United States Air and Radiation EPA420-R-01-058 

Environmental Protection November 2001 

Agency M6.STE.003 

Determination of Start 

Emissions as a Function of 

Mileage and Soak Time for 

1981-1993 Model Year 

Light-Duty Vehicles

 Printed on Recycled 

Paper 
EPA420-R-01-058 

November 2001 

Determination of Start Emissions as a Function of
 

Mileage and Soak Time for 1981-1993 Model Year
 

Light-Duty Vehicles
 

M6.STE.003
 

Ed Glover
 

Penny Carey
 

Assessment and Standards Division
 

Office of Transportation and Air Quality
 

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 that 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.
 
 

 

 

 

 

 

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1.0 INTRODUCTION
 

MOBILE6 will allocate vehicle exhaust emissions to either the “extra” emissions 

associated with engine start (start emissions) or the “base” emissions associated with travel 

(running emissions). This distinction is to some extent an artificial constraint since in 

reality “start” emissions in part will be released while the vehicle is in motion. However, 

it is a useful constraint and not too far from physical reality, in at least warm weather 

driving conditions, when the extra emissions from a start end after only 1 or 2 minutes of 

driving. This split allows the separate characterization of start and running emissions for 

correction factors such as fuel effects and ambient temperature. It also allows a more 

precise weighting of these two aspects of exhaust emissions for particular situations such 

as morning commute, parking lots and freeways. This document describes the 

methodology used to calculate start emissions as a function of mileage and soak time for 

use in MOBILE6. The results for model year 1981-1993 light-duty cars and light-duty 

trucks are presented. The deterioration of running emissions are addressed in a separate 

document (EPA Report Number M6.EXH.001). 

This document is organized into six sections. The first section is the short 

introductory section. Section 2 describes the FTP data sources and the model year and 

technology groups which are used. Section 3 provides a definition of start emissions in 

mathematical terms and shows the relationship between start emissions and the Federal 

Test Procedure (FTP) emissions. This includes a description of the FTP cycle, the Hot505 

cycle, and a definition of cold start and hot start emissions. Section 4 describes the 

methodology used to predict start emissions as a function of soak time. Section 5 shows 

the algorithm used to predict start emissions versus mileage. Section 6 contains an example 

of the final start emission results as used in MOBILE6 as a function of both deterioration 

and soak time. Appendix A briefly discusses the methodology and rational for using a high 

emitter correction factor and presents the fleetwide high emitter penetration fractions. 

2.0 DATA SOURCES USED 

The basic datasets used to determine in-use deterioration are based on FTP testing. 

(I/M data from Dayton, Ohio were also used to correct the results for recruitment bias 

which EPA believes affects the FTP test samples - see Appendix A). Three FTP data 

sources were used: 1) the test results from the EPA laboratory in Ann Arbor, Michigan, 2) 

the data received from AAMA (American Automobile Manufacturers Association) based 

on testing conducted in Michigan and Arizona, and 3) the API (American Petroleum 

Institute) data collected in Arizona. The model years in the dataset range from 1981 

through 1993, and contain both cars and trucks. Table 1 gives a breakdown by vehicle type, 

model year, and technology for the three datasets combined. 
-3­

Table 1
 

Distribution of Vehicles by Model Year and Technology*
 

Cars Cars Cars Cars Cars Trucks Trucks Trucks Trucks 

MYR OPLP CL TBI PFI ALL MYR OPLP CL TBI PFI 

Carb Carb 

81 367 657 15 29 1068
 81 124 82 71 71 74 8 224
 82 45 83 63 57 127 62 309
 83 8 3 84 5 30 46 35 116
 84 26 22 1 85 24 74 56 66 220
 85 33 30 13 6 86 7 34 60 92 193
 86 14 9 23 41 87 1 17 76 106 200
 87 6 4 88 15 69 113 197
 88 89 22 38 103 163
 89 90 160 250 410
 90 144 1 91 91 426 517
 91 141 144 92 57 347 404
 92 92 92 93 29 366 395
 93 90 93 

ALL 538 977 898 2003 4416
 ALL 250 64 509 382 

* No entry indicates no data available for that model year/technology type in the FTP dataset used for t

M6.STE.003 
 

 

 

 

-4­

In general, most of the 1990+ model year vehicle data were supplied by AAMA, 

and most of the pre-1990 data were supplied by the EPA laboratory testing. The API 

sample is a relatively small sample (99 cars and trucks). Its chief appeal is that the vehicles 

all have generally higher mileage readings than the rest of the sample (all over 100,000 

miles). The other general trend in the data is toward PFI technology, and away from the 

others. This is seen in the 1990+ vehicles which are predominately PFI with some TBI still 

present. The 1981 - 1989 model years start with a high percentage of carbureted and some 

open loop, but end with mostly TBI and PFI technology. Although not explicitly shown 

in the tables, new catalyst technology was phased slowly into the fleet starting in the mid 

1980's. 

For analysis, the cars and trucks were placed into the model year/technology groups 

shown below. Trucks were separated into five different groups by pollutant due to 

differences in certification standards. 

CARS 

MY Group / Technology Type 

HC/CO/NOX 

1988-93 PFI 

1988-93 TBI 

1983-87 FI 

1986-93 Carb 

1983-85 Carb 

1981-82 FI 

1981-82 Carb 

TRUCKS 

MY Groups / Technology Type 

HC/CO/NOX 

1988-93 PFI 

1988-93 TBI 

1981-87 FI 

1984-93 Carb 

1981-83 Carb 

The technology groups are closed-loop ported fuel injection (PFI), closed-loop throttle body 

injection (TBI), and carbureted (CARB). FI refers to a combination of PFI and TBI. 

CARB includes both closed-loop and open-loop vehicles which are carbureted. These 

model year/technology grouping boundaries were selected on the basis of changes in 

emission standards or the development/refinement of new fuel metering or catalyst 

technologies. It is assumed that as of 1990, carbureted technology had a very tiny market 

share, and are included with the previous carbureted group. Because of the relatively large 

amount of 1988-93 fuel injected data, the category was split into PFI technology and TBI 

technology for both cars and trucks. This produces separate deterioration functions based 

on this fuel delivery technology and allows the modeling of the future penetration of PFI 

technology into the in-use fleet. 
 

