Effect of Engine Speed
on
Undiluted HC and CO Emissions
by
Gordon J. Kennedy
March 1977
Notice
Technical support reports do not necessarily represent the final EPA
decision on regulatory issues. They are intended to present a technical
analysis of an issue and recommendations resulting from the assumptions
and constraints of that analysis. Agency policy considerations or data
received subsequent to the date of release of this report may alter the
recommendations reached. Readers are cautioned to seek the latest analysis
from EPA before using the information contained herein.
Characterization and Applications Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Abstract
Increased interest in motor vehicle emission inspection/maintenance programs
and specifically on the warranty provisions of section 207(b) of the Clean
Air Act has focused attention on the short exhaust emission tests. Lack
of concurrence on the approach to specifying engine speed requirements dur-
ing these short tests has prompted this study in xvhich a small number of
vehicles were tested at MVEL to determine the levels of undiluted IIC and
CO emissions at different engine speeds. This study was conducted to
discover what pattern, if any, exists which might relate HC and CO
exhaust emission levels to the vehicle engine's speed as this speed is
changed under no-load conditions.
It was found in most cases, especially in the lower idle-speed ranges,
that a small increase in engine speed results in a sharp drop in emission
levels. It is apparent, therefore, that a "failed" vehicle could easily
be caused to pass its idle test by merely increasing the idle speed suf-
ficently to drop its emission levels below the established standards.
With emission levels dependent on engine speed, it behooves those involved
with state inspection/maintenance programs to apply some degree of
uniformity to engine idle settings prior to commencing idle emission
tests. For the low-speed idle setting, the adjustment least likely to
result in controversy xrould be to set the engine speed to the manufacturer's
specification. For vehicles equipped with automatic transmissions, in
which the idle speed is specified with the transmission in drive, the
engine should be so adjusted prior to shifting the transmission to
neutral for the emission tests. Since manufacturers' low-idle specifica-
tions are usually near minimum levels for smooth engine operation,
tolerance limits for low-speed idle should be plus or mimus 100 RPM.
All idle emission tests should be performed in neutral (unloaded) to
allow a uniform basis for vehicles equipped with either automatic or
manual transmissions. It is mandatory that the high-speed idle tests be
performed in neutral due to the increased stress and non-linear load
x^hich would otherwise occur with automatic transmissions. An unloaded
high-idle speed of 2,500 RPM is being generally adopted by states as an
optimum choice for inspection/maintenance high-speed undiluted emission
tests. Due to the appreciable rate of emission changes with engine
speed, exhibited by certain vehicles, even in the higher idle speed
ranges, the tolerance limits should be no more than plus or minus 100
RPM.
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I. Purpose
The purpose of this study is to determine whether relatively small
changes in a vehicle's engine speed under no-load conditions can have a
significant effect on its HC and CO emission levels. The results from
this study xdLll then be used to determine the advisability of first
checking the engine idle speeds on all vehicles against some uniform set
of specifications prior to performing the idle emission tests.
II. Introduction
With the advent of vehicle emission inspection/maintenance programs at
the state level, considerable attention has been devoted to the simplest
and least expensive emission test procedure—the undiluted idle test for
HC and CO. Some states are stipulating a high-speed idle test in the
range of 2,500 RPM in addition to the low-speed idle test. However,
there seems to be a lack of concurrence regarding a uniform basis for
setting the engine speed for the low-speed idle test, as well as a lack
of guidance on what would constitute an optimum high-speed setting.
III. Discussion
Six pre-1975 vehicles and two 1976 vehicles, one of which was maladjusted
prior to testing, were analyzed for undiluted HC and CO exhaust emission
levels while being operated at various no-load engine speeds ranging
from 500 RPM to 3,000 RPM. These tests were performed on February 22-
25, 1977. Table 1 in the Appendix identifies the vehicles tested.
A
The Model EET-947 (Serial No. 248-08722) Analyzer, manufactured by Sun
Electric Company was used to measure the undiluted HC and CO levels, as
well as the engine speed for each test. This analyzer was calibrated on
the first day of these tests by the MVEL Calibration and Maintenance
Group. Ambient conditions were normal for this time of year in the
laboratory.
