- Draft -
Trip Length Activity Factors for Running Loss and
Exhaust Running Emissions
Report Number M6.FLT.005
Edward L. Glover
David J. Brzezinski
U.S.EPA Assessment and Modeling Division
1.0 INTRODUCTION
MOBILE6 will compute and report (as a user option) hourly emission factors for
start, running, and evaporative emissions. These will be in addition to the standard daily
emission estimates which are currently calculated by MOBILES. The hourly emission
factors will allow the MOBILE6 model to provide more precise output that accounts for the
time of day that vehicle emissions occur. The temporal distribution of emissions is an
important factor in the formation of diurnal evaporative and start emissions.
The hourly emission estimates require considerable vehicle activity information and
analysis. The term "activity" refers to the vehicle's operating mode such as running, idling,
parked (soaking), etc. The specific activity information includes soak durations, time of
soak, trip lengths, time of trip, and other information. This document (M6.FLT.005)
discusses the issue of vehicle run time as it pertains to running loss emissions and exhaust
emissions. Other activity estimates needed to develop daily emission factors for hot soak,
diurnal, start or resting loss emissions will be documented in other MOBILE6 documents
listed with the report numbers "M6.FLT.XXX."
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2.0 DATA SOURCES USED
The primary data source for this analysis is an EPA instrumented vehicle study
conducted in Baltimore and Spokane. In these studies, instrumentation to monitor vehicle
usage was installed with the motorists' permission on 168 randomly selected vehicles while
they were tested at an Inspection / Maintenance (I/M) station. The motorists returned one
or two weeks later to have the instrumentation removed. Information from more than
8,500 vehicle-trips was recorded. The raw data collected from the study were processed
by the Radian Corporation under EPA contract to create a "trip characteristics" file. This
processed file was used to develop the hourly soak time distributions. For more details
regarding the instrumented vehicle study and the data processing, please refer to the
document "Travel Trip Characteristics Analysis" Final Report under EPA Contract 68-C1-
0079 WA 2-05 with Sierra Research.
3.0 METHODOLOGY FOR RUNNING LOSS EMISSION ACTIVITY
This section describes the basic methodology to develop the trip length activity
estimates used to calculate running loss emissions. The process consisted of several steps.
These are discussed below.
3.1 Definitions
3.1.1 Running Loss Emissions
Running loss emissions are evaporative emissions which have escaped from a
vehicle while the engine is operating. These emissions may appear from numerous
locations on a vehicle such as the evaporative canister, the fuel inlet, the top of the gas tank,
and other spots where the integrity of the evaporative system has broken down or the purge
system has become inoperative. Running loss emissions may be an artifact of a particular
evaporative system design or the result of poor maintenance. In severe cases the emissions
can become quite high as the result of large thermal gradients around a vehicle's fuel and
evaporative system. In addition, because of greater heating of the fuel and evaporative
system on longer trips, running loss emissions are not constant throughout a trip. Instead,
the rate is assumed to continually increase as a function of trip length until it reaches a
plateau at a trip length of about 50 to 60 minutes.
3.1.2 Running Loss Trip
For the purpose of activity estimates for running loss emissions, a trip is defined as
the duration of time or distance between a vehicle key on and key off (excluding engine
stalls, which were removed from the database). Throughout this document the time
duration of a trip will be referred to as the "trip time length". The duration of a trip in miles
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will be referred to as the "trip distance length".
3.2 Hourly Intervals
The 24 hour day was divided into 14 different hourly groups. Thirteen of these
groups have a duration of one hour. These start at 6:OOAM and run through 7:59:59PM.
The fourteenth group contains the remaining nighttime hours as one interval. Collapsing
these hours into one group was done for three reasons: (1) the emissions contributed during
the night have a relatively smaller impact on daily ozone or CO formation than those
contributed during the morning or day, (2) there were relatively little data for these time
periods, and (3) what data were available produced results which showed very little hour
to hour variance. The hourly intervals are shown in Table 1. In addition to their use for the
running loss activity estimates presented here, the same hourly group intervals are used in
the calculation of activity estimates for start emissions, running emissions, hot soak
emissions, resting loss emissions, and diurnal emissions.
Table 1
Hourly Ranges
Group Name
6
7
8
9
10
11
12
13
14
15
16
17
18
24
Hourly Range
6-7
7-8
8-9
9- 10
10- 11
11 - 12
12- 13
13- 14
14- 15
15- 16
16- 17
17- 18
18- 19
19 -24 and 24 -5
Time
6 am to 7 am
7 am to 8 am
8 am to 9 am
9 am to 10 am
10 am to 11 am
1 1 am to noon
noon to 1 pm
1 pm to 2 pm
2 pm to 3 pm
3 pm to 4 pm
4 pm to 5 pm
5 pm to 6 pm
6 pm to 7 pm
7 pm to 6 am
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3.3 Weekday Versus Weekend
For a number of the activity parameters a significant difference existed between the
value for the weekday and the value for the weekend. Conceptually this make sense since
most motorists have different usage patterns for their vehicles on weekdays than on
weekends. Differences may also exist for the various days of the week; however, the
database was too small to reliably discern these differences.