 

 

 

 

-5­

3.0 DEFINITION OF START EMISSIONS 

3.1 Overview of the Federal Test Procedure (FTP) 

The Federal Test Procedure (FTP) is a test cycle which is used to certify new 

vehicles to emission performance standards (see 40 CFR Part 86, Subpart B, Section 

86.144). The FTP consists of a cold start segment (Bag 1), a hot stabilized segment (Bag 

2), and a hot start segment (Bag 3). Initially, the vehicle is stored for a minimum of 12 

hours before testing to simulate a 12 hour overnight soak period. The vehicle is then driven 

over the cold start segment, which lasts 505 seconds over a length of 3.59 miles, and the 

emissions collected as Bag 1. The latter part of the driving in Bag 1 occurs with the engine 

and catalyst in a hot stabilized condition. Bag 2 emissions are then immediately collected 

from the hot stabilized segment, which lasts 867 seconds over a length of 3.91 miles. After 

a 10 minute soak, the 505 seconds of the start segment is then repeated and the emissions 

collected as Bag 3. 

The FTP composite emission rate is a weighted combination of the three measured 

bags to represent two trips. The first trip is a cold start trip after a 12 hour soak, and the 

other is a hot start trip after a 10 minute soak. Each trip is a “LA4" cycle, which is a 

combination of the 505 cycle (either Bag 1 or Bag 3) and the Bag 2 cycle. In a typical FTP 

test, the Bag 2 is only measured once and the results are used for both trips. Since the 505 

cycle is 3.59 miles long and the Bag 2 cycle is 3.91 miles long, each LA4 trip is 7.5 miles 

long. The cold start trip is weighted at 43% and the hot start trip weighted 57%. If the cold 

start trip is 43% of the driving, then the vehicle miles traveled (VMT) in Bag 1 (containing 

the cold start) is: 

FTP Bag 1 VMT Weighting = 43% * ( 3.59 miles / 7.5 miles ) = 0.206 

The hot start trip is 57% of driving, and the VMT weighting for Bag 3 (containing the hot 

start) is: 

FTP Bag 3 VMT Weighting = 57% * ( 3.59 miles / 7.5 miles ) = 0.273 

The remaining VMT is from stabilized driving, represented by Bag 2. Since Bag 2 is used 

for both the cold start and hot start trips, it uses VMT weighting from both. 

FTP Bag 2 VMT Weighting = ( 43% + 57% ) * ( 3.91 miles / 7.5 miles ) = 0.521 

The standard VMT weighting of the bags reported in grams per mile for the full FTP are: 
 

 

 

 

 

-6­

FTP = (Bag 1 * 0.206) + (Bag 2 * 0.521) + (Bag 3 * 0.273) 

where the fractions represent the amount of vehicle miles traveled within the three modes 

during the FTP trip, and Bag1, Bag2, Bag3 and the FTP emissions are in grams per mile 

(g/mi). 

3.2 Overview of the Hot Running 505 and Its Use 

The FTP testing method outlined above does not allow the precise separation of 

start and running emissions, since Bags 1 and 3 contain both start and running emissions. 

Bag 2 of the FTP does not contain an engine start; however, the driving cycle used in the 

second bag is significantly different from the cycle used for Bags 1 and 3. Thus, to 

estimate the amount of FTP emissions that can be allocated to engine start, the concept of 

the Hot Running 505 (HR505) is needed. 

The HR505 is an extra 505 cycle performed immediately following bag 3 of the 

FTP. It uses an identical driving cycle as the first and third bags of the FTP, but does not 

include an engine start. For more information, refer to the document “The Determination 

of Hot Running Emissions from FTP Bag Emissions”, report number M6.STE.002. With 

a HR505 emission result, it is possible to compare the results obtained from the HR505 to 

the results from Bags 1 and 3 of the FTP to determine the portions of Bags 1 and 3 

attributable to start emissions following a 12 hour soak and start emissions following a 10 

minute soak, respectively. 

Since the HR505 has not historically been included in FTP test programs, a method 

of estimating the HR505 was developed, as described in report M6.STE.002. Briefly, 

HR505 emissions were measured from a sample of 77 vehicles tested under EPA contract. 

The results from this vehicle sample were used to develop a correlation between the HR505 

and FTP bag data. This correlation was then used to estimate HR505 results for the FTP 

dataset used for this analysis. 

3.3 Basic Start Emission Rate 

For MOBILE6, the basic unit of engine start emissions is defined as a start after a 

12 hour soak. The units for engine start emissions will be grams, instead of grams per mile, 

since start emissions will not be allocated by vehicle miles traveled. The engine start basic 

emission rate can be determined by subtracting the HR505 emission rate from the Bag 1 

emission rate (in grams per mile) using the nominal distance traveled in the 505 driving 

cycle: 

Basic Start Emission Rate (grams) = [Bag 1(g/mi) - HR505(g/mi)] * 3.59 miles 
 

 

 

 

 

-7­

For illustration purposes, the average basic start emission rates (in grams) after a 

12 hour soak were calculated for each model year and are shown in Tables 2 and 3 for cars 

and trucks. 

Start emissions after a 10 minute soak can also be estimated from the Bag 3 and 

HR505 emission rates, analogous to the basic start emission rate: 

Start Emissions after 10 minute soak (grams) = [Bag 3(g/mi) - HR505(g/mi)] * 3.59 miles 

The average start emissions after a 10 minute soak are also shown in Tables 2 and 

3 for each model year and for cars and trucks. 

For some FTP tests of some cars, the predicted HR505 emissions are higher than 

Bag1 and/or Bag3 emissions. This causes the start emissions to be negative. This probably 

is due to intermittent emission control system defects. Except in some cases of very small 

samples, the negative values were retained in the analysis. Sample sizes are shown in Table 

1. 

4.0 Basic Start Deterioration with Mileage 

4.1 Definition of Categories 

The basic modeling concept behind the start emission factor methodology is that 

the fleet can be represented as two types of vehicle emitter categories. These two types are 

termed “high” emitters, and “normal” emitters. The “high” emitters have FTP average 

emission levels which are considerably higher than the overall mean emission levels, and 

significantly higher than their FTP standards, indicating that they have problems with their 

emission control systems. The “normal” emitters are low and average emitting vehicles 

with emission control systems which are generally functioning properly. The overall fleet 

emission factor is a weighted average of the high and normal emitters. 

The high/normal emitter modeling concept is also used in the estimation of running 

emissions, and is discussed in depth in other reports. For both start and running emissions, 

the high/normal concept allows for corrections due to recruitment bias against higher 

emitting vehicles, and for more accurate estimates of the effects of I/M programs, fuel 

effects, temperatures, etc. 

