United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/S4-90/032 Mar. 1991
iSrEPA Project Summary
Evaluation of a Remote
Sensor for Mobile Source
CO Emissions
Donald H. Stedman and Gary A. Bishop
Carbon monoxide (CO) emission
measurements of thousands of vehicles
per day are possible with a recently
evaluated remote sensor developed at
the University of Denver. Funded by
the Environmental Monitoring Systems
Laboratory - Las Vegas (EMSL-LV) In-
novative Research Program, the evalu-
ation has demonstrated the compara-
bility of volume concentration mea-
surements made by this method with
traditional emission monitoring Instru-
mentation. Measurements are made
unobtrusively as vehicles pass through
an infrared light beam directed across
one traffic lane about 25 centimeters
above the pavement. A video camera
records the vehicle registration num-
ber of each vehicle as its CO emissions
are measured so that characteristics of
Individual vehicles and vehicle fleet
categories can be associated with each
measurement. Determining appropriate
applications and monitoring protocols
for this technology is the second phase
of this Innovative Research Project.
Similar remote sensing technology for
monitoring mobile hydrocarbon and ni-
trogen oxide emissions can be devel-
oped to address the urban ozone non-
attainment problem.
This project summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
This project report describes an EPA-
sponsored evaluation of a device designed
to measure the carbon monoxide (CO)
emissions from a passing automobile by
means of remote sensing. The University
of Denver developed the device with sup-
port from the Colorado Office of Energy
Conservation as an approach to promote
improved vehicle fuel economy. High CO
emissions are indicative of incomplete fuel
combustion caused by out-of-adjustment
or defective fuel and emission systems.
Surveillance for CO emissions will identify
vehicles with potential fuel efficiency ben-
efits if repaired. As a result of this original
purpose, the CO emissions monitoring
system is called FEAT, for Fuel Efficiency
Automobile Testing.
The FEAT, shown schematically in Fig-
ure 1, consists of three basic units: an
infrared (IR) light source, an IR detector
and a personal computer. IR absorption is
used to determine the amounts of CO and
CO, emitted by a passing automobile. The
IR fight source, located on one side of a
roadway, sends a collimated beam into a
gas filter radiometer equipped with two
liquid-nitrogen-cooled indium antimonide
photovoltaic detectors. A 4.3 micron filter
isolates the CO region before one of the
two detectors. The resulting beam passes
through a rotating gas filter wheel, half of
which contains a CO and H2 mixture and
the other half N2. The rotating wheel
modulates the signal and provides both a
reference channel and a CO data channel.
The system is installed across a single-
lane highway with the IR beam located at
Printed on Recycled Paper
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Carbon Monoxide Remote Sensing
Computer.
Calibration
Figure 1. Schematic diagram.of the University of Denver CO remote sensor.
the height of most exhaust pipes (about
25 centimeters above the pavement).
When a vehicle enters the optical path a
drop in reference voltage signals the
vehicle's presence. Span voltages from
each of the three signal channels (CO,
CO2, and reference) are acquired before
the vehicle enters the beam, and zero
correction voltages for each channel are
acquired while the vehicle is completely
blocking the beam. As the vehicle exits
the beam, a one half second voltage ver-
sus time trace from each of the three
channels is obtained.
The voltage values obtained as a ve-
hicle exits the beam are corrected for the
ambient measurements obtained in front
of the vehicle. Voltages are converted to
path-averaged concentrations with a linear
calibration equation. Calibration constants
are determined periodically while in use
by measurements of certified compressed
gas mixtures to allow path-averaged CO
and CO.J concentrations to be determined.
The concentrations of the CO and CO2 in
the exhaust plume change jointly as they
rapidly disperse in the turbulent wake be-
hind the vehicle. The computer calculates
a ratio of CO to CO2 that corresponds to
the slope of a best fit line for a scatter plot
of these two gases. The confidence limits
of the slope must be better than 20% for
the software to accept the measurement
as valid.
Using the combustion reaction equa-
tions, the ratio of CO to COa is converted
to exhaust percent CO. The results are
stored on floppy disk and printed on a
video tape together with a freeze-frame
video image of the rear of the vehicle at a
rate as fast as one vehicle per second. At
some locations emissions measurements
of over one thousand vehicles per hour
have been made.
