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U n i o n o f C o n c e r n e d S c i e n t i s t s Diesel Passenger Vehicles and the Environment 1

Diesel Passenger Vehicles and the Environment

Diesel cars and light trucks are receiving heightened

attention as a near-term strategy to meet fuel economy

and climate change goals. But amidst growing interest in

diesels for passenger vehicles, there is also concern over

the public health implications of expanding diesel’s mar-

ket share.

While pollution from diesel passenger vehicles has been

cut by a factor of 5–10 over the past two decades, it must

be reduced another factor of 10 within the next few

years in order to meet near-term pollution standards.

Significant emission reductions appear technologically

possible, but the engineering, economic, and infrastruc-

ture challenges are large. The ten-fold reductions

needed to meet next-generation standards will require a

very high degree of technical success.

In addition to conventional pollution reductions, diesel

engine research must also address evolving public health

concerns, including emissions of ultrafine particles and

toxics. These emerging issues must be dealt with if diesel

is to meet increasingly stringent environmental needs.

An extremely rapid introduction of diesel passenger ve-

hicles could yield carbon emission savings within the

decade of up to 4 percent versus the base case. Such re-

ductions could provide headway in the struggle against

global warming, but are far from enough. Furthermore,

these potential benefits must be appropriately balanced

against the air quality risks posed by diesel.

iesel engines power most of the nation’strucks, buses, trains, ships, and offroad ma-chinery. Roughly one-fifth of the US trans-

portation sector’s energy is consumed in these en-gines (Davis 1998), and diesel fuel use is currentlygrowing faster than gasoline consumption (DeCiccoand Mark 1998).

Diesel Passenger Vehicle MarketsDiesel passenger vehicles comprise a small share ofthe US fleet, accounting for approximately two per-cent of the fuel consumed by cars and light trucks to-day (Davis 1998). Currently, diesel cars make up amodest 0.1 percent of automobile sales, while dieselhas now captured roughly 4 percent of light trucksales1 (Davis 1998; AAMA 1996).

US Trends. US diesel automobile sales peaked dur-ing the early 1980s in the wake of two major oilshocks (Figure 1). Forecasts at that time projectedsales would reach 20 percent by 1990 (DOT 1991).But fluctuations in the price of diesel fuel, declininggasoline prices, and vehicle performance problems allled to the diesel auto’s decline in the United States(Sperling 1988; Cronk 1995).

Diesel light truck sales also experienced a boomin the early 1980s, although recent increases have farsurpassed previous sales as the entire light truck mar-ket has grown quickly. US factory sales2 of diesellight trucks have more than doubled in the past five 1 Vehicles up to 10,000 pounds gross vehicle weight(GVW). Traditionally, light trucks have been catego-rized as vehicles up to 10,000 pounds GVW; however,air quality regulations define light trucks as vehicles upto 8,500 pounds GVW.2 Actual sales numbers were not available. Factorysales constitute the majority of, but not all, light truckssold in the United States.

D

2 Diesel Passenger Vehicles and the Environment U n i o n o f C o n c e r n e d S c i e n t i s t s

years, especially in the heavier light truck segment(6,000–10,000 pounds gross vehicle weight) (AAMA1996). Ford currently makes about 200,000 lighttrucks per year powered by Navistar diesel engines.Chrysler sells 60,000 Dodge Ram pickup trucks an-nually with Cummins diesel engines and may soonbuild sport-utility vehicles with Detroit Diesel en-gines. GM has recently announced plans to ramp upproduction of diesel engines for large light truckswith Isuzu (Nauss 1998; Konrad 1998).

For diesel engine manufacturers, the light truckmarket is an opportunity for companies that currentlysell hundreds of thousands of engines to begin pro-ducing millions of engines. At present, most of theseengines are scaled-down versions of larger units builtfor heavy trucks. But most engine makers are pursu-ing even smaller engines that could be introducedmore broadly throughout the market (Cummins1998).

%#(' 5JQTVHCNNU� Automakers’ renewed interestin diesel engines for the US market has been spurredin large part by fuel economy regulations—the Cor-porate Average Fuel Economy (CAFE) standards.Despite the fact that light trucks are held to a lowerstandard than automobiles,3 domestic automakers arefalling short of their truck CAFE requirements

3 The automobile CAFE standard is 27.5 miles per gal-lon (mpg), while that for light trucks up to 8,500pounds gross vehicle weight is 20.7 mpg (Davis 1998).

(Eisenstein 1997). These shortfalls are largely due torecord sales of the most profitable but least efficientvehicles, particularly sport-utility vehicles and heavypickups.4 If light truck fuel economy does not im-prove in the coming years, domestic auto makers willbe unable to accrue credits to offset the substantialdebits that have accumulated during the past 2–5years (Figure 2). By delivering a fuel economy im-provement of up to 30–50 percent, diesel engines arean attractive strategy for complying with the stan-dards.

