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Part VIII – Fuels and Fuel eff ects on Emissions 2103555 Engine and Emissions Control

Fuels and Fuel effects on Emissions

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Part VIII – Fuels and Fuel

effects on Emissions

2103555 Engine and Emissions Control

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Fuel Types

• Gasoline and diesel fuel are both produced from

crude oil. Together, gasoline and diesel fuel powerapproximately 99% of the motor vehicle fleet.

• However, alternative fuels are being used more andmore to reduce vehicle emissions.

• This section briefly explains various fuel types

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1.1 Indolene-Indolene is used as the standard gasoline

emission test fuel for spark ignition engines.

Indolene is a well refined gasoline with low levels of sulfur, phosphorus, and vapor pressure.

1.2 Diesel Fuel - The diesel fuel is commonly used inrelatively large displacement compression ignitionengines.

Diesel fuel is used in a broader range of engine sizes inEurope and other areas of the world.

 The average molecular weight and boiling point of dieselfuel is greater than that for gasoline, which makes itsuitable for use in compression ignition engines,characterized by higher in-cylinder temperatures andpressures.

1 Fuel Types

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1.3 California Phase-2 Gasoline - California phase-2

(CF2) is a gasoline blend designed specifically formileage accumulation and emissions testing of vehicles

to be sold in California. The blend contains 11.0% by volume MTBE (anoxygenate), very low sulfur, and controlled levels of benzene and aromatics.

CF2 volatility is adjusted for the summer and winterseasons (6.8 psi in the summer vs. 11.0 psi in the winter).

1 Fuel Types

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1.4 Reformulated Gasoline - Reformulated gasoline issimilar to CF2 gasoline.

 The reformulated blends usually contain an oxygenateadditive such as MTBE, EtOH, or ETBE.

1.5 Compressed Natural Gas - Compressed naturalgas (CNG) is comprised primarily of methane (CH4).

CNG vehicles generally produce lower emissions thantheir gasoline counterparts.

However, there are tradeoffs in engine power andefficiency.

1 Fuel Types

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1.6 Methanol (CH3OH) - Methanol is a promisingalternative fuel because it generally produces lowertailpipe emissions than gasoline and can be manufactured

at prices comparable to gasoline.A blend of 85% methanol and 15% unleaded gasoline(M85) is typically used. However, M85 vehicles arevirtually phased out of new vehicle manufacture in Brazil.

Vehicles that operate on methanol consume more fuelthan if they were operating on 100% gasoline because itsenergy content (calorific value) is less.

1 Fuel Types

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1.7 Ethanol (C2H5OH) - Ethanol is an importantcomponent of automotive fuel used in Brazil.

A mixture of 22% ethanol with gasoline (E22) is

commonly used.Ethanol is also used in the USA as an octane enhancerfor gasoline (up to 10%).

It is also used for flexible fuel vehicles as a blend of 85%

ethanol and 15% unleaded gasoline.Ethanol is produced from corn, sugar cane or other cropsbut is currently more expensive than gasoline.

1 Fuel Types

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2 Fuel Properties

• Fuel properties and compositions are regulated by mostgovernments.

• U.S. Federal requirements for test fuels are specified in

the Code of Federal Regulations (CFR).• The following subsections briefly describe some

important fuel characteristics and their influence on

engine performance.• Subsequent to these sections, Tables 1 and 2 providesome fuel properties for liquid and gaseous fuels.

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2.1 Fuel Volatility-

Volatility is loosely defined as thetendency of a liquid to evaporate.

Fuel for engines must conform to strict specifications

regarding their volatility characteristics to ensuresatisfactory operation.

 The fuel must contain a large enough proportion of highlyvolatile components to ensure good cold starts, but the

volatility must not be so high as to impair operation(vapor lock) and starting when the engine is hot.

2 Fuel Properties

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2.2 Distillation Curve - Three areas on the distillation orboiling curve have a pronounced effect on an engine'soperating behavior. They can be defined according to thepercent of fuel evaporated at three temperatures during astandard ASTM distillation test.

a] The volume of fuel that evaporates up to 70° C must beadequate to ensure good cold starting, but not so large as

to cause vapor lock when the engine is hot.b] The vaporized volume at 100° C should be high enoughto minimize dilution of the engine's lubricating oil.

c] The percentage of fuel which evaporates at 180° C

determines the engine's warm up qualities, as well as it'sacceleration and response characteristics once it hasreached normal operating temperature.

2 Fuel Properties

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2.2 Distillation Curve -

Figure 1 illustrates the effect of changing the shape of the

distillation curve.

