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TRB 12-4674
REAL-WORLD MEASUREMENT AND EVALUATION OF HEAVY DUTY TRUCK
DUTY CYCLES, FUELS, AND EMISSION CONTROL TECHNOLOGIES
Gurdas S. Sandhu
Graduate Research Assistant
Department of Civil, Construction, and Environmental Engineering
North Carolina State University
Raleigh, NC 27695-7908
Telephone 919-600-0490, Fax 919-515-7908
Email gurdas_sandhu@ncsu.edu
H. Christopher Frey, Ph.D.*
Professor
Department of Civil, Construction, and Environmental Engineering
North Carolina State University
Raleigh, NC 27695-7908
Telephone 919-515-1155, Fax 919-515-7908
Email frey@ncsu.edu
* Corresponding author
Submitted for Consideration for Presentation and Publication at the 91st Annual Meeting of the
Transportation Research Board
Submission Date: November 3, 2011
Text words 6231 plus 1,250 words for 3 Tables and 2 Figures = 7481 Words
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ABSTRACT
The purpose of this paper is to assess the robustness of relative comparisons in emission rates
between fuels and technologies to differences in real-world duty cycles based on in-use
measurements of five heavy duty diesel vehicles (HDDVs). The paper briefly reviews prior
comparisons of biodiesel versus ultra low sulfur diesel (ULSD) with respect to emissions, recent
changes in emission standards applicable to HDDVs, and typical emission control technologies
used in HDDVs. The study methodology includes field measurements with a portable emission
measurement system (PEMS) and related instruments and sensors for five selected HDDVs
operated in normal service by professional drivers on multiple roundtrip routes within North
Carolina. Duty cycles and emission rates are quantified based on manifold absolute pressure
(MAP), which is an indicator of engine load. Variability in engine load for each observed
roundtrip is quantified based on the cumulative distribution function of normalized MAP. The
effect of variability in duty cycles on fuel-based emission rates for NO, CO, hydrocarbons, and
particulate matter is evaluated. Comparisons are made for emissions of three trucks operated on
each of B20 biodiesel and ULSD. Furthermore, comparisons are made among five trucks with
model years ranging from 1999 to 2010 to illustrate the impact of different emission standards
and emission control technologies on real world emission rates. A key finding is that relative
comparisons pertaining to fuels and technologies are robust to variability in observed duty
cycles.
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INTRODUCTION
A third of Nitrogen oxides (NOx) emissions and a quarter of Particulate Matter (PM)
emissions from mobile sources are from heavy-duty trucks and buses (1). Ultra low sulfur diesel
(ULSD) enables the use of post-combustion controls that would have been poisoned by higher
exhaust sulfur levels. Post-combustion emissions control devices reduce PM and NOx emissions
from heavy-duty diesel vehicles (HDDVs) by more than 90 percent (2). These emissions
reductions are estimated to prevent 8,300 premature deaths, 5,500 cases of chronic bronchitis,
and 17,600 cases of acute bronchitis in children annually (2). These reductions translate to
annual benefits of over $290 billion with a benefit to cost ratio of 19:1 (3).
Over the last four years, a unique dataset has been accumulated based on in-use measurements of
five combination trucks. These data are based on repeated measurements of these trucks on
several study routes, enabling assessment of inter-route variability and its impact on the
robustness of relative comparisons between fuels and technologies. For three trucks,
measurements were made for both soy-based B20 biodiesel and ULSD. The five trucks
represent 1999 to 2010 model years, thereby covering a wide range of emission control
technologies. The main research objectives of this paper are to: (1) quantify inter-run variability
in emission rates; (2) compare fuels; (3) compare technologies; and (4) assess the robustness of
relative comparisons between fuels and technologies taking into account run-to-run variability.
BACKGROUND ON TRUCK FUELS AND EMISSION CONTROL TECHNOLOGIES
This section provides a brief review of truck emissions for biodiesel versus ULSD, HDDV
emissions standards, and the related emission control technologies.