+PVGTPCVKQPCN )TQYVJ� Overseas markets fordiesel passenger vehicles have historically been muchlarger than in the United States. Many countries inEurope and parts of Asia levy significantly highertaxes on motor fuels, particularly gasoline, that havespawned consumer interest in higher-efficiency dieselcars. Recent trends in Europe have been towards in-creasing diesel sales, where roughly one-quarter of allnew automobiles are currently diesel powered (Walsh

4 For example, the New York Times reports profits forFord’s Expedition and Lincoln Navigator at $12,000–15,000 per unit (Bradsher 1999) and the 1998 DodgeDurango was reported to provide $8,000 in profit perunit (Bradsher 1997).

FIGURE 2US Manufacturer Light Truck CAFECredits and Debitsa

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a. A calculated fine in a given year does not necessarily meana manufacturer owes that amount in a given year, since creditsfrom up to three years before of after the year may be used tooffset the penalty.Source: National Highway Traffic Safety Administration

FIGURE 1US Sales of Diesel Vehicles

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Light Trucks = 10,000 lbs. Gross Vehicle Weight and lessSource: Davis 1998


1997, Krieger et al. 1997). France, where roughlyhalf of all new sales are diesel, has a particularly highproportion of diesel cars in its fleet (Wang et al.1997). The trend does not appear to be restricted toEurope, however. In Japan, the number of diesel pas-senger vehicles appears to have tripled in the pastdecade (based on Walsh 1997).

As a result of increasing globalization, mostautomakers selling in the US market now also pro-duce diesels for international markets, some on theirown and some with foreign partners. This permitsmany companies to transfer research, development,and even production experience among global divi-sions. Traditionally, the tighter US air quality stan-dards have made the transfer of diesel technology toNorth America difficult without additional develop-ment costs. But the worldwide trend towards tighteremission standards is forcing technology improve-ments that may put more diesels in reach of the USstandards. Conversely, research aimed at meeting theUS targets will eventually pay off in foreign markets.

%NKOCVG %JCPIG� The renewed interest in die-sels is also driven by growing attention to climatechange as a major environmental concern. Thehigher efficiency of diesel vehicles results in loweremissions of carbon dioxide, the chief gas responsiblefor global warming. While automakers officially op-posed the recent Kyoto Protocol that established le-gally-binding cuts in greenhouse gas emissions, manyhave demonstrated high-efficiency prototypes pow-ered by diesel as climate change mitigation technolo-gies.5

Federal Support. Over the past five years, theUS federal government’s interest in diesel vehicleshas increased considerably, with consequent in-creases in research funding. Many federal agenciesare sponsoring research on diesel vehicles, but themajority of research for passenger vehicles is fundedby the Department of Energy, whose diesel budgetshave been on the rise in recent years (Figure 3).

Federal interest in diesels stems largely from theAdministration’s focus on global warming and energy

5 For example, a recent Toyota advertisement in Timemagazine reads: “If You’re Concerned About GlobalWarming You Should Be Interested in Diesel Engines.”

security as environmental policy priorities. Diesel isa priority component of the joint government-industryresearch project, the Partnership for a New Genera-tion of Vehicles (PNGV). Launched in 1993,PNGV’s primary goal is to develop a production-ready prototype of an 80 mile-per-gallon automobileby 2004. While several promising technologies areunder consideration by PNGV, the lead option is ahybrid vehicle powered by a diesel engine.

In addition to funding research on diesel passen-ger cars under PNGV, the federal government is alsoengaged in research to assist diesel engines in cap-turing a larger share of the light truck market. Under

FIGURE 3US Department of Energy Budget forDiesel Passenger Vehicle Research

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Source: FY2000 DOE Budget request (February 1, 1999)

TABLE 1Gasoline vs. Diesel Fuel Economy,Efficiency, and Carbon Comparisons

diesela gasolineb diesel vs.gasoline

Fuel ParametersHeating Value (Btu/gal)c 128,500 115,500 +11%Carbon Content (gCO2/Btu) 0.095 0.094 0%

Example: 1998 VW Jettae

Fuel Economy (mpg)f 43.6 26.7 +63%Efficiency (mi/mmBtu) 339 231 +47%Carbon Emissions (g/mi) 280 408 -32%

a. Conventional diesel fuel (Wang 1998).b. Conventional gasoline fuel (Wang 1998).c. Lower heating value.d. Full fuel cycle carbon emissions (Wang 1998).e. 1.9-liter manual transmission diesel Jetta vs 2.0-liter gasoline versiof. Combined city/highway fuel economy (adjusted).


a Light Truck Clean Diesel (LTCD) program, the De-partment of Energy is working with industry to de-velop cleaner light-duty diesel engines.

