Environ. Sci. Techno/. 2003, 37, 630-637 

Quantifying the Emission Benefits of 

Opacity Testing and Repair of 

Heavy-Duty Diesel Vehicles 

ROBERT L. MCCORMICK,* 

MICHAELS. GRABOSKI , 

TERESA L. ALLEMAN.t AND 

JAVIER R. ALVAREZ 

Colorado Institute for Fuels and Engine Research. 

Colorado School of Mines. Golden. Colorado 80401-1887 

K. G. DULEEP 

Energy and Environmental Analysis, Inc., 

Arlington. Virginia 22209 

The objective of this study was to begin to quantify the 

benefits of a smoke opacity-based (SAE J1667 test) inspection 

and maintenance program. Twenty-six vehicles exhibiting 

visible smoke emissions were recruited: 14 pre-1991 

vehicles and 12 1991 and later model year vehicles. Smoke 

opacity and regulated pollutant emissions via chassis 

dynamometer were measured, with testing conducted at 

1609 m above sea level. Twenty of the vehicles were then 

repaired with the goal of lowering visible smoke emission, 

and the smoke opacity testing and pollutant emissions 

measurements were repeated. For the pre-1991 vehicles 

actually repaired, pre-repair smoke opacity averaged 39% 

and PM averaged 5.6 g/mi. NOx emissions averaged 22.1 

g/mi. After repair, the average smoke opacity had declined 

to 26% and PM declined to 3.3 g/mi, while NOx emissions 

increased to 30.9 g/mi. For the 1991 and newer vehicles 

repaired, pre-repair smoke opacity averaged 59% and PM 

averaged 2.2 g/mi. NOx emissions averaged 12.1 g/mi. 

After repair, the average opacity had declined to 30% and 

PM declined to 1.3 g/mi, while NOx increased slightly to 

14.4 g/mi. For vehicles failing the California opacity test at 

>55% for pre-1991 and > 40% for 1991 and later model 

years, the changes in emissions exhibited a high degree 

of statistical significance. The average cost of repairs was 

$1088, and the average is very similar for both the pre1991 and 1991 + model year groups. Smoke opacity was 

shown to be a relatively poor predictor of driving cycle PM 

emissions. Peak CO or peak CO and THC as measured 

during a snap-acceleration were much better predictors 

of driving cycle PM emissions. 

Introduction 

A number of states have been concerned about the contribution of heavy-duty diesel vehicle (HDDV) emissions to 

emission inventories. This is because HDDVs are important 

sources of fine particulate matter (PM) and nitrogen oxide 

• Corresponding author present address: National Renewable Energy Laboratory, 1617 Cole Blvd., MS 1633, Golden, 

CO 80401; phone: (303)275-4432; fax: (303)275-4415; e-mail: 

robert_mccormick@nrel.gov. 1 Present address: National Renewable Energy Laboratory, Golden, 

co 80401. 

6 3 0 • ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 37, NO.3, 2003 

(NOx) air pollutants (1, 2). Fine carbon or primary PM can 

lead to reduced visibility (3, 4), and potentially mutagenic 

and carcinogenic polycyclic aromatic hydrocarbons (PAH) 

are associated (1, 5) with these respirable particles (6-9). 

Gas-phase taxies such as formaldehyde are also emitted. The 

U.S. Environmental Protection Agency (U.S. EPA) has proposed new national ambient air quality standards for PMz.s. 

Nitrogen oxides contribute to the formation of visibilityreducing and respirable secondary PM (J {}J as well as groundlevel ozone (J 1) . Furthermore, a broad spectrum of other 

organic compounds can also be found in the gaseous fraction 

of diesel exhaust (1, 12). 

To limit or reduce pollutant emissions from diesel vehicles, 

the U.S. EPA regulates both the quality of on-road diesel fuel 

and the pollutant emission levels from engines under the 

authority of the Clean Air Act. Additionally, smoke opacity 

tests are used by many states as part of inspection programs 

for control of PM emissions. Opacity is defined as the 

percentage a flight transmitted from a source that is prevented 

from reaching a light detector. In smoke opacity measurements from a diesel vehicle, a beam of light is transmitted 

across the exhaust plume to a light detector. High opacity 

may indicate engine malfunction and increased emissions 

of air pollutants, primarily unburned fu el hydrocarbons 

(emitted as an aerosol) or soot particles (13). It is well-known 

that diesel engine malfunction or maladjustment can result 

in increased emissions of pollutants (J 4), and repair of high 

opacity emitters may therefore result in a decrease in the 

contribution of diesel vehicles to the pollutant inventory. 

The measured value of smoke opacity is highly dependent 

upon the test procedures, including ambient conditions, 

engine operating mode, measurement configuration, and 

instrumentation. In recent years, the SAEJ1667 test procedure 

has been suggested as a standard test (J 5). This test was 

developed specifically to identify gross emitting heavy-duty 

trucks and buses. 

To date, no study has quantified the actual emission 

benefits of smoke opacity testing. The objective of this study 

is to begin to quantify the benefits of a smoke opacity-based 

inspection and maintenance (I/ M) program. To that end, a 

number of vehicles exhibiting visible smoke emissions were 

recruited. Smoke opacity and regulated pollutant emissions 

were measured. The vehicles were then repaired with the 

goal of lowering visible smoke emissions, and the smoke 

opacity testing and pollutant emissions measurements were 

repeated. Because of cost and time limitations and because 

relatively few vehicles exhibit high opacity, no effort was 

made to test a representative sample of the in-use heavyduty vehicle neet in Colorado. Instead, all vehicles emitting 

visible smoke that could be obtained during the time period 

of the study were tested. 

The study was conducted in Denver, CO, at 1609 m above 

sea level. At this altitude, the barometric pressure is typically 

82.6 kPa versus 98.9 kPa at sea level. This change in altitude 

has been estimated to increase engine transient test PM 

emissions by from 50 to more than I 00% in several studies 

(16- 18). During short duration accelerations, engine mechanisms that sense air pressure may not adequately compensate for the reduced air density at altitude, leading to a 

decrease in air/ fuel ratio and increased emissions of PM. 

Maximum speed in the snap-acceleration test cycle is reached 

in 1-1.5 s,leading to similarly low air/ fuel ratio and increased 

smoke opacity at altitude. The SAE has recommended that 

a correction factor be applied to smoke opacity measured on 

theJ1667 test conducted at high altitude (15). The calculated 

correction is quite large for typical values of dry air density 

10.1021/es0256919 CCC: S25.00 
TABLE 1. Properties of Vehicles Tested 

vehicle no. mileage (mi) GVW(Ib) test weight model year engine model engine family engine hp vehicle use 