B20 Biodiesel and Petroleum Diesel (PD)
Diesel engines can accommodate biodiesel (BD) without major modifications (4). Neat biodiesel
(B100) is typically blended in a 20:80 volume ratio with ULSD to create B20 biodiesel (B20).
The U.S. Energy Independence and Security Act of 2007 places emphasis on using biodiesel
blends to reduce dependence on foreign oil and improve US energy security.
Engine dynamometer tests compiled by the U.S. Environmental Protection Agency
(EPA) indicate an average 2% increase for NOx emissions, 20% decrease for HC, and
approximately10% decrease for CO and PM for B20 (5). These results are comparable to chassis
dynamometer test results by National Renewable Energy Laboratory (NREL) (6). However, realworld in-use measurements of more than 30 onroad and nonroad HDDV made using a Portable
Emissions Measurement System (PEMS) show average reductions in exhaust emissions of NOx,
HC, CO, and PM for B20 (7-11). Another study reported about 20% reduction in HC, CO, and
PM and about 1% increase in NOx (12). Variations in comparisons could be partly attributable to
whether the measurements are based on real world activity. There is a need for additional data
for real-world comparisons of B20 and ULSD to further expand the existing database.
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Emissions Standards
EPA has put in place increasingly stringent exhaust emissions standards for HDDV engines (13).
Diesel engines emit relatively high uncontrolled levels of NOx and PM compared to other
sources. A key research need is for real-world verification of the efficacy of the standards.
From 1998 to 2003 the NOx standard was 4.0 g/bhp-hr. From 2004 to 2006, NOx and
non-methane hydrocarbons (NMHC) were regulated jointly. A typical acceptable NOx emission
rate under this standard was approximately 2.0 g/bhp-hr, in combination with a NMHC rate of
0.5 g/bhp-hr. During 2007 to 2010, a NOx emission limit of 0.2 g/bhp-hr (0.075 g/MJ) was
phased in. From 1998 to 2006 the PM standard was 0.1 g/bhp-hr. From 2007 onwards the PM
standard has been 0.01 g/bhp-hr (0.0037 g/MJ) (13).
Emissions Control Technologies
HDDVs have different combinations of emissions control technologies depending on their year
of manufacture. Model year 2010 and newer trucks typically have a combination of exhaust gas
recirculation (EGR), diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and
selective catalytic reduction (SCR). Each is briefly described.
Exhaust Gas Recirculation
EGR is an in-cylinder method to reduce NOx emissions. Mixing the exhaust with intake air
lowers the peak flame temperature, thus lowering NOx production. However, there is a penalty of
higher PM emissions, decrease in engine power output by 3-5%, and subsequently increase in
fuel consumption. To combat the penalty, Cooled EGR technology was introduced in which the
exhaust being fed back is first cooled. EGR systems were introduced by heavy duty diesel
(HDD) manufacturers in mid-2002 in response to the 1998 Consent Decree with EPA under
which the 2004 NOx emissions standards were moved ahead to October 2002 (14-16).
Diesel Oxidation Catalysts
DOC is a post-combustion device that has a porous honeycomb structure coated with catalyst.
The catalyst oxidizes CO, HC, and the soluble organic fraction (SOF) of PM. DOCs reduce PM
emissions by 20-40 percent, CO by 10-60 percent, and HC by 40-75 percent. DOCs provided
adequate PM control to meet emission standards up to 2006; however, 2007 standards for PM
required emissions reduction beyond DOC capability (17-19).
Diesel Particulate Filter
DPFs are aftertreatment devices that can trap PM. Periodically, filter regeneration oxidizes
trapped PM to ash, carbon dioxide (CO2) and water vapor (H2O). “Active” regeneration relies on
fuel burners or catalytic burners and may involve a fuel consumption penalty. All on-road HDD
engines use active regeneration technology. According to EPA and California Air Resources
Board (CARB), NO2 emissions are allowed to increase by no more than 20% as a result of use of
DPFs. DPFs typically remove 85 to 90 percent of the exhaust PM and achieve up to 70 to 90
percent reduction in HC and CO emissions (1, 20, 21)
Selective Catalytic Reduction
SCR systems inject Diesel Exhaust Fluid (DEF) into the hot exhaust (800-1000 oF) upstream of a
catalyst. DEF is a solution of 32.5% pure urea in 67.5% deinonized water. The DEF decomposes
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to CO2 and ammonia (NH3) through hydrolysis, which, in the presence of the catalyst, converts
NOx to nitrogen (N2) and water vapor. SCR systems are often used downstream of a DOC and/or
DPF. SCR is used in response to the 2010 NOx standard. It is typically used in combination with
EGR. SCR systems typically achieve 65-75% reduction in NOx emission levels (14, 22, 23).