In the data analysis, vehicles were defined as normal emitters for a specific pollutant 

if their FTP HC emissions were less than twice the applicable new car certification 

standard, or their FTP CO emissions were less than three times the applicable new car 
-8­

Table 2
 

Mean Estimated Start and FTP Emission Levels by Model Year for Light-Duty 

in the FTP Dataset 

Basic start (after 12 hour soak) Start (after 10 min soak) Composite 

grams grams grams/m

MYR HC CO NOx MYR HC CO NOx MYR HC CO 

81 4.002 46.419 1.373 81 0.610 5.115 0.041 81 0.706 9.6

82 2.445 36.378 1.237 82 0.373 4.843 -0.045 82 0.789 8.3

83 2.399 26.112 1.264 83 0.400 3.827 0.150 83 0.431 5.07

84 2.950 34.827 1.190 84 0.513 3.418 0.047 84 0.756 9.9

85 3.468 30.353 1.204 85 0.506 4.737 0.095 85 0.533 6.9

86 2.526 26.639 1.432 86 0.298 2.082 0.241 86 0.926 10.

87 2.712 20.030 1.376 87 0.597 2.104 0.170 87 0.656 8.3

88 2.831 19.716 1.419 88 0.406 1.147 0.223 88 0.406 4.5

89 2.254 18.610 1.434 89 0.379 2.524 0.216 89 0.311 3.9

90 2.169 18.677 1.930 90 0.332 2.219 0.611 90 0.274 3.6

91 2.183 19.494 1.443 91 0.275 2.132 0.530 91 0.237 3.1

92 2.271 18.878 1.645 92 0.304 2.595 0.485 92 0.267 4.3

93 2.312 21.030 1.801 93 0.310 2.564 0.392 93 0.225 2.5

M6.STE.003 
-9­

Table 3
 

Mean Estimated Start and FTP Emission Levels by Model Year for Light-DutyT

in the FTP Dataset
 

Basic start (after 12 hour soak) Start (after 10 min soak) Compos

grams grams grams

MYR HC CO NOx MY HC CO NOx MY HC C

R R 

81 7.342 107.501 1.055 81 1.212 14.211 0.385 81 1.275 18

82 7.909 116.584 -0.119 82 1.489 14.189 0.006 82 1.732 16

83 6.537 104.817 0.796 83 1.577 18.657 -0.209 83 1.361 13

84 5.219 95.893 0.299 84 1.098 20.057 0.004 84 0.802 10

85 4.766 84.621 0.457 85 0.854 7.742 0.102 85 1.281 14

86 3.752 41.196 0.729 86 0.607 2.148 0.128 86 0.823 8

87 3.352 26.635 1.266 87 0.566 1.433 0.017 87 0.401 4

88 88 88 

89 89 89 

90 4.705 45.331 4.683 90 0.930 7.037 0.765 90 0.800 9

91 3.521 41.128 2.761 91 0.878 7.129 0.519 91 

92 3.656 41.446 3.054 92 0.654 5.746 0.656 92 

93 3.644 40.557 2.736 93 0.589 4.634 0.676 93 0.420 5

M6.STE.003 
-10­

certification standard, or their FTP NOx emissions were less than twice the new car 

certification standard. 

Vehicles were defined as high emitters for a specific pollutant if their FTP HC 

emissions or FTP CO emissions exceeded twice or three times the applicable new car 

certification standard, respectively, or their FTP NOx emissions were two times the new 

car certification standard. Because high NOx emissions often occur with low HC and/or 

low CO emissions, and sometimes even HC can be high and CO normal, the three 

categories were kept separate. Thus, a vehicle could be a high HC emitter, but a normal 

CO and NOx emitter. 

4.2 Calculation of Start Emission Rates for Normal Emitters 

Emission rates for normal emitters were calculated by least squares regression of 

the emissions of the normal emitters versus mileage. The regression was done for each 

pollutant / model year / technology group. The start emission regression coefficients for 

cars are shown in Table 4a and for trucks in Table 4b. The column labeled ZML 

contains the zero mile coefficients, and the column DET contains the deterioration 

coefficients (slope) from the regressions. Appendix B Tables B-3 through B-8 show the 

regression statistics for normal emitter cars and trucks for each pollutant. 

Table 4a 

Regression Coefficients for START Emissions from Normal Emitter CARS 

MY 

Group 

Tech 

Group HC Coefficients CO Coefficients NOx Coefficients 

ZML DET ZML DET ZML DET 

1988-93 PFI 1.9987 0.006830 18.972 0.00703 1.444 0.00220 

1988-93 TBI 1.9019 0.002679 19.233 0.00000 2.300 0.00000 

1983-87 FI 2.3589 0.001388 19.949 0.00000 1.461 0.00141 

1986-89 Carb 1.4934 0.018238 24.698 0.10947 1.405 0.00000 

1983-85 Carb 1.5892 0.009408 24.442 0.10577 0.748 0.00524 

1981-82 FI 2.3543 0.008533 20.038 0.22673 1.530 0.00059 

1981-82 Carb 2.1213 0.013610 28.637 0.22673 1.601 0.00000 
-11­

Table 4b 

Regression Coefficients for START Emissions from 

Normal Emitter Light Trucks 

MY 

Group 

Tech 

Group HC Coefficients CO Coefficients NOx Coefficients 

ZML DET ZML DET ZML DET 

1988-93 PFI 2.873 0.00000 32.178 0.0168 1.597 0.00000 

1988-93 TBI 4.073 0.01309 42.456 0.1411 4.294 0.00324 

1981-87 FI 2.599 0.00964 23.497 0.0613 1.384 0.00000 

1984-93 Carb 3.916 0.00854 78.286 0.2564 0.143 0.00436 

1981-83 Carb 6.817 0.00154 98.432 0.3240 1.082 0.00000 

4.3 Calculation of Start Emission Rates for High Emitters 

High emitters are the vehicles in the fleet which likely have problems with their 

emission control systems, as evidenced by emission levels which are considerably 

higher than the FTP standards. In the analysis they were defined as those vehicles 

exceeding either twice FTP standards for HC or NOX or three times FTP standards for 

CO. The emissions line is a flat horizontal line because the emissions of a high emitter 

were not a statistically significant function of mileage. In addition, the relatively small 

sample sizes of high emitters make regression determined mileage coefficients 

unreliable indicators of actual behavior. Table 5a shows the average emissions of the 

high emitters (cars only) for the pollutant / model year / tech groups, and Table 5b 

shows the analogous results for light trucks. Appendix B Tables B-1 and B-2 show the 

sample size and standard deviation statistics for the High emitter cars and trucks. It 

should be noted by the reader that the data were combined across model year and 

technology group in some cases to produce a larger sample of high emitters. 