1999-1 86 671 25 000 20 000 1995 Navistar X4L SNV466D6DARA 190 rental 

1999-2• 33 649 34 000 28 000 1988 DOC 6V-92 JDD0552FZG8 350 bus 

1999-3 112 436 50 000 35 000 1986 Caterpi liar 3208 GCT0636DAA2 210 delivery 

1999-4• 129 345 50 000 39 000 1987 Caterpillar 3208 E6HT -6007 -FH 215 delivery 

1999-5 160 817 80 000 39 000 1989 Cummins NTC315 G93E 315 class 8 

1999-5b 160 817 80 000 39 000 1989 Cummins NTC315 G93E 315 class 8 

1999-6 99 024 50 000 39 000 1986 Caterpi liar 3208 GCT0636DAA2 210 delivery 

1999-7 116 004 50 000 39 000 1986 Caterpi liar 3208 GCT0636DAA2 210 delivery 

1999-8 122 360 80 000 52 000 1990 Cummins NTC315 093E 315 class 8 

1999-9 23 519 32 000 26 000 1989 International DT466 unknown 210 school bus 

1999-10 191 525 80 000 52 000 1989 Cummins NTC315 093E 315 class 8 

1999-10< 191 525 80 000 52 000 1989 Cummins NTC315 093E 315 class 8 

1999-11 128 292 50 000 39 000 1987 Ford 8000 HFM07-8FPA3-01 210 delivery 

1999-12 119 280 54 000 43 000 1987 International DT466 unknown 210 delivery 

1999-13• 52 031 42 000 30 000 1994 International DT466 unknown 250 delivery 

1999-14 83 000 54 000 39 000 1990 Cummins LTA10 unknown 240 delivery 

1999-15• 47 511 50 000 39 000 1993 Cummins C8.3 413C 210 delivery 

1999-16 131 349 50 000 39 000 1989 Ford KEM708FPD1 210 delivery 

1999-17 125 127 50 000 39 000 1989 Ford KFM078FD1 210 delivery 

2000-1 108 490 30 000 22 244 1995 Cummins 85.9 SCE35906DAAA 210 box truck 

2000-2 43 151 15 000 12 500 1994 GMC 6.2L RGM6.5C6DAA 190 flat bed 

2000-3 36 872 16 000 13 715 1998 Cummins 85.9 WCEXH0359 175 step van 2000-4 50 319 16 000 13715 1999 Cummins 85.9 unknown 175 step van 2000-5 84 039 16 000 13 715 1997 Cummins 85.9 TCE35906DARW 160 step van 2000-6 189 155 11 050 11 625 1991 lsuzu 4BD2TC NSZ0235FAAX 135 box truck 

2000-7 d 145 596 15 000 12 500 1995 GMC TSA6.5L RGM6.5C6DAA 190 box truck 

2000-8 162 005 62 000 52 000 1992 Cummins L 10 unknown 250 garbage 

2000-98 106 567 60 000 37 310 1996 Cummins M11 TCE661 EJ DARB 280 class 8 

• Vehicles rejected after initial tests because of low smoke opacity. b Vehicle 5 was repaired and tested twice. ' Vehicle 10 was repaired and 

tested twice. "Vehicle not tested after repair because the owner could not make it available. 

encountered at Denver. A vehicle whose measured smoke 

opacity at Denver exceeds 70% points would have an 

""adjusted" smoke opacity at reference conditions (0.0755 

lb/ft3 air density) of around 40%. 

Recently the U.S. EPA has recommended that the states 

use smoke opacity failure points developed by the State of 

California (55% for pre-1991 and 40% for 1991 and newer 

engines) to ensure uniformity across state lines {19). In a 

1997 analysis performed for the California Air Resources 

Board (CARB), EEA and CARB reviewed the diesel liM 

programs currently in place in the United States (20). Many 

other states were using or planning to use the 55/ 40% points. 

In some states there are random roadside smoke inspections 

while others require the opacity test annually as a part of 

vehicle registration. Additionally, some states merely require 

vehicles that fail to be repaired while others issue a fine in 

addition to requiring repair. 

Very little information is available on the performance of 

HDDV liM programs other than failure rates. For the 

California program in place before 1997 (a snap-acceleration 

with smoke opacity measurement by SAE 1243 procedure), 

the failure rate was 34% from spring of 1989 through autumn 

of 1991 but declined to 18.5% in 1993 (20). This decline was 

presumably caused by vehicle owners repairing their vehicles 

to avoid being cited and fined. The current California opacity 

program experiences a failure rate of about 8% (for the HDVIP 

in March 2000; 21). NESCAUM (Northeast States for Coordinated Air Use Management) performed a pilot study of 

SAE 11667 during 1996 (22). On the basis of a sample of 781 

heavy-duty vehicles, the failure rates (at 40/55%) were 4% 

for 1991 and later model years and 25% for pre-1991 model 

years. 

There appear to be no studies of the impact of 1/ M 

programs on actual pollutant emissions from HDDVs. 

However, in the 1980s Ragazzi and co-workers conducted a 

study of the impact of an 1/M program on emissions from 

light -duty diesel vehicles (LDDVs; 23). Repair of these vehicles 

produced, on average, a 37% decrease in PM, a 50% decrease 

in CO, a 34% decrease in HC, and a 21% increase in emissions 

of NOx. Several of the vehicles actually exceeded the NOx 

standard after repair. While the NOx increase was not 

discussed in depth, this is apparently a manifestation of the 

well-known NOxiPM tradeoff where factors (such as injection 

timing) that decrease PM cause an increase in combustion 

temperature and hence emissions ofNOx. At least two studies 

oflight-duty IM240 programs have also shown NOxincreases 

following repair of high CO-emitting gasoline automobiles 

(24, 25). 

A number of different engine malfunctions can cause 

retarded or delayed injection timing, which would increase 

PM emissions while decreasing NOx (26). These include fuel 

pump and injector wear as well as tampering with pump 

settings and smoke-limiting throttle controls. Other malfunctions and maladjustments can apparently have the same 

result, as was demonstrated by Ullman and co-workers {1 4). 

In their study, two 1979 model year engines were tested with 

various maladjustments including increased intake air 

restriction, intake air leak, improperly set injector lash, 

disabling of throttle delay, and injection timing retard. One 

engine was tested new and with a set of fuel injectors that 

had accumulated 50 000 mi of service in a city bus. All 

maladjustment configurations produced a doubling in PM 

emissions and a decrease in NOx emissions of roughly 25-

30%. In a second study, Ullman and Human examined 

maladjustment of a 1986 transit bus engine (27). Retarded 

fuel injection timing, increased intake air restriction, and 

reduced throttle delay all resulted in a decrease in NOx 

emissions. All of these studies taken together indicate that 

diesel engines deteriorate with use to have higher PM 

emissions but generally lower NOx emissions. 

Methods 

Vehicles Tested. Table !lists Lhe properties of the 26 vehicles 

recruited: 14 pre-1991 and 12 of 1991 and later model year. 

VOL. 37, NO.3, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 631 
The only criterion for vehicle selection was the emission of 

visible levels of smoke during acceleration; no attempt was 

made to test a fleet representative of in-use smoking vehicles. 

Thus, the study results cannot be directly applied to inventory 

calculations of the impact of smoke opacity testing and repair. 

Five vehicles were rejected for repair after initial testing 

as not being acceptable candidates (smoke opacity too low 

and reasonably low emissions), but emissions results are 

included for comparison. Two vehicles, nos. 1999-5 and 1999-

10, were repaired twice with emissions testing after each 

repair. These vehicles both had high smoke opacity and PM 

emissions, and initial repairs were not successful at reducing 

emissions. Vehicle no. 2000-7 could not be made available 

for final testing in a timely manner and so was not tested 

following repair. 

Decisions regarding what repairs to perform were largely 

left to shop technicians. However, repair shops were requested to use the following sequence of checks in order to 

determine the cause of smoke: 

(i) Intake air restriction or malfunctioning turbocharger. 

(ii) Malfunctioning or maladjusted throttle controls. 

(iii) Fuel pump or fuel injector malfunction or maladjustment. 

No fixed limit to repair costs was used. Reduction of the 

smoke opacity for many of the vehicles required replacement 

of all fuel injectors, which typically cost in excess of $1500. 