METHODOLOGY
HDDV emissions measurements are often done using engine dynamometers, chassis
dynamometers, tunnel studies, remote sensing, and on-board measurements (24-27). Engine
dynamometer measurements are reported in units of g/bhp-hr, which are not directly relevant to
in-use emissions estimation. Many engine dynamometer test cycles are steady-state modal
profiles that do not capture real world activity patterns. Further, standardized transient test cycles
are not likely to be representative of real-world operation of a particular vehicle.
Chassis dynamometer tests provide emission factors in grams of pollutant emitted per
mile of vehicle travel, which can be multiplied by estimated vehicle miles traveled to arrive at an
inventory. However, they may not be representative of real world operation, are relatively
expensive, and there are few facilities capable of performing such tests (28, 29).
Tunnel studies provide site-specific measurements averaged over many vehicles. They
may not be representative of emissions for real-life duty cycles and have limited capability to
differentiate between vehicle types (30). Remote sensing measurements are a snap shot of
vehicle activity at a particular location, and thus may not characterize an entire duty cycle (31).
On-board emission measurements quantify real-world in-use emissions over entire duty cycle.
Compared to a chassis dynamometer, on-board systems are easier to setup thus making it
possible to test multiple vehicles per day and for each collect several hours of in-use data. The
cost of commercially available PEMS is far smaller than that of a chassis dynamometer facility.
Commercially available on-board systems first appeared in the late-nineties and have gained
more acceptance over the last decade (32-38). On-board measurement is used here.
Vehicles
Measurements were made on combination tractor trailer trucks with model years 1999, 2005,
2007, 2009, and 2010. The technical specifications of each truck are given in Table 1. These
trucks are owned and operated by the North Carolina Department of Transportation (NCDOT)
for the purpose of delivering highway maintenance supplies to field sites at various locations in
the state. The trucks were operated by NCDOT drivers on regular routes during the
measurements. All trucks pulled 48-foot long trailers. The weight at the start of a run was
always more than the weight at the end. The trucks delivered cargo originating from the Raleigh
depot. On occasion, back-hauled cargo was transported from a field site to Raleigh depot. The
total weight of the trucks varied from 33,300 lbs to 44,800 lbs. The weight at the end of the trip
was typically 80% to 90% of the weight at the start of the trip. The Gross Vehicle Weight
(GVW) of these trucks ranged from 53,200 lbs to 60,600 lbs. Thus, the loads were relatively
small.
Instrumentation
Measurements in 2008 and 2009 used the OEM-2100 Montana portable emissions measurement
system (PEMS) from Clean Air Technologies International (CATI, Buffalo, NY) and
measurements in 2011 used CATI’s OEM-2100AX Axion PEMS. Both systems are similar with
regard to sensors. They differ mainly with respect to software used.
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The PEMS have two parallel operation 5-gas analyzers to measure exhaust gas
concentrations of HC, CO and CO2 using nondispersive infrared (NDIR), nitric oxide (NO) and
O2 using electrochemical sensors, and PM concentrations using light scattering (32, 39, 40).
Two point calibration is used for the gas analyzers. While in-use, the gas analyzers are “zeroed”
using ambient air as a reference to recalibrate the oxygen sensors to ambient concentration and
the CO2, CO, HC, and NO concentrations to baseline values to prevent instrument drift. The
levels of the latter in ambient air are negligible compared to tailpipe exhaust. Both benches zero
every 10 minutes staggered to provide uninterrupted data recording. Span calibration is done in
the lab using a BAR 97 low calibration gas mixture.
