Comparison of Start Emissions
in the LA92 and ST01 Test Cycles
Report Number M6.STE.001
Phil Enns
David Brzezinski
U.S. EPA Assessment and Modeling Division
Introduction
As part of the MOBILE inventory model revision, an effort has been undertaken to model
separately start emissions occurring at the beginning of a trip. A special start driving cycle,
described below, has been developed for the purpose of measuring this portion of emissions in
laboratory testing. However, sufficient data from such tests is not available for use in MOBILE
at this time. The Federal Test Procedure (FTP) and its California replacement the LA92, also
described below, do include engine start, but use different driving patterns than the special start
driving cycle.
This document reports on a comparison of start emissions for two test cycles using data
from a sample of five vehicles tested at the EPA National Vehicle and Fuel Emissions
Laboratory in Ann Arbor, Michigan. The purpose of the analysis is to determine if, during the
start portion of the cycles, there is a significant difference between the cycles in their excess
emissions attributable to a cold start conditio'n.
Vehicle Sample and Testing
The two cycles used in this study were developed recently to serve the needs of revised
emissions testing. The 258-second "ST01" cycle was developed as part of EPA's Revised FTP
project. It is designed to simulate typical driving during the beginning of a trip and is comprised
of observed speed segments of real driving collected as part of that project. The "LA92" cycle
(also called the Unified Cycle) was created by the California Air Resources Board to replace the
FTP cycle. It also is constructed from segments of actual driving, recorded in the Los Angeles
area, and includes elements of driving that are more "aggressive" than any found on the FTP.
While the full LA92 cycle lasts 1,436 seconds, only the first 298 seconds are considered in this
study. This includes the elapsed time of the ST01 cycle plus the time needed for the vehicle to
return to idle. Figure 1 displays the speed traces for the two cycles.
Determining when the cold-start portion of a trip ends is not a trivial problem. A typical
pattern of modal emissions for identical driving under cold-start and (warm) no-start driving is
depicted in Figure 2. In the absence of test variation, the two graphs converge at a specific time
. M6FVCAR.WPD 1 May 30, 1997
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which can be defined as the end of the cold start portion. In practice, this point of convergence is
not obvious because of random fluctuations from one test to another. However, in the current
study if it is assumed that the cold-start and no-start emissions eventually converge within the
first 258 seconds, then knowledge of the exact time of that convergence is not needed in order to
compute relative emissions from the two operating modes. When the difference between the cold
and no-start emissions on a given cycle is calculated, the post-convergence values cancel.
Using this assumption, the primary null hypothesis can be stated as follows: the average
difference between cold-start and no-start emissions is the same for the ST01 and LA92 cycles.
In the current experiment, each of five vehicles was driven over the two cycles in both a cold-
start and warm no-start condition, a total of twenty tests. (A third "hot-start" test also was
performed in which the warmed-up vehicle started from engine off mode.) Duplicate tests were
not done. From these tests, a simple paired difference test can be constructed by first computing
the excess of cold-start over no-start emissions for each vehicle on each cycle, and then
differencing these values by vehicle.
Table 1 gives characteristics of the five vehicles employed in the study. Figure 3 shows
the second-by-second HC emissions for one of these vehicles on the first 298 seconds of the
LA92 cycle in the cold- and no-start conditions. Also shown are the cumulative emissions for the
two tests. The difference in cumulative emissions at 258 seconds (the end of the shorter ST01
test) becomes the basic data measurement on which the cycle comparison is based. Table 2 lists
all the cumulative values for CO, HC, and NOx; excess cold-start emissions by start condition
and resulting cycle differences appear in Table 3.
Results and Conclusions
Table 3 shows final t-test results for the hypotheses that the average difference between
cold-start and no-start emissions is the same for the ST01 and LA92 cycles. These small sample
tests are non-significant for all three pollutants. In other words, they support the idea that, on
average, excess emissions from cold-start operation are no different for the LA92 cycle than for
the ST01 cycle. The variability of the emission results, especially for NOx emissions, was very
high. Further analysis may be warranted to investigate the reasons for this variability or to
increase the sample size.