Smoke Opacity Testing. Snap-idle tests were performed 

with the warmed-up vehicle in neutral using a Wager Digital 

Smoke Meter, model6500. The accelerator is rapidly pushed 

to the floor held there for 5 s or until the engine reaches 

maximum (governed) speed, while smoke opacity is measured. The maximum opacity observed is reported. For the 

SAE ]1667 test, three practice tests are first performed. This 

is followed by three real tests, which are averaged to obtain 

the reported value. The three tests must meet an allowable 

spread criterion, and the percentage of smoke opacity is 

corrected for stack diameter using an extinction coefficient 

specific to the instrument. The correction factor for ambient 

temperature, humidity, and barometric pressure specified 

in the ]1667 procedure was applied. 

Chassis Dynamometer Testing. The chassis dynamometer system employed in this work has been described in 

detail in previous publications (28, 2f!). The system is suitable 

for operating at vehicle speeds up to 60 mph (97 km/h) at 

up to 52 000 lb (23 586 kg) vehicle inertial weight. Inertia is 

simulated mechanically using flywheels, while wind and road 

friction are simulated electrically. The vehicle is driven on 

twin 40-in. rolls. The driver manages the vehicle speed. The 

cycle is displayed for the driver using a driver's aid prompt 

that shows the driver his current speed and approximately 

30 s into the future so he can anticipate shifting. For quality 

control purposes, ±2 mph error bands are displayed for the 

heavy-duty driver. This study used the Urban Dynamometer 

Driving Schedule for Heavy-Duty Vehicles (30), except for 

vehicle 1999-2 (a transit bus), which was tested using the 

Central Business District Cycle (31). 

All testing was performed using certification diesel fuel 

obtained from Phillips Chemical Company. While five 

different lots of certification diesel were used over the course 

of the study, every vehicle was tested before and after repair 

on fuel from the same lot. The fuel lots employed were W-642, 

D434 , 9CP05201, 9HP5202, and OKP05202. 

Emissions Measurement. The system for emissions 

measurement includes supply of conditioned intake and 

dilution air, exhaust dilution system. and capability for 

sampling of particulate and analysis of gaseous emissions. 

All components of the emissions measurement system meet 

the requirements for heavy-duty engine emissions certification testing as specified in Code of Federal Regulations 40, 

Part 86, Subpart N. 

632 • ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 37, NO. 3. 2003 

Engine and Dilution Air. Engine intake and dilution air 

are supplied to the chassis test cell through a conditioning 

system at 77 ± 9 °F at 0 ± 0.5 in. of water column. The air 

is filtered to ASHRAE 80% minimum. Humidity is controlled 

to produce a NOx humidity correction factor of I ± 0.03. 

Humidity is measured continuously in the conditioned air 

inlet by two independently calibrated methods: a dew point 

meter and a polymer membrane sensor. The relative humidity 

obtained is converted to absolute humidity using the intake 

air temperature. 

The emissions measurement system includes an 18-in. 

dilution tunnel for exhaust conditioning. Engine exhaust and 

dilution air are mixed at the tunnel inlet with a typical dilution 

ratio of 20 (dilution ratio varies in a narrow range with the 

size of the engine being tested). Exhaust emission samples 

are removed through a secondary sampler to the emission 

bench 10 tunnel diameters downstream. The total flow is 

controlled with a critical flow venturi (CFV) system. A 

centrifugal blower provides suction. 

Gaseous Emissions Measurement. A Pierburg emissions 

bench provides total hydrocarbon from a heated FID, NOx 

from a chemiluminescence analyzer (CLA), and CO and C02 

from infrared detectors. All instruments meet or exceed CFR 

(Title 40, Part 86, Subpart N) requirements for response and 

accuracy. The gas analysis bench is calibrated against U.S. 

EPA protocol gases. NOx and hydrocarbon analyses are 

performed on wet gas while the CO and C02 analysis is for 

a dried gas sample. 

Gas samples are also collected in an automated bag 

sampler through a small sampling CFV rated at about 5 Lim in 

and maintained at the same temperature and pressure as 

the main exhaust CFV to ensure proportionality. The sample 

line is equipped with a Balston filter to remove particulate 

matter and most of the water in the sampled gas. The bag 

sampler gives an integral average emission directly. Dilution 

air is also collected in the bag sampler system. Correction for 

background emissions is performed using the dilution air 

analysis. 

Particulate Matter Sampling. Particulate matter is sampled 

in a Pierburg secondary dilution tunnel with sampling 

temperature maintained below 52 °C. Particulate is proportionally collected in filtering units equipped with two filter 

coupons in series. The particulate weighing is conducted in 

a temperature and humidity controlled room meeting CFR 

specifications. The weighing room is kept at a temperature 

of71 ±I °F, a nominal relative humidity of 50%, and a dew 

point of 48 ± 3 °F. Weighing is accomplished on a Sartorius 

R200D semimicro-balance meeting CFR specifications. The 

loading of particulate on the filter coupons ranged from I to 

8 mg. depending upon vehicle and test cycle. 

Results and Discussion 

Smoke Opacity. The study was performed in two phases 

during 1999 (phase 1) and 2000 (phase 2). Smoke opacity 

(via SAE ]1667) test results are reported in Tables 2 and 3, 

respectively, for each phase. Also noted are visual observations regarding smoke (i.e., if the vehicle was a white smoker) 

and a very brief description of repairs made to the engine. 

Nine of the 26 vehicles emitted white smoke. Of the pre-1991 

vehicles, four would have failed the California IIM test level 

of 55%. Other pre-1991 vehicles that were repaired exhibited 

visible smoke at lower opacity levels. Of the 1991 + model 

year vehicles. five failed the opacity cutpoint of 40% used in 

California. 

In most cases repair resulted in an apparent reduction in 

opacity. although not always to a passing level. For several 

of the vehicles, additional repairs were probably needed in 

order to pass the opacity test. but this could not be done 

because of cost and demands of the vehicle owners for return 

of their vehicles. 
TABLE 2. Average Emissions and Fuel Economy for Phase 1 Vehicles before and after Repair (g/mi) 

