United States

Environmental Protection

Agency

Air and Radiation EPA420-R-02-025

October 2002

Study of Exhaust

Emissions from Idling

Heavy-Duty Diesel

Trucks and Commercially

Available Idle-Reducing

Devices

 Printed on Recycled Paper
EPA420-R-02-025

October 2002

Study of Exhaust Emissions from

Idling Heavy-Duty Diesel Trucks and

Commercially Available Idle-Reducing Devices

Han Lim

Certification and Compliance Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data that are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
Page 1 of 10

ABSTRACT

Heavy duty diesel truck idling contributes significantly to

energy consumption in the United States. President

Bush’s May 2001 National Energy Policy tasks the U.S.

Environmental Protection Agency (EPA) and the U.S.

Department of Transportation (DOT) to reduce truck

idling. Consequently, the EPA initiated a study that

would quantify long duration idling emissions and fuel

consumption rates. 

The idling study was conducted over a two year period at

the U.S. Army’s Aberdeen Test Center (ATC). A short

introductory study with five tests was done in June 2001

and a larger study with 37 tests in May 2002. In the

larger study, EPA worked with Oak Ridge National

Laboratory (ORNL) and Rowan University with funding

from the New Jersey Department of Transportation

(NJDOT). 

In total, ATC performed 42 tests on nine class-8 trucks

(model years ranging from 1980's to 2001). Two of those

trucks were equipped with 11 hp diesel auxiliary power

units (APU’s), and one was equipped with a diesel direct

fired heater (DFH). The APU powers electrical

accessories, heating, and air conditioning, whereas a

DFH heats the cab in lieu of truck idling. All tests were

run in a climate controlled chamber, where the trucks

idled at high and low RPMs in the following

environments: 90OF with air conditioning on, 0OF with the

heater on, and 65OF with no accessories on. ATC test

technicians adjusted the air conditioning or heater to

maintain a target cab temperature of 70OF. Each idling

test was run for approximately three hours. 

EPA’s ROVER (Realtime On-road Vehicle Emissions

Reporter) and ORNL laboratory emissions instruments

were simultaneously used for measuring fuel

consumption rates, HC (hydrocarbons), NOx (nitrogen

oxides), CO (carbon monoxide), CO2 (carbon dioxide), O2

(oxygen), and PM (particulate matter) (ORNL only)

during the May 2002 test runs. Only the ROVER was

used during June 2001. The test data showed that,

based on the data obtained from this study: (a) on

average, a typical 1980s-2001 model year idling truck

emits 144 g/hr of NOx and 8224 g/hr CO2 and consumes

about 0.82 gal/hr of diesel fuel; (b) there is good test

repeatability when measuring idling emissions; and (c)

use of an APU can reduce idling fuel consumption by

50% to 80% and reduce NOx by 89% to 94% whereas

use of a DFH can reduce fuel consumption by 94% to

96% and reduce NOx by 99%.

INTRODUCTION

In May 2001, President Bush issued the National Energy

Policy directing EPA and DOT to work with the trucking

industry to establish a program to reduce emissions and

fuel consumption from long-haul trucks. Responding to

this directive, EPA initiated a comprehensive program

aimed at reducing idling. This includes organizing

workshops, issuing grants, implementing demonstration

projects, and most importantly, closely examining idling

fuel consumption and exhaust emissions. 

EPA ran a two-phase test program. Phase 1 consisted

of a small sample of trucks and one APU, which was

completed in June 2001. The results of this phase

prompted EPA to develop a more comprehensive test

program (Phase 2), completed in May 2002.

Certain stakeholders reviewed and commented on the

May 2002 test protocol and actual field tests, including

the American Trucking Association (ATA), Rowan

University, and the 21st Century Truck Partnership (a

government-industry partnership). The phase 2 test

program was funded by the New Jersey Department of

Transportation (NJDOT) using the DOT Congestion

Mitigation and Air Quality (CMAQ) funds. The final test

protocol that was used for the May 2002 testing

incorporated comments from the above mentioned

organizations, as well as ATC and ORNL.

Long haul truck drivers idle their engines during their rest

periods to provide heat or air conditioning for the sleeper

compartment, keep the engine warm during cold

weather, and to maintain adequate battery voltage while

using electrical appliances such as a microwave oven or

television set. Other reasons cited by truck drivers for

idling include safety (i.e., keeping the windows closed,

thereby needing cooling or heating) and habit (i.e.,

protecting the engine by not turning it off).

The precise number of long-duration idling trucks is not

known. Estimating the potential number of idling trucks

requires examining several sources. The U.S. Census
Page 2 of 10

Bureau, Vehicle Inventory and Use Survey (VIUS)

(1997), estimates that approximately 500,000 heavy duty

trucks (> 26,001 lbs) travel more than 500 miles as their

average daily trip length. Driving this distance may

require an eight hour rest period, and during this rest

period truck drivers may idle their engines.

The uncertainty on idling trucks extends to the hours and

number of days over a year spent idling. Some reports

indicate 6-8 hours per day over 250-300 days per year.