&KGUGN�'PIKPGU�CPF�)NQDCN�9CTOKPIA vehicle using a state-of-the-art diesel engine, theso-called compression ignition direction injection(CIDI) engine (see Box 1), offers a substantial fueleconomy gain over today’s gasoline engines. How-ever, a portion of the higher fuel economy is a resultof diesel fuel’s higher density: each gallon of dieselcontains roughly 11 percent more energy. Correctingfor fuel density, today’s best diesel cars are over45 percent more efficient than their counterparts.This higher efficiency translates directly into30–35 percent savings of carbon emissions, the chiefgas responsible for global warming (Table 1).

Diesel Fuels. Future diesel fuel may need to bemodified or replaced with alternatives to assist dieselengines in meeting air quality goals. Compared toconventional diesel, some of these fuel changes willyield higher greenhouse gas emissions. For example,Fischer-Tropsch diesel, a fuel manufactured fromnatural gas feedstocks, has the potential to loweremissions of key air pollutants but would increasecarbon emissions by over 20 percent comparedto conventional diesel (Figure 4).6,7 In contrast,

6 This estimate is preliminary, since commercial facili-ties are not yet in production. The efficiency ofFischer-Tropsch production may increase in the futurecompared to the 68% conversion efficiency assumedhere. Furthermore, the sale of co-product steam or theuse of otherwise flared natural gas could change theresults.7 Compared to gasoline vehicles, this would reducediesel’s carbon benefits to 15–20 percent, rather thanthe 30–35 percent achievable with the best diesel en-gines available today.

BOX 1

Diesel Technology Basics

State-of-the-art diesel passenger vehicles are powered by compression ignition direct injection (CIDI) engines.

Compression Ignition (CI) vs. Spark Ignition (SI). Traditional gasoline vehicles are powered by spark ignition (SI)engines, which use a spark to initiate combustion. In the compression ignition (CI) engines used in diesel vehicles,the fuel and air mixture spontaneously ignites as it is compressed in the engine’s cylinders. CI engines are moreefficient during ideal conditions because they operate at higher compression ratios, permitting them to get moreuseful work out of each cycle. Diesel combustion can also be designed to occur away from the cylinder walls, re-ducing heat loss. As a result, the peak thermal efficiency of CI engines can be typically 10–30 percent higher thanfor SI engines. Diesel engines are also more efficient under real-world driving conditions, when a vehicle’s powerdemand fluctuates extensively. SI engines restrict the flow of the fuel and air mixture entering the cylinders whenfull power is not required, creating additional “throttling” losses and pumping losses. In contrast, CI engines re-duce only the fuel supplied in order to lower power output. Accounting for these additional driving cycle benefits,diesel engines can be a total of 25–50 percent more efficient than gasoline engines (Arcoumanis and Schindler1997).

Direct Injection (DI) vs. Indirect Injection (IDI). Most heavy vehicles today use direct injection (DI) diesel en-gines, while until recently most automobile-size diesel engines have used indirect injection (IDI) technology. DIengines inject fuel and air directly into the cylinder, while an IDI engine uses a prechamber to help mix the fueland air before entering the main cylinder. The IDI system comes with a 15 percent efficiency penalty compared tothe DI because the prechamber permits additional energy losses (Ashley 1997; Arcoumainis and Schindler 1997),but its superior fuel and air mixing has been essential for diesel passenger vehicles. The small, high-speed enginesused in automobiles require fuel and air to mix 10 times faster than in larger engines (Heywood 1988), somethingthat has been difficult to achieve without a prechamber. Only recently have diesel engine developers overcomethis mixing limitation with DI engines.


biodiesel fuels manufactured from agricultural feed-stocks would reduce carbon emissions by three-fourths.

Potential Carbon Savings. The widespread intro-duction of higher-efficiency diesel engines into theUS passenger vehicle market has the potential to de-liver important carbon savings. In the absence ofhigher fuel economy standards or a major fuel priceincrease, however, per-vehicle efficiency gains do notyield fleetwide fuel economy benefits per se. Overthe past decade, engine and drivetrain efficiency im-provements have translated into performance andweight gains rather than actual on-road fuel effi-ciency improvements.8

8 For example, passenger car weight has increased 8percent on average over the past decade while 0–60acceleration time has improved 19 percent and fueleconomy has remained stagnant. Light truck weighthas increased 15 percent on average while accelerationhas improved 17 percent. (Heavenrich and Hellman1996).