Battelle (41) compared the CATI PEMS to standard testing equipment using 40 CFR Part
86 reference methods. The tests were conducted on a chassis dynamometer and used FTP and
US06 test cycles. Linear regression slopes for measurements from the PEMS and reference
facility ranged from 0.97 to 1.03 for CO2, 0.95 to 1.05 for CO, and 0.92 to 1.03 for NOx,
indicating that CO2, CO, and NOx measurements from the PEMS are accurate to within 10% of
reference measurements. HC measurements are biased low by a factor of approximately two
because the PEMS used NDIR and reference method used Flame Ionization Detection (FID) (41,
42). PM measurements are analogous to opacity and used for relative comparisons.
An engine sensor array is used to record engine revolutions per minute (RPM), intake air
temperature (IAT), and manifold absolute pressure (MAP). RPM is measured using an optical
sensor in combination with reflective tape placed on a part that rotates at the same rate as the
crankshaft. The engine intake air sensor is a thermocouple installed in the intake air flow path.
An MAP sensor is installed on a port typically available after the turbocharger.
Data Collection
The time to install the instrument on the study trucks is typically two hours. The measurement
system is installed a day before the test. On the day of the test, the PEMS is warmed up for at
least 45 minutes before the start of driving. During testing, periodic checks of the system status
are conducted. This is done by determining whether engine data is updated on the instrument
display in an appropriate manner and whether the gas concentrations are reasonable.
The data collection also includes: topping off the fuel tank at start and at end of test and
recording the fuel used; weighing the truck at start and at end of test; and recording the odometer
reading at start and at end of test. The refueling of trucks is done from the gas pump owned and
operated by NCDOT at their Raleigh depot.
Quality Assurance and Estimation of Emission Rates
The quality assurance procedure includes: (1) imputing missing seconds of data when 1 or 2
seconds are missing; (2) verification of PEMS internally synchronized concentration and engine
data; (3) checking for errors in IAT, RPM, and MAP values and substituting with corrected
values when possible; (4) correcting influence of ambient air concentration during lead-in and
lead-out when a gas bench is zeroing; (5) checking and correcting when possible consecutive
seconds of concentration data that show no change even though engine parameters change; (6)
correcting or removing seconds of data that report negative concentrations; (7) correcting or
removing seconds of data where the two parallel gas analyzers report significantly different
concentrations; and (8) comparing estimated fuel use with gas pump fuel use.
After quality assurance, intake airflow, exhaust flow, and mass emissions are estimated
using the speed-density method reported by Vojtisek-Lom (43). The method is based on several
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key steps. Air flow through the engine is estimated based on the ideal gas law, taking into
account the pressure and temperature of air entering the cylinder, the compression ratio, the
engine RPM, and a parameter referred to as volumetric efficiency. Volumetric efficiency is the
ratio of actual mass flow through the engine to the theoretical mass flow based on piston
displacement in each cylinder. Based on the exhaust gas composition, the air-to-fuel ratio can be
inferred irrespective of mass air flow. From these estimates, the exhaust flow rate is estimated.
Mass emission rates are estimated based on exhaust concentrations of each pollutant and the
exhaust flow rate. The mass flow calculations are evaluated by comparison of estimated fuel
flow accumulated over a trip to the amount of fuel needed to refill the fuel tank.
Routes and Duty Cycles
The trucks make round trip runs originating at the NCDOT depot in Raleigh, NC and make stops
at other NCDOT depots where they unload and/or load cargo. The only change to the regular
driving cycle made during the measurement study was to keep the truck running during
loading/unloading activity. This was done to maintain continuous operation of the PEMS, and in
turn collect idling data. The typical loading/unloading activity duration at each depot is 30-45
minutes.
Fuel use rate is directly correlated to engine load. A few studies have reported good linear
correlation between fuel use and Manifold Absolute Pressure (MAP) (7, 44). In a study of motor
graders and another study of cement mixers, MAP had an average rank correlation exceeding
93% to fuel use rate (11, 45). Thus, MAP is a good surrogate for engine load with higher MAP
values corresponding to higher power demand.