For NOX start emissions, the normals and the highs were combined together, 

and the emissions were regressed versus mileage. This has the effect of eliminating the 

NOX high emitter group for start emissions. This combination was done for two 

reasons. First, for many of the model year / tech groups the average NOX start 

emissions of high emitters were found to not be statistically significantly different from 

start emissions from the normals. This was found to be the case even when different 

definitions of a high emitter were tried (1X FTP NOX, 1.5X FTP NOX, and 2X FTP 

NOX). This phenomenon is consistent with the mechanisms of NOX formation - higher 

emissions under lean high temperature / load FTP conditions, and lower during rich and 
-12­

cooler start conditions. Second, the sample sizes for NOX high emitters were smaller in 

both an absolute sense, and in comparison to the HC and CO high emitter sample. 

Table 5a 

Mean START Emissions of High Emitter CARS 

MY Group 

Tech 

Group HC Mean CO Mean NOx Mean 

1988-93 PFI 4.829 38.06 Same as Normals 

1988-93 TBI 4.829 38.06 Same as Normals 

1983-87 FI 5.313 65.31 Same as Normals 

1986-89 Carb 10.520 92.82 Same as Normals 

1983-85 Carb 10.520 92.82 Same as Normals 

1981-82 FI 5.313 92.82 Same as Normals 

1981-82 Carb 10.520 92.82 Same as Normals 

Table 5b 

Mean START Emissions of High Emitter Trucks 

MY Group 

Tech 

Group HC Mean CO Mean NOx Mean 

1988-93 PFI 5.212 83.862 Same as Normals 

1988-93 TBI 5.212 83.862 Same as Normals 

1981-87 FI 5.826 60.319 Same as Normals 

1984-93 Carb 9.406 162.115 Same as Normals 

1981-83 Carb 17.865 179.549 Same as Normals 

The other anomally in the results was the HC and CO high emitter emission 

levels for the 1990-93 TBI group. In both of these cases the average start emission 

levels of the few cars tested were judged to be unrealistically low. For the case of CO, 

the value was -123.84 grams, and for HC it was 0.0356 grams. These low average 

levels are the result of small sample size, and the possibility of negative values when the 
-13­

hot running 505 on a particular car is greater than Bag1 of the FTP. Rather than insert 

negative values in the MOBILE6 model, the HC and CO high emitter emission levels 

from the 1988-93 PFI group were substituted in the 1988-93 TBI group. This is a 

reasonable assumption since these vehicles are generally about the same age and model 

year vintage, and have reasonably similar emission control technology. 

4.4 Fraction of High and Normal Emitters in the Fleet 

The basic start emission factor is computed from a weighted average of the highs 

and normals, and the fraction of high emitters in the fleet is the weighting factor for the 

highs and normals. The fraction of high emitters in the fleet and the definition of a high 

emitter are based on FTP emission results. Since similar definitions of a high emitter 

were not available in running and start units, the consistent FTP high emitter definition 

and high emitter fraction values were used for both start and running, but only for HC 

an CO emissions. For NOx emissions, the high emitter fraction for running emissions is 

the same as the FTP fraction, but the start fraction is not. The start NOx high fraction 

is assumed to be zero because NOx emissions rarely occur in high concentration during 

start operation. A quick analysis of the data using various definitions of a high emitter 

for start and running indicated that this was a reasonable choice. However, from a 

vehicle engineering perspective, start emission problems sometimes are not well 

associated with running operation emission problems (i.e., choke system malfunctions). 

Appendix A presents the fraction of HC and CO high emitters in the fleet at 

selected mileages / ages for each pollutant (see document M6.IM.001 for further 

details). The fraction of NOX high emitters is not shown because for NOX because the 

Normals and Highs are assumed to have the same emission rate (no start NOX highs are 

assumed to exist). Appendix A also provides a brief discussion of the high emitter 

adjustment factor used to correct for recruitment bias inherent in FTP data type 

sampling. This adjustment factor generally increased the rate of high emitters in the 

fleet over the fraction that would be predicted based on the FTP sampling. 

4.5 Calculation of Basic Start Emission Rates 

The basic start emission rate is calculated for each combination of vehicle type / 

pollutant / model year group / technology group. The units are start emissions in grams. 

Equation 1 is used to calculate the basic (mean) start emission rate from the high and 

normal emitter emission values and the rate of high emitters in the fleet. For NOX 

emissions a special case of this equation is used where the normal and high emission 

rates are set equal to each other. 
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START = ST_High_ave * Highs + ST_Norm_ave * Normals Eqn 1 

Where: 

Highs = fraction of High emitters 

Normals = fraction of Normal emitters 

START is the basic (mean) emission rate 

ST_High_ave is the high emitter start emission average 

ST_Norm_ave is the normal emitter start emission average 

Where: 

Highs + Normals = 1 Eqn 2 

5.0 Start Emissions Versus Soak Time 

Start emissions will now be modeled as a function of soak time in MOBILE6. 

As such, the model will be able to account for the entire distribution of soak times 

observed in the fleet rather than just two soak time points (10 and 720 minutes) that 

were implicit in the FTP test procedure and in MOBILE5. The model will allow the 

soak time to range from a minimum of zero minutes up to a 12 hour soak period (720 

minutes). Soak periods exceeding 12 hours will be assumed to be the same as for a 12 

hour soak. 

The MOBILE6 relationship between start emissions and soak time was 

developed by using the FTP database, and a California soak time and engine start 

model (CARB model). The FTP start emission data were available only at the soak 

time periods of 10 minutes (hot start), and 720 minutes (cold start). The CARB model 

predicted start soak emission effects for the entire range of soak time lengths (0 to 720 

minutes). The details of CARB model are documented in the report “Methodology for 

Calculating and Redefining Cold and Hot Start Emissions”. 

The start emissions model developed for MOBILE6 is really just an adjusted or 

calibrated version of the CARB model. It uses the FTP start emission data from the two 

FTP soak times to adjust the CARB model curves at the ten minute level. This has the 

effect of forcing the CARB model curves through these two points, but retaining the 

general shape of the CARB model curve. The start emission data points at 10 minutes 

and 720 minutes are derived from the FTP dataset described earlier. The California 
 

 

-15­

interpolation curves (California Soak Function) is a function of pollutant and catalyst 

type. 

