THE EFFECT OF EXHAUST SYSTEM
BACKPRESSURE ON HC , CO, AND NOx EMISSIONS
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Table of Contents
Abstract
I. Introduction
Purpose
Objectives of the Test Program
II. Description of Test and Maintenance Sequence
III. Discussion of Results
IV. Conclusions
Attachments
I. Vehicle Descriptions
II. Pressure Range of Transducer(s) Used For
Each Test
III. Test Sequence
IV. Average Gauge Pressures Measured During
Steady State Tests and Approximate Ranges
of Difference in Backpressure Between M-Tl
and M-3 Tests
V. Emission Results
VI. Integral Exhaust Pressure Modulated EGR
Valve - Positive Control Pressure -
Description of Operation
VII. Integral Exhaust Pressure Modulated EGR
Valve - Negative Control Pressure -
Description of Operation
Figures
I. Location of Pressure Transducer for
Each Test
3
4
4
5
5
8
13
14
15
16
17
18
19
21
23
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Abstract
The exhaust systems of three light-duty passenger cars
and one light duty truck were altered and the vehicles were
tested for exhaust emissions using the Federal Test Procedure.
To determine, tne effect of the alterations on backpressure/
exhaust system pressure was measured for four of the five
tests run on each vehicle. When the catalyst was replaced
by a straight, unrestricted pipe, two of the four vehicles
exceeded the Federal standard for NOx emissions, one exceeded
the standard for CO, and all vehicles exceeded the standard*,
for HC. When dual exhaust systems were installed in place
of the single exhaust systems all vehicles failed NOx.
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I. Introduction
Purpose
The purpose of this report is to present the results
of a test program conducted at the Virginia Testing.
Laboratory for the Field Operations and Support. Division.
(FOSD). The test program involved measurement of exhaust
pressure during Federal Test Procedure (FTP) testing
of in-use vehicles. The purpose of the program was
to (1) determine the impact of a decrease in exhaust
system backpressure on HC, CO, and NOx emissions from
vehicles with and without backpressure-actuated EGR
systems, and, (2) separate the effects [on emissions] of
the following:
(1) the restriction to flow caused by the catalytic
converter, and,
(2) the combined effect of the catalytic material
and the restriction to flow caused by the catalytic
converter.
The effects on exhaust emissions of the converter
restriction and the catalytic material plus the
converter restriction can be separated by determining
the emissions with the catalyst in place and the
emissions with the catalyst replaced by a restriction
which simulates the restriction caused by the
converter but which employs no catalytic material.
For this program, a gate valve was used to simulate
the restriction caused by the converter.
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Objectives of the Test Program
The objectives of the program were to: (1) obtain
measurements of the exhaust pressure at a point upstream
of the catalytic converter, and at that same point with
the catalyst replaced by (a) a straight, unrestricted
pipe, and, (b) the catalyst and original exhaust system
replaced by a dual exhaust system equipped with a muffler
on each exhaust pipe, and (2) determine the effect, of
these changes [(a) and (b)] on the FTP measured HC, CO,
and NOx emissions of the vehicles.
II. Description of Test and Maintenance Sequence
This test program involved FTP testing of three
passenger cars and one van. . A.description of the vehicles
is given in Attachment I. Five FTP'S were run on each
vehicle. The exhaust system on each vehicle was altered
prior.to every test except the first two tests. Each
vehicle underwent the same alterations. The configuration
for each vehicle for.each test was as follows:
M-l
as received configuration
M-Tl
as -received configuration with
pressure transducer installed
M-2
converter removed and straight pipe
'[fitted with a gate valve] installed;
gate valve partially closed to simulate
catalyst backpressure.
M-3.
straight pipe installed; gate valve
in wide-open position
M-4
dual exhaust system installed; muffler
in each pipe; transducer' installed
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For Vehicles 119/0001, 119/0004, and 119/0005 catalytic
converter inlet pressure was measured and recorded throughout
the exhaust portion of the second FTP (i.e., that portion of
the M-Tl. test (as received configuration with pressure transducer
installed) which involves sampling the exhaust, as opposed
to other portions of the FTP such as preconditioning, fueling,
etc. ) . This was done .so that the difference in pressure
between the M-Tl test (as received configuration with pressure
transducer installed) and M-3 test (catalyst replaced by
gate valve - gate valve in wide-open position) could be determined.
See Figure I for the location of the pressure transducer for
each test. For the M-2 tests (catalyst replaced by partially
closed gate valve) and M-3 tests on vehicles 119/0001, 119/0004,
and 119/0005, the pressure was measured at the same point as.
it was for the second test, but the converter was not present.