It should be noted, of course, that these results do not conclusively prove that excess
emissions for a cold start are the same on ST01 as for any other driving pattern. Imputing that
conclusion to other cycles (such as the FTP cycle) should be done with caution. Moreover,
because the number of vehicles tested was small, the power of the t-test to detect a difference
between cycles is limited.
Nevertheless, this study supports the concept that the increment in emissions caused by
engine start is reasonably independent of the underlying driving cycle. For the MOBILE6 model,
the emissions caused by engine start will be extracted from existing FTP and LA92 emission
M6FVCAR.WPD 2 May 30, 1997
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testing data. It is proposed, for purposes of modeling with MOBELE6, that the emissions from
engine start will be assumed to be the same, i-egardiess of the driving which occurs after engine
start. Other factors, such as temperature, fuel composition and soak time since the last engine
running will still be used to affect the emissions from engine start for particular, user specified
scenarios.
M6FVCAR.WPD 3 May 30, 199?
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Table 1: Test Vehicle characteristics
VEHICLE
5174
5177
5181
5182
5183
MY
91
94
94
94
94
MAKE
CHEVROLET
FORD
OLDSMOBILE
BUICK
SATURN
MODEL
CORSICA
THUNDERBIRD
ACfflEVA
ROADSTER
SATURN
VIN
1G1LT53G3ME142337
1FALP6240RH1 10885
1G3NL15D7RM029502
1G4BN52P3RR420339
1G8ZJ5574RZ301364
ENGINE FAMILY
M1G2.2V5JFG3
RFM3.8V8GAEA
R1G2.3VHGFEA
R1G5.7V8GAEE
R4G1.9VHGBEA
VEHICLE
5174
5177
5181
5182
5183
CID
134
231
139
350
145
DRIVE TRAIN
FWD
RWD
FWD
RWD
FWD
CATALYST
3-WAY
3-WAY
3-WAY
3-WAY
3-WAY
CYL
4
6
6
4
8
FUELINJ
TBI
PFI
PH
PFI
PFI
TRANS
AUTOMATIC
AUTOMATIC
AUTOMATIC
AUTOMATIC
MANUAL
M6FVCAR.WPD
May 30, 1997
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Table 2
258-SECOND CUMULATIVE EMISSIONS
Total Emissions (grams)
NO START
COLD START
CO | HC
1
NOX | CO | HC
1 -------- 1
NOX
VEHICLE|CYCLE
5174 |LA92 | 5.74| 0.42| 0.49| 24.75| 1.96| 1.16
|ST01 | 6.17| 0.48| 0.67| 23.49| 2.25| 2.41
5177 |LA92 | 9.22| 0.36| 0.76| 19.32| 1.92) 1.87
|ST01 | 15.76| 0.34| 0.48| 28.05| 2.08| 0.51
5181 |LA92 | 3.58| 0.05| 0.26| 12.76] 1.31J 0.88
JST01 | 5.30| 0.05| 0.24| 14.21| 1.26| 0.97
5182 |LA92 | 0.39| 0.02| 0.02J 9.09| 1.13| 0.29
|ST01 | 0.78| 0.02| 0.09| 12.92| 1.20| 0.53
5183 |LA92 | 5.91| 0.21| 0.121 19.25| 1.71| 0.49
|ST01 | 4.95| 0.11| 0.13| 17.59| 1.39| 0.19
M6FVCAR.WPD
May 30; 1997
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Table 3
EXCESS OF COLD-START OVER NO-START EMISSIONS
BY VEHICLE AND CYCLE with PAIRED DIFFERENCE (grams)
VEHICLE | CYCLE
5174
LA92
ST01
DIFFERENCE
5177
LA92
ST01
DIFFERENCE
5181
LA92
ST01
DIFFERENCE
5182
LA92
ST01
DIFFERENCE
5183
LA92
ST01
DIFFERENCE
| CO | HC
1
| 19.01
1.54
| 17.32| 1.76
| 1.69
| 10.10
| 12.29
| -2.19
| 9.18
| 8.91
| 0.27
| 8.70
j 12.14
| -3.44
| 13.34
| 12.64
| 0.70
-0.22
NOX
0.67
1.74
-1.07
1.56| 1.10
1.75
0.04
-0.19| 1.07
1.26
1.21
0.06
1.11
1.18
-0.07
1.50
1.28
0.22
0.62
0.73
-0.11
0.26
0.44
-0.17
" 0.37
0.06
0.31
M6FVCAR.WPD
May 30, 1997
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Table 4
PAIRED DIFFERENCE T-TESTS
for DIFFERENCES IN Table 3
MINIMUM (grams)
MAXIMUM (grams)
MEAN DIFFERENCE
STD DEVIATION
STANDARD ERROR
T
PROB>T
CO
-3.44
1.69
-0.59
2.14
0.96
-0.62
0.57
HC
-0.22
0.22
-0.04
0.18
0.08
-0.49
0.65
NOX
-1.07
1.07
0.01
0.78
0.35
0.02
0.99
M6FVCAR.WPD
May 30, 1997
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Figure I
LA92 SPEED TRACE
100 200
TIHE ISECOKDSJ
ST01 SPEED TRACE
100 200
TIKE (SECONDS!