Basic Start Soak Equation 

The general form of the MOBILE6 start emissions as a function of soak time 

calculation is shown in Equation 3. 

Start Emissions (@ soak time) = 	 Basic Start Emissions (@ 12 hour soak) * 

Soak Function(@soak time) Eqn 3 

At a given soak time, it is the product of the basic 12 hour cold start emission 

factor and the soak function. The Basic Start Emissions are the start emission factors 

discussed previously in this document. The Soak Function is a multiplicative correction 

factor that converts the basic 12 hour start emission factor into a cold start emission 

factor at any soak time length. Mathematically, the Soak Function is defined in 

Equation 4. 

Soak Function = California Soak Function * [Ratio+(1-Ratio)*((SoakTime-10)/(X-10))] Eqn 4 

The terms used in the equation re defined below. 

California Soak Function: 

The California Soak Function is the basis of the MOBILE6 start soak time 

model. It is an empirical relationship derived by the California Air Resources Board 

(CARB), and used in the EMFAC model to calculate start emissions as a function of 

soak time. Upon review of the CARB document “Methodology for Calculating and 

Redefining Cold and Hot Start Emissions”, EPA found it to be the only available 

empirical model for this purpose. 

Mathematically, the California Soak Function has the following general form: 

California Soak Function = a + b*soak length + c*soak length ^ 2 Eqn 5 

where soak length is the time in minutes since the last engine operation, and a, b, and c 

are soak function coefficients. These are shown in Table 6 for each pollutant, three 

different catalyst types (the coefficients for catalyst-equipped vehicles are for the model 

year/technology groups examined in this report), and by soak time domain. The soak 

time domain is divided into two groups (a low domain and a high domain) which are 

listed in Table 6. 
-16­

Table 6
 

Coefficients for Adjusting Engine Start Emissions for Soak
 

Time
 

(from “Methodology for Calculating and Redefining Cold and Hot Start Emissions”, CARB) 

Non-Catalyst Vehicles 

HC 

Curve 1 

HC 

Curve 2 

CO 

Curve 1 

CO 

Curve 2 

NOX 

Curve 1 

NOX Curve 

2 

Constant 0.38067 0.43628 0.43803 -0.08541 1.31568 2.48061 

minutes -0.00163 0.00078 -0.00998 0.00303 0.02752 -0.00018 

minutes2 6.64E-05 0 7.01E-05 -2.11E06 -0.00015 -2.6E-06 

domain(min) 0-52 53-720 0-119 120-720 0-119 120-720 

Catalyst Equipped Vehicles 

HC 

Curve 1 

HC 

Curve 2 

CO 

Curve 1 

CO 

Curve 2 

Nox 

Curve 1 

NOx Curve 2 

Constant 0 0.57130 0 0.70641 0.11796 1.12983 

minutes 0.01272 0.00072 0.01195 0.00033 0.02967 2.21E-05 

minutes2 -6.30E05 -1.76E-07 -4.76E05 1.00E-07 -0.00021 -3.04E-07 

domain(min) 0-89 90-720 0-116 117-720 0-61 62-720 

Electrically Heated Catalyst Equipped Vehicles 

HC 

Curve 1 

HC 

Curve 2 

CO 

Curve 1 

CO 

Curve 2 

Nox 

Curve 1 

NOx Curve 2 

Constant (a) 0 0.50641 0 0.44733 1.05017 1.37178 

minutes (b) 0.00561 0.00069 0.00707 0.00162 0.00362 0.00027 

minutes2 (c) -5.09E06 0 -1.33E05 -1.18E06 -5.57E06 -1.09E-06 

domain(min) 0-117 118-720 0-107 108-720 0-113 114-720 

California Soak Function = a + b * minutes + c * minutes2 (where minutes is time since last engine operation (i.e., soak time)) 

The Soak Function is the grams per soak time i divided by the grams per overnight soak (720 minutes or 12 hours) 
 

-17­

Ratio 

The Ratio is the variable shown in Equation 4. It is a ratio of two ratios, and is 

shown mathematically in Equation 6. 

EPA Start Emissions at 10 minutes 

EPA Start Emissions at 720 minutes 

Ratio = _____________________________ Eqn 6 

CARB Start Emissions at 10 minutes
 

CARB Start Emissions at 720 minutes
 

or 

Ratio = EPA Ratio / CARB Ratio 

The EPA ratio for each pollutant was derived empirically from FTP hot and cold 

start test data by calculating a hot start emission factor and a cold start emission factor 

using the equations in Section 3.3, and dividing the Start @10 minutes (hot start) by the 

Start @ 720 minutes (cold start). One value for each pollutant was developed that 

included all technologies and vehicle types. These values, used in the numerator of the 

equation, are: 

EPA Ratio HC = 0.160 

EPA Ratio CO = 0.112 

EPA Ratio NOx = 0.204. 

The statistics from the data analysis used to generate these ratios is provided in 

the attached document STAT.lst. 

The CARB ratios were computed by inserting a soak time of 10 minutes and a 

soak time of 720 minute into Equation 5 for catalyst equipped vehicles. For catalyst 

equipped vehicles, the ratios were for each pollutant. 

CARB Ratio HC = 0.1209 

CARB Ratio CO = 0.1147 

CARB Ratio NOx = 0.3937 

Differences in these ratios reflect differences in the hot and cold start data from 

EPA and California. Note there are only small differences between the EPA and CARB 
-18­

results for CO start emissions. 

The final ratios used in Equation 4 is the ratio of the EPA and CARB ratios. These are 

shown below for each pollutant. 

Ratio HC = 1.3234 

Ratio CO = 0.9765 

Ratio NOx = 0.5182 

SoakTime 

The soak time is the time duration between successive engine starts in minutes. 

It can range from zero minutes to 720 minutes. 

X term: This term is defined to be zero for soak times from 0-10 minutes. For the range 

from 10 minutes to 720 minutes, it is set equal to the highest minute in the domain of 

the California Soak Function. For example, HC emissions from catalyst equipped 

vehicles have two time domains in Table 6. These are 0-89 minutes and 90-720 

minutes. Thus, for this example, X = 0 for times of 10 minutes or less, and X = 89 for 

times from 11 minutes through 89 minutes. No soak adjustment is applied for the 

remaining soak period of 90 minutes through 720 minutes. Only the California Soak 

Function is used. 

6.0 START EMISSION RESULTS 

Start emissions are both a function of vehicle deterioration represented by 

mileage, and soak time. In previous sections the results were shown separately. In this 

section, examples of the results are shown with both effects combined. 