The remainder of this document sum-
marizes the evaluation of the FEAT. It is
organized in four sections which corre-
spond to the objectives of the project.
These objectives are:
1. to evaluate the theory used to con-
vert the CO to CO ratio to percent
CO and to grams CO per gallon of
fuel;
2. to validate the theory using existing
data from laboratory-grade gas
monitors;
3. to inter-compare FEAT and labora-
tory-grade emissions measurements
of vehicles on a dynamometer; and
4. to discuss potential applications for
remote sensing of vehicle emissions.
Theory
The FEAT depends on the carbon and
oxygen balances of the combustion reac-
tion equations to convert the measured
CO to CO2 ratio into percent CO and grams
CO per gallon of fuel burned. This entails
certain assumptions concerning the
chemical formula for the fuel and the
presence of unburned hydrocarbons.
Complete derivation of the theory used by
FEAT and an analysis of the sensitivity of
the FEAT results to the required assump^
tions is the subject of this part of the
evaluation project.
The conversion equation from the mea-
sured ratio to exhaust percent CO de-
pends on a knowledge of the carbon to
hydrogen ratio in the fuel. However the
error introduced by assuming a typical
value is very small, particularly for low
emission vehicles (less than 4% CO).
About 70% of the vehicles measured by
FEAT have percent CO emissions of less
than 1% CO. Even considering a very
high CO emitting vehicle with the most
extreme cases of fuel carbon to hydrogen
ratios, 1:0 for pure carbon (e.g., coal) and
1:4 for methane, the FEAT reported per-
cent CO of 8.8 would be 26% low for the
coal-fired vehicle and 33% high for the
methane vehicle.
When FEAT readings are converted to
mass emissions in grams CO per gallon
of fuel, hydrocarbon emissions may either
be neglected, or corrected using an aver-
age hydrocarbon emissions factor. In either
case the corrections are small, particularly
for the low emission vehicles. For most
vehicles (i.e., about 95%) the uncorrected
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grams CO per gallon values are within 6%
of the corrected values.
Presently under development is a hy-
drocarbon remote sensing monitoring
channel for FEAT which will eliminate this
small source of inaccuracy when reporting
grams CO per gallon of fuel. This hydro-
carbon channel will also enable remote
sensing to report hydrocarbon exhaust
emissions directly,
Theory Validation
Ratios of CO to CO2 from conventional
exhaust system probe measurements can
be directly compared to FEAT measure-
ments of the ratio. However, a direct
comparison of percent CO can not be
done without correcting the probe value
for dilution air. Dilution of vehicle exhaust
by air which is not part of the combustion
process (e.g., as introduced by an air
pump) results in a lower CO concentration
at the probe. FEAT exhaust measurements
are unaffected by dilution air. Probe read-
ings are adjusted upwards to take into
account any dilution caused by the excess
air by using the probe exhaust oxygen
(O2) measurement as an indicator. Unless
stated otherwise, all exhaust probe results
used in comparisons have been adjusted
with the oxygen correction for dilution air.
A data set obtained from the Chrysler
Corporation allowed a FEAT-independent
evaluation of the FEAT theory. Measure-
ments included real-time CO, CO2, and O2
vehicle emissions concentrations before
and after an exhaust system catalyst. The
malfunctioning vehicle was operated on a
dynamometer for the test. Catalyst effi-
ciency was near 100% for low percent CO
emissions but dropped to about 50% when
presented with high percent CO emissions.
The same equations that FEAT uses for
calculation of percent CO where applied
to the ratio of CO to CO2 calculated from
the Chrysler data.
Figure 2 shows the resulting FEAT
theory calculated percent CO (the lines)
compared to the Chrysler measured per-
cent CO (the boxes and X's). The agree-
ment is excellent, except for a slight in-
ability of the FEAT theory to respond
completely to the highest CO peaks, which
may be due to lags in laboratory instru-
mentation. The FEAT theory applies as
well after the catalyst as before it, indicat-
ing that the catalyst follows the same
combustion reaction equation as the en-
gine.