smoke 

vehicle pre/post opacity THC NO, co PM 

no. repair (%) (g/mi) (g/mi) (g/mi) (g/mi) mpg comments 

1999-1 pre 72 43.6 15.3 28.9 5.29 5.32 black and white smoke 

1999-1 post 11 2.4 15.4 12.3 1.18 6.11 replace one injector 

1999-2 nr 6 1.0 44 .3 24.4 1.59 3.61 

1999-3 pre 21 2.2 41.7 26.8 3.61 5.56 black smoke 

1999-3 post 8 2.8 35.1 54.5 4.58 5.37 broken throttle pedal, reset pump timing 

1999-4 nr 8 1.5 29.8 14.8 2.33 5.09 

1999-5 pre 82 2.7 20.4 58.8 6.99 4.39 black and white smoke 

1999-5 post 77 2.2 21 .5 44.5 5.84 4.54 6 new injectors, black smoke 

1999-5 2nd post 29 2.2 26.1 22.4 3.25 4.49 new fuel pump 

1999-6 pre 13 1.3 25.3 42.3 5.14 5.40 low level of black smoke 

1999-6 post 7 1.2 32.9 54.6 4.43 5.36 major tune up, pump timing 

1999-7 pre 13 0.8 15.8 37.5 6.82 4.84 low level of black smoke 

1999-7 post 13 1.2 29.8 55.6 5.00 5.09 major tune up 

1999-8 pre 76 2.4 19.1 73.7 9.98 4.19 black and white smoke 

1999-8 post 34 2.8 24.3 15.3 2.64 4.00 reset tampered fuel pump 

1999-9 pre 19 1.6 16.6 17.0 2.32 6.70 low levels of black smoke 

1999-9 post 23 2.1 36.8 21.3 2.01 5.57 new injectors, rebuild fuel pump 

1999-10 pre 59 60.2 28.2 79.4 16.4 3.95 black and white smoke 

1999-10 post 36 58.9 26.2 79.2 15.5 3.97 replace fuel pump 

1999-10 2nd post 33 2.8 25.4 20.2 5.39 4.04 replace one fuel injector 

1999-11 pre 31 1.1 25.7 17.6 2.56 5.18 low level of black smoke 

1999-11 post 35 1.6 54.1 26.2 1.86 5.21 fuel pump, overhead, governor control 

1999-12 pre 31 1.8 26.6 41 .2 4.69 4.48 black and white smoke 

1999-12 post 30 1.3 29.7 38.9 3.98 5.05 replace ir]jectors 

1999-13 nr 11 0.36 13.6 8.1 1.08 5.77 

1999-14 pre 76 7.5 14.3 18.6 3.62 5.11 black and white smoke 

1999-14 post 49 4.4 21 .4 17.2 2.32 5.05 new injectors and camshaft 

1999-15 nr 4 0.96 14.5 3.2 0.89 5.58 

1999-16 pre 30 1.4 16.6 15.3 2.68 4.90 black smoke 

1999-16 post 25 1.4 26.3 14.7 1.75 5.48 rebuilt throttle linkage 

1999-17 pre 21 1.4 15.2 13.1 2.00 5.10 black smoke 

1999-17 post 24 1.1 29.2 18.3 2.03 4.88 set pump timing, repair throttle linkage 

• nr,=not repaired. 

TABLE 3. Average Emissions and Fuel Economy for Phase 2 Vehicles before and after Repair (glmi) 

smoke 

vehicle pre/post opacity THC NO, co PM 

no. repair (%) (g/mi) (g/mi) (g/mi) (g/mi) mpg comments 

2000-1 pre 31 0.33 13.81 8.94 1.59 7.04 very large volume of white smoke 

2000-1 post 31 0.22 14.27 6.1 0 1.06 7.04 replace fuel injectors and thermostat 

2000-2 pre 82 0.98 6.62 24.44 2.78 9.99 very heavy black smoker 

2000-2 post 40 0.08 5.88 3.66 0.77 10.76 replace fuel injectors 

2000-3 pre 37 0.27 13.34 6.46 0.52 9.50 black and white smoke 

2000-3 post 8 0.23 13.36 5.46 0.44 9.58 replace cracked intercooler 

2000-4 pre 41 0.32 11.72 5.64 0.70 9.54 moderate black smoke 

2000-4 post 12 0.35 11 .65 4.79 0.44 9.54 moderate adjustment to fuel pump 

2000-5 pre 33 0.31 11.39 5.41 0.35 10.25 black and white smoke 

2000-5 post 10 0.27 11.05 6.27 0.50 10.32 replace fuel injectors, pressure release valve 

2000-6 pre 70 0.23 4.94 10.1 5 1.46 11 .42 very heavy black smoke 

2000-6 post 57 0.13 4.84 6.21 0.96 11 .62 replace fuel injectors, recalibrate fuel pump 

2000-7 pre 38 0.19 5.35 6.50 0.95 10.65 moderate black smoke 

2000-8 pre 80 3.52 26.45 61 .93 6.17 3.38 heavy black smoke 

2000-8 post 69 2.24 38.56 73.72 4.76 3.32 replaced fuel injectors 

2000-9 nr• 11 0.28 14.13 27.26 2.71 5.08 low smoke, vehicle rejected 

• nr,= not repaired. 

Regulated Emissions. Table 2 reports average regulated data for on the order of 100 trucks. Both of the studies are 

emissions and fuel economy for each phase I vehicle before believed to include almost entirely data for properly funcand after repair, and the same results are reported in Table tioning vehicles, although this cannot be proven. 

3 for phase 2 vehicles. Raw data can be obtained from the The vehicles in both model year groups have PM, THC, 

study final report (32). Table 4 reports average emission values and CO emissions well above the averages from the earlier 

for all of the vehicles in this study by the two opacity cutpoint studies. The pre- 1991 vehicles examined in this study are on 

model year ranges, both before and after repair. For average likely to be high PM emitters with an average of 

comparison, average emission values from the Northern more than 5 g/mi as compared to 2.8 g/mi for NFRAQS and 

Front Range Air Quality Study (NFRAQS; 28) and average 2.0 g/mi for vehicles covered in the review. Average PM 

emission values from a recent review of all heavy-duty vehicle emissions from the 1991 and later vehicles are also more 

emissions data (33) are included. This later study included than two times higher than the average values from the 

VOL. 37, NO.3, 2003/ ENVIRONMENTAL SCIENCE & TECHNOLOGY • 633 
..---, Opacrty 

c:::::J THC 

-NO, 

r:s:s! co - PM 

1991+ (>40%) 

c=:::::::::::::::::::::::::::::::==1- --

Pre-1991 (>55%) 

·80 -60 ·40 -20 0 20 40 

Percent Change in Emission 

FIGURE 1. Average percent change in opacity and g/mi emissions after repair for failure points of 55% (pre-1991) and 40% (1991 and later). 

Failing vehicles only. 

TABLE 4. Average Emissions Levels from This Study Compared 

with Other Stuc1ies 

repaired vehicles (pre) 

repaired vehicles (post) 

repaired vehicles (pre) 

repaired vehicles (post) 

smoke 

opacity 

(%) 

Pre-1991 

39 

26 

THC NO, CO PM 

(g/mi) (g/mi) (g/mi) (g/mi) 

7.0 22.1 36.8 5.6 

2.1 30.9 29.9 3.3 

1991 and Later 

59 5.5 12.1 17.6 2.2 

30 0.74 14.4 14.8 1.3 

NFRAQS Study (211) 

pre-1991 36 2.3 23.2 30.1 2.8 

1991 and later 22 0.77 18.4 9.4 1.0 

Review Study (33) 

pre-1991 2.6 28.1 15.1 2.0 

1991 and later 1.2 25.9 9.0 1.0 

previous studies. Emissions from the 1991 and later model 

year vehicles are on average much lower than for the older 

vehicles. In part this is because the vehicles are newer. 

However, these vehicles are on average much lower gross 

vehicle weight and hence lower horsepower, which also leads 

to lower grams per mile emissions. 

Four of the pre-1991 vehicles failed the 55% failure point 

used in California and recommended by the U.S. EPA. The 

average change in emissions for this group of opacity test 

failing vehicles is shown in Figure I. Apparent benefits in 

terms of reduced smoke opacity. PM, CO, and hydrocarbon 

emissions are observed. NOx emissions appear to have 

increased by more than 20%. Additionally, five of the 1991 + 

model year vehicles exhibited greater than 40% opacity before 

repair, the failure point used in California and recommended 

by the U.S. EPA. The average change in emissions is shown 

in Figure I, and this group also shows apparent benefits in 

reduced opacity, PM, CO, and hydrocarbon emissions. NOx 

appeared to increase by about 6% for this group. 

A statistical analysis of the data was performed to allow 

a determination as to whether observed differences in 

emissions after repair were significant. This analysis employed a two-sample t-test comparing mean emission before 

and after repair. The t-test tool in Microsoft Excel was used 

under the assumptions of equal variance, two tailed tdistribution, and hypothesized mean difference of zero. 