Shown in the linked EXCEL spreadsheets (CAR_BER.xls and TR_BER.xls) is a 

sample calculation of the basic emission start factors for the various model year groups 

and pollutants. It includes calculations for both start and running emissions. The 

calculations in the spreadsheet use Equation 1 in this document, and show the 

magnitude of the start emission factors that will be used in MOBILE6. 

The statistical results and output are too voluminous to present directly in this 

document. However, they are available in the linked document (stat.lst). The statistical 
-19­

software SPSS was used to perform the linear regressions and compute the means. In 

general, the regression correlation coefficients (r-squared) are not high (< 0.10), and 

reflect the tremendous scatter in emissions data. However, virtually, all of the 

regression coefficients of the normal emitters are significant at least at a 90 percent 

confidence level. On the other hand, the confidence intervals around the average start 

emission levels of the high emitters are quite large due to high scatter and small sample 

sizes. 

Shown below for illustration purposes is a sample calculation of start emissions. 

It illustrates the soak function equations and methodology shown in Section 5. 

Example:	 Calculate HC start emissions at a soak time of 88 minutes for a 1991 

model year PFI-equipped car with 60,000 miles. 

Start Emissions (@88min) = Basic Start Emissions (@12hr) * Soak Function 

From Tables 4a, 5a and A-1: 

Basic Start Emissions = High_ave * Highs + Norm_ave * Normals 

START = 4.829*0.0987 + (1.999 + 0.00683*60)(1.0 - 0.0987) = 2.647 g HC 

Soak Function = California Soak Funct * [Ratio+(1-Ratio)*((SoakTime-10)/(X-10))] 

From Table 6, using the coefficients for catalyst-equipped vehicles:
 

California Soak Funct = 0.000 + (0.01272)*(88) + (-6.30E-05)*(88)2 = 0.63149
 

Ratio = (Start@10min / Start@12hr) / California Soak Funct@10min

 = 1.3234 for HC as given in Section 5.0 

SoakTime = 88 minutes 

X = 89 minute HC time domain (from Table 6). 

Soak Function = 0.63149 * [1.3234 + (1-1.3234) * ((88-10) / (89-10))] = 0.63407 

Start Emissions(@90min) = 2.647 * 0.63407 = 1.679g HC 
-20­

New Data Update 

No new data have been incorporated into this analysis since the initial Draft release 

of this document. 

Response to Stakeholder and Peer Review Comments 

Significant stakeholder comments were not received for this document. However, 

three separate paid and independent peer reviewers were used, and provided the following 

comments. Their comments were either addressed directly in the document or are discussed 

below. 

1.	 One reviewer questioned the inclusion of the API vehicle test data solely on the 

basis of its higher mileage nature. He felt that the different geographical location 

of the testing and sampling, and potentially different recruitment techniques may 

add uncertainty to the results. 

In response, EPA feels that these geographical differences should be minimal, 

and that API followed reasonable and traditional recruitment and testing 

methods. In addition, the inclusion of these data provide largely unique and 

sorely needed information on higher mileage vehicles. 

2.	 In regards to the Start Soak Adjustment, one reviewer expressed concern about 

the use of two data points to adjust the entire continuum of soak time emission 

effects. 

This is certainty a valid concern. However, the only data outside of the special 

CARB study which were available are at the FTP soak time values of 10 minutes 

and 720 minutes. To help overcome this lack of data, the CARB study model is 

used to fill in these blanks. 

3.	 One reviewer stressed the need for the regression statistics to be reported in an 

easily readable table. This will allow subsequent reviewers to determine the 

uncertainty in the final results. 

These are now provided in Appendix B in Tables B-1 through B-8. 

4.	 The remaining comments presented by the Peer reviewers which could not be 

incorporated into this document largely pertain to suggestions that more testing 

and data analysis are needed. In particular more information is needed to better 
-21­

characterize the behavior of high emitters, and develop more robust statistics. 

They also stressed that additional data should be obtained on start soak periods 

between the extremes of 10 minutes and 720 minutes to better validate the model. 

EPA agrees on these points. However, such data collection efforts and analysis 

will likely have to wait for the next generation of MOBILE models. 
 

 

 

 

 

-22­

Appendix A
 

Fraction of High and Normal Emitters in the Fleet
 

This appendix shows the fraction of HC and CO high and normal emitters in the 

fleet for start emissions. High emitter fractions for NOx emissions are not shown because 

only normal emitters are assumed to exist for NOx start emissions (data suggested that high 

NOx emissions rarely occur during cold start). This Appendix also briefly discusses the 

methodology and rational for the high emitter adjustment factor for recruitment bias. 

Tables A-1 and A-2 show the fraction of HC and CO high emitters in the fleet after 

they have been adjusted for recruitment bias. The use of these to develop start emission 

factors are discussed in Section 4.4 and Section 4.5. The derivation of these high emitter 

fractions and the average High and Normal emitter emission factors is discussed in detail 

in EPA document M6.IM.001. However, a brief description and the mathematical equation 

is shown below. 

The number of High and Normal emitters is calculated at each age point for each 

combination of vehicle type / pollutant / model year / technology group using the following 

general equations. 

Where:
 

‘Highs’ is the fraction of High emitters.
 

‘Normals’ is the fraction of Normal emitters.
 

‘RLA4' is the average running emission rate, after adjustment based on IM240 data from
 

Dayton, OH.
 

‘High_ave’ is the high emitter running emission average estimated from the FTP data.
 

‘Norm_ave’ is the normal emitter running emission average estimated from the FTP data.
 

Highs + Normals = 1 Eqn A-1 

and 

RLA4 = High_ave * Highs + Norm_ave * Normals Eqn A-2 

Solving for the variables Highs and Normals produces: 

Highs = (RLA4 - Norm_ave) / (Highave - Norm_ave) Eqn A-3 

Normals = 1 - Highs Eqn A-4 
 

 

 

 

-23­

Recruitment Bias Discussion 

The theoretical justification for the FTP recruitment bias rests on anecdotal 

observations from test programs over the years that sample vehicles recruited by EPA or 

a major automaker will be disproportionately lower in high emitters. This is because 

owners who will participate in a test program are less likely to tamper or deliberately malmaintain their vehicle than are owners who decline to participate. This may result from 

the fear exposure by a Federal agency for an act which is illegal (tampering the emission 

controls on a vehicle). The other bias which is likely present in the EPA and automaker 

data is the tendency to recruit relatively low mileage vehicles at the expense of high 

mileage vehicles. These low mileage vehicles are frequently a test priority for an automaker 

due to warranty and recall concerns. This type of sampling bias leads to a well 

characterized sample with mileages less than 50,000 miles, but a more poorly characterized 

sample with higher mileages. 