For the last test the pressure was measured on both sides of
the dual exhaust system and in the same relative position
(relative to the chassis of the vehicle) as it was for the
second test.
For vehicle 119/0003^, exhaust system pressure was meaisured
in a straight section of pipe downstream from the left bank
of cylinders but upstream from the "Y" connection where the
exhaust from both banks of cylinders meet (see Figure I).
This was done because a sufficient length of pipe did not
exist between the "Y" connection and the inlet to the catalyst.
It was believed that a true reading of the pressure of the
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stabilized exhaust flow from both banks of cylinders would
not have been obtained had a measurement been made at the
converter inlet.
Pressure transducers in two different pressure ranges
were used. These ranges were .0-1.25 psi and 0-3.2 psi. The
3.2 psi transducer was used because the 1.25 psi transducer
was damaged after exposure to pressures higher than 1.25 psi.
Attachment II shows the pressure range of the transducer used
for. each test.
A flowchart showing the test sequence is shown in
Attachment III.
Following the Mt-T1 test (pressure transducer installed)
on each vehicle a series of steady state tests were run.
The purpose of the steady states was to measure the catalytic
converter inlet pressure (gauge pressure) so that this
pressure could later be approximated using a gate valve in
place of the catalytic converter. The steady state tests are
a means of finding an adjustment of the gate valve which
should cause the pressure*" sensed by the' EGR backpressure
transducer to be similar to that pressure sensed when the
catalytic converter is in place. Other means, such as running
accelerations representative of the accelerations in the
driving cycle, or running complete LA-41s (an LA-4 is phases
1 and 2 of the EPA Urban Dynamometer Driving Schedule (UDDS))
at different gate valve adjustments, could have been used.
The steady states were run on a dynamometer and involved
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operating the engine at several constant speeds (20, 40, and
60 mph or 20, 40, and 55 mph). The speeds were chosen to
cover the range of speeds in the UDDS. The reason that the
55 mph steady state was used for vehicle 119/0001 was because
the pressure at 60 mph was off-scale with the 1.25 psi full-
scale range transducer. The output of a pressure transducer,
exposed to the catalytic converter inlet pressure was recorded
on a stripchart. (A metal tube approximately 12 inches in
length was placed between the hole in the exhaust pipe at
the inlet to the converter and the pressure transducer so
that the hot exhaust gases, would not damage the transducer.)
Attachment IV shows the steady state speeds at which each
vehicle was run and the average gauge pressure measured
during each steady state.
The restriction.to flow of the catalytic converter was
simulated, by means of a gate valve, for one test on each
vehicle. Pipes were attached to the inlet and exit of the
gate valve and the assembly was installed in place of the
catalyst.
Following the M-2 maintenance (catalyst replaced by
partially closed gate valve) a second series of steady
state tests were run. The gate valve was adjusted
so that the average pressure as read from the stripchart
approximated, as closely as possible, the pressure
recorded during the steady states conducted after this
M-Tl test (pressure transducer installed). The gate
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valve was adjusted so that the average pressure was similar
for as many of the steady state speeds as possible' (see
Attachment IV). It was assumed that this adjustment of the
valve should cause the pressure sensed by the EGR back-
pressure transducer to be similar to that pressure sensed
when the catalytic converter was in place.
III. Discussion of Results
A. Summary of Results
With one exception, the objectives of the test program
were met. This exception was that complete pressure data
was not obtained for the M-4 test (dual exhaust system installed)
on vehicle 119/0004.
For all vehicles, pressure was less for the M-3 test
(converter replaced by unrestricted pipe) than for the M-Tl
test (as received). Also, pressure was less for the M-4
test than for the M-3 test. (See the attached pressure
traces.)
For all vehicles, HC and CO emissions increased when
the catalyst was replaced by an unrestricted straight piece
of pipe. NOx emissions increased for vehicles 119/0001,
119/0003, and 119/0004. The emission results are given in
Attachment V.
B. Comparison of As-Received and Post M-2 Tests (Gate
Valve Installed)
M-2 FTP (catalyst replaced by partially closed gate
valve) NOx emissions for vehicles 119/0001 and. 119/0003
increased from the as-received values (M-l and M-Tl). This
is not what would be expected assuming that the partially
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closed gate valve simulates the restriction of the catalyst.