2 Fuel Properties

Figure 1 The Effects of Altering the Fuel Distillation Curve

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2.2 Disti llation Curve - From Figure 1,

• Raising the left end, by reducing the amount of highly

volatile (low boiling point) compounds in the blend,degrades cold starting.

• Lowering the left end can cause vapor lock, poor hotstarts, and high evaporative losses.

• Raising the right end, by adding compounds with highermolecular weights, leads to increased oil dilution anddeposit formation.

• Lowering the right end increases fuel consumption during

warm engine operation.• Raising the middle degrades warm-up driveability, causes

rough accelerations, and degrades fuel economy duringshort trips.

2 Fuel Properties

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2.3 Vapor Pressure - Fuel vapor pressure curves arestrongly influenced by fuel composition.

 The curves for fuels containing alcohol, for instance, are

much steeper than those for pure hydrocarbon mixtures. The result is that fuels with alcohol display a greatertendency to vaporize and form vapor lock, thus impairingoperation at higher temperatures.

Since 1992, federal emission laws have regulatedmaximum RVP to reduce evaporative and exhaustemissions, particularly in California.

2 Fuel Properties

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2.4 Vapor/Liquid Ratio - This specification provides anindex of a fuel's tendency to form vapor bubbles (vaporlocks).

It is based on the volume of vapor generated by aspecific quantity of fuel at a set temperature.

Assuming a constant temperature, a larger volume of vapor will form at low pressures (i.e., at high altitudes)

than at higher pressures (i.e., at sea level). This phenomenon can lead to vapor-lock problems whendriving in mountainous areas.

 The addition of alcohols, methanol in particular, raisesthe vapor/liquid ratio.

2 Fuel Properties

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2.5 Driveability Index - The driveability index (DI) isused often by engineers to predict engine performancebased on three points from a fuel’s distillation curve.

 These points are referred to as the T10, T50 and T90temperatures, and correspond to temperatures in °F atwhich 10%, 50%, and 90% of the volume of a fuel hasevaporated.

 The index is calculated using Equation 1:DI = 1.5 T10 + 3.0 T 50 +T 90. (1)

2 Fuel Properties

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2.5 Driveability Index –

• Note that more emphasis is placed on the T50temperature since this is the approximate temperature

at which the fuel enters the combustion chamber.• A larger DI results from a less volatile fuel and vice

versa.

• Typical DI’s for gasoline range from 1,000 to 1,200.

• Fuels with a DI greater than 1200 may causehesitation, stumbles and misfires, contributing tohigher emissions.

2 Fuel Properties

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2.6 Octane/Cetane Rating–

 The octane rating defines a gasoline's ignition quality(engine "knock resistance") for gasoline engines.

 The cetane number carries similar meaning but fordiesel engines.

However, the two numbers have inverse relationships

with respect to knock resistance; the resistance toengine knock increases with increasing octane numberand decreasing cetane number.

2 Fuel Properties

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2.7 Knock Inhibitors - The most effective knockinhibitors are organic lead compounds. These can raisethe octane number by several points.

Environmental concerns have lead to a steady reductionin the amount of lead in fuels.

A compound developed to replace lead compoundsmethylcyclopentadienyl manganese tricarbonyl (MMT) is

currently used in Canada and adversely affectsemissions.

In U.S. gasoline, refiners have not currently used MMT.Data is being generated to effect a ban of MMT by the

U.S. EPA.

2 Fuel Properties

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2.8 Calorific Value - The specific values for the net(lower) and gross (higher) calorific values, provide anindex for the energy content of fuels.

When fuels (such as methanol), with a lower calorificvalue are used, greater quantities will be required toachieve comparable engine output.

2 Fuel Properties

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3 Fuel Composition

Fuels vary widely in chemical composition, depending on

the source of crude and on the methods used in refining.

In every case, however, the fuel consists almost entirelyof a mixture of hydrocarbon compounds having differentmolecular weights and different types of structure

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3.1 Paraffins - Paraffins are straight chainedhydrocarbons, also called alkanes. Some examples arepropane and butane. Isoparaffins have a branched chain

structure.3.2 Aromatics - Aromatics are high octane blendinghydrocarbons that have a benzene ring in their molecularstructure. Examples are benzene, toluene, xylene.

3.3 Olefins - Olefins are gasoline hydrocarbons resultingfrom several refining processes. Examples are ethylene,propylene, butylene. Olefins often contribute to the

formation of gum and deposits in engines and theinduction system. Olefins are also called alkenes.

3 Fuel Composition

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3.4 Oxygenates - Oxygenates are an octane component

containing hydrogen, carbon, and oxygen in theirmolecular structure.

Includes ethers such as MTBE and alcohols such asethanol and methanol.