Since DOT mandates a rest period of 8 hours after a

maximum of 10 hours of driving, it is possible to conclude

idling times of 1,500-2,400 hours per year.

While exact numbers, hours, and days are currently

unavailable, the objective of the test program was to

better understand the fuel consumption and exhaust

emissions associated with idling trucks under different

weather and accessory loads.

METHODOLOGY

To measure actual emissions from a truck’s tailpipe, EPA

used the EPA-developed ROVER (Realtime, Onhighway Vehicle Emissions Reporter) system. ROVER

allows mass emissions data to be obtained

simultaneously with vehicle/engine parameter data from

the truck’s diagnostic port along with vehicle location and

speed data using an integrated global positioning system

(GPS). Other instruments can also be used by

communicating through auxiliary analog channels or

serial ports. 

Initially, ATC was directed to perform a 6 hour idling test

to determine the point when the pollutants reach steady

state conditions. Initial tests showed steady state

emissions at 3 hours. Therefore, every subsequent test

was run for 3 hours. Once the data were collected, EPA

staff analyzed the data and evaluated the baseline idling

emissions for these nine trucks, two APU’s, and one

DFH.

Truck Selection

Trucks of various ages that represent highway trucks

from the 1980's to present day were obtained for testing.

ATC used class-8 trucks that were available at the

Aberdeen Test Proving Grounds and commercially

available truck rentals such as Ryder that ranged from

mid-1980's to 2001. Various model year trucks were

selected including three representing the model years

near the pivotal highway diesel engine standards change

years 1997 (5.0 g/bhp•hr NOx standard) to 1998 (4.0

g/bhp•hr NOx standard). Table 1 below shows the

descriptions of each truck. The grey blocks in Table 1

represent trucks and APU tested in June 2001, the white

blocks represent trucks, APU, and DFH tested in May

2002.

Table 1: Truck and Engine Identification

Truck and Engine Rated HP and

engine 

displacement

Additional Truck

Information as available

1985 Volvo White** 

1990 Volvo NTC-400

400 @ 2100 rpm 

855 CID (14.0 L)

Engine # 11611170, date

of mfr. 12/90

1992 Ford* 

1992 Caterpillar 3406B

425 @ 2000 rpm

890 CID (14.6 L)

Engine Family

NCT0893FPB9 

Serial No. 3ZJ23694

1995 International

(Navistar) 1995

Detroit Diesel S60 

470 @ 2100 rpm

774 CID (12.7 L)

Engine Family:

SDD12.EJDARA 

approx. age: 650,000miles

1997 International

(Navistar) 1997

Caterpillar 3406

410 @ 1800 rpm 

890 CID (14.6 L) 

Engine Family:

VCP893EZDARX 

Serial No. 6TS09138

1998 Freightliner

1997 Cummins

370/435 @ 1800

rpm

Engine Family:

VCE855FJDARA 

Serial No. 11861993

1999 Volvo 

1999 Detroit Diesel S60

470 @ 2100 rpm

774 CID (12.7 L)

Engine Family:

XDDXH12.7EGL 

Serial No. 06R0-547668

1999 KenworthT800

1998 Caterpillar 3406

410 @ 1800 rpm

890 CID (14.6 L)

Engine Family:

WCPXH0893ERK

2000 Volvo 2000 Detroit

Diesel Series 60

470 @ 2100 rpm

774 CID (12.7 L)

Engine Family

YDDXH12.7EGL 

4V4NC9RH31N247483

2001 Freightliner

2000 Detroit Diesel

Series 60

500 @ 2100 rpm

774 CID (12.7 L)

Engine Family

YDDXH12.7EGL 

Serial No. 06R0612467

Idle Reducing Devices:

1995 Pony Pack APU:

Kubota Z200

2000 Pony Pack APU:

Kubota Z482

11 @ 3600 rpm 

29.23 CID 

(0.482 L)

Engine Family

YKBXL.719KCB 

Serial # YC4323

1990's Espar Diesel

Direct Fired Heater 

Model D1LC

NOTES: Grey blocks represent June 2001; elsewhere May 2002 *

Only truck without a sleeper cab nor ECM data port **

No ECM (engine control module diagnostic port) available 

The horsepower ratings are obtained from the engine label (advertised hp)

Each truck was equipped with a sleeper cab (except as

noted in Table 1) air conditioning and heating.

Trucks were not subjected to any special maintenance

procedures (e.g., oil changes, tune-ups, etc.). All trucks

were tested as received or as rented, including the

standard diesel fuel (standard filling station fuel) that all

trucks were equipped with. 

Test Conditions / Scenarios / Variables

All tests were run in a climate controlled chamber,

approximately 40 ft x 40 ft x 24 ft, where the trucks were
Page 3 of 10

idled at high and low RPMs in the following

environments: 90OF with air conditioning operating, 0OF

with the heater operating, and 65OF with no accessories

operating. Table 2 shows all the test scenarios for phase

1 (June 2001) and phase 2 (May 2002). 