Assuming that increasing diesel sales yield actualon-road fuel efficiency gains, the total fleetwide car-bon savings are limited by several additional factors:(a) the speed of market entry, (b) the efficiency pen-alty associated with engine modifications or after-treatment devices needed to meet air quality targets,and (c) the carbon content of the diesel fuel used.Figure 5 illustrates one potential pathway for dieseli-zation, in which diesel passenger vehicle sales in-crease from 3 percent in 2001 to 50 percent by 2010(and constant thereafter). This represents an ex-tremely aggressive scenario, in which diesel wouldovertake the passenger vehicle market more than twotimes faster than occurred in France, which has one ofthe most heavily dieselized car markets.9

If conventional diesel is used in this new dieselfleet, passenger vehicle carbon emissions would be4 percent lower than in the base case by 2010 and

9 France’s fuel prices are also 2.4 to 3 times higher thanthose in the United States (Davis 1998).

FIGURE 4Carbon Dioxide Emissions from the Production, Distribution, and End Use of Fuels(grams of CO2 per Btu consumed)

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Notes:1. Source: based on modeling runs of GREET1.4 (Wang 1998) for CO2 only2. Tailpipe emissions are from the combustion of fuels in the end-use vehicle’s engine3. Upstream emissions include the production and distribution of fuels4. Reformulated gasoline refers to federal RFGII5. Net tailpipe emissions for biodiesel are near zero to take credit for carbon taken from the atmosphere by the fuel’s feedstock.


12 percent lower by 2020.10 If a cleaner diesel isrequired, such as Fischer-Tropsch diesel, carbonbenefits may be substantially attenuated (Figure 6).Passenger vehicle carbon emissions must be reducedby over 30 percent in 2010 to return to pre-1990 lev-els and meet the targets established in the Kyoto

10 For comparison, equivalent carbon emission savingscould be achieved in 2010 by improving new gasolinepassenger vehicle fuel economy less than 2 percent an-nually beginning in 2001.

Protocol. Thus, additional measures and alternativepathways will be required for the transportation sec-tor (UCS 1998).

&KGUGN�'PIKPGU�CPF�#KT�2QNNWVKQPAs with fuel use, heavy vehicles and nonroad equip-ment account for the majority of existing diesel pol-lution, rather than the relatively few diesel-poweredpassenger vehicles on the road today. If diesel en-gines were to capture a large share of the car and lighttruck market, however, the impact of diesel enginescould increase. Critically important is how the envi-ronmental performance of diesel passenger vehicleswould compare with that of the displaced gasolinevehicles.

Health Effects. Collectively, diesel-powered vehi-cles account for nearly three-quarters of all directparticulates (PM) from US transportation and overhalf of all nitrogen oxides (NOx)—a precursor to bothsmog and fine particles (EPA 1998a).11 In urbancenters, where exposure to diesel exhaust may be es-pecially high, diesel engines can be a dominantsource of particulates (Walsh 1997). Furthermore,diesel exhaust is increasingly being scrutinized as apotential human carcinogen.

Diesel vehicles offer clear benefits for one keypollutant, carbon monoxide (CO). Although prob-lems persist in some urban regions, roughly threetimes more US citizens live in areas not currentlymeeting the federal ozone standards than in areas notattaining CO standards (EPA 1999). Thus, while COcontinues to be an important motor vehicle pollutant,this analysis focuses on the more significant hazardsof urban ozone, particulates, and toxics.

Urban Ozone. NOx and hydrocarbon emissionsfrom motor vehicles contribute to ozone (smog), themajor ingredient in the smog engulfing major cities.High up in the stratosphere, ozone shields us fromharmful ultraviolet (UV) rays. Yet at ground level,ozone irritates the respiratory system, causing

11 The NOx estimates from diesel highway vehicles islikely to increase as new information regarding highemissions during highway operation (Walsh 1998) isincorporated into official inventories. In addition, newinformation about high PM emissions from malfunc-tioning gasoline vehicles (Cadle et al. 1998) may affectfuture estimates of PM inventories.

FIGURE 5Diesel Passenger Vehicle MarketIntroduction Rate

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FIGURE 6US Passenger Vehicle Carbon EmissionsRelative to 1990 Baselinea,b,c

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a. Full fuel cycle carbon emissions calculated using UCS in-housenational vehicle stock model calibrated to the Energy InformationAdministration’s Annual Energy Outlook 1999 (EIA 1998). Carboncoefficients from Wang (1998) for gasoline (base case), 100%Fischer-Tropsch diesel, and conventional diesel (base diesel).b. Diesel vehicles (cars + light trucks) are assumed to be 35% moreefficient than base gasoline passenger vehicles, per DOE’s lighttruck efficiency goal (OHVT 1997).c. Higher diesel vehicle efficiency is assumed to directly translateinto real-world fuel economy gains, rather than increased weight orpower.


coughing, choking, and reduced lung capacity. Urbanozone pollution has been linked to increased hospitaladmissions for respiratory problems such as asthmaand daily mortality, even at levels below the currentstandard (ATS 1996). Some early studies suggestthat long-term exposure to ozone may yield chronic,irreversible impacts on lung function (Tashkin et al.1994). More recent work has tended to support thisconclusion in preliminary studies (Kunzli et al. 1997).