The MOBILE6 model will distinguish between weekend and weekday in terms of
activity and emissions, and a user input will be required to tell the model which one is to
be reported. The default will likely be the "weekday." Also, since the default MOBILE6
hourly activity estimates are based exclusively on 168 vehicles, and cannot possibility
reflect all geographical areas, times, or other variables, the user will have the option of
providing running loss activity data into the MOBILE6 model from an external file.
3.4 Trips per Car-Day
The first necessary parameter in the model is the estimate for trips/car-day. Four
different estimates were developed. There is one estimate for each combination of car or
truck and weekday or weekend. These are average values obtained from the instrumented
vehicle database. The values are shown in Table 2a.
Table 2a
Trips per Car per Day
Cars
Weekday
7.28
Weekend
5.41
Trucks
Weekday
8.06
Weekend
5.68
These estimates are based on the instrumented car data, and are subject to revision
pending completion of a thorough analysis of national trip data by an EPA contractor. The
values could potentially vary considerably from those presented here.
3.5 Daily Trip Distribution by Hourly Group
Table 2b contains distributions of vehicle trips by hourly group. One of the
distributions is based on vehicle miles travelled (VMT) of the trips, and the other is based
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on the distribution of trip counts. Distance estimates and count-based estimates are
provided for each of the fourteen groups, and separate estimates are provided for weekends
and weekdays. The trip count distribution is shown for comparison purposes, since it is
expected that the MOBILE6 model activity distribution for running losses will be based on
trip distances in miles. The distance-based distribution will be used because it is directly
proportional to the VMT accumulated during the hourly group. This is important because
in the MOBILE6 model running loss emissions are a function of miles, and are presented
in units of grams / mile.
Table 2b
Daily Distribution of Weekday Trips by Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
3.67
7.29
8.18
4.75
4.59
5.40
6.10
7.05
7.97
8.14
8.94
8.19
6.28
13.45
Trip Count Based
1.983
5.461
5.872
4.744
5.217
6.757
8.237
7.352
8.069
9.015
8.603
7.779
6.025
14.887
Significant differences in the VMT based and the trip count based distribution can
occur. For example, Table 2b shows that approximately 4.155 percent of the weekday
daily trip VMT occured during the period from 6:00 AM to 6:59:59 AM; however, this
VMT represented 1.983 percent of the total daily trips. The data which underlie Table 2b
were obtained from the instrumented vehicle database. Each column sums to 100 percent.
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Table 2c
Daily Distribution of Weekend Trips bv Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
0.90
1.70
2.84
4.57
6.20
7.62
8.79
9.98
7.64
9.19
9.04
6.88
6.57
18.07
Trip Count Based
0.905
1.962
3.068
6.590
6.992
7.998
10.312
7.294
8.803
7.294
7.998
7.042
6.087
17.656
3.6 Running Loss Trip Distance Length Distribution by Hourly Group
A distribution of running loss trip distance lengths was developed for each of the
28 hourly group / weekday-weekend groups. The methodology used to analyze the data,
and the results of the analysis are presented below. This particular methodology for
estimating running loss activity was chosen because it fits the running loss emission values
that are available in MOBILES (these will not be updated in MOBILE6). The only
enhancement for the new model is that these activity parameters will be available for each
hourly group and for weekday/weekend categorization (28 groups) rather than just an
overall average set which represents all hours and days of the week. The overall set of
MOBILES running loss activity estimates are shown in Table 2d (the percentages shown
in Table 2d add up to 100%). For additional details regarding the MOBILES activity
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parameters for running losses see the EPA report EPA-AA-AQ-AB-94-01 May 94 entitled
"User Guide to MOBILES."
TABLE 2d
MOBILES Running Loss Activities
Trip Duration
Range (min)
Percentage
0- 10
minutes
6.744
11-20
minutes
18.507
21 -30
minutes
16.775
31 -40
minutes
13.108
41 -50
minutes
8.335
51+
minutes
36.531
3.6.1 Methodology
The analysis consisted primarily of categorizing each of the 8,500 vehicle trips in
the database into a particular hourly / weekday-weekend group. This was done by
examining the day of the week of the trip, and the starting and ending time of each trip. A
vehicle trip was classified as a weekday trip if it started on Monday through Friday. It was
a weekend trip if it started on Saturday or Sunday.