Each vehicle was allowed to warm-up to normal operating temperature
prior to testing. During testing, for each speed setting, the engine
was allowed to operate at that particular speed sufficiently long to
obtain stabilized readings for HC and CO.
Subsequent to these emission tests, those test vehicles equipped with
automatic transmissions were again normalized to standard operating
temperatures. The idle speed of each vehicle was then adjusted to the
manufacturer's specifications with the transmission in drive. The
transmission was then shifted to neutral, and the increased idle speed
recorded in each case. A portable tachometer was used for these speed
checks (Universal Tach Dwell Meter, manufactured by Mac Allied Tools
Corp., serial number 001513).
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—2—
IV. Results
Tables 2 through 9 list the HC and CO values which correspond to the
various engine speed settings for Vehicles A through H, respectively.
Table 10 lists the manufacturer's idle-speed specification to which the
engine idle speed of each vehicle equipped with an automatic transmission
was subsequently set, as well as the speed to which the engine increased
when the transmission was shifted to neutral.
Figures 1 and 2 are plots of HC and CO emissions, respectively, versus
engine speeds, taken from data listed in Tables 2 through 9.
V. Conclusions
From the plots in Figures 1 and 2, it is obvious that idle HC and CO
emission levels vary considerably as engine speeds change. It can be
seen that, in most cases, especially for HC from a vehicle being emission
tested at an idle speed below 1,000 RPM, a "failed" vehicle could easily
be caused to pass merely by increasing its idle speed. In some cases,
either an increase or a decrease in engine speed could result in the
vehicle passing. Vehicle A, for example, might fail to meet a CO standard
at 600 RPM, but it might pass at either 550 or 650 RPM. Conversely,
Vehicle A might pass a CO standard at 700 RPM, and fail at either 650 or
750 RPM.
Table 10 shows the disparity between idle engine speeds as measured in
drive versus their increased values x^hen those vehicles, equipped with
automatic transmissions, are shifted to neutral. These changes in idle
speeds, for this reason alone, could easily mark the difference between
passage and failure on individual vehicles.
VI. Recommendations
For a vehicle requiring idle undiluted HC and CO exhaust emission measure-
ments, it is recommended that the vehicle's engine idle speed be first
checked to determine whether it meets its manufacturer's specification,
by performing the following steps:
1. Allow the vehicle to be normalized to its standard operating
temperature.
2. Perform any diagnostic checks or maintenance items which may
be required prior to testing (depending upon requirements in
the test program involved).
3. Check the engine low-speed idle setting against the vehicle
manufacturer's specification. For a vehicle equipped with
automatic transmission, make sure the speed is checked in
drive or neutral, per the manufacturer's specification.
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4. If the engine low-speed idle setting conforms to the manu-
facturer's specification; within tolerance limits of plus or
minus 100 RPM; proceed with the exhaust emission tests. If
the idle setting does not conform, arrange with the owner to
have the idle setting corrected before proceeding with the
emission tests.
5. For the high-speed idle emission test, ..increase the idle speed
adjustment gradually to 2,500 RPM (within tolerance limits of
plus or minus 100 RPM), and allow engine to stabilize at this
speed before proceeding with taking emission measurements.
6. At the completion of emission tests, decrease the idle speed
adjustment gradually to the manufacturer's low-speed idle
specification before returning the vehicle to its owner.
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APPENDIX
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Table 1
Test Vehicles
Veh. Description VIN CID Trans. Garb. Bbls,
A
B
C
D
E
F
G
H
76 Chev. Impala
70 Plymouth Valiant
73 Ford Capri .
71 Chev. Vega
71 Ford LTD
70 Chev. Impala
70 Ford Falcon
76 Ford LTD
1L69V6J139469
VL41COB215566
GAECNP52810
141151U195333
1J72N125679
164690U193074
OH27L159045
6B74S228258
350
225
122
140
429
350
250
400
A
A
M
M
A
A.