Particulates. Motor vehicles create particulatematter pollution by emitting combustion particles di-rectly to the air and by releasing pollutants, notablyNOx and hydrocarbons, that form secondary particlesin the atmosphere.12 Particulates irritate the eyes andnose and aggravate respiratory problems. Children,the elderly, people with pre-existing heart or lungdisease, and asthmatics are particularly at risk fromexposure to particulates (EPA 1997a).

Fine particulates, those less than 2.5 microns indiameter (PM2.5),

13 have also been directly associatedwith an increased risk of premature death (EPA1996a; ATS 1996). In one recent study, researchersfollowed more than 8,000 people in six different lo-cations for 17 years. They found that the risk of pre-mature death in areas with high levels of fine parti-cles was 26 percent greater than in areas with lowerlevels (Dockery et al. 1993). Based on extrapolationsfrom a larger study of premature mortality and par-ticulates, EPA estimates that its new health standardsfor PM2.5 will save 15,000 lives each year (EPA1997a).

At present, the specific mechanism by which fineparticulates increases mortality risk is unknown (EPA1996a). As a result, while regulations are based onthe total mass of particulates less than 10 or 2.5 mi-crons (see Table 2), other characteristics such as par-ticle size, surface area, number, chemical composi-tion, or physical shape might also be important(Sawyer and Costantini 1997). As a result, controlstrategies focused solely on reducing the mass of par-

12 Motor vehicles are also responsible for particulatepollution from road dust that they kick up and from tireand brake wear. While an important part of the totalparticulate inventory, these sources are generally inde-pendent of engine type and are much larger particlesthan combustion byproducts.13 A micron, or micrometer, is one-millionth of a meter.The average human hair is about 100 microns thick.

ticulates may not proportionally reduce public healthrisks. In fact, emerging evidence indicates that his-toric efforts to reduce PM10 mass from diesel engineshave significantly increased the number of ultrafineparticles (those less than 0.1 microns) (Bagley et al.1996; Walsh 1998; HEI 1997). Smaller particlesmore readily evade the body’s physical defenses,penetrating further into the lungs, and are theorized tocause more health damage (EPA 1996b; ATS 1996).

Carcinogenesis. In addition to its contribution tomainstream air pollution problems, major publichealth agencies also consider diesel exhaust a poten-tial human carcinogen (Table 3). While diesel ex-haust contains over 40 compounds thought to causecancer (CalEPA 1998a), most public health studies ofdiesel exhaust have focused on the aggregate emis-sions rather than on specific compounds. In its recentruling, however, the California Air Resources Boardvoted to list only diesel exhaust particulates as atoxic, rather than whole diesel exhaust, which con-tains both particulates and vapor-phase emissions(CARB 1998b).

Studies of humans routinely exposed to dieselexhaust indicate a greater risk of lung cancer. For ex-ample, occupational health studies of railroad, dock,trucking, and bus garage workers exposed to highlevels of diesel exhaust over many years consistentlydemonstrate a 20–50 percent increase in the risk oflung cancer or mortality (HEI 1995; Bhatia et al.1998).

Even at the average rates of exposure experiencedby most people, diesel exhaust poses a potential can-cer risk. Extrapolating from epidemiological studies,

TABLE 2Diesel Particulates: Size

SizeDescription

SizeRangea Also known as:

Nanoparticles 0–0.05 µmb Nuclei ModeUltrafine 0.05–0.1 µm Nuclei ModeFine 0.1–2.5 µm Accumulation ModePM-10 2.5–10 µm Coarse Mode

a. Size range based on particulate areodynamic diameter.b. A micron or micrometer (µm), is one-millionth of a meter

(1/106 meters). The average human hair is about 100 mi-crons thick.

Source: Kittleson 1998


at current exposure levels it is estimated that up to450 of every million Californians are at risk of con-tracting lung cancer as a result of lifetime exposure todiesel exhaust, or over 14,000 residents.14

Diesel Exhaust vs. Gasoline Exhaust. Becauseemissions from motor vehicles continue to improve inresponse to new standards, it is difficult to develop anabsolute picture of diesel vs. gasoline vehicle emis-sions. In addition to technology improvements, emis-sion measurement techniques continue to evolve, andemerging public health information is pointing re-searchers to evaluate new exhaust constituents. Us-ing a combination of measured in-use, certificationtesting, and modeled emissions, however, a snapshotof today’s best gasoline and diesel technology can beconstructed.