Table 2: Test Matrix

JUNE 2001 TEST SCENARIOS

A/C On

Chamber

Temp=90OF

Heater On

Chamber

Temp=0OF

No Accessory

Chamber

Temp=65OF

1985 Volvo White - 1 test 750 RPM N/A N/A

1995 International - 1 test 600 RPM N/A N/A

1999 Kenworth - 1 test 600 RPM N/A N/A

2000 Volvo - 1 test 1000 RPM N/A N/A

1995 APU - 1 test 3600 RPM N/A N/A

MAY 2002 TEST SCENARIOS

1992 Ford - 6 tests 600 RPM 600 RPM 600 RPM

1200 RPM 1200 RPM 1200 RPM

1997 International 

6 tests

700 RPM 700 RPM 700 RPM

1100 RPM 1100 RPM 1100 RPM

1998 Freightliner - 8 tests*

600 RPM 600 RPM 600 RPM

800 RPM* N/A N/A

1000 RPM* N/A N/A

1200 RPM 1200 RPM 1200 RPM

1999 Volvo - 6 tests plus 2

repeats**

600 RPM 600 RPM 600 RPM x3**

1200 RPM 1200 RPM 1200 RPM

2001 Freightliner - 6 tests 600 RPM 600 RPM 600 RPM

1200 RPM 1200 RPM 1200 RPM

2000 APU - 2 tests 3600 RPM 3600 RPM N/A

1997 DFH - 1 test N/A High Heat N/A

NOTES: Total number of tests = 42 

A/C = air conditioning; none = no accessories active 

* Performed additional tests at the intermediate speed to examine emissions/

fuel consumption rates with varying engine RPM. 

** Performed two repeat tests at 600 RPM with no accessories active to

assure good test to test repeatability

The APU tests were run with the air conditioning

operating and heater on in the 90OF and 0OF

environments, respectively. However, the DFH test was

run only in the 0OF environment. ATC test technicians

adjusted the air conditioning or heater to maintain a

target cab temperature of 70OF. EPA staff developed a

test matrix to examine the following primary test

variables: engine RPM, truck age, and accessory load.

The engine RPM was usually dialed in via electronic

onboard controls in the truck cab, except for the 1985

Volvo White truck which did not have the electronic

control feature. The 1985 Volvo truck was only tested at

one rpm. Most trucks were capable of idling from 600 to

1200 rpm. The RPM was set at one value throughout a

three hour test.

Test Equipment

ATC staff used the EPA-developed ROVER system

which is comprised of an IBM PC based data acquisition

system running EPA developed software, a Snap-OnTM

gas analyzer, an optional Horiba MEXA-120 zirconia

NOx sensor, and specially designed exhaust gas

flowmeter modules of various sizes. 

ROVER is used to measure fuel consumption rate, HC,

CO, CO2, and NOx emissions. In the Snap-On MT3505

analyzer, non-dispersive infrared technology is used to

measure HC, CO, and CO2, whereas an electrochemical

sensor is used to measure NOx and O2. Generally, the

electrochemical and zirconia based NOx measurements

showed good comparability, however during the second

phase of this testing program, there was a zero-drift error

in the electrochemical sensor which could not be readily

rectified. Hence, only the Horiba NOx results are

presented. Furthermore, a zirconia NOx instrument

such as the Horiba MEXA-120 was appropriate for the

experiments in this study. SAE Paper 2001-01-3619

contains information that shows a zirconia NOx

instrument has a 4 to 1500 ppm linear calibration at ± 5%

measured point accuracy versus primary standards

(Schenk et al, 2001). The appropriate gas flowmeter

module was attached to the exhaust pipe of the

specimen truck. 

Since the Pony PackTM engine was equipped with a

smaller diameter exhaust pipe than the main engine, a

smaller flowmeter was used. The appropriate flowmeter

module also provided the gas sample to the gas analyzer

and temperature and pressure measurements needed

for mass determinations. Fuel consumption rate was

determined by carbon balance. Systems checks were

performed before and after a set of tests by the zero and

span gas method using calibration gases. The flowmeter

was calibrated using a laminar flow element. The

ROVER instruments were placed near the driver’s seat

during idling tests (see Figure 1). The flowmeter module

mounting was different for various exhaust pipe

configurations. Figure 2 shows the typical horizontal

mounting of the flowmeter, which replaced the normal

tailpipe on a class-8 truck. ATC technicians adapted to

various configurations by making support jigs specific to

an exhaust pipe orientation or exhaust pipe size (i.e.,

smaller flowmeter for Pony PackTM engine). Since this

idling study was a joint effort with ORNL, ORNL staff

measured the same pollutants with a separate sampling

line that was connected to their own instrumentation. 
Page 4 of 10

Figure 1: ROVER system installed on the passenger seat

Figure 2: Mounting of ROVER flow pipe / sampling lines

Note: Figure is NOT to scale.

Although PM measurements were obtained by ORNL

staff, only the ROVER test data will be presented, with

the focus on fuel consumption rates and NOx and CO2

emissions. ORNL staff will present their PM results in

their own SAE paper. ORNL PM data are not presented

in this paper. 