TABLE 1 Engine and Emissions Control Specifications for Combination Trucks
Truck
Model
Year
Odoa
(mi)
Make
5715
1999
303,744
International
6415
2005
235,202
International
6667
2007
61,008
International
9200I
0009
2009
72,831
International
0121
2010
29,229
Mack
a
b
Disp.
(L)
HP@RPM
Emissions
Controlb
2574 6X4 Cummins ISM-370
10.8
370@2100
-
9400I 6X4 Cummins ISX-500
15.0
500@2100
EGR
Cummins ISX-500
15.0
500@2000 EGR, DOC, DPF
9200I
Cummins ISX-500
15.0
CHU613
Mack MP8-445C
12.8
500@2000 EGR, DOC, DPF
EGR, DOC,
445@1500
DPF, SCR
Model
Engine
Miles on the odometer on test days corresponding to results in Table 3
DOC = Diesel Oxidation Catalyst; DPF = Diesel Particulate Filter; EGR = Exhaust Gas Recirculation; SCR
= Selective Catalytic Reduction
Since the minimum and maximum MAP values are different between truck models, the
MAP values need to be normalized before they can be plotted as frequency distribution to
compare the power demand distribution between routes. Typical idling MAP for all trucks is 98
kPa. The typical peak power demand MAP is 260 kPa for truck 5715, 310 kPa for trucks 6415
and 6667, and 340 kPa for trucks 0009 and 0121. Normalized MAP at time t is equal to the
difference between MAP value at time t and minimum MAP of the route divided by the span of
MAP values for the route. For purposes of comparing driving cycles, the portions of the cycles
associated with idling were excluded. About 97% of the removed data represent time when the
truck is parked at the NCDOT depot. On a non-measurement run, the truck would have been
turned off during such time.
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RESULTS
Five combination trucks were measured for duty cycles, fuel use, and emission rates on a total of
15 days, representing model years from 1999 to 2010. These trucks have engine output ranging
from 370 to 500 hp and are similar in gross vehicle weight and tare weight. As shown in Table
1, all trucks except the 1999 truck have EGR. The 2007 and 2009 trucks also have DOC and
DPF. The 2010 truck also has SCR. Thus, the five trucks represent a wide variety of applicable
allowable emission rates and include combinations of the most typical technologies for NOx and
PM control. All trucks have 6 cylinder engines, 13-speed manual transmissions, and 5 axles
including the trailer. Trucks 5715, 6415, and 6667 were tested in 2008 using petroleum diesel
fuel and in 2009 with soy-based B20 biodiesel fuel. Trucks 0009 and 0121 were tested in 2011
with soy-based B20 biodiesel fuel. The B20 fuels were procured by NCDOT under strict
specifications based on B100 blend stock that is in compliance with applicable ASTM standards.
The trucks operated on a variety of roundtrip routes to destinations within North Carolina, as
shown in Figure 1. Over the period of study from 2008 to 2010 the trucks travelled on 5 routes.
Figure 1 shows the geographic locations of the routes and stops. The routes, depot stops, and
typical distances are:
• Route 1 : Raleigh to North Wilkesboro to Winston-Salem to Raleigh, 320 miles
• Route 2 : Raleigh to Greensboro to Raleigh, 150 miles
• Route 3 : Raleigh to Wilson to Selma to Raleigh, 130 miles
• Route 4 : Raleigh to Castle Hayne to Burgaw to Clinton to Raleigh, 270 miles
• Route 5 : Raleigh to Manns Harbor to Hertford to Raleigh, 430 miles
N. WILKESBORO
1
HERTFORD
GREENSBORO
WINSTON-SALEM
5
2
RALEIGH
WILSON
3
MANNS HARBOR
SELMA
NORTH CAROLINA
CLINTON
4
#
24
25
Route ID Number
BURGAW
CASTLE HAYNE
FIGURE 1 Driving routes from Raleigh to destinations within North Carolina.