634 • ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 37, NO. 3, 2003 

TABLE 5. Significance of Emissions Changes after Repair 

Reported as p Value 

fa iling 

vehicle vehicles no. HC NO, co PM (%) 

1999-1 0.0003 0.926 <0.0001 0.0001 >40 

1999-3 0.015 0.0002 O.Q18 0.065 

1999-5 0.0200 0.0002 0.0420 0.0065 >55 

1999-6 0.757 <0.0001 0.003 0.932 

1999-7 <0.0001 <0.0001 0.00063 0.075 

1999-8 0.018 0.267 0.051 0.076 >55 

1999-9 0.085 0.0007 0.027 0.06 

1999-10 <0.0001 0.012 <0.0001 0.0067 >55 

1999-11 0.012 <0.0001 0.0026 0.0087 

1999-12 0.06 0.0003 0.21 0.202 

1999-14 <0.0001 <0.0001 0.042 0.032 >55 

1999-16 0.854 0.0006 0.482 0.0022 

1999-17 0.0069 <0.0001 0.024 0.845 

2000-1 0.089 0.051 0.138 0.316 

2000-2 0.018 0.044 0.0039 0.0045 >40 

2000-3 0.119 0.683 0.274 0.545 

2000-4 0.121 0.775 0.28 0.134 >40 

2000-5 0.013 0.41 0.202 0.022 

2000-6 0.0194 0.342 0.027 0.032 >40 

2000-8 0.0057 <0.0001 0.178 0.094 >40 

no. <1 16/20• 14/20 13/20 14/20 

no. <1, >55 4/4 3/4 4/4 4/4 

no. <1, >40 4/5 2/5 3/5 4/5 

avg 0.11 0.18 0.10 0.17 

avg failing >55 0.0095 0.070 0.034 0.030 

avg failing >40 0.0323 0.42 0.098 0.053 

• Numbers of vehicles for which p < 0.1 are noted as a fraction of 

the total number of vehicles in each group. 

Results of this analysis are reported in terms of p value in 

Table 5. A low p value indicates a higher probability that the 

change in emissions was significant. For example. a p value 

of 0.01 corresponds to a significant change with 99% 

confidence. If we arbitrarily select 90% confidence as the 

significance level. about 75% of the repaired vehicles showed 

a significant change in emissions of all four criteria pollutants. 

All of the pre-199 !-vehicles failing the opacity test exhibited 

significant reductions in emissions of HC, CO. and PM 

following repair. Two out of four of these vehicles exhibited 

a significant increase in emissions ofNOxwhile one exhibited 

a significant decrease. For the 1991 + vehicles, 4 out of 5 

exhibited significant reductions in HC and PM, and 3 out of 

5 exhibited a significant reduction in CO. NOx emissions 
increased significantly for one and decreased significantly 

for a second one out of these 5 vehicles. Examination of the 

average p values for all 20 repaired vehicles indicates that, 

on average, changes in emissions after repair are significant 

with 80-90% confidence. However, this increases to greater 

than 90% confidence when the average is taken only for pre1991 vehicles with greater than 55% opacity before repair, 

including a significant increase in NOxemissions. For those 

vehicles failing the 1991 and later model year opacity cutpoint 

of 40%, changes in HC, CO, and PM were on average 

significant with greater than 90% confidence while the change 

in NO xis not on average significant for this group as a whole. 

It would be desirable to estimate the magnitude of the 

changes associated with a diesel I/M program. However, since 

a distribution function for j-1667 smoke opacity based on a 

large sample does not exist for the Colorado fleet, it is not 

possible to estimate how many vehicles an IIM program 

would impact. 

Types of Repairs and Repair Costs. Excessive smoke is 

caused by operation at a fuel/air ratio above the smoke limit, 

and thus repairs typically involve components that affect 

fuel/ air ratio. Virtually all repairs were to the injectors, fuel 

pumps, fuel pump calibration, and injection timing. Injection 

pump work was performed on 10 out of 20 engines. Eleven 

outof20 engines had injectors replaced, of which 9 required 

replacement of all injectors. Injection timing and governor 

adjustment was the most common repair for pre-1991 

vehicles, and injector replacement was the most common 

repair for 1991 and later vehicles. Three vehicles exhibited 

a maladjusted throttle linkage. There was also one replacement of a cracked intercooler. Repair costs ranged from $85 

to $2053 with an average of $1088, including both repairs as 

a single cost for the vehicles that were repaired twice. Average 

repair cost for the pre-1991 opacity test failing group was 

$1202 and for the 1991 +opacity test failing group was $991 . 

Comparison to NO. Emission Standards. NOxemissions 

were observed to increase for 12 of the 20 repaired vehicles. 

To understand the cause of this increase, it is useful to 

compare the observed NO. emissions to the NOx standard 

in effect for each model year. Engine certification testing is 

performed on an engine dynamometer, and emissions are 

reported in units of grams of pollutant emitted per unit work 

(g/ bhp-h). To compare vehicle emissions with the emissions 

standards, which are likewise in g/ bhp-h, a method for 

converting the g/ mi emissions to g/ bhp-h is needed. The 

conversion of vehicle emissions in grams per mile to engine 

emissions in units of grams per bhp-h can be performed on 

a fuel consumption basis using eq I: 

I . __g__ bhp-h g-fuel g m1 = X X , bhp-h g-fuel m1 (l) 

The term g-fuel/ bhp-h is the engine brake-specific fuel 

consumption (BSFC). The BSFC for new engines over the 

engine transient cycle is available from the manufacturers, 

as measured from the certification test. In general, the 

presence or absence of mal performances alters the BSFC by 

less than ±5% unless the malperformance is very severe. 

A comparison of measured NOx emissions, converted to 

g/ bhp-h using eq 1, with the emissions standards is shown 

in Figure 2. For the pre-1991 vehicles, all but one was below 

the NOx standard before repair, and all but one vehicle (a 

different one) was below after repair. Vehicle 1999-3 had 

high NOx emissions (above the standard) before repair, and 

the injection timing may have been set incorrectly for the 

vehicle as received. Adjustment of timing resu lted in decreased NOx and increased PM emissions. Vehicle 1999-11 's 

repairs also involved changing or resetting injection timing 

and it is possible that timing was set inaccurately resulting 

in the high g/ bhp-h NO. emissions. NOx emissions were 54 

~ 16 

c. 

~ 

.Q 

0,12 

r:: 

.Q 

(/) 8 0 (/) 

·e 

w 

>< 4 • 0 

z 

Q Vthlcle11tV·11 • Pre-Repair 

0 Post.Repalr 

e vehic .. 1!iiH..J - NOx Standard 

0 

0 

0 

• • • :l 0 

i . • 

0 

CL-_..;.... __ _,e1-...,L_ o 0 ~ i . 

+---~-- --~--~--~--~--~~~ 

1984 1986 1988 1990 1992 1994 1996 1998 2000 

Model Year 

FIGURE 2. Brake-specific NO. emissions before and after repair 

as compared to the NO. emission standards. 

g/mi (28 1 g/ gal) after repair for this vehicle. These values are 

very high in comparison to other heavy-duty vehicles of this 

vintage (33). For the 1991 and later model year vehicles on 

average, pre and post repair NO. emissions are near the 

standard , although the 1998 and 1999 model year vehicles 

fa ll slightly farther above the line. The increase in NOx 

emissions for these newer vehicles is smaller and not evident 

in every case. Since BSFC is a function of speed and load, 

using an average value to estimate emissions is only an 

approximation. Additionally, it is not known if the emissions 

certification for any of these engines included any allowance 

for emissions banking or trading. Even with these considerations, the results suggest that in many cases repair results 

in an increase in NOx. but to levels that are still near or below 

the standards for the engine's model year. 