Test Procedure

For the five tests performed during the June 2001 runs,

an initial 10 to 15 minute ‘warm-up’ drive was performed

before the idling test began. An ATC driver drove the

truck on the highway at about 55 miles per hour, then

parked the truck in the climate chamber with the ROVER

system fully attached. The ROVER data acquisition

program was started once the truck was in motion. The

test technician noted when the idling portion of the test

began. The chamber technician ensured that the test

chamber maintained a steady temperature of 90oF, 0oF,

or 65oF, then shut the chamber doors and began the

idling test. For safety, the ATC technicians ran flexible

exhaust hoses from the flowmeters to ports available in

the test chamber to remove unwanted exhaust gases.

The same procedure was used for the May 2002 tests,

except the warm up drive portion was not done. EPA

staff determined from the June 2001 data that the drive

portion did not appear to affect the emissions or fuel

consumption data. 

During the idling tests, the ATC technicians performed

routine instrumentation checks every 30 minutes to

ensure the highest quality data even under the relatively

low exhaust flow rate under idle conditions. This was

performed by temporarily applying no-flow conditions to

the flowmeter module transducers while recording their

outputs. Each pollutant and the volume flow rate during

these half-hourly checks show up as zeros or close to

zero in the ROVER data files. 

All tests for the first phase were performed over a three

day period, where two climate controlled chambers were

used to test two trucks per day. The order of the tests

was randomized to minimize bias in the data collection

(i.e., a 1990's truck was tested on the same day as a

1980's truck; two 1990's trucks were not tested the same

day). Tests for the second phase were run in a single

climate controlled chamber over a three week period.

Similar to phase 1, the order of tests was randomized to

the extent practicable.

Data Post-processing and Analysis

The ROVER portable emissions measurement system

has the capability to create a text file with columns of

data for time, HC, CO, CO2, NOx, engine speed, fuel

consumption rate, etc. The data from each test were

sampled every second (1 Hz). The ROVER data text

files were imported into Microsoft ExcelTM for data

analysis.

RESULTS AND DISCUSSION

Idling emissions data were collected for nine class-8

heavy duty diesel trucks, two APU Pony PackTM engines,

and one EsparTM DFH. The first part of this section will

discuss sample calculations that show how the

emissions and emissions reductions were obtained; the

second part will discuss the data analyses of the June

2001 testing; the third part will discuss the data analyses

of the May 2002 testing.

Sample Calculations

Determining the masses for the pollutants was important

for determining the tons of emissions. The ROVER

program was set up to determine mass emissions in real

time. The gas concentrations in parts per million (ppm)
Page 5 of 10

or percentage (%) for the various pollutants, CO, CO2,

NOx, and HC, the measured exhaust gas flow rate, and

the gas analyzer transport and response time delays are

used. Table 3 shows the densities and other constants

used for performing various mass and fuel consumption

rate calculations in real time.

Table 3: Constants used in Emissions Calculations

CO CO2 HC NOx O2

Density

(g/ft3

)

32.97 51.81 16.33 54.16 37.18

Density of diesel fuel = 3212 g/gal; 

Weight fraction of C in diesel fuel = 12.011 = 

 [12.011 + (1.80 • 1.008)]

= 0.869 from the relation CH1.80

Mass of C in 1 gal. of diesel fuel = 3212 g/gal • 0.869 = 2791 g

C/gal

An example record from the ROVER data file is shown in

Table 4.

Table 4: Sample excerpt from a ROVER Excel Data File

 Delta t (s) 1.001 Lambda 9.450

 EngineLoad % 0.000 AnalyCorFac 1.015

FUEL RATE G/H 0.900 GPS_Speed(mph) 0.000

 ENGINE RPM 1,199 Accel(mph/s) 0.000

 OIL TEMP F 209 Road Grade(%) 0.000

 Ann_dP ("H2O)L 3.706 Climb Rate(ft/s) 0.000

 Ann_dP ("H2O)H 3.734 Altitude(ft) 0.000

 Exh_Temp (F) 223.202 Latitude(deg) 0.000

 Amb_P ("Hg) 30.237 Longitude(deg) 0.000

 Exh_dP ("Hg) 0.008 #_Satellites 0.000

 Chamber_T (F) 62.798 PDOP 0.000

 Vmix'(scfm) 259.813 T_Heading(deg) 0.000

 Vmix_CF 1.000 GPS_Time

 Mode MT GPS_Status Waiting for

GPS Time

 HC(ppm) 10.000 miles driven 0.000

 CO(%) 0.010 HC(mg) 8.373

 CO2(%) 1.510 CO(mg) 14.087

 O2(%) 18.580 CO2(mg) 3,342.595

NOx(ppm) 239 O2(mg) 29,912

 TEMP 82.800 mpg 0.000

 Lambda_SO 0.000 mgal 0.332

 Lambda(B) 9.320 NOx (mg) 55.306

 A/F 136.96 

Values in the white blocks are used in the sample calculation.