To experimentally verify the potential high emitter recruitment bias, a large amount 

of in-use IM240 data from the Dayton, Ohio I/M program (211,000 initial tests) were 

obtained. These data were used to investigate and correct the recruitment bias believed to 

be inherent in the FTP type data. Since the Dayton sample contains virtually the entire 

vehicle fleet (or a randomly selected 50 percent) of the city these issues of recruitment bias 

should be minimal. For example, the sample size is quite large (211,000 vehicles). Even 

if high emitters are just a few percent of this sample, they should be well characterized. 

Also, I/M is not a voluntary program in Ohio; thus, a motorist cannot simply decline to 

participate; however, they can illegally evade the program by registering outside of the 

program boundary. It was suggested by some reviewers, that the program evaders could 

range from 1 percent to 5 percent of the total fleet. 

Although the mileage data from Dayton are highly suspect and not useful, the 

vehicles of a given model year vintage are older than the vehicles of the same model year 

vintage in the EPA and AAMA databases because the Dayton testing was done subsequent 

to the EPA and AAMA testing. Thus, the Dayton data should provide a more 

representative cross-section of the in-use fleet. The derivation of the average running 

emission rates, with the adjustments based on the Dayton IM240 data, are discussed in EPA 

document M6.EXH.001. 

The high emitter recruitment adjustment factor is documented in EPA report 

M6.EXH.001 in detail. In terms of start emissions it had a tendency to increase the fraction 

of high emitters in the fleet by a few percent. The impact of this was slightly higher on start 