Urban Ozone PrecursorsNitrogen Oxides. Diesel passenger vehicles re-

cently certified in the United States have achievedNOx emissions of 0.6–0.9 g/mi (EPA 1997b, CARB1997). Many gasoline cars being sold today haveNOx certification levels less than half the dieselvalues, and some are certifying at less than one tenthdiesel levels (CARB 1997).

14 The independent Scientific Review Panel of the Cali-fornia EPA has proposed a reasonable estimate of can-cer risk from diesel exhaust to be 0.0003 for every mi-crogram of diesel exhaust per cubic meter of air (3 x 10-

4 (µg/m3)-1) (CalEPA 1998b). The current average expo-sure rate is 1.5 µg/m3, resulting in an average lifetimerisk of 4.5 x 10-4.

While gasoline vehicles may certify to relativelylow NOx standards, they will typically emit severaltimes those values in the real world as they age. Die-sel passenger vehicles have not been studied as exten-sively, but their emissions are thought to degrade lessthan gasoline cars because they do not currently usecatalytic converters to control emissions (Fairbanks1997). This conclusion may change if exhaust con-trol technologies are installed on diesel vehicles inthe future.

Emissions modeling suggests that lifetime aver-age NOx emissions from diesel vehicles are roughlytwice that of gasoline cars being sold in some statesby 1999 and the rest of the country by 2001 (Table 4).These results must be viewed with caution, however,because the EPA model used to construct these esti-mates is based on relatively little diesel vehicle emis-sions data.

Hydrocarbons. Hydrocarbons are the other keyprecursor to urban ozone. During the early 1990s,vehicle regulations focused aggressively on loweringhydrocarbon emissions as the chief ozone-reductionstrategy. In recent years, the focus of new vehicleregulations aimed at ozone reduction has shifted moreto NOx reductions.

Nonetheless, diesel cars typically yield loweremissions of hydrocarbons than gasoline vehicles.This is due in large part to the fact that diesel fueldoes not evaporate as readily as gasoline. However,hydrocarbon emissions from gasoline vehicles areexpected to be cut to near-diesel levels as NLEV ve-hicles are introduced (Table 4).

TABLE 3Cancer Risk Assessments of Diesel Exhaust

Organization Year Conclusion

National Institute for Occupational Safety and Health 1988 potential occupational carcinogenInternational Agency for Research on Cancer (World Health Org.) 1989 probable human carcinogenState of California 1990 known to cause cancerUS Environmental Protection Agency (Draft) 1998 “highly likely” human carcinogenCalifornia EPA (Staff Recommendation) 1998 “may cause an increase in the likelihood of cancer”California Air Resources Board 1998 diesel particulate emissions are toxic air contaminant

SourcesNIOSH and IARC: HEI 1995, p.19State of California: listing under the Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65).US EPA: EPA 1998b, p.12–29Cal EPA: CalEPA 1998a, p. ES-27CARB: CARB 1998b


ParticulatesDirect Particulates. Recent data indicates that

diesels emit at least 10 and perhaps as much as 300times more PM mass than properly operating moderngasoline vehicles (Table 5). Many factors—such asvehicle age, condition, temperature, and driving cy-cle—can impact these vehicle-to-vehicle compari-sons. For example, gasoline cars that are malfunc-tioning can increase PM emissions by a factor of 100.But even emissions from malfunctioning gasolinecars appear to be several times lower than from dieselvehicles.

There may also be key differences in the sizedistribution of particulates emitted by diesel versusgasoline vehicles, although research has only recentlyturned to such comparisons. These studies indicatethat diesel engines emit larger numbers of smallerparticles.15 For example, Walsh (1998) presents re-sults from a UK study indicating that current indirectinjection diesel technology emits nanoparticles and

15 Additional study of this issue is needed, as some con-cerns about techniques for sampling such small parti-cles persist.

ultrafines at perhaps 100 times the rate of moderngasoline cars over European driving cycles.

More recently, the British oil research consortiumCONCAWE tested modern direct injection diesel andconventional gasoline-powered passenger vehicles(Hall et al. 1998). At high speeds, the researchersdemonstrated that gasoline vehicles emit roughly thesame number of ultrafine particles as the dieselstested and that a higher percentage of gasoline par-ticulates fall into the smaller size ranges. However,their most significant result is that—over the Euro-pean driving cycles—diesel cars emitted 10–100times more nanoparticles and ultrafines than moderngasoline vehicles equipped with catalytic converters.

Secondary Particulates. The higher NOx emis-sions of diesel vehicles may yield higher levels of ni-trates, an acid aerosol that is an important component

TABLE 4Modeled Lifetime Average Emissionsfrom Diesel vs. Gasoline Vehicles(g/mi)a,b

HCc CO NOx

Diesel Car 0.40 1.09 1.10

Gasoline Car NLEV base cased 0.30 2.35 0.41 NLEV no I/Me 0.51 4.11 0.57a. Based on model runs of MOBILE5b (14-Sep-96) using guid-

ance memo for NLEV emissions analysis (July 1998). Runsare based on mileage-weighted average values for a fleet ofvehicles all meeting the same emission standard in calendaryear 2020. Emissions over the urban driving cycle (19.6 mphaverage speed) were combined with emissions over thehighway driving cycle (48.2 mph average speed) using a55/45 split.

b. Modeled values are adjusted post-hoc to account forplanned changes to the MOBILE model that adjust for newemission deterioration rates, off-cycle emissions, fuel effects,and fleet characteristics per EPA (1998c).

c. Includes tailpipe+evaporative emissions.d. Base case includes National Low Emission Vehicle (NLEV)

program vehicles, Federal Phase 2 reformulated gasoline (6.7psi), and inspection/maintenance program per LEV programcredit guidance.

e. No inspection/maintenance program assumed.

TABLE 5Measured PM Emissions from ProperlyOperatinga Gasoline Vehicles vs. DieselVehicles

Study Average(g/mi)

GasolineEnvironmental Research Councilb 0.001Coordinating Research Council summerc 0.00282Coordinating Research Council winterd 0.00351Cadle 1996e 0.005

DieselCoordinating Research Council summerc 0.811Coordinating Research Council winterd 0.460Certification Valuesf 0.04–0.06

a. Gasoline vehicle PM emissions for “smokers” can be 100times that of properly operating modern vehicles (Cadle1997; Cadle et al. 1998).

b. The ERC Study conducted by the Big 3 using late-model,high-production vehicles in both low and high mileage usingCalifornia Phase II reformulated gasoline. Range: 0.0006–0.0016 g/mi. Source: Cadle 1997, p.451.

c. The CRC Project data for 20 vehicles from model years1991–1996 in Denver summer testing (no oxygenates). 10diesel vehicles were tested from model years 1980–1994.Source: Cadle et al. 1998, p.ii. Range: 0.0011–0.0096 g/mi.Source: Cadle 1997, p.455.

d. See footnote c. 10 gasoline and 12 diesel vehicles weretested during the winter. Tests were completed indoors; out-door tests revealed average PM emissions of 0.025 g/mi forthe gasoline vehicles.

e. Source: HEI 1997, p.15f. Environmental Protection Agency and California Air Re-

sources Board certification data for 1997 and 1998 modelyear vehicles. Sources: EPA 1997; CARB 1997, p.8.

10Diesel Passenger Vehicles and the Environment U n i o n o f C o n c e r n e d S c i e n t i s t s

of particulate pollution, especially in the WesternUnited States.16

In the case of hydrocarbons, the specific chemicalcomposition of the emissions is likely to be importantin determining the contribution of diesel or gasolineexhaust to secondary particle formation. It is cur-rently unclear how important motor vehicle-relatedhydrocarbons are as a secondary particulate source.Since diesel vehicles emit fewer hydrocarbons thangasoline cars, hydrocarbon-related secondary par-ticulate formation may well be lower.

Toxics. Comprehensive and detailed compari-sons of the carcinogenic properties of diesel exhaustversus gasoline exhaust have not been made, but earlyevaluations suggest that important differences mayexist. For example, the International Agency for Re-search on Cancer has classified diesel engine exhaustas “probably carcinogenic” but assigned gasoline ex-haust a lower risk of “possibly carcinogenic” (IARC1989).

More detailed and recent research into gasolinehas focused on specific toxic compounds, such asbenzene or 1,3 butadiene, rather than whole gasolineexhaust.17 Current evidence suggests that diesel ex-haust is more potent than these individual toxic con-stituents (as measured by its unit risk); however, acomplete risk assessment would need to comparepublic exposure to these compounds as well as theirpotency relative to diesel (Table 6).

Emission Regulations. Today’s diesel passengervehicles are allowed to emit more of two key pollut-ants, nitrogen oxides (NOx) and particulate matter(PM), than their gasoline counterparts under existingregulations, the Federal “Tier 1” standards (Table 7).This loophole is largest for diesel automobiles andsome small pickups and sport-utility vehicles, forwhich NOx standards are over two times higher thanfor gasoline cars. Although gasoline vehicles are notrequired to meet a PM standard, their emissions are

16 Nearly 16 percent of PM2.5 is thought to be nitrate inthe Western U.S. (EPA 1996b). In a more recent studyof the Denver metropolitan area, ammonium nitrateaccounted for roughly one-quarter of urban area PM2.5and almost half of PM2.5 in non-urban regions duringthe winter (Watson et al. 1998)17 Many of these compounds are also found in dieselexhaust.

typically 16 times lower than the PM standards fordiesel cars.