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0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Route 1
Route 1
Route 1
Route 1
Route 1
Route 2
Route 2
Route 3
Route 3
Route 4
Route 4
Route 4
Route 4
0
1
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0.1
0.2
0.3
0.4
0.5
0.6
0.7
Normalized MAP
0.8
0.9
1
Route 5
Highway
Arterial
FIGURE 2 Frequency distributions of Manifold Absolute Pressure for real-world duty
cycles based on round-trips originating in Raleigh, NC.
These routes typically involved a high proportion of miles travelled on interstate highways,
especially for the trips to Winston-Salem (via I-40), Greensboro (via I-40), and Castle Hayne
(via I-40). Portions of the roundtrip for Route 4, for the segment from Burgaw to Clinton,
involve travel on a rural arterial with low density of signalized intersections. The trip from
Raleigh to Wilson involves travel on U.S. 64, much of which is limited access divided highway.
The trip from Raleigh to Manns Harbor also involves travel on limited access divided highway
and rural arterials with low density of signalized intersections.
The driving cycles for each roundtrip observed from 14 days of measurements are shown
as cumulative distribution functions of normalized MAP for non-idle operation in Figure 2.
There was one additional day of measurement on Route 3 for which insufficient data were
obtained to characterize the round-trip driving cycle. Among the observed driving cycles,
average normalized MAP ranges from 0.33 to 0.48. At the 10th percentile, normalized MAP
among the observed cycles range from approximately 0.01 to 0.20. At the median (50th
percentile), normalized MAP ranges from approximately 0.3 to 0.5. At the 90th percentile,
normalized MAP ranges from approximately 0.6 to 0.9. Thus, each duty cycle has a wide range
of variability in engine load. The standard deviations of normalized MAP for each duty cycle
are in the range of 0.2 to 0.3.
The cycle-to-cycle variability in the normalized MAP cumulative distribution as shown
in Figure 2 leads to differences in the cycle average rate of fuel consumption and emissions. To
quantify such variability, two cycles were selected for analysis that approximately bound the
observed cycles. One is based on driving on arterial roads (ARs) as extracted from one of the
trips on Route 1, with an average normalized MAP of 0.33. The other is based on driving only
on highways (HWs) from the same trip, with an average normalized MAP of 0.52. Thus, the AR
and HW cycle represent examples of low and high average engine load duty-cycles compared to
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those observed in real-world driving of these trucks. The AR and HW cycle are comprised of
991 seconds and 5397 seconds of data, respectively.
The normalized MAP modal time-based average emissions and fuel use rates are
multiplied with the time spent in each mode, and summed over all modes for the AR and HW
duty cycles to arrive at cycle average emissions and fuel use. The cycle average emission is
divided by cycle average fuel use to arrive at fuel based (g/gal) results.
The effect of variation among cycles on fuel-based emission rates is indicated in Table 2.
For NOx and PM, there is only 5 percent or less difference in the emission rate between the AR
versus HW cycles among the three trucks that were tested for both biodiesel and petroleum
diesel fuel. For HC, all of the cycle average measurements are based on a significant portion of
second-by-second exhaust measurements that are below the detection limit of the HC gas
analyzer; thus, any apparent differences are not statistically significant. For CO, the emission
rate was consistently higher for the AR cycle.
Emission rates for driving cycles are also compared in Table 3 for all five measured
trucks, based on the use of B20 biodiesel, including the two newest of the trucks. For NOx, the
2009 model year truck had a 6 percent higher emission rate on the AR cycle versus the HW
cycle. The 2010 model year truck had approximately twice the emissions on the AR cycle as
the HW cycle, but the emission rates for both were extremely low at only 1 to 2 g/gal, versus 110
g/gal for the 1999 model year truck. Thus, the absolute difference in the NOx emission rate for
the 2010 truck between the cycles was small. There was no significant difference in PM or HC
emission rates between the cycles for the newer trucks. Similar to the older trucks, the CO
emission rate was somewhat higher on the AR cycle.