The policy and regulatory implications of the observed 

NOxincrease are not clear at this time. The U.S. EPA pollutant 

inventory models do not consider emissions deterioration 

for heavy-duty diesels, so these models are already counting 

the increased NOx emissions that would result from a 

widespread opacity based IIM program. However, in some 

urban areas the increase in actual NOx. as opposed to modelpredicted NOx. could have a significant impact on air quality 

and lead to violation of the National Ambient Air Quality 

Standards for ozone and fine particles. Both the U.S. EPA's 

guidance document and all current diesel IIM programs are 

based on the belief that repair of high opacity vehicles will 

have only air quality benefits. Given data that this is not the 

case, it is recommended that deterioration be incorporated 

into inventory models and that these models be used to 

determine the overall air quality impact of diesel I/M. A great 

deal of additional data will clearly be required to model 

deterioration accurately. 

Prediction of High PM Emitters. Early work on smoke 

opacity found some correlation between smoke and PM 

emissions. McGuckin and Rykowski (34) observed good 

correlation when comparing smoke opacity with steady-state 

emissions of PM. Alkidas (35) found that Bosch smoke 

number correlated reasonably well with total PM but that 

the correlation was better with the nonvolatile (soot) fraction 

of PM. A comparison of smoke opacity and PM emissions is 

presented in Figure 3. The data from this study has been 

combined with results from two previous studies (28, 36) of 

properly functioning vehicles in order to examine the 

correlation between smoke opacity and PM emissions and 

to examine the potential for using other measurements to 

predict what vehicles are high PM emitters. Note that these 

previous studies used uncorrected opacity (not corrected 

for ambient temperature, humidity, or barometric pressure), 

and thus Figure 3 includes uncorrected opacity results from 

this study as well. 

VOL. 31 , NO. 3, 2003/ ENVIRONMENTAL SCIENCE & TECHNOLOGY • 635 
18 

• 1999 liM Study, Pre-repair 

16 0 1999 l/M Study, ost-n~pair • .. 2000 liM Study, Pre-repair 0 

'V 2000 liM Study, Post-t'8pair 

14 • Reference 36 

0 Reference 28 

- Linear Regression 

·e 12 lntercept=O. 79 

Slope=0.052 

c;, r 2

=0.23 

vi 10 • c: 

0 

'iii 

Ill 8 

·e 

w • • 

::!! 6 0.. 

4 

2 

0 

0 20 40 60 80 100 

J1667 Smoke Opacity (uncorrected) 

FIGURE 3. PM emissions as a function of smoke opacity for this 

study and two previous studies. 

18 

• Phase 1 1/M Study, Pre-repair 

0 Phase 1 1/M Study, Post-repair 

16 .. Phase 2 1/M Study, Pre-repair • 'V Phase 21/M Study, Post-repair 0 

• Reference 36 

14 0 Reference 28 

- Linear Regression 

lntercept=-0.006 

12 Slope=0.115 

r 2=0.67 

.E 10 • c, 

::i 8 Q, 

• 6 0 .. 0 

0 • • 0 

e " 4 • • 0 0 0 

2 • 

0 

0 20 40 60 80 100 

CO, g/mi 

FIGURE 4. Correlation of PM and CO emissions. 

Clearly there is a relationship between PM and smoke 

opacity, although smoke opacity is not a particularly good 

predictor of PM emissions (f! = 0.23, note that removal of 

the two > 14 g/ mi PM points increases f! to 0.40). For the 

results of this study only, f! values are about 0.1 for both 

uncorrected and corrected opacity. Examining pre-repair data 

from this study only, smoke opacity is still a poor predictor 

of PM emissions with an f! of 0.21. The fact that several 

vehicles with relatively high PM emissions exhibit low smoke 

opacity indicates that smoke opacity measurements may fail 

to identify all high emitters. One reason for this is probably 

emission of white smoke. which is primarily unburned fuel. 

White smoke is typically caused by fuel pump or fuel injector 

malfunctions but can also be caused by disabled or malfunctioning throttle controls. 

Figure 4 compares CO and PM resu lts from this study 

along with results from the previous studies. For all of the 

data taken together (pre- and post-repair), there is a 

reasonably good correlation (f! = 0.67. pvalue on slope much 

636 • ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 37 , NO.3, 2003 

less than 0.01). However, considering only the pre-repair 

data from this study, f! improves to 0.85 and the coefficient 

for CO is significant at well above the 99% confidence level 

(p value much less than 0.0 1). Hydrocarbon emissions are 

a slightly better predictor of PM emissions than smoke opacity 

(f! = 0.48, p < 0.01); however, correlation of PM emissions 

with both CO and hydrocarbon produces an f! of 0.94 with 

highly significant coefficients for both independent variables. 

Thus, CO and hydrocarbon emissions for driving under load 

can be an excellent predictor of PM emissions. 

For diesel inspection and maintenance purposes, CO and 

hydrocarbon emissions while driving a cycle under load may 

not be obtainable because of high testing equipment capital 

and operating costs. Measurement of CO and hydrocarbon 

during the snap-acceleration (as performed for smoke opacity 

measurement) would be a much easier measurement to make 

in the fie ld. During the phase 2 study, concentrations of 

gaseous pollutants were monitored during snap-accelerations. To accomplish this, the snap-accelerations were 

performed repeatedly while the veh icle exhaust was connected to the dilution tunnel. Results for regression ofUDDS 

cycle PM emissions as a function of snap-acceleration peak 

THC and CO emissions indicate that peak CO is a reasonable 

predictor of PM emissions (f! = 0.74, p value < 0.01) and 

multiple regression of both snap-CO and -THC is even better 

(f! = 0.86, p value < 0.01). Use of either CO or CO and THC 

from a snap-acceleration test has significantly better predictive capability for high PM emitters than does smoke opacity 

for the vehicles examined in this study. 

Acknowledgments 

This work was supported by the United States Environmental 

Protection Agency through a State and Tribal Agency Grant 

to the Colorado Department of Public Health and Environment. This work was also supported directly by the Colorado 

Department of Public Health and Environment. Air Pollution 

Control Division. 

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(I) Sawyer, R. F.; johnson,]. H. Diesel Exhaust: A Critical Analysis 

of Emissions, Exposure. and Health Effects; Health Effects 

Institute: Cambridge, MA, 1995; pp 67-81. 

(2) U.S. EPA. National Air Pollutant Emission Trends Update. 1970-

1997; EPA-454/ E-98-007; U.S. Government Printing Office: 

Washington, DC, 1998. 

(3) Larson, S. M.; Cass, G. R.; Gray, H. A. Aerosol Sci. Techno]. 1989, 

/0, 118-130. 

(4) Larson, S. M.; Cass. G. R. Environ. Sci. Techno]. 1989, 23, 281-

289. 

(5) Nauss, K. M. Diesel Exhaust: A Critical Analysis of Emissions. 

Exposure, and Health Effects; Health Effects Institute: Cambridge. MA. 1995; pp 11-61. 

(6) Miguel, A. H.; Friedlander, S. K.Atmos. Environ. 1978, 12, 2407-

2413. 

(7) Venkataraman, C.; Lyons,]. M.; Friedlander, S. K. Environ. Sci. 