A vehicle trip was classified into a particular hourly group if any part of the trip
duration was in a given hourly group. A given vehicle trip could potentially be classified
into one, two, or even three different hourly groups depending on the duration of the trip,
and how many group interval boundaries it crossed. This classification could fit two
general cases which are best illustrated with an example. (1) If a vehicle trip was from 8:20
AM to 8:40 AM it was classified as a twenty minute trip in Hourly Group 8. (2) If the trip
was from 7:51 to 8:15, its contribution had to be split between two hourly groups (7 and
8 hour). Thus, a nine minute trip was assigned to the Hour 7 group, and a nine to twenty-
five minute trip was assigned to the Hour 8 group. The nine to twenty-five minute label
was assigned to the hour 8 group rather than a fifteen minute label, because the vehicle had
been operating for nine minutes prior to the 8th hour, and would nominally have higher
running losses than a vehicle which was just starting its trip. This is reflected in the
MOBILE6 model because running losses are a function of the duration of the trip.
After labelling each of the trips in the database using the method above, each trip
was classified into one of six trip duration categories based on trip duration in minutes (see
Table 3). For example, the trip which lasted from 8:20 to 8:40 AM would fall into category
#2 and hour 8. The trip which lasted from 7:51 to 8:15 AM would be treated as two trips.
The first phase (nine minutes) would fall into the category #1 and hour 7. The second
phase would fall into category #3 and hour 8. Category #3 is used because the upper
duration of the trip is 25 minutes, and the maximum length of the trip is used to determine
M6FLT005.WPD DRAFT
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February 18, 1998
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category number. Running loss emissions from trips are also measured in 10 minute
intervals, and are keyed to the upper duration of the trip.
Table 3
Trip Duration Categories
Category Number
1
2
3
4
5
6
Trip
0 -
11 -
21 -
31 -
41 -
51+
Duration Range (in Minutes)
10 minutes
20 minutes
30 minutes
40 minutes
50 minutes
minutes
After classifying the trips and phases of trips by hourly / weekday-weekend group,
and by category number, the duration in miles of each trip and trip phase was found. In
cases where the trip contained only one phase the trip distance in miles was readily
available. In cases where two phases were present, the mileage was split according to the
length of the trip in time. This assumes that the average speeds in both phases were equal.
After obtaining the mileage for each trip and trip phase, the mileages were summed
for each hourly / weekday-weekend group and for each category within a hourly /weekday-
weekend group. From the sums, percentages contributions were calculated for each
category within a group. Tables 4a and 4b show these percentages for weekdays and
weekends.
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TABLE 4a and 4b
Mileage (VMT) Distribution by Trip Length (in time)
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
0-10 Min
14.89%
16.06%
14.56%
27.40%
28.47%
32.94%
33.41%
28.89%
27.08%
26.79%
24.91%
21.09%
26.80%
20.95%
0-10 Min
18.98%
26.89%
20.60%
29.83%
25.57%
27.61%
25.85%
18.37%
23.71%
21.16%
25.66%
24.31%
18.01%
15.69%
TABLE 4a
11 -20 Min
22.70%
31 .05%
33.48%
32.71%
42.99%
32.78%
39.64%
46.80%
39.31%
41 .84%
40.78%
34.84%
32.23%
37.26%
21-30 Min
29.44%
40.00%
18.37%
22.20%
17.06%
14.96%
15.19%
13.93%
18.29%
24.28%
18.79%
29.41 %
25.10%
24.51 %
TABLE 4b
11 -20 Min
60.63%
42.40%
41 .68%
38.26%
42.08%
36.42%
35.51%
19.41%
39.19%
25.39%
25.90%
37.71%
38.43%
27.42%
21-30 Min
20.39%
30.71 %
18.82%
23.09%
16.58%
13.25%
28.61 %
22.48%
12.26%
25.46%
12.34%
27.30%
18.27%
15.74%
31 -40 Min
20.76%
9.13%
18.49%
4.75%
7.92%
7.53%
1 1 .76%
9.27%
3.18%
3.90%
10.87%
9.23%
1 1 .98%
7.52%
31 -40 Min
0.00%
0.00%
0.00%
8.82%
15.77%
10.31%
3.54%
9.86%
14.93%
4.07%
6.60%
3.91%
20.83%
9.57%
41 -50 Min
12.22%
3.75%
3.01 %
0.00%
3.56%
5.34%
0.00%
1.10%
1 .38%
2.20%
1 .76%
5.42%
3.05%
6.37%
41 -50 Min
0.00%
0.00%
18.90%
0.00%
0.00%
3.38%
6.49%
4.32%
9.91 %
0.00%
15.03%
0.00%
4.46%
2.58%
51+ Min
0.00%
0.00%
12.10%
12.94%
0.00%
6.47%
0.00%
0.00%
10.76%
0.99%
2.89%
0.00%
0.84%
3.38%
51+ Min
0.00%
0.00%
0.00%
0.00%
0.00%
9.03%
0.00%
25.56%
0.00%
23.92%
14.48%
6.76%
0.00%
28.99%
3.6.2 Results
Tables 4a and 4b show the running loss activity estimates for all 28 groups and 6
categories. Each of the cells in Tables 4a and 4b show the percentage of vehicle miles
travelled (VMT) for a given hourly / weekday-weekend group which falls in a given trip
duration category. For example, 14.89 percent of the VMT in the 6 AM to 7 AM time
interval on a weekday occurs in trips which are 0 to 10 minutes in duration. Also, each of
the rows in the tables sums to 100 percent.