emissions factors. 
-24­

Table A-1
 

Estimated Fraction of HC High Emitters in the Fleet
 

HC HC HC HC HC HC HC 

MILEAGE 88-93 PFI 88-93 TBI 83-87 FI 86-89 Carb 83-85 Carb 81-82 FI 81-82 Carb 

2.142 0.0184 0.0239 0.0223 0.0052 0.0232 0.0203 0.0282 

12.823 0.0227 0.0251 0.0157 0.0197 0.0158 0.0654 0.0543 

29.335 0.0422 0.0270 0.0406 0.0526 0.0047 0.1613 0.1580 

50 0.0800 0.0386 0.1003 0.1042 0.0917 0.2861 0.2906 

60.006 0.0987 0.0458 0.1298 0.1296 0.1348 0.3485 0.3560 

74.239 0.1260 0.0561 0.1723 0.1661 0.1972 0.4393 0.4503 

87.786 0.1525 0.0661 0.2078 0.2012 0.2578 0.5275 0.5416 

100.01 0.1770 0.0753 0.2346 0.2334 0.3135 0.6094 0.6253 

112.948 0.2036 0.0851 0.2634 0.2678 0.3737 0.6986 0.7152 

124.625 0.2280 0.0940 0.2898 0.2992 0.4290 0.7812 0.7976 

135.738 0.2518 0.1026 0.3153 0.3295 0.4826 0.8620 0.8772 

146.315 0.2748 0.1110 0.3400 0.3586 0.5345 0.9407 0.9539 

156.38 0.2972 0.1190 0.3638 0.3866 0.5847 1.0000 1.0000 

165.96 0.3189 0.1267 0.3868 0.4135 0.6332 1.0000 1.0000 

175.077 0.3398 0.1341 0.4089 0.4393 0.6801 1.0000 1.0000 

183.753 0.3601 0.1412 0.4303 0.4641 0.7253 1.0000 1.0000 

192.01 0.3798 0.1480 0.4508 0.4879 0.7690 1.0000 1.0000 

199.869 0.3988 0.1546 0.4706 0.5108 0.8111 1.0000 1.0000 

207.349 0.4171 0.1609 0.4896 0.5327 0.8516 1.0000 1.0000 

214.466 0.4348 0.1669 0.5079 0.5537 0.8907 1.0000 1.0000 

221.241 0.4519 0.1727 0.5255 0.5738 0.9284 1.0000 1.0000 

227.688 0.4683 0.1782 0.5425 0.5931 0.9646 1.0000 1.0000 

233.823 0.4842 0.1836 0.5587 0.6116 1.0000 1.0000 1.0000 

239.663 0.4994 0.1887 0.5743 0.6293 1.0000 1.0000 1.0000 

245.22 0.5141 0.1936 0.5893 0.6462 1.0000 1.0000 1.0000 

250.509 0.5283 0.1982 0.6036 0.6624 1.0000 1.0000 1.0000 
-25­

Table A-2
 

Estimated Fraction of CO High Emitters in the Fleet
 

CO CO CO CO CO CO CO 

MILEAGE 88-93 PFI 88-93 TBI 83-87 FI 86-89 Carb 83-85 Carb 81-82 FI 81-82 Carb 

2.142 0.0093 0.0552 0.0180 0.0103 0.0130 0.0119 0.0508 

12.823 0.0082 0.0553 0.0123 0.0388 0.0093 0.0511 0.1102 

29.335 0.0241 0.0553 0.0357 0.0929 0.0473 0.1334 0.2441 

50 0.0458 0.0554 0.0889 0.1741 0.1783 0.2466 0.4138 

60.006 0.0566 0.0555 0.1150 0.2140 0.2430 0.3024 0.4969 

74.239 0.0721 0.0555 0.1496 0.2715 0.3364 0.3830 0.6163 

87.786 0.0872 0.0556 0.1765 0.3270 0.4271 0.4611 0.7312 

100.01 0.1010 0.0556 0.2012 0.3778 0.5102 0.5327 0.8360 

112.948 0.1159 0.0557 0.2276 0.4323 0.5998 0.6097 0.9479 

124.625 0.1296 0.0558 0.2518 0.4822 0.6819 0.6802 1.0000 

135.738 0.1429 0.0558 0.2751 0.5302 0.7614 0.7484 1.0000 

146.315 0.1556 0.0559 0.2976 0.5764 0.8381 0.8141 1.0000 

156.38 0.1680 0.0559 0.3193 0.6210 0.9121 0.8775 1.0000 

165.96 0.1799 0.0560 0.3402 0.6638 0.9836 0.9387 1.0000 

175.077 0.1914 0.0560 0.3602 0.7050 1.0000 0.9976 1.0000 

183.753 0.2025 0.0561 0.3795 0.7445 1.0000 1.0000 1.0000 

192.01 0.2132 0.0561 0.3981 0.7825 1.0000 1.0000 1.0000 

199.869 0.2235 0.0561 0.4159 0.8191 1.0000 1.0000 1.0000 

207.349 0.2334 0.0562 0.4330 0.8541 1.0000 1.0000 1.0000 

214.466 0.2429 0.0562 0.4495 0.8877 1.0000 1.0000 1.0000 

221.241 0.2521 0.0562 0.4653 0.9200 1.0000 1.0000 1.0000 

227.688 0.2609 0.0563 0.4804 0.9510 1.0000 1.0000 1.0000 

233.823 0.2693 0.0563 0.4949 0.9806 1.0000 1.0000 1.0000 

239.663 0.2774 0.0563 0.5089 1.0090 1.0000 1.0000 1.0000 

245.22 0.2852 0.0564 0.5222 1.0363 1.0000 1.0000 1.0000 

250.509 0.2927 0.0564 0.5350 1.0623 1.0000 1.0000 1.0000 
-26­

Appendix B 

Statistical Detail: Standard Errors, P values and Standard Deviations 

Table B-1 

Standard Deviations of Means 

START Emissions of High Emitter CARS 

MY Group 

Tech 

Group 

HC 

Sample 

Size 

CO 

Sample 

Size 

HC 

Standard 

Deviation 

CO 

Standard 

Deviation 

NOx 

Standard 

Deviation 

1988-93 PFI* 25 30 5.84 73.73 N/A 

1988-93 TBI 25 30 5.84 73.73 N/A 

1983-87 FI*** 103 124 10.39 92.75 N/A 

1986-89 Carb** 371 350 23.89 83.21 N/A 

1983-85 Carb 371 350 23.89 83.21 N/A 

1981-82 FI 103 124 10.39 83.21 N/A 

1981-82 Carb 371 350 23.89 83.21 N/A 

* 1988-93 PFI and TBI combined together for a total sample size of 3 

** 1986-89 Carb, 1983-85 Carb and 1981-83 Carb combined together for a total sample size of 350 or 371 

*** 1983-87 FI and 1981-82 FI combined together for a total sample size of 124 or 103 

Table B-2 

Standard Deviations of Means 

START Emissions of High Emitter TRUCKS 

MY Group 

Tech 

Group 

HC 

Sample 

Size 

CO 

Sample 

Size 

HC 

Standard 

Deviation 

CO 

Standard 

Deviation 

NOx 

Standard 

Deviation 

1988-93 PFI 3* 3 5.10 68.29 N/A 

1988-93 TBI 3 3 5.10 68.29 N/A 

1981-87 FI 4 18 5.30 115.30 N/A 

1984-93 Carb 12 19 8.81 93.90 N/A 

1981-83 Carb 17 23 13.72 116.09 N/A 

* 1988-93 PFI and TBI combined together for a total sample size of 3 
-27­

Table B-3 

Regression Statistics from Normal Emitting Cars - CO EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

CO CO CO CO CO 

1988-93 PFI 1591 0.01618 0.6688 0.1897 0.000 

1988-93 TBI 433 0.02424 1.0846 0.9624 0.000 

1983-87 FI 641 0.01600 0.9760 0.7870 0.000 

1986-89 Carb 94 0.06987 4.4534 0.1206 0.000 

1983-85 Carb 234 0.06328 2.4836 0.0960 0.000 

1981-82 FI 108 0.09004 4.4260 0.0038 0.000 

1981-82 Carb 816 0.03257 1.3539 0.0000 0.000 

Table B-4 

Regression Statistics from Normal Emitting Cars - HC EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

HC HC HC HC HC 

1988-93 PFI 1583 0.00115 0.0475 0.0002 0.000 

1988-93 TBI 436 0.00205 0.0914 0.1092 0.000 

1983-87 FI 623 0.00100 0.0860 0.3450 0.000 

1986-89 Carb 92 0.00351 0.2172 0.000 0.000 

1983-85 Carb 234 0.00376 0.1469 0.0130 0.000 

1981-82 FI 105 0.00391 0.1948 0.0313 0.000 

1981-82 Carb 839 0.00184 0.0775 0.000 0.000 
-28­

Table B-5 

Regression Statistics from Normal Emitting Cars - NOX EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

NOX NOX NOX NOX NOX 

1988-93 PFI 1611 0.00122 0.0509 0.1064 0.000 

1988-93 TBI 441 0.00328 0.1467 0.5472 0.000 

1983-87 FI 694 0.00100 0.0760 0.2460 0.000 

1986-89 Carb 95 0.00345 0.2215 0.5311 0.000 

1983-85 Carb 248 0.00328 0.1347 0.1113 0.000 

1981-82 FI 108 0.00368 0.1808 0.8725 0.000 

1981-82 Carb 974 0.00157 0.0708 0.0000 0.000 
-29­

Table B-6 

Regression Statistics from Normal Emitting TRUCKS - CO EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

CO CO CO CO CO 

1988-93 PFI 330 0.00410 0.1993 0.2870 0.0000 

1988-93 TBI 464 0.00363 0.1550 0.0003 0.0000 

1981-87 FI 76 0.00477 0.3028 0.0467 0.0000 

1984-93 Carb 115 0.00857 0.4228 0.3220 0.0000 

1981-83 Carb 157 0.01256 0.5563 0.9020 0.0000 

Table B-7 

Regression Statistics from Normal Emitting TRUCKS - HC EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

HC HC HC HC HC 

1988-93 PFI 329 0.0514 2.496 0.744 0.0000 

1988-93 TBI 465 0.0542 2.314 0.009 0.0000 

1981-87 FI 90 0.0844 5.783 0.407 0.0000 

1984-93 Carb 122 0.2009 10.434 0.204 0.0000 

1981-83 Carb 163 0.1920 8.671 0.094 0.0000 
-30­

Table B-8 

Regression Statistics from Normal Emitting TRUCKS - NOX EMISSIONS 

MY 

Group 

Tech 

Group 

Sample 

Size 

S.E 

Slope 

SE 

ZML 

Sig T 

Slope 

Sig T 

ZML 

NOX NOX NOX NOX NOX 

1988-93 PFI 331 0.00282 0.137 0.179 0.0000 

1988-93 TBI 466 0.00541 0.233 0.549 0.0000 

1981-87 FI 93 0.00315 0.219 0.020 0.0000 

1984-93 Carb 132 0.00498 0.276 0.382 0.6044 

1981-83 Carb 166 0.00642 0.293 0.081 0.0003