Dynamometer Testing
An interface was required which would
enable the FEAT unit to measure the
emissions from a vehicle on a dynamom-
eter since the normal FEAT mode of op-
eration requires a passing vehicle to trigger
its measurement cycle. The interface con-
sisted of a system for compressing raw
exhaust into a two liter stainless steel
canister from which there were two exits.
At the bottom of the canister a small leak
eliminated the water which had condensed,
and maintained ventilation of the tubing.
At the side of the canister a short tube led
through a computer controlled solenoid
valve to a ring of holes normally used as
the intake for calibration gas to the FEAT
system. A small rotating blade was used
to simulate the presence of a vehicle and
to trigger the release of exhaust into the
beam. Since the blade rotated every two
seconds, a reading of the exhaust was
available every two seconds when the
FEAT was operated in this mode. The
hardware and software were otherwise
identical to those used by the FEAT dur-
ing normal on-road operation.
Comparison testing was performed at
the Environmental Testing Corporation
(ETC), a private automobile emissions
testing facility that specializes in high alti-
tude certification tests for several automo-
bile manufacturers. ETC is equipped with
dynamometer facilities, constant volume
sampling systems, and a full suite of ex-
haust gas analytical instrumentation.
Tests were conducted on November 1,
1989 and November 15, 1989. Single blind
and double blind test protocols were em-
ployed to compare FEAT to laboratory CO
measurements. The majority of the tests
were conducted in a steady-state mode
where a vehicle on a dynamometer was
allowed to stabilize at a specific speed for
a specific load prior to calling for the mea-
sured values from the FEAT and ETC
monitors. In one test, the emissions from
a cold vehicle were monitored as the ve-
hicle warmed up, thus allowing the chang-
ing emissions from the period before,
during, and after catalyst warm-up to be
monitored. In that test, O2 data from ETC
was unavailable to allow dilution correc-
tion to their values.
Comparison Between FEAT Equation
and Vehicle Data
10
G Chrysler angina out
FEAT prediction
T T' r~ 1 "IT T
20 30 40 50 60
TIME (sec. )
80
+ Chyrsler tailpipe out
FEAT prediction
Figure 2. Chrysler data on a malfunctioning vehichle. Upper line is the FEA T equation prediction from the engine emissions. Points are the oxygen corrected
data. Lower line and points are the equivalent data from the tailpipe. wxyyeuwreciea
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Measurement Comparison
D Open comparison
4- Blind comparison
Fit
ETC % C O
Figure 3. Uncorrected FEAT readings against ETC readings. Blind comparisons conducted 11/1/89 and 11/15/89. Open comparison data obtained on 11/
15/89 is dilution corrected. All data not corrected for calibration cylinder discrepancies.
Figure 3 is a scatter graph of the FEAT-
and ETC-measured percent CO along with
a best fit line for the steady state tests.
The slope of the line is 0.94, the intercept
0.011% CO and the R2 is 0.99. The FEAT
data are reported as raw data. If the cali-
bration data are used from the day on
which the measurements were taken, the
FEAT readings would be increased by a
factor of approximately 1.04. For the non-
steady-state test the vehicle started at CO
levels too high for ETC to read, then as
the engine and catalyst warmed up the
emissions decreased to very low values.
Comparison for this test also showed
agreement between the two approaches.
The agreement between the FEAT and
laboratory-grade instrument indicates that
the FEAT correctly measures the instan-
taneous emissions of on-road vehicles.
Precision was determined to be better than
one third of one percent CO for a range of
measured emissions up to greater than
ten percent CO.
Possible Applications
FEAT has been shown to make fast
and accurate measurements of the in-
stantaneous CO emissions from vehicles
as they pass by the instrument. Broadly
speaking, this technology has application
for characterization of CO emissions from
individual vehicles or characterization of
fleet CO emissions by aggregation of indi-
vidual measurements.