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3.7 Using the Hourly Running Loss Activities in MOBILE6
3.7.1 Hourly Running Loss Calculation
The average hourly running loss emissions are calculated from four parameters.
These are: (1) the six running loss emission values (one value for each trip duration
category). These values will not be updated for MOBILE6, and can be found in EPA
document AP-42 for MOBILES. (2) the six trip duration by hourly group running loss
activity values shown in Table 4a or 4b. (3) the number of trips per day per vehicle values
shown in Table 2a, and (4) the daily distribution of trips by hourly group (shown in Tables
2b and 2c).
MOBILE6 will multiply each of the six running loss emission values with each of
the corresponding six running loss activity values. The six products (emissions x
activities) are then summed to produce an estimate of running losses on a per trip basis.
The calculation is done once for each of the 28 groups, to produce a value for each of the
28 groups. These 28 hourly running loss emission values are per trip estimates, and are the
result of an intermediate calculation in the MOBILE6 model.
The second calculation takes the 28 hourly running loss emission values in per trip
units, and converts them into average hourly running loss emission values. This is done by
multiplying the 28 hourly per trip running loss values by the number of trips per day (shown
in Table 2a) and by the appropriate value from the daily distribution of trips by hourly
group table (shown in Table 2b or 2c). The daily distribution of trips by hourly group
apportions the overall trips per day value into hourly group values.
3.7.2 Daily Running Loss Calculation
An overall daily running loss average is computed by weighting together the
average running loss values of the 28 groups. The weighting factors are the daily
distribution of trips by hourly group values shown in Tables 2b and 2c. The number of
trips per day per vehicle from Table 2a is used to calculate the final daily running loss
emissions.
4.0 METHODOLOGY FOR RUNNING EXHAUST EMISSION ACTIVITY
This short section describes the basic methodology to develop the trip length
activity estimates used to calculate running exhaust emissions. The process and the results
are virtually identical to those shown in the running loss activity estimate sections, and are
shown here for completeness.
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4.1 Definition
Exhaust running emissions are the emissions which exit a vehicle's tailpipe while
the vehicle is operating in a warmed up condition. Excluded from the definition of exhaust
running emissions are those emissions which occur during vehicle start (cranking the
engine) and warm-up. Standardized test procedures such as the Federal Test Procedure
(FTP), and test equipment have been extensively developed to measure exhaust emissions.
The exhaust running emissions in the MOBILE6 model are based on such FTP data.
Detailed information on the definition of start emissions, the methodology used to
develop the start emission estimates, and the projected MOBILE6 light duty vehicle start
emission factors are provided in the EPA reports M6.STE.003 entitled "Determination of
Start Emissions as a Function of Mileage and Soak Time for 1981-1993 Model Year
Light-Duty Vehicles", and M6.STE.002 entitled "The Determination of Hot Running
Emissions from FTP Bag Emissions." Also, detailed information on the determination of
running exhaust emission factors for MOBILE6 can be found in EPA report M6.EXH.001
entitled "Determination of Running Emissions as a Function of Mileage for 1981-1993
Model Year Light-Duty Vehicles."
4.2 Activity Estimates
The activity estimates for exhaust running emissions required for the MOBILE6
model are minimal, since we assume exhaust running emissions are not a function of the
trip length or the preceding soak period. Thus, the only activity estimates needed to
calculate hourly running exhaust emissions in MOBILE6 are the trips per day per vehicle
estimates and the daily trip distribution by hourly group. The trips per day per vehicle
value is an estimate of overall daily vehicle usage. It is shown in Table 2a. The trip
distribution by hourly group allocates the trips per vehicle over all 14 hourly groups. It is
shown in Tables 2b and 2c.
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COMMENTS
Comments on this report and its proposed use in MOBILE6 should be sent to the attention
of the author, and submitted electronically to mobile@epamail.epa.gov, or by fax to
(313)741-7939, or by mail to MOBILE6 Review Comments, US EPA Assessment and
Modeling Division, 2565 Plymouth Road, Ann Arbor MI 48105. Electronic submission
of comments is preferred, since we will make any comments available on our web site. In
your comments, please note clearly the document that you are commenting on including the
report title and the code number listed. Please be sure to include your name, address,
affiliation, and any other pertinent information.
M6FLT005.WPD DRAFT 12 February 18, 1998
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