For illustration purposes, sample calculations are

provided below:

Mass of CO2 = [ß • V • C • dt] / F

where: 

ß = Density of CO2 = 51.81 g/ft3 

V = Volume Flow Rate = 259.813 ft3/min 

C = CO2 concentration = 1.51% 

F = water condensation in the sample line factor = 1.01

dt = delta time = 1 second

Plugging in the values shown above, yields a mass of

CO2 value equal to 3.342 g = 3342 mg, as shown in

Table 4. 

Also, using the relation 0.273 grams of C in 1 gram of

CO2 yields the following:

[3.342 g CO2 • (0.273 g C/1 g CO2)] / [(2791 g C/1 gal)] =

0.332 mgal = instantaneous milligallons in 1 second.

To determine the gallons per hour fuel consumption rate,

a half hour’s worth of instantaneous milligallon numbers

(not shown in Table 4) are summed up and calculated to

yield 1.25 gal/hr.

Data Analyses of June 2001 Tests

As mentioned earlier, the ATC technicians performed

routine instrumentation checks every 30 minutes by

zeroing the gas analyzer, the MEXA-120, and by

checking the zero of the exhaust flowmeter module

transducers to detect any drifts in the emission

measurement system.

The HC and CO data showed low level emissions that

are typical of diesel engines. Although HC and CO data

were collected, these data were not relevant to this

study, and therefore detailed analyses of HC and CO are

not presented in this paper. NOx showed some

dynamics during the first few hours of idling, then

reached steady state. Therefore, the NOx data were

used as the criteria for determining steady state

conditions. 

In the idling tests, NOx emissions typically exhibited the

behavior seen in Figure 3. After about three hours of

idling, the emissions reach a steady state condition.

When the Kenworth truck was tested the first day, ATC

staff ran the test for six hours. It appeared that a three

hour idle test would be adequate for determining the

steady state emission rates from an idling truck.

CO2 remains at a fairly constant steady state condition

throughout an idling test. However, some tests involving

no accessory loads and heater loads showed an initial

high CO2 value in the same manner as the NOx curve in

Figure 3 and then reached steady state in about 3 hours.

This phenomenon is perhaps due to a ‘cold engine
Page 6 of 10

Figure 3: NOx versus time for 1999 Kenworth Truck

Figure 4: Details/close-up of Figure 3

starting’ scenario where the increased work initially

needed to pump the cold engine oil would result in a

higher initial CO2 and then a leveling off (steady state

pattern) over time as the engine warms up.

For illustration purposes, Figure 4 shows the detailed

steady state NOx patterns for the last hour of the idle

test. The emission data (in average grams/hour) for the

five trucks are presented in Table 5. As mentioned

before, the averaged values were calculated over a

steady state portion of the idling data file, typically the

last hour or half hour of the idling test.

In Phase 1, the idle speeds were not adjusted for any of

the tests, except for the 2000 Volvo. The 2000 Volvo

truck has a feature that will shut down the engine if the

on-board controls detect that the engine is idling at less

than 1000 rpm; therefore, the 2000 Volvo was manually

set to idle at 1000 rpm. The idle speed data were

obtained either automatically via diagnostic tool port or a

direct reading of the dashboard tachometer. 

Table 5: Emission Data Summary for 2001 Test Runs

Idling

RPM

NOx

g/hr

CO2

g/hr

NOx*

tons/yr

CO2*

tons/yr

gal/

hr

1985 Volvo

White

750 19.80 4830 0.05 12.78 0.49

1999

Kenworth

600 81.40 4650 0.22 12.30 0.46

1995

International

600 84.54 4256 0.22 11.26 0.42

2000 Volvo 1000 112.50 8078 0.30 21.37 0.80

1995 Pony

Pack Engine

3600 10.13 2199 0.03 5.82 0.22

* These figures were calculated assuming a truck idles at 2400 hrs/year per

truck (using the assumptions described in the Introduction section, 8 hr/day,

300 days/yr)

The engine idle speed had a significant effect on the fuel

consumption rate, and consequently, CO2 emissions.

The 2000 Volvo truck, which was run at 1000 rpm,

produced nearly twice the CO2 and nearly double the fuel

consumption rate compared to a truck run at 600 or 750

rpm. This result is not surprising, considering that if an

engine is idling at a faster speed, fuel will be injected into

the engine many more times per second, which will lead

to a higher fuel consumption rate.

The Pony PackTM engine is a commercially available

idling reduction technology used in thousands of heavy

duty diesel trucks in the United States. The Pony PackTM

is a 2-cylinder KubotaTM diesel engine, model Z-200 that

practically replaces the main engine’s functions during

idling periods. It powers the air conditioning, heating,

and electrical accessories. The emission data in Table 5

show that there are significant reductions in fuel

consumption and emissions when compared to the 1995

International Truck with 1995 Detroit Diesel engine or the

other engines tested for this study. There is about an

88% reduction in NOx and 50% reduction in CO2. When

the Pony PackTM engine emissions are compared to

other engines, the reductions can be higher. For

example, if a Pony PackTM is used for idling instead of

the stock 2000 Volvo engine, EPA staff estimates a 73%

reduction in CO2 and 90% reduction in NOx could be

achieved.