Since the catalyst produces some NOx,< and it is assumed
that exhaust gas recirculation would occur on vehicles
119/0001 and 119/0003 at the same times as with the catalyst
in place, it would be)'expected that NOx emissions would
decrease from the as-receivted values. The increase is.
greater than the maximum + 33% (for NOx) test-to-test variability
cited in the literature-*- and could be due to the fact that
the gate valve does not correctly simulate catalyst backpressure
under all engine operating conditions.
M-2 FTP (catalyst replaced by partially closed gate
valve) NOx emissions for vehicle 119/0004 decreased from
the as-received values as expected. NOx emissions for
vehicle 119/0005 stayed approximately the same. NOx would
be expected to stay the same or decrease for this vehicle
(119/0005).
Iwiplove K. Juneja, David D. Horschler, and Harold M. Haskew,
"A Treatise oh Exhaust Emissions Test Variability", SAE
Paper 770136.
C. Don Paulsell and Ronald E. Kruse, "Test Variability
of Emission and Fuel Economy Measurements Using The 1975
Federal Test Procedure", SAE Paper 741035.
Martin Fock, Karl-Heinz Lies, and Laszlo Pazsitka, "Critical
Study of the United States Exhaust Emission Certification
Test - Error and Probability Analysis", SAE Paper 750 678.
Douglas Berg, "Survey of Sources of Test Variability in
the 1975 Federal Test Procedure", internal EPA report,
August, 1978.
R.D. Lawrence, "Emission Data Variability", internal EPA
memo to R. E. Harrington.
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M-2 FTP HC and CO emissions increased, in part,, because
of the absence of the catalyst.
C. Comparison of As-Received and Post M-3 Tests (Unrestricted Pipe)
M-3 FTP. (unrestricted pipe) NOx emissions either increased
from or remained approximately equal to.the as-received values.
These trends are expected. Increases are expected because o-f
higher combustion chamber temperatures and pressures. Higher
temperatures and pressures would exist because of a decreased
amount of residual gas in the cylinder as the piston would be
friee to expel more burned gases on the exhaust stroke, due
to the decreased backpressure. It is also believed, that the
greater NOx increase for vehicles with positive backpressure
EGR valves (as compared to those with negative backpressure
EGR valves) is due to the positive backpressure EGR valves
being affected to a greater degree by changes in backpressure
than the negative backpressure EGR valves.
D. Comparison of M-2 and M-3 Tests (Gate Valve Installed
vs. Unrestricted Pipe)
It would be expected that M-3 NOx values would be greater
than M-2 NOx values. For vehicles with backpressure controlled
EGR this was expected because of decreased amounts of residual
gases and decreased EGR, both impacts due to decreased backpressure.
For vehicles without backpressure controlled EGR a NOx increase
is expected because of the decrease in residual gases. Even
though the gate valve does not exactly simulate the catalyst
backpressure, a NOx increase is expected because backpressure
dees decrease. NOx increased for two vehicles, decreased for one
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vehicle and remained the same for one vehicle. Test-to-test
variability and the backpressure effect (i.e., decreased
amounts of residual gases and decreased EGR (where applicable)
due to decreased backpressure) are possible reasons for the
difference in NOx.
M-3 FTP (unrestricted pipe) HC and CO emissions decreased
from their M-2 FTP values for vehicles 119/0001, 119/0004,
and 119/0005, HC emissions for vehicle 119/0003 decreased,
and CO emissions for vehicle 119/0003 increased. (The
non-rounded CO emissions for the M-2 and M-3 FTP's on vehicle
119/0003 were 11.591 and 12.131, respectively. Both of
these values round to 12, which is the number shown in Attachment
V. ) The reason for the CO increase for vehicle 119/0003 is
unknown. The HC and CO decrease for vehicles 119/0001,
119/0004, and 119/0005, and the HC decrease for vehicle
119/0003 could be attributed to a decreased throttle opening
due to decreased backpressure or more complete burning due
to the decreased amount of residual gas in the cylinder.
E. Description of Dual Exhaust System Test
The last test on each vehicle was run with the vehicle
equipped with a dual exhaust system. A pressure measurement
was made on both sides of the dual exhaust system (see Figure
I). (For vehicle 119/0004, no stripchart recording of the
pressure was obtained for the the right bank of cylinders.
This might have been due to plugging of the hole in the
exhaust pipe where the pressure was measured.) Except for
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the second "hill" (that portion of the driver's trace between
the second and third idle periods) of test 5535 (the M-4
test on vehicle 119/0005.) the two pressures measured were
approximately•equal in each case. The reason that the
pressures measured for the second hill of test 5535 were not
approximately equal is unknown.