For three of the trucks, a comparison of emission rates for B20 biodiesel versus
petroleum diesel is possible, as shown in Table 2. The PEMS used here measured nitric oxide
(NO). The results indicate that NO emissions (reported as equivalent mass of NO2) are lower by
approximately 10 to 20 percent on a fuel basis for B20. This finding is consistent for both the
AR and HW driving cycles, and is similar to findings reported previously based on in-use
measurements of dump trucks (8). Furthermore, although a 2002 EPA report is often quoted to
imply that NOx emissions increase by a few percent for B20 compared to petroleum diesel, a
more careful review of the data that comprises the EPA report reveals substantial inter-engine
variability in the results, with some engines having lower and others having higher emission
rates (5). Thus, the results obtained here are not inconsistent with prior results, but of course
represent only a small number of trucks. As expected, there are reductions in the CO emission
rate for the oxygenated B20 versus the non-oxygenated petroleum diesel. The results for PM
were somewhat variable, with one truck having approximately 15 percent lower emission rate on
B20 while the other two had approximately 10 to 25 percent higher emission rates, but the latter
is based on very low emission rates on both fuels. The higher PM emission rates for two of the
trucks are contrary to expectations based on comparison to literature, but may merely reflect
inter-vehicle variability. From an emission inventory perspective, the main concern is the
average difference in emission rate over a large fleet of in-use vehicles, rather than differences
merely for an individual vehicle.
The comparison of emission rates among different model year vehicles given in Table 3
provides clear indication of the efficacy of increasingly stringent emission control standards, as
reflected in successively lower emission rates for NO, CO, and PM from the 1999 to 2010 model
year vehicles tested. The only minor exception is that the 2005 model year truck appears to have
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TABLE 2 Effect of Fuel Type on Fuel-based Emissions Rate
HC [g/gal]b,c
NOx [g/gal]
Truck
5715
6415
6667
2
3
4
5
6
7
8
9
10
11
12
13
14
HW
AR
a
b
c
Fuel
a
PM [g/gal]
HW
AR
HW
AR
HW
AR
HW
AR
B20
110
110
5.8
6.2
6.5
8.6
0.21
0.22
PD
130
130
1.9
2.3
7.6
10.5
0.25
0.25
B20/PD
0.87
0.88
0.86
0.82
0.84
0.87
B20
30
31
2.2
2.7
9.3
12
0.50
0.50
PD
40
42
1.1
1.4
12.3
15
0.45
0.45
B20/PD
0.76
0.73
0.76
0.82
1.11
1.11
B20
17
17
3.8
4.1
0.5
2.0
0.02
0.02
PD
19
19
2.6
3.0
1.3
3.8
0.02
0.02
B20/PD
0.91
0.89
0.39
0.54
1.26
1.28
Highway Duty Cycle
Arterial Duty Cycle
B20 = soy-based B20 biodiesel. PD = ultra-low sulfur petroleum diesel. The composition of B20 is 84.5
weight percent carbon, 13.3 weight percent hydrogen, and 2.2 weight percent oxygen. The composition of
petroleum diesel is 86.4 weight percent carbon and 13.6 weight percent hydrogen.
HC modal average concentrations were below detection limit for at least 6 out of 10 MAP modes for each
fuel and each truck; hence, ratios between the fuels are not meaningful and thus are not reported.
HC results are bias corrected by a factor of 2.0 because NDIR-based HC measurements are biased low by a
factor of approximately two (42).
TABLE 3 Effect of Emissions Control Technologies on Fuel based Emissions Ratea
NOx [g/gal]
15
16
17
18
19
20
21
22
23
24
CO [g/gal]
Truck
Model Year
Data Size
5715
1999
6415
d
HC [g/gal]b,c
CO [g/gal]
PM [g/gal]
HW
AR
HW
AR
HW
AR
HW
AR
16814
110
110
5.8
6.2
6.5
8.6
0.21
0.22
2005
21095
30
31
2.2
2.7
9.3
12
0.50
0.50
6667
2007
25074
17
17
3.8
4.1
0.5
2.0
0.02
0.02
0009
2009
30965
16
17
3.5
3.8
0.3
1.5
0.01
0.01
36120
1
2
4.4
4.8
0.0
0.2
0.01
0.01
0121
2010
HW
Highway Duty Cycle
AR
Arterial Duty Cycle
a
Trucks operated with soy-based B20 biodiesel fuel
b
HC modal average concentrations were below detection limit for at least 6 out of 10 MAP modes for each
truck.
c
HC results are bias corrected by a factor of 2.0 because NDIR-based HC measurements are biased low by a
factor of approximately two (42).
d
Number of seconds of test data used in calculating time-based modal average emissions rate.