A
A
2
1
2
1
4
2
1
2
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Vehicle A
Table 2
76 Chevrolet Impala, 1L69V6J139469,
350 CID, Auto., 2 Bbl Garb.*
RPM HC (ppm) CO (%)
500 360 3.9
600 365 4.7
700 350 3.3
800 350 4.1
900 310 4.0
1000 270 3.9
1500 155 2.9
2000 130 2.2
2200 100 1.2
2400 60 0.15
2600 40 0.05
2800 30 0.04
3000 25 0.03
Carburetor maladjusted: Fuel-air mixture screws turned counterclockx^ise
outward 4 3/4 turns from manufacturer's specification for richer mixtures.
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Vehicle B
Table 3
70 Plymouth Valiant, VL41COB215566,
225 CID, Auto., 1 Bbl Carb.
RPM HC (ppm) CO (%)
500 470 7.3
.600 370 6.2
700 360 5.7
800 280 5.3
900 280 4.9
1000 325 4.3
1500 315 2.8
2000 125 0.25
2200 110 0.25
2400 105 0.35
2600 110 0.45
2800 105 0.60
3000 100 0.65
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Vehicle C
Table 4
73 Ford Capri, GAECNP52810,
122 CID, Manual, 2 Bbl. Carb.
RPM HC (ppm) CO (%)
550 300 3.4
600 310 .4.1
700 290 5.1
800 250 4.6
900 200 3.2
1000 150 2.3
1500 90 1.2
2000 80 1.1
2200 50 0.5
2400 45 0.4
2600 40 0.3
2800 30 0.2
3000 30 0.15
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Vehicle D
Table 5
71 Chevrolet Vega, 141151U195333,
140 CID, Manual, 1 Bbl. Carb.
RPM HC (ppm) CO (%)
550 500 0.5
600 510 1.0
700 400 0.2
800 130 0.15
900 80 0.15
1000 70 0.13
1500 45 0.10
2000 40 0.12
2200 40 0.12
2400 40 0.22
2600 40 0.26
2800 45 0.45
3000 50 0.55
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Vehicle E
Table 6
71 Ford LTD, 1J72N125679,
429 CID, Auto., 4 Bbl. Carb.
RPM HC (ppm) CO (%)
500 550 3.9
600 280 0.9
700 205 0.95
800 160 0.2
900 130 0.18
1000 90 0.18
1500 80 0.18
2000 80 0.25
2200 100 0.35
2400 110 0.50
2600 110 0.60
2800 120 0.75
3000 115 0.95
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Vehicle F
Table 7
70 Chevrolet Impala, 164690U193074,
350 CID, Auto., 2 Bbl. Carb.
RPM HC (ppm) CO (%)
500 450 1.3
600 180 1.2
700 160 1.3
800 150 1.3
900 140 1.8
1000 120 1.7
1500 95 1.5
2000 65 1.55
2200 55 1.8
2400 85 2.4
2600 85 2.3
2800 80 1.7
3000 60 1.2
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Vehicle G
Table 8
70 Ford Falcon, OH27L159045,
250 CID, Auto., 1 Bbl. Carb.
RPM HC (ppm) CO (%)
500 900 4.3
600 790 4.2
700 710 4.1
800 510. 3.5
900 420 2.9
1000 360 2.2
1500 130 0.2
2000 1.25 0.4
2200 140 0.65
2400 160 1.35
2600 150 1.5
2800 155. 1.55
3000 . 165 1.7
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Vehicle H
Table 9
76 Ford LTD, 6B74S228258,
400 CID, Auto., 2 Bbl. Garb.
RPM HC (ppm) CO (%)
500 2000 6.8
600 1300 7.4
700 810 . 6.6
800 600 5.7
900 490 4.8
1000 450 3.9
1100 440 3.3
1200 450 ; 2.6
1300 . 440 1.9
1400 360 1.3
1500 220 1.2
2000 80 0.1
2200 70 0.06
2400 60 0.05
2600 60 0.03
2800 60 0.03
3000 60 0.03
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Table 10
Veh.
A
B
E
F
G
H
Mfr's. Idle Speed Specs. (RPM)
(Veh. with Auto, Trans, in Drive)
600 D
650 D
600 D
. 600 D
500 D
600 D
Idle Speed When-
Shifted to Neutral
800 N
840 N
740 N
760 N
580 N
760 N
NOTE: The manufacturer's idle speed specification for Vehicle C
is 750 RPM, and for Vehicle D 850 RPM, both of which are
measured with transmission in neutral.
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