F. Comparison of M-3 and M-4 Tests (Unrestricted Pipe vs.
Dual Exhaust System)
M-4 FTP (dual exhaust system installed). NOx emissions
increased from their M-3 FTP (unrestricted pipe) values.
Possible reasons for the increase are reduced EGR caused by
reduced backpressure, and higher combustion chamber temperatures
and pressures, which are in turn due to less residual gas in
the cylinder. The reasons stated above related to varying
increases based on the presence of a positive or negative
backpressure EGR valve also apply to the comparison of these
two tests (M-3 and M-4).
M-4 FTP (dual exhaust system installed) HC and CO
emissions increased from their M-3 FTP (unrestricted pipe)
values for two vehicles and decreased for one vehicle. For
the fourth vehicle M-4 FTP HC emissions increased and CO
emissions remained approximately the same. HC and CO increases
could be attributed to (1) a richer mixture due to decreased
amounts of recirculated exhaust gas in the cylinder, and/or,
(2) decreased oxidation of unburned mixture in the exhaust
manifold, due to less residence time in the manifold.
HC and CO decreases could be attributed to higher combustion
chamber temperatures and pressures.
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IV Conclusions
It can be concluded that> for the vehicles tested,
exhaust system pressure decreased when the catalytic
converter was removed and an unrestricted, replacement
pipe was installed. NOx emissions for every vehicle
equipped with a dual exhaust system exceeded the NOx standard.
In order that restrictions used in future test programs
simulate as closely as possible the restriction of the
catalyst, we would attempt to use as the restriction a
catalyst containing no active catalytic coating on the
ceramic substrate.
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Attachment I
Vehicle Control No.
Model Year
Manufacturer
Model
Engine Disp.
No.- of Cylinders
Engine Family
VIN
Trans. Type
Air Cond. (Yes/No)
EGR Valve Type
119/0001
1979
Pord
Econoline 150 (van)
351 cu. in.
8
5.8W "D" (]X150)
E14HBEC4-916
Automatic
Yes
Integral backpressure
transducer; positive
pressure
Vehicle Descriptions
119/0003
1979
GM
Buick Electra Limited
350 cu. in.
8
940J4U
4X69X9H453297
Automatic
Yes
Integral backpressure
transducer; positive
pressure
119/0004
1979
GM
Chevrolet Caprice S.W.
350 cu. in.
8
910L4
1L35L9C110551
Automatic
Yes
Integral backpressure
transducer; negative
pressure
119/0005
1978
GM
Monte Carlo
305 cu. in.
8
810Y2
1Z37U8B496656
Automatic
Yes
Vacuum
modulated
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Attachment II
Pressure Range of :Transducer(s) Used for Each Test
(Ranges are given in pounds per square inch (psi).)
Veh icle
Maintenance 119/001 119/0003 119/0004 119/0005
M-Tl
0 -
- 1.25
0 -
- 3.2
0 -
- 3.2
0
- 3.2
M-2
0 -
- 1.25
0 -
- 3.2
0 -
- 3.2
0
- 3.2
M-3
0 -
- 1. 25
0 -
- 3.2
0 -
- 3.2
0
-3.2
M-4
0 -
- 3.2
0 -
- 3.2
0 -
- 3.2
0
-3.2
(2 transducers
used)
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Attachment III
Test Seguenee
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Attachment IV
Average Gauge Presaute9 Moasuced Putiny Steady State Tests and Approxlmate
Ranges~oT Difference ~ln Backpressure Uetween M-.T1 and M-3 Tests
Column A _ Column B
Gauge Pressures Tin "l^O) Gauge Pressures (In "M2O) Approximate llangou of
Measured During Post M-Tl Measured During Post M-2 1>1 f f er encu . i n Backpressure
FTP Steady States (with
Maintenance Steady State;;
Column A Minus
Between M-T'l jnd M-3
Vehicle
catalyst)1
(with restriction)*
Column B (4)
(in "llp(J) 2
119/0001
3. 32
3 . 50
-0.18
2.1 to 6.9
12.59
1 3.26
-0.67
31.21
31.31
-0.10
119/0003
1. 34
1. 34
0.00
1.0 to 4.4
4.44
3. 58
+0.86
16.77
15. 10
4-1.6 7
119/0004
2.68
2. 24
+ 0.4 4
3.5 to (1.9
12.50
15.97
-3.47
33.58
45.95
H 2. 37
119/0005
1.79
1 .79
0.00
1.3 to U.O
7.14
7.14
0 .00
17.74
25.66
-7.92
^The gauge pressures given correspond to 20, 40, and 55 inph, respectively (e.g., 3.32 "H2O corresponds to 20 nipli, 12.59 "II2O
corresponds to 40 niph, and 31.21 "II2O.corresponds to 55 niph) for vehicle 1 19/0001, and 20, 40, and 60 inph, respectively, for
all other vehicles.