The value of individual vehicle monitor-
ing is the identification of the small fraction
of the vehicles that are responsible for the
majority of the mobile emissions (i.e.,
roughly 20% of the vehicles produce 60%
of the emissions). Fleet emissions charac-
terization might produce improved emis-
sions input for air quality predictive models,
or more quickly identify a subset of the
fleet with systematic failures that can be
addressed through actions such as recall
or tampering investigations. However, to
fully realize specific applications requires
additional research. This section discusses
some of the applications of the technology
and the associated issues which must be
addressed. Vehicle emissions are variable
and depend upon many factors including
engine/control system condition, vehicle
load, driving mode (e.g., rate of accelera-
tion/ deceleration, etc.), and engine and
catalyst temperature. The traditional ap-
proaches for emissions monitoring attempt
to minimize the variations by controlling
the influential factors. The Federal Test
Procedure (FTP), which is used to certify
new vehicle emissions, does this by mak-
ing integrated emissions measurements
on vehicles operated on a dynamometer
under very carefully controlled conditions
during a prescribed operating and vehicle
load cycle. The FTP emission measure-
ments are on a mass basis as opposed to
volume concentration measurements.
Though considerably less well controlled
than the FTP, most Inspection and Main-
tenance (I & M) programs use steady-
state tests conducted on warmed engines
under idle and fast idle, no-load conditions.
I & M emissions are measured on a vol-
ume concentration basis. The present new
vehicle emission .control standards and
the maintenance requirements for the on-
road fleet are designed to be responsive
to these two test procedures.
The remote sensor has less control over,
or capability to monitor the factors which
influence emissions than the traditional
approaches. In addition, it makes a "snap
shot" type of measurement which could
catch a vehicle during an anomalous point
in its emission cycle. Judicious selection
of the monitoring location will allow some
control over load as influenced by road
grade and position with respect to road
conditions (i.e., highway entrance or exit
ramps) and traffic signs (i.e. stop signs).
Use of radar (recently integrated into the
FEAT data, system) will permit vehicle
driving mode to be monitored.
The remote sensor has two powerful
advantages over the traditional ap-
proaches. It measures emissions of ve-
hicles as they are in the real-world, under
load in which a traditional I & M test does
not. A single system can make thousands
of measurements per day in a cost effi-
cient way.
Fleet and individual vehicle applications
have different monitoring requirements. For
fleet monitoring applications, the large data
sets that can be easily gathered make the
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aggregated results immune from errors
resulting from emission variability, though
still subject to any bias resulting from non-
representative emissions conditions. If bias
cannot be sufficiently minimized or deter-
mined, it could prevent intercomparisons
between emissions in different parts of
the country. However, bias would have
little effect in the intercomparison in rela-
tive terms of various segments of the same
fleet (e.g., vehicle age, make and model,
etc.) monitored at a single location and
time. Individual vehicle monitoring is sub-
ject to both emissions bias and variability.
It should be possible to minimize variabil-
ity by selection of monitoring sites that
increase the chances of a vehicle being in
a steady-state mode of operation. Simi-
larly, bias may be controlled by picking
monitoring locations that will result in rep-
resentative emissions. Multiple measure-
ments of the same vehicle is another ap-
proach to decrease problems caused by
variability.
There are a number of possible goals
that could be established as part of an
individual vehicle monitoring program.
Obvious gross emitting vehicles could be
required to improve emissions through re-
pair or adjustment; yehicles with high test
results could be subject to additional tests;
or obviously clean vehicles could be given
an exemption from periodic I & M tests.
Research is required to establish the re-
mote sensor measurement value that cor-
responds to the action level desired.
Notice
The information in this document has
been funded wholly or in part by the United
States Environmental Protection Agency
under Cooperative Agreement CR-815778-
01-0 to the University of Denver. It has
been subject to the agency's peer and
administrative review, and it has been ap-
proved for publication as an EPA docu-
ment. Mention of trade names or commer-
cial products does not constitute endorse-
ment or recommendation for use.
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DonaldH.Stedman and'GaryA. Bishop are with the University of Denver, Denver,
CO 80208.
Marc L Pttchford is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of a Remote Sensor for Mobile Source CO
Emissions," (OrderNo. PB91-148320/AS; Cost: $17.00, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
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Penalty for Private Use $300
EPA/600/S4-90/032
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