Technol. 1994, 28, 555-562. 

(8) Venkataraman, C.; Friedlander, S. K. Environ. Sci. Technol. 1994, 

28, 563- 572. 

(9) Allen, ]. 0 .; Dookeran, N. M.; Smith, K. A.; Sarofim. A. F. ; 

Taghizadeh, K.; Lafleur, A. L. Environ. Sci. Techno/. 1996, 30, 

1023-1031. 

(10) Atkinson, R. Air Pollution, the Automobile, and Public Health; 

National Academy of Sciences Press: Washington, DC. 1988. 

(II) Seinfeld, ]. H. }. Air Waste Manage. Assoc. 1988, 38, 616- 645. 

(12) Winer. A. M.: Busby. W. F. Diesel Exhaust: A Critical Analysis 

of Emissions, Exposure. and Health Effects; Health Effects 

Institute: Cambridge, MA, 1995; pp 83 - 105. 

(I 3) Williams, G.; Hartman, F. W., Jr.: Mackey. G. R. SA£ Tech. Pap. 

Ser. 1973, No. 730660. 

(14) Ullman, T. L.; Hare, C. T.; Baines, T. M. SA£ Tech. Pap. Ser. 

1984, No. 8404/6. 

(IS) SAE. Surface Vehicle Recommended Practice }1667, SnapAcceleration Smoke Test Procedure for Heavy-Duly Diesel 

Powered Vehicles; 1996. 
(16) Human, D. M.; Ullman, T. L.; Baines, T. M. SA£ Tech. Pap. Ser. 

1990, No. 900883. 

(17) Chaffin, C. A.; Ullman, T. L. SAETech. Pap. Ser.1994, No. 940669. 

(18) Graboski, M.S.; McCormick, R. L. SA£ Tech. Pap. Ser. 1996, No. 

961947. 

(19) Oge, M. T. Guidance to States on Smoke Opacity Cutpoints to 

be used with the SAE} 1667 In- Use Smoke Opacity Test Procedure; 

U.S. Environmental Protection Agency: Washington, DC, 

February 1999. 

(20) California Air Resources Board and Energy and Environmental 

Analysis, Inc. Regulatory Amendments to California's HeavyDuty Vehicle Inspection Program (HDVIP) and Peroidic Smoke 

Inspection Program (PSIP); August 1997. 

(21) jacobs, P. E.; et al. March 2000 Mobile Source Enforcement Branch 

Monthly Status Report, April 10, 2000. 

(22) NESCAUM. Final Report: Northeast Diesel Smoke Testing Pilot 

Program; May 1997. 

(23) Ragazzi, R.; Gallagher, G. L.; Barrett, R. A. SAE Tech. Pap. Ser. 

1986, No. 860297. 

(24} McClintock, P.M. The Denver Remote Sensing Clean Screening 

Pilot, Final Report prepared for Colorado Department of Public 

Health and Environment, December, 1999. 

(25) Wenzel, T. Human Behavior in liM Programs; Presented at the 

15th Annual Mobile Sources/Clean Air Conference, Snowmass, 

CO, September 1999 (as cited In ref 23). 

(26) Weaver, C. S.; Balam, M.A.; Broderick, C. j. Modeling Deterioration in Heavy-Duty Diesel Particulate Emissions; Final Report 

submitted to U.S. Environmental Protection Agency, Contract 

8C-SI12-NTSX; September 30, 1998. 

(27) Ullman, T. L.; Human, D. M. SA£ Tech. Pap. Ser. 1991, No. 

910735. 

(28) Yanowitz, j.; Graboski, M. S.; Ryan, L. B. A.; Daniels, T. L.; 

McCormick, R. L. Environ. Sci. Techno/. 1999, 33, 209-216. 

(29) Clark, N. N. M.; Gautam, B. L.; Rapp, D. W.; Lyons, M.S.; Graboski, 

R. L.; McCormick, T. L.; Alleman, P. Norton SAE Tech. Pap. Ser. 

1999, No. 1999-01 -1469. 

(30) Code of Federal Regulations, Vol. 40, Subpart M. 

(31) SAE Recommended Practice}1376, Fuel Economy Measurement 

Test for Trucks and Buses; 1982. 

(32) McCormick, R. L.; Graboski, M.S.; Alleman, T. L.; Alvarez,]. R.; 

Kado, N. Y.; Kuzmicky, P. A.; Kobayashi, R.; Duleep, K. G. 

Quantifying the Emission Benefit of Opacity Testing and Repair 

of Heavy-Duty Diesel Vehicles; Final Report to Colorado Department of Public Health and Environment, july 200 I (this report 

can be obtained from the author). 

(33) Yanowitz, ].; McCormick, R. L.; Graboski, M. S. Environ. Sci. 

Techno!. 2000, 34, 729- 740. 

(34) McGuckin, j.; Rykowski, R. Correlation between Smoke and 

Particulate Emissions from Heavy-Duty Diesel Engines; U.S. EPA, 

Office of Mobile Sources Technical Report: Washington, DC, 

1981. 

(3Sj Alkidas, A. C. SAE Tech. Pap. Ser. 1984. No. 840412. 

(36) McCormick, R. L.; Alleman, T. L.; Graboski, M. S. Emissions 

TesUngofHeavy-Duty Vehicles: Phase I; Final Report to Colorado 

Department of Public Health and Environment and Regional 

Air Quality Council; Colorado Institute for Fuels and High 

Altitude Engine Research: june 1999. 

Received for review April 1, 2002. Revised manuscript received October 23, 2002. Accepted December 2, 2002. 

ES0256919 

VOL. 37 . NO. 3, 2003 I ENVIRONMENTAL SCIENCE & TECHNOLOGY • 637 
QUANTIFYING THE EMISSIONS BENEFIT OF OP

TESTING AND REPAIR OF HEAVY -DUTY DIESEL V

Robert L. McCormick, MichaelS. Graboski, Teresa L. Alleman, Jav

Colorado Institute for Fuels and Engine Research 

Colorado School of Mines 

Golden, CO 80401-1887 

K.G. Duleep 

Energy and Environmental Analysis, Inc. 

Arlington, VA 22209 
Acknowledgetnents: 

This study was sponsored by the Air Pollution Control D

the Colorado Department of Public Health and Environ

This study was also sponsored by the Office of Transport

Air Quality of the U.S. Environmental Protection Agenc

authors wish to thank these entities for their support. 

The authors also wish to thank the many truck owners, larg

small, that allowed their vehicles to be used in this study. 
Study Objectives: 

•Begin to quantify the benefits of a smoke opacity (SAE J1

based inspection and maintenance program. 

• Provide data useful for selection of opacity failure points. 

Approach: 

• Identify smoking trucks by various means (calling fleets, s

vehicle hot line, Colorado snap test, ...... Obtain permissio

•Test these vehicles on HD chassis dynamometer via UDD

cycle with measurement of THC, CO, NOx, PM. 

• Repair vehicles to have reduced opacity. Repairs perform

OEM local reps or other authorized facilities. 

• Repeat chassis dynamometer tests. • Best possible I/M scenario-close monitoring of technician

that real repairs were performed. 
Scope and Lintitations: 

•Twenty six vehicles tested before repair 

• Seventeen pre-1991, four exhibited low opacity and w

repaired, two were repaired twice. 

• Nine 1991 and later, one exhibited low opacity and wa

repaired, a second was not made available by the ownetesting. 

•Twenty vehicles in total repaired and tested a second time. 