^For all vehicles, the difference In pressure sometimes exceeded the upper limits of these ranges. The upper limits shown
were given to represent an average over the entire driving cyclr., Including the large differences between some pressure
peaks would give a larger range which would not be typical of the overall driving cycle. For vehicles 119/000), 119/0004,
and 119/0005 only a small difference (.44 "IliO) in Idle period pressure could be seen on the strlpchart recordings. Thin
is partly due to the fact that transducers of a larger range .(IVliger than what was used .on vehicle 119/0001j w
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Attachment V
Bnlsslon Results
(Bnlsslons are given In grams/toile. Failing values ate underlined)
Vehicle Control Ho.
BGR Valve Type
119/0001
Integrai/Rjsltlve"
119/0003
Integral/Positive"
119/0004
119/0005
Integral/Negative"
Vacuum Modulated
Maintenance
M-l - as received
M-Tl - As received
exhaust pressure
measured throughout
exhaust portion of
FTP
M-2 - Catalyst replaced
by partially closed
gate valve
M-3 - same as
M-2 except gate
valve fully opened
M-4 - dual exhaust
system installed) one
muffler In each
exhaust llnej no
catalyst.
hc oo Mat re
0.4 3.6 1.0 12.2
0.4 3.6 1.6 12.1
2.2 29 2.9 12.2
1.9 21* 2.5 13.1
2.3 31 5.2 12.5
jic go
0.9 7.7
0.7 8.0
NOx FE
1.9 16.6
1.9 16.4
2.4 12 2.7 16.9
2.2 12 3.0 17.0
2.4 13 5.0 16.3
HC CO Ufa FE
0.6 6.5 1.2 13.9
0.6 6.1 1.2 13.8
4.0 9.1 1.4 14.4
3.7 7.4 2.2 15.7
IIC CO FE
0.5 6.6 2.0 17.5
0.5 5.7 1.9 17.8
4.9 11 1.1 13.7 2.4 14 1.9 18.5.
2.1 11 1.9 18.7
2,2 11 2.1 IB.4
Bnlsalon Standards (g/mlle)
HC
CO
MOk
1970
1.5
15
2.0
1979
1.5
15
2.0
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Attachment VI
Integral Exhaust Pressure Modulated EGR Valve -Positive Control
Pressure -Description of Operation
Exhaust pressure modulated EGR uses a transducer
responsive to exhaust pressure to. mod"Ulate the vacuum signal
to the EGR valve. The vacuum signal source is a port above
the.throttle valve. The transducer is located within the EGR
valve. Under conditions when exhaust pressure is below the
control value, the transducer diaphragm spring is expanded
against the transducer diaphragm and causes the vacuum signal
to be reduced by the air bleed. No. EGR is obtained under .
these . conditions. Under conditions when exhaust ..pressure is
above .the control value, the transducer diaphragm spring is
compressed and the air bleed "is closed,, causing the valve to
open if the vacuum signal is strong enough.
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Attachment VII
Integral Exhaust Pressure Modulated EGR Valve - Negative
Control Pressure - Description of Operation
The negative control pressure EGR valve, :like the. positive
control pressure valveuses a transducer, and a vacuum signal
source located above the throttle valve. When either a positive
pressure (a pressure above atmospheric pressure) or a negative
pressure (vacuum) of small magnitude (small enough that the
transducer diaphragm spring .holds the transducer diaphragm
tight against the bleed hole) exists at point A, the bleed ho»le
is closed and EGR occurs if the vacuum signal is strong enough.
When a. negative pressure of sufficient magnitude exists at
point A (as occurs when the valve is open and intake manifold
vacuum causes the pressure at.point A to drop) the transducer .
diaphragm spring is compressed and the bleed hole is open causing
the vacuum signal to be reduced and EGR to be cut off.
-22-
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Figure I.
Location of Pressure Transducer for Each Test
Vehicle
Test
Location of Pressure.Transducer
119/0001, 119/0004, M-Tl
119/0005
119/0001, 119/0004, M-2, M-3
119/0005
OOT Pl/T
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Q-*-. -rcflNsooceR
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119/0001, 119/0004,. M-4
119/0005
119/0003
M-Tl
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