Data Analyses of May 2002 Tests

The May 2002 tests were run in a similar manner to that

of the June 2001 tests, except more trucks and idle

reducing devices were tested with other variables

including heating loads, air conditioning loads, and

various RPMs. The following table shows a summary of

the test results from the May 2002 tests:
Page 7 of 10

Table 6: Test Results from May 2002

1992 Ford Truck with 1992 Caterpillar Engine NCT0893FPB9:

Test # RPM chmbr. temp (OF) NOx (g/hr) CO2 (g/hr) gal/hr

21 600 0 76.64 5965 0.60

22 1200 0 63.99 11404 1.15

9 600 65 69.41 4653 0.46

10 1200 65 55.76 11342 1.13

11 600 90 75.50 6040 0.60

12 1200 90 69.32 14711 1.47

1998 Freightliner Truck with 1997 Cummins Engine VCE855FJDARA:

20 1200 0 328.59 12599 1.26

13 600 65 154.49 5348 0.53

14 800 65 164.29 7335 0.73

15 1000 65 163.13 9452 0.94

16 1200 65 199.05 12433 1.26

17 600 90 174.68 6813 0.68

18 1200 90 251.42 16577 1.65

2001 Freightliner Truck with 2000 Detroit Diesel Engine YDDXH12.7EGL:

Test # RPM chmbr. temp (OF) NOx (g/hr) CO2 (g/hr) gal/hr

30 600 0 136.23 6848 0.69

29 1200 0 193.57 10460 1.07

23 600 65 77.69 4392 0.44

24 1200 65 135.31 9787 0.98

25 600 90 94.36 4787 0.48

26 1200 90 186.95 12090 1.21

1997 International Truck with 1997 Caterpillar Engine VCP893EZDARX:

34 700 0 137.02 7099 0.71

33 1100 0 196.76 10232 1.02

31 700 65 146.01 5596 0.55

32 1100 65 214.62 10043 0.99

36 700 90 174.17 7262 0.72

37 1100 90 267.27 13575 1.34

The test numbers denote the sequence in which the tests

were run. Test instrumentation issues were worked out

early in the testing by running triplicate identical tests to

assure good test repeatability as can be seen with test

numbers 1, 2, and 4 (see double lined entries in Table 6).

Table 6 (continued):

1999 Volvo Truck with 1999 Detroit Diesel Engine XDDXH12.7EGL:

Test # RPM chmbr.

temp (OF)

NOx

(g/hr)

CO2

(g/hr)

gal/hr

6 600 0 54.83 5878 0.61

7 1200 0 170.28 10877 1.17

1 600 65 80.98 4102 0.41

2 600 65 86.64 3968 0.40

4 600 65 83.24 3915 0.39

3 1200 65 240.98 9251 0.92

5 600 90 104.58 5042 0.50

8 1200 90 288.42 11679 1.16

2000 Pony Pack APU Engine YKBXL.719KCB (for 2001 Freightliner Truck):

28 3600 0 7.27 2053 0.20

27 3600 90 10.01 2353 0.23

Espar Direct Fired Heater Model D1LC (for 1997 International Truck):

38 N/A 30 0.21 402 0.04

NOTES: “Chmbr. temp” = Chamber Temperature 

“RPM” = Engine idling RPM set during a three hour test 

Tests for the 1997 International truck were run at 700 and 1100 RPMs since

these were the high and low engine speeds available. 

Test with 1998 Freightliner at 0 OF was omitted due to instrumentation

problems (excessive freezing on the sample line). 

Tests 1, 2, and 4 are repeat tests intended to demonstrate repeatability of

test instrumentation (outlined in double lines in this table). 

For tests 1, 2, and 4, the standard deviations for NOx,

CO2, and fuel consumption were 4.85 g/hr, 96 g/hr, and

0.01 gal/hr, respectively. The coefficients of variation

(standard deviation divided by the mean) for NOx, CO2,

and fuel consumption were 0.06, 0.02, and 0.025. These

values indicate low variability and good test repeatability.

Several tests on the 1998 Freightliner truck showed a

good linear relationship between fuel consumption rate

and engine rpm. Upon analyzing the data at the various

rpm, EPA staff concluded that running tests on a high

and low rpm would be adequate to predict intermediate

fuel consumption rates at intermediate rpm’s. Figure 5

shows a graphical representation of the 1998 Freightliner

truck data shown in Table 6. An R2 value of 0.99 shows

a strong linear relationship. All tests in Figure 5 were run

without any accessories active (at 65oF). Different

accessories produced different NOx emission patterns

during the idling tests. Tests with no accessories active

or heater active showed flat line responses as shown in

Figure 6. 
Page 8 of 10

Figure 5: Linear relation between gal/hr and rpm

Figure 6: Example Plot with No Accessories Active

Figure 7: Example Plot of Air Conditioning Test

Figure 8: NOx data sorted by RPM for 2002 Tests

Air conditioning tests, however, showed cyclical patterns,

which is caused by the engine fan and the air conditioner

electromagnetic clutch engaging during the tests (see

Figure 7). Furthermore, Figure 7 shows that the engine

rpm, engine load, and NOx emissions are affected by the

air conditioner compressor load. The NOx, rpm, and

load spikes and peaks appear to occur at the same time.