Adding an oxygenate to gasoline results in enleanmentthereby improving combustion and reducing tailpipeemissions.

3 Fuel Composition

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Table 1 Typical Properties of Liquid Fuels and Hydrocarbons

3 Fuel Composition

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Table 2 Typical Properties of Gaseous Fuels and Hydrocarbons

3 Fuel Composition

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3.5 Additives - Chemicals are added to gasoline in very

small quantities to improve and maintain gasolinequality.

Examples include: detergents which prevent or clean upcarburetor and fuel injector deposits, corrosion inhibitors,

antioxidants to reduce gum formation in storage, acetoneor similar water absorbents, and ethers such as methyltertiary butyl ether (MTBE) for increased octane levels.

3 Fuel Composition

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3 Fuel Composition

3.5.1 Effects of fuel additives

Fuel additive serve two distinct purposes.

– to improve combustion and pollutant emissions,

– to ensure reduced wear and limit deposit formationduring the engine life cycle of several hundreds of thousands of miles

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3 Fuel Composition

3.5.1 Effects of fuel additives

Figure 2 shows some important fuel-relevant problemareas in a spark-ignition engine

Figure 2 Problem areas related to fuel in a spark ignition engine.

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3.5.2 Additive for gasoline engine

Most important additives that have a direct effect

on combustion are “anti-knock products”.Additives containing metals are designed toimprove combustion (combustion enhancers).

Deposits will accumulate on the valve headsthat act like a sponge during cold starting andcause the air-fuel mixture to become leaner.

At full throttle, the deposits reduce the cylindercharge and thus cause power losses.

3 Fuel Composition

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Figure 3 Effect of deposit formation on pollutant emissions

3 Fuel Composition

Figure 3 shows how exhaust emission are affectdadversely by deposit formation and residues inside thecombustion chamber and on the inlet valves. Additive

will help avoid these problems.

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3.5.3 Additive for diesel engines

Additives, for diesel engine, concentrates on improvingcombustion, reducing deposit formation duringcombustion of the fuel and keeping the injectors clean.

 Three group of additives are used for diesel engines:\

Cetane number enhancer s: Increasing the cetanenumber by using cetane number enhancers means that

ignition lag is reduced, i.e. ignition is advanced.Extensive research has proven that increased additiveconcentration help to reduce HC, CO and particulateemission by a significant degree without causing NOxemission to increase.When the cetane number was increased by ten units,HC emissions dropped by up to 80%, CO emissionswere up to 50% lower and particulate emissions werereduced by up to 30%. Combustion noise is also

reduced considerably.

3 Fuel Composition

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Combustion enhancers: Combustion enhancers actdirectly on the combustion process. This effect is achieved both in clean and in cokedengines, Unburned hydrocarbons and particulates are

reduced by a significant degree whereas CO and NOxemissions remain unchanged.

Multifunctional diesel additive: This additive packincludes components for corrosion protection,

demulsification and reducing the foaming tendency.Oxidation inhibitors and detergents are used to keep theinjectors clean.After run with the additive pack, HC emissions had

dropped and CO and particulates were more lowerwithout causing any significant increase of NOxemissions.

3 Fuel Composition

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4 Influence of fuel on emissions

 The emission characteristic of an internal combustion

engine are dictated by the type of fuel used.

 The fuel composition is reflected by a spectrum of exhaust components typical for a particular type of fuel.

 Therefore, emission criteria will play an increasing role inthe production of future fuels.

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4.1 GasolineGasoline for S.I. engines consists of a mixture of hydrocarbons that is liquid at room temperature andcovers a boiling range of approx. 30 – 215 C.

 To improve cold starting, it is enriched with easily volatileconstituents in winter.

4 Influence of fuel on emissions

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4.1 Gasoline The following criteria are of importance for exhaustemissions:

– Good starting across a wide temperature range (-40 C

to + 50 C),

– Good transient characteristics (load changes) andtransient operation,

– Low vapor-lock tendency,– High energy density,

– High knock resistance.

In order to meet the above criteria, specific fuel propertiesare required.

 Table 3 shows some important quality criteria for spark

ignition engine fuels and their effect on engine operation.

4 Influence of fuel on emissions

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Table 3 Quality criteria for gasoline fuels

4 Influence of fuel on emissions

4 I fl f f l i i

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4.1 GasolineAlcohols and esters are used to improve knockresistance of gasoline.

 The limit for adding alcohol is approx. 10% by volume,not least of all because of limited availability.

 The use of components containing oxygen is also criticalas this increases fuel volatility and may lead to drivability

problems cause by vapor locks.In addition, exceeding the admissible vapor pressