be 

selected for this analysis a vehicle had to meet the following set of criteria: 

�	 Model year 1981 and later (I/M credits for pre-81 will not be updated in MOBILE6) 

�	 Gasoline vehicles only 

�	 LDV or LDT only 

�	 Not tested in California or Colorado (because of different standards and effects of higher 

altitudes respectively) 

�	 Failed its “as received” test and passed its after-repair test 

�	 Vehicle has both I/M240 and corresponding FTP results ( FTP results are necessary to 

evaluate cold start emissions, i.e., emissions after 12+ hour soak) 

The only available data meeting these criteria were in EPA’s Emissions Factor Database. After 

eliminating vehicles that did not meet the above criteria, the remaining vehicles, with their FTP 

and IM240 results, created the database used in this analysis. This database contained 369 LDVs 

and 41 LDTs with model years ranging from 1983 through 1993. 

Analysis 

Calculating Start and Running Emissions 

In order to examine repair effects on start emissions, it is necessary to determine what start 

emissions are. The FTP results were split into start and running fractions using the methodology 

explained in detail in report number M6.STE.002 and report number M6.STE.003. 

Determining Emitter Categories 

All the vehicles included in this analysis were classified as meeting “Tier 0" certification 

standards and were defined as either normal or high emitters. Normal emitters were defined as 

vehicles with FTP emission rates less than or equal to two times the standard for hydrocarbons 

(HC) and oxides of nitrogen (NOx) and FTP emission rates less than or equal to three times the 

standard for carbonmonoxide (CO). High emitters were defined as vehicles with FTP emission 

rates greater than two times the standard for hydrocarbons or oxides of nitrogen or FTP emission 

rates greater than three times the standard for carbon monoxide. The cutpoints used in defining 

an emitter category can be found in Table 1. 

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Table 1: Cutpoints used in determining emitter category 

Vehicle 

Type 

Model 

Year 

2 x HC Stand. 

(g/mi) 

3 x CO Stand. 

(g/mi) 

2 x NOx Stand. 

(g/mi) 

LDV 81 - 93 .82 10.2 2.0 

LDT 81 - 87 1.6 30 4.6 

LDT1* 

LDT2 

88 - 93 1.6 30 2.4 

3.4 

*Light Trucks have different NOx standards based on vehicle weight 

For the purpose of this analysis, additional criteria have been used for determining emitter-class 

designation. In cases where a vehicle exceeds the emission cutpoint for either HC or CO, it is 

considered an HC/CO high emitter. In this report, this emitter category will be known as “Emitter 

Class A.” Vehicles which exceed the NOx cutpoint are considered NOx high emitters. This 

emitter category will be referred to in this report as “Emitter Class B.” It is then possible for a 

vehicle to be a normal emitter for Emitter Class A and a high emitter for Emitter Class B or vice 

versa. 

Determining Pass or Fail with Cutpoint Combinations 

Determining whether a vehicle passed or failed its as-received or after-repair tests depends on the 

cutpoint for the test. The EPA developed six different sets of cutpoint combinations for this 

analysis that reflected the range of cutpoints currently being used in I/M lanes. These 

combinations are listed in Table 2. Different cutpoint combinations were analyzed to determine 

whether cutpoint selection would greatly affect the amount of emission reduction.(Of course, as 

described in M6.IM.001, cutpoints do affect identification rates and thus I/M credits) These 

cutpoints were applied to both LDVs and LDTs for all model years. As in actual I/M tests, a 

vehicle had two chances to pass for HC and CO pollutants. As set by IM240 standards, “if the 

composite emission level is equal to or below the composite standard or if the Phase 2 gram per 

mile emission level is equal to or below the applicable Phase 2 standard, then the vehicle shall 

pass the test for the exhaust component.”(EPA Report Number: EPA-AA-EPSD-IM-93-1) Phase 

2 in this context means the average emissions in the last 129 seconds of the IM240 test. An 

example of this pass/fail technique would be if a vehicle’s IM240 composite HC emissions were 

1.0 grams per mile and its Phase 2 emission were 0.5 grams per mile and the cutpoint 

combination being used was combination #2 in Table 2, the vehicle would fail the composite 

standard but would pass the Phase 2 standard therefore passing the test. This same vehicle would 

fail the test if it were examined using cutpoint combination #1. Each vehicle in the data set was 

given a pass or fail for each cutpoint combination. 

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Table 2: Cutpoint Combinations Used to determine Pass or Fail for Composite (C) and 

Phase 2 (P2) test data. 

Comb. #1 Comb. #2 Comb. #3 Comb. #4 Comb. #5 Comb. #6 

C P2 C P2 C P2 C P2 C P2 C P2 

HC 0.6 0.4 0.8 0.5 1.0 .63 1.2 .75 2.0 1.25 2.4 1.5 

CO 10 8 15 12 20 16 30 24 60 48 78 60 

NOx 1.5 2.0 2.5 3.0 4.0 6.0 

For this analysis, the EPA examined those vehicles that failed their as-received test and passed 

their after-repair test based on HC, CO and NOx cutpoints and both emitter Class A and Class B. 

Each vehicle’s percent reduction for start and running emissions were calculated. 

Calculating Percent Emission Reduction 

Percent emission reduction can be calculated using a vehicle’s before- and after-repair emission 

rates. Emission reductions for each pollutant were calculated using the following equation. 

Percent Emission Reduction = EBR - EAR X 100

 EBR 

Where EBR = Emissions Before Repair and EAR = Emissions After Repair 

Results 

Table 3 and 4 contain the results of this evaluation. Vehicles were divided into categories based 

on their vehicle type and emitter category. Results showed that when repairing vehicles that were 

classified as HC/CO high emitters, there is a noticeable increase in NOx emissions. This would 

seem to indicate that I/M programs that test and repair vehicles based only on HC and CO 

failures (i.e., HC/CO high emitters) would on average increase the fleet NOx emissions. This 

phenomenon has been called NOx disbenefit and is discussed in more detail in EPA report 

number M6.IM.001. 

ANOVA 

After examining the start emission results from this table, it appears that on average, no matter 

which cutpoint combination was chosen, the same average after-repair emission levels were 

achieved. Based on this hypothesis, an analysis of variance (ANOVA) was performed on the 

data, examining model year, vehicle type and cutpoint combination as independent variables. 

Interest in model year and vehicle type significance comes from the fact that we want to be able 

to split the start emission rates in MOBILE6 by model year and vehicle type. Table 5 contains 

the results of this analysis. 

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TABLE 5: Analysis of Variance Results 

Analysis of Variance Results (ANOVA p-values)* 

HC/CO High Emitters 

Variables HC CO NOx 

Model Year .0188 .0001 .0001 

Vehicle Type .0001 .0014 .6968 

Model Year x Vehicle Type .0504 .0184 .9311 

Cutpoint Combination .4640 .6959 .7630 

NOx High Emitters** 

Model Year .0087 .0001 .0001 

Vehicle Type .6155 .0116 .0006 

Cutpoint Combination .9887 .9049 .9740 

*in log space 

** model year x vehicle type not applicable here because of lack of data 

The statistical analysis showed that repair effects vary significantly by model year and also vary 

by vehicle type for all but NOx emissions of HC/CO high emitters and HC emissions of NOx 

high emitters. This analysis also verified that cutpoint combination was not a significant variable 

for all three pollutants. 

Conclusions 

After reviewing the results of this analysis EPA has chosen to use one cutpoint combination 

when determining the repair effects on start emissions. Cutpoint combination #2 at .8 g/mi. HC / 

15 g/mi. CO / 2 g/mi. NOx was chosen because it is felt to be the most common cutpoint 

combination used by I/M programs because it is the cutpoint combination EPA suggests in the 

I/M test procedure guidance, EPA Report Number EPA-AA-EPSD-IM-93-1. Based on the 

results of the ANOVA analysis, the data was stratified by model year group based on EPA 

standards. Using cutpoint combination #2, the mean cold start after-repair emission levels were 

determined for LDVs and LDTs for Emitter Class A & B. These cold start after-repair emission 

levels, found in Tables 6 and 7, will be used to develop repair effects that will be applied to all 

starts in MOBILE6. It is these newly developed repair effects that will be used to compute I/M 

credits for start emissions. The methodology used to compute repair effects on starts and to 

calculate I/M credits can be found in EPA report number M6.IM.001. 

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