These spikes and peaks produce an additive effect on

the NOx emission rates. As discussed in the June 2001

data section, higher rpm’s will produce higher emissions

and fuel consumption rates as evidenced in the data

presented in Table 6. 

Figure 8 shows all of the average NOx data in grams per

hour for the 2002 tests. Note that there is a general

trend where NOx increases with chamber temperature

and RPM. The data in Figure 8 are also presented in

Table 6. The occasional low NOx values are from the 

1992 Ford Truck with a Caterpillar engine. This truck

appears to have the lowest NOx values and seems least

affected by engine RPM or ambient temperature or

accessories active. 

Clearly, air conditioning tests will produce higher NOx

values, especially with the additive cyclical NOx emission

patterns and higher engine load from the air conditioner

compressor. 

The matrix of tests run during 2002 showed a wide range

of fuel consumption and emissions results by varying

engine age, engine rpm, and ambient temperature

(accessories on or off). Table 7 summarizes the high,

low, and average values.
Page 9 of 10

Table 7: High, Low, and Average Emissions and Fuel

Consumption Rates for 2002 Test Data

NOx (g/hr) CO2 (g/hr) gal/hr

ARITHMETIC MEAN FOR ALL TESTS

High Value 329 16,578 1.65

LowValue 55 3,915 0.39

Average Value 144 8,224 0.82

Standard Deviation 72 3571 0.40

Coefficient of Variation 0.5 0.43 0.43

Low RPM avg. (600 - 800 rpm) 114 5805 0.58

High RPM avg. (1000 - 1200 rpm) 190 11815 1.18

WEIGHTED AVERAGE VALUES (60% High RPM, 40% Low RPM):

Weighted Average Value: 160 9411 0.94

WEIGHTED AVERAGE VALUES (70% High RPM, 30% Low RPM):

Weighted Average Value: 167 10012 1.00

As it turns out, the average fuel consumption rate is the

same as the highest rate obtained from the 2001 testing.

The average NOx of 144 g/hr, however is somewhat

higher than the 112 g/hr obtained from the 2001 testing,

which seems reasonable considering tests at higher

rpms such as 1200 rpm were performed. One should

keep in mind that the average values in Table 7 are

calculated with over 30 unique test scenarios that span

extreme ambient temperatures, engine rpms, and

parasitic loads (A/C and heating), which would explain

the coefficients of variation in the 0.4 to 0.5 range.

It is general practice to idle the engine at 1000 rpm or

higher when heavy accessory loads are present.

Truckers can reasonably be expected to operate their

heaters on days when the temperature drops below 50oF

and to operate their air conditioning on days when

temperatures exceed 70oF. National Oceanic and

Atmospheric Administration (NOAA) temperature data

indicate that these temperature ranges can be expected

to occur nationwide between four and nine months of the

year (NOAA, 2000). Operation of accessory loads

would occur during these months.

With that said, for illustration purposes only, weighted

average values were calculated in addition to the overall

arithmetic mean values, to reflect use of accessory

loads. Table 7 shows that with the high rpm tests

weighted at 60% and low rpm tests weighted at 40%, the

weighted average NOx value is 160 g/hr. If the high rpm

weighting factor is increased to 70% high rpm, with a

30% low rpm, the weighted NOx is 167 g/hr. These are

only example weighted averages. However, EPA staff

conservatively estimate that on average, an idling class-8

truck can emit 144 g/hr of NOx based on the data

collected in this study.

Emissions and Fuel Consumption Reductions Achievable

by Using Idle Reducing Devices

Several types of technologies exist that will effectively

reduce long-duration idling. EPA maintains a list of idle

reduction technologies that can be accessed at the

following website: http://www.epa.gov/otaq/retrofit/

idlingtech.htm. EPA makes no claims as to the

effectiveness or operation of these products, and the list

is for informational purposes only. Two idle technologies

were selected to determine the emissions associated

with these products, and their potential emission

reductions when compared to baseline idling emissions

from the test vehicles. One technology, an auxiliary

power unit by Pony Pack, Inc. has the ability to provide

heat, air conditioning, and electrical power. The other

technology is a direct fired heater by Espar, Inc., which

provides heat only. These two products were selected

for their ability to provide air conditioning and/or heat.

While other similar products exist on EPA’s list of

technologies, these two were selected arbitrarily. 

The following discussion compares the idle reduction

device to the truck to which that device is attached. For

the direct fired heater (DFH), NOx emission rates and

fuel consumption rates from the 1997 International Truck

with Caterpillar engine are compared to the DFH. With

the DFH producing 0.21 g/hr of NOx and the Caterpillar

engine emitting 137 to 197 g/hr (700 to 1100 rpm,

respectively at a chamber temperature of 0oF with the

main engine heater on), a 99% reduction in NOx appears

to be achievable. Similarly, Table 8 shows percent

reductions achievable from a DFH or an APU.