• Pre-repair opacity ranged from 23 to 99%. 

• Engine model years ranged from 1986-1999. 

•GVW from 11,000 to 80,000 lb. 

• Engines manufactured by DDC, Caterpillar, Cummins, Isu

International, GMC. 
Summary Results: 

Smoke THC, NOX, co, 

Opacity, 0/o g/mi g/mi g/mi PM, g/mi 

Pre-1991: 

Repaired vehicles (pre) 54 7.0 22.1 36.8 5.6 

Repaired vehicles (post)* 39 2.1 30.9 29.9 3.3 

1991 and Later: 

Repaired vehicles (pre) 66 5.5 12.1 17.6 2.2 

Repaired vehicles (post) 39 0.74 14.4 14.8 1.3 

NFRAQS Studyi 24 1.3 21.0 16.8 1.7 

Review of Diesel -- 2.2 24.4 11.0 1.6 

Emissions 11 

After all repairs for vehicles undergoing two rounds of repair. 

i Yanowitz, J., Graboski, M.S., Ryan, L.B.A., Daniels, T.L., McCormick, R.L. Envir

Techno/. 1999, 33, 209-216. 

ii Yanowitz, J., McCormick, R.L., Graboski, M.S. Environ. Sci. Techno/. 2000, 34, 7
Brake specific NOx emissions before and after re

compared to the NOx emission standards: 

.c 16 

I 

c. 

.c 

.c 

........ Cl 12 

"" s:::: 

0 

-

8 

·-

E 

w 

>< 4 0 

z 

-Except for the newest engines, NOx goes up after re

-but to levels that are close to the emission standards 

0 

• 

0 

0 

0 Q 

• • • I 

i 0 • • 0 

0 

• 

• Pre-Repair 

o Post-Repair 

- NOx Standard 

.a i . 

0 

£.._ 0 

l 

Mechan

govern

deterior

higher 

lower N

1984 1986 1988 1990 1992 1994 1996 1998 2000 

Model Year 
(The well-known) NOx/PM Trade-Off: 

Engine operating strategies that lower NOx cause a loweri

combustion temperature and increase in PM 

-deterioration of injectors, pumps, etc lowers combustion 

temperature, reducing engine efficiency and lowering NO

130 120 110 Naturally Aspirated 

0 

100 

0... 

90 

I 

80 I Turbocharged lntercooled 

60 65 70 75 80 85 90 95 100 105 110 115 120 

NOx 0/o 
Opacity Testing/Repair Observations : 

• Most repairs involved fuel injectors, fuel pumps, fuel pump 

calibration, throttle controls and injection timing. 

•Average repair cost was $1,088 (range $699-$2053) and was 

for both pre-1991 and 1991 + engines. One outlier cost $85 t

opacity from 50 to 17% and reduce PM by 35%. 

• Most vehicles were out of service for 2 days due to repair, b

required as long as two weeks-usually for multiple repairs. 

•Repair ofpre-1991 vehicles exceeding 45% opacity reduced 

emissions by 45-50%. A PM benefit was observed for every 

repaired. NOx increased by about 35% on average. 

• of 1991 + vehicles exceeding 40% opacity reduced PM 

emissions by 25-30%. For one vehicle there was no PM ben

NOx increased by 7% on average. 
Relationship Between PM and Opacity o

18 18 

• 1999 1/M Study, Pre-repair • 1999 1/M Study, Pre-repair 

16 -l 0 1999 1/M Study, Post-repair • 0 1999 1/M Study, Post-repair 

2000 1/M Study, Pre-repair 0 16 .. 2000 1/M Study, Pre-repair 

'V 2000 1/M Study, Post-repair 'V 2000 1/M Study, Post-repair 

14 -1 • McCormick, et al., 1999 • McCormick, et al., 1999 

0 NFRAQS 14 0 NFRAQS 

-- Linear Regression -- Linear Regression 

12 lnt=0.79 I nt=-6. Oe-3 

E Slope=0.052 12 Slope=0.115 

- r 2=0.23 r 2=0.67 C) 

Ill 10 • 

s:::: E 10 -1 

0 

Ill 

-

C) 

8 

E :!! 8 

w • • a. 

:2: 6 0 _____;-n 6 • a. .. 0 

0 

4 -l 

------

0 • • 0 0 n I 

: j •n•/ 6 

2 u I 0 n 0 oO 0 • 

0 yv' I 

0 

0 20 40 60 80 100 0 20 40 60 

J1776 Smoke Opacity (uncorrected) CO, g/mi (various c
Predicting PM with Snap Acceleration Pea

-CO is well correlated with PM. 

-Opacity and idle emissions of CO are poorly correlated w

-This suggests a simple tail-pipe CO measurement to iden

PM emitters. 

-gaseous emissions measured during snap-test identical to 

-1991 + vehicles only (7 vehicles) 

-peak CO during snap predicts PM with r2==0.74 

-peak CO is a much better predictor of PM than opacity 
Conclusions: 

• Repair of high opacity vehicles significantly reduced P

emissions in essentially every instance. 

•Opacity is a poor predictor of PM. While considerabl

would be required, a simple tail-pipe measurement of 

during a snap acceleration might be a more accurate p

• In general, repair of high opacity vehicles also causes 

emissions to increase. 
Driving Cycle and Chassis DynaDloDleter: 

70 

60 

50 

"E. 40 

E 

"0 30 

Q) Q) 

c.. 

en 20 

10 

0 

Heavy-Duty Transient Truck Cycle 

0 200 400 600 800 1000 
SAE J1667 Opacity Test: 

• Vehicle warmed up and in neutral. 

• Accelerator is rapidly pushed to the floor held for 5 secon

the engine reaches maximum (governed) speed, while sm

opacity is measured. 

•The maximum opacity observed is reported and the avera

tests is reported. 

•Opacity is corrected for stack diameter. Altitude correctio

of the procedure but was not used in this study as it is not 

accurate or meaningful. 

• EPA recommends that states use opacity failure points de

the State of California (55% for pre-1991 and 40% or 199

newer engines) to insure uniformity across state lines. 1 

10ge, M.T., Guidance to States on Smoke Opacity Cutpoints to be used with the SAE 

Smoke Opacity Test Procedure U.S. Environmental Protection Agency, February, 199
Average percent change in g/mi emissions 

with repair, pre-1991: 

.-

c '(5 <45% 

a. 

::s 

nJ 

LL 

45% 

0 

nJ a. 

0 

C1) 

55% 

(J) 

!:SS.:S1 co 

rzzzl NOX 

-THC 

c::=J Smoke Opacity V//////////////////////// 

...... 1///////////////A 

"'' '''"' ''-''' '' '' ////////.11 

-60 -40 -20 0 20 40 60 

Percent Change After Repair (Average) 

55°/o as recommended by EPA and 45°/o as recommended in the st

Inc. performed for State of Colorado. 
Average percent change in g/rni emissions 

with repair, 1991 +: 

rs;s:ssJ co 

.... 

c: 

NOX 

-THC 

0 

a. c:::=:::J Smoke Opacity 

Q) 

... 

::::s LX 

n:s 

L1. ""'""'" """ '"" ""' "'-.'\.. ........ 40% tzzJ 

C) 

n:s 

a. 

0 I 

Q) 

0 

E 

en 

..so -40 ..JO -20 -10 0 10 20 

Percent Change After Repair (Average) 

40°/o as recommended by EPA. All vehicles also failed 33°/o as recom

the study by EEA, Inc. performed for State of Colorado.