300
300
M6FVCAR.WPD
May 30, 1997
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Figure 2
E
H
I
S
S
I
0
N
S
B9
TIME
-------
Figure 3
HC Emissions — Vehicle 5174
0.03
0.02
0.01
o .
II I I II
M I •« II
»—. nil. i -
ll ', J II lA_(
100 200
SECONDS
COLD_STA NO_START
0
START
Cumulative HC Emissions — Vehicle
2 . 0
i . 5:
i. o
300
M6FVCAR.WPD
10
May 30,1997
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Appendix
Test Plan
Evaluation of the Effects of Driving Cycles
on Cold Start Emissions
BACKGROUND
A theory has been presented that cold start emissions are independent of specific driving
circles. In other words, no matter what driving occurs immediately after a vehicle is started the
emissions over the time it takes for the engine to warm up are the same.
PURPOSE
The purpose of this testing is to gain some insight into whether or not the cold start
emissions of a vehicle are independent of driving patterns. This is a preliminary investigation
which could result in further testing.
RECRUITMENT
Five vehicles shall be selected to receive a series of cold start tests after the normal
Emission Factor Indolene test sequence. Because the data are needed quickly, the next five
available EF vehicles shall be used. The vehicles selected shall be 'normal' emitting. No high
emitting vehicles are to be included in this sample. If it is determined, after testing begins, that a
vehicle is a high emitter it shall be removed from this program and another vehicle selected in its
place.
TESTING
This testing shall be performed after the normal EF sequence on Indolene. The sequence
shall be as shown on the attached flowchart. The cycles to be used for this testing will be the
ST01 (first 258 seconds of the SC03 cycle) and the first 298 seconds of the LA92 (Unified
Cycle).
Each of these cycles will be performed modally both as a cold start, hot start, and running start
test.
TEST SEQUENCE
The test sequence shall begin no sooner than four hours after the Indolene testing has
been completed. The sequence shall begin by draining the fuel and filling the tank to 40%, by
volume, with Indolene test fuel. An LA4 prep cycle shall be driven and vehicle soaked for a
minimum of 12 hours. An effort shall be made to soak the vehicle approximately the same
M6FVCAR.WPD 11 May 30, 1997
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length of time after each prep, before each cold start. The vehicle will then receive a cold start
ST01 with second by second dilute modal emission measurements and a bag sample. An
unmeasured hot LA4 will then be driven followed by a 10 minute soak and a hot start ST01 and a
running ST01 after the hot start test (back to back ST01 cycles).
No sooner than four hours later, an LA4 prep cycle shall be driven and the vehicle soaked
for a minimum of 12 hours. The vehicle will then receive a cold start with second by second
dilute modal emission measurements and a bag sample of the first 298 seconds of the LA92
cycle. An unmeasured hot LA4 will then be driven, followed by a 10 minute soak and a hot start
298 second test and a running 298 second test (back to back 298 second tests).
The order of testing shall be alternated for each vehicle. The first vehicle will have the
ST01 sequence first, the second vehicle will have the 298 second sequence first. The third
vehicle, the ST01 first, and so on.
DATA COLLECTION
All data collected on these cycles will be second by second dilute modal and be
simultaneously collected in a bag.
M6FVCAR.WPD 12 May 30, 1997
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