The data from the May 2002 testing shows results

comparable to those obtained in June 2001. The 1995

APU was capable of reducing NOx by 88% and CO2 by

50%, whereas a 2000 APU was capable of reducing NOx

by 89 to 96% and CO2 by 52 to 81%. 

Although the data obtained from June 2001 provided a

glimpse of the idling data characteristics, EPA staff

based their data analyses and emissions estimates on

the May 2002 data, which provided a more

comprehensive and consistent set of data.

Nevertheless, the data from 2001 and 2002 are

comparable. 
Page 10 of 10

Table 8: Percent Reductions Achievable with the Use of

an Idle Reducing Device

1997 International Truck with Caterpillar Engine with the truck

heater on

gal/hr NOx (g/hr) CO2 (g/hr)

Truck @ 700 rpm 0.71 137 7100

Truck @ 1100 rpm 1.02 197 10232

DFH 0.04 0.21 402

Percent Reduction 94-96% 99% 94-96%

2001 Freightliner Truck with Detroit Diesel Engine with the truck

heater on

gal/hr NOx (g/hr) CO2 (g/hr)

Truck @ 600 rpm 0.69 136 6848

Truck @ 1200 rpm 1.07 194 10640

APU w/ heater on 0.20 7.3 2053

Percent Reduction 71-81% 94-96% 71-81%

2001 Freightliner Truck with Detroit Diesel Engine with the truck air

conditioning on

gal/hr NOx (g/hr) CO2 (g/hr)

Truck @ 600 rpm 0.48 95 4787

Truck @ 1200 rpm 1.21 174 12090

APU w/ AC on 0.23 10.0 2353

Percent Reduction 52-80% 89-94% 52-80%

Using the data in Tables 7 and 8 it is possible, with some

assumptions about the idling characteristics of the fleet,

to make estimates of the idling emissions of the nation’s

truck fleet. As an example, if the fleet consists of

500,000 trucks and those trucks idle 8 hours per day,

300 days per year while consuming 0.8 gal of fuel per

hour then those trucks produce 10.9 million tons of CO2

per year (21.7 tons/year per truck) and 190,476 tons of

NOX per year (0.38 tons/year per truck). Under this

example, those trucks would consume 960 million

gallons of diesel fuel while idling. 

CONCLUSION

The purpose of EPA’s idling test program was to closely

examine long-duration idling emissions on a diverse

group of heavy duty trucks under various engine speeds,

ambient temperatures, and accessory loads. Prior to this

test program, long-duration idling tests had not been

conducted. EPA and other organizations and institutions

have examined idling for brief periods of time, but these

tests did not reflect the more realistic long-duration idling

periods.

Based on the emissions and fuel consumption data

generated from this study, the test data showed that (a)

on average, a class-8 truck could emit 144 g/hr of NOX

and 8224 g/hr of CO2, and could consume about 0.82

gal/hr of diesel fuel and (b) the use of idle reduction

technologies can reduce fuel consumption and emissions

significantly.

The idling test program supports EPA’s continued

commitment in working with the trucking industry and its

stakeholders to reduce idling. To date, EPA has

organized workshops, funded demonstration projects,

and created a grant program to assist trucking

companies. Future plans include identifying idling

emissions within mobile models and inventories, and

creating incentives for states to reduce idling.

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Avallone, Eugene A. and Theodore Baumeister III, Marks’

Standard Handbook for Mechanical Engineers Ninth Edition,

McGraw-Hill Publishers, 1987

Brodrick, C.J. et al, Potential Benefits of Utilizing Fuel Cell

Auxiliary Power Units in Lieu of Heavy Duty Truck Engine

Idling, Institute of Transportation Studies, University of

California - Davis, 2001.

National Archives and Records Administration (NARA), Code

of Federal Regulations Title 40 Part 86: Control of Emissions

from New and In-Use Highway Vehicles and Engines, Office of

the Federal Register, July 2001. 

National Energy Policy Development Group, National Energy

Policy, President of the United States of America, May 2001.

National Oceanic and Atmospheric Administration, National

Climate Data Center, U.S. Climatological Averages and

Normals, Normal Daily Maximum and Minimum Temperatures,

Degrees F (1971 - 2000); Calculated by month (January -

December) for major metropolitan areas, 2000. 

Schenk, Charles R., Joseph F. McDonald, and Christopher A.

Laroo, High Efficiency NOx and PM Exhaust Emission Control

For Heavy Duty On-Highway Engines - Part Two. SAE Paper

Number: 2001-01-3619, Society of Automotive Engineers

International, 2001.

Stodolsky, Frank, Linda Gaines et al, Analysis of Technology

Options to Reduce Fuel Consumption of Idling Trucks, Center

for Transportation Research, Argonne National Laboratory,

Paper No. ANL/ESD-43, 2000.

U.S. Census Bureau, VIUS 1997, Vehicle Inventory and Use

Survey, Department of Commerce, Washington, D.C. 1997.

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