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higher PM emission rates than the 1999 truck, but otherwise the emission rate decreases for
newer vehicles. The comparisons for HC are not statistically significant.
As expected based on the applicable emission standards, the 2005 truck has substantially
lower NOx emission rate than the 1999 truck. Except for the 1999 truck, the other four trucks are
within the useful life over which they are required to maintain compliance with the applicable
standards. The 2007 and 2009 trucks have similar NOx emission rates that are approximately
half that of the 2005 truck. The 2010 truck, which has SCR, has NOx emissions that are
approximately 99 percent lower than that of the 1999 model year truck. For trucks without DPF,
NO2 typically comprises 5 percent of total NOx emissions. For trucks with DPF, the fraction of
NO2 in NOx may increase to approximately 20 percent. Even if an increase in the proportion of
NO2 in the NOx emissions had occurred, the net reduction in total NOx emissions would be
above 98 percent when comparing the 2010 truck to the 1999 truck. The PM emission rates of
the 2007 to 2010 trucks are substantially lower than those of the 1999 and 2005 trucks. The CO
emission rates of the three newer trucks are also substantially lower than those of the two older
trucks. Thus, there is a clear difference in the 2007 and newer trucks versus their older
counterparts, with the 2010 truck having extremely low emission rates of NOx, CO, and PM
compared to the others. The HC exhaust concentrations for all trucks were typically below
detection limit and thus comparisons are not statistically significant.
CONCLUSIONS
Differences in fuel-based emission rates among the cycles were typically modest at 5 percent or
less for NOx and PM and statistically insignificant for HC. The CO emission rate was typically
higher on the arterial versus the highway cycle. Furthermore, relative differences in emission
rates, such as between fuels or between trucks, were not substantially different for the arterial
versus the highway cycle. Thus, comparisons of emission rates by fuel or by vehicle technology
are at least somewhat robust to the choice of driving cycle from among the observed real world
cycles. An implication is that it is possible to obtain reliable comparisons between fuels or
technologies regardless of which of the routes were driven by a given truck on a given day, based
on use of modal emission rates and standardization of the comparison to a few duty cycles.
For three of the trucks, a comparison was possible for soy-based B20 biodiesel versus
petroleum diesel. As expected, CO emission rates are lower for the biodiesel. PM emission
rates were lower for one truck, higher for another, and approximately the same for a lowemitting truck. NO emission rates were lower for all three of the trucks. The results here
illustrate inter-vehicle variability, such as for PM. The NO comparison is similar to that for inuse measurements of other vehicles, and thus might be attributed in part to differences in duty
cycles compared to other studies that are based on engine dynamometer tests.
A comparison among the 1999 to 2010 model years represented by the five trucks
provides empirical support that successively more stringent emissions standards are efficacious
in reducing real-world emissions under actual driving conditions. Although the vehicle sample
sizes here are too small to support conclusions regarding trends in fleet average emission rates by
model year, they are consistent with a hypothesis that in-use emissions are substantially lower
for 2010 model year trucks than any prior model year, and that trucks manufactured since 2007
have substantially lower emission rates than those from prior model years.
A key methodological finding of this work is that comparisons between trucks and fuels
are robust to variations among driving cycles observed during real-world data collection. This
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finding provides support for the use of a limited number of ‘bounding’ cycles to characterize
cycle-to-cycle variability.
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ACKNOWLEDGEMENTS
North Carolina Department of Transportation provided fuels and drivers for each tested truck.
This material is partly supported by the National Science Foundation under Grant No. CBET0853766. Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the authors and do not necessarily reflect the views of NCDOT or the
National Science Foundation.
TRB 2012 Annual Meeting
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TRB 2012 Annual Meeting
Paper revised from original submittal.