National Low Emission Vehicle (NLEV). Be-ginning in 1999 in the Northeast and 2001 elsewhere,new standards are coming on line that lower emis-sions from cars and light trucks up through 6,000pounds gross vehicle weight (GVW). These stan-dards do not affect heavier light trucks, nor do theyeliminate the ability to sell Tier 1 vehicles (for whichdiesel waivers exist).18

New California Standards (LEV II). In Califor-nia, regulators have recently adopted new standardsfor 2004 and beyond called the Low-Emission Vehi-cle II (LEV II) program. California’s LEV II estab-lishes tighter standards for all passenger vehicles andremoves loopholes that currently (a) permit lighttrucks to emit more than automobiles and (b) allowdiesel vehicles to emit more pollution than gasolinecars. Under LEV II, diesel passenger cars and lighttrucks up through 8,500 gross vehicle weight (GVW)sold in California must meet the gasoline automobilestandards.

New Federal Standards (Tier 2). Some statesmay choose to opt into the California LEV II emis-sions program. For the remaining states, federalemission standards will apply. “Tier 2” standards arenow under development and could come into effect asearly as model year 2004. In addition to requiringemission reductions from all passenger vehicles, the

18 However, sales of vehicles dirtier than the averageNLEV requirement (such as diesel Tier 1 vehicles)would have to be offset by cleaner vehicles elsewherein the fleet.

TABLE 6Cancer Potency of Diesel Exhaust vs.Gasoline Exhaust Constituents

CompoundUnit Riska

(µg/m3)-1Range(µg/m3)-1

Diesel Exhaust 3 x 10-4 1.3 x 10-4 to 2.4 x 10-3

1,3 Butadiene 1.7 x 10-4 4.4 x 10-6 to 3.6 x 10-4

Benzene 2.9 x 10-5 7.5 x 10-6 to 5.3 x 10-5

Formaldehyde 6.0 x 10-6 2.5 x 10-7 to 3.3 x 10-5

Acetaldehyde 2.7 x 10-6 9.7 x 10-7 to 2.7 x 10-5

a. Unit risk refers to the upper bound risk of contracting cancerover a human lifetime at a standard concentration of toxicmaterial (one microgram of toxic per cubic meter of air).

Source: CalEPA 1998b, p.7 (Appendix II)

U n i o n o f C o n c e r n e d S c i e n t i s t s Diesel Passenger Vehicles and the Environment11

Tier 2 standards may also close the historic loopholethat has permitted diesel vehicles to emit more ofsome pollutants. As in California, the federal Tier 2standards could therefore require major reductions indiesel passenger vehicle emissions.

Research Targets. While many of these newstandards will come on line within the next five years,federal research into both diesel automobiles andlight trucks continues to focus on less stringent emis-sion targets. While federal research targets may besubject to change, the current official goals for dieselpassenger cars (PNGV) and light trucks (the LightTruck Clean Diesel program) are 3–7 times higherthan California’s new emissions standards (Table 7).

TABLE 7Passenger Vehicle Emission Standards and Research Goals (grams/mile @ 100,000 miles)

Vehicles ≤6000 lbs GVWa Vehicles 6,001–8,500 lbs GVWCars, Light Trucks≤3750 lbs LVWb

Light Trucks>3750 lbs LVW

Light Trucks ≤5750 lbs LVW

Light Trucks>5750 lbs LVW

NOx PM NOx PM NOx PM NOx PMFederal Tier 1 Standard (through 2003) gasoline 0.6 ---c 0.97 --- 0.98 --- 1.53 --- diesel 1.25 0.10 0.97 0.10 0.98 0.10 1.53 0.12National LEV Standard (1999/2001+) gasoline 0.3 --- 0.5 --- diesel 0.3 0.08 0.5 0.10California LEVII Standard (phase in 2004–2006)d

LEV, ULEV 0.07 0.01 0.07 0.01 0.07 0.01 0.07 0.01 SULEV 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01PNGV Research Goal original goals 0.2 0.04 “stretch objective” 0.2 0.01DOE Research Goale

diesel 0.5 0.04 0.5 0.04 0.5 0.04 0.5 0.04

a. Gross vehicle weight, which includes the vehicle curb weight plus full payload.b. Loaded vehicle weight, which includes the curb weight plus 300 pounds (lbs.).c. Gasoline vehicles are not subject to a PM standard, but data suggests values are ≤0.005 g/mi (Table 5)d. Standard at 120,000 miles. Applies to all vehicles, gasoline or diesel. 4% of vehicles are allowed to emit 0.1 g/mi of NOx at 120,000 miles.e. Based on DOE (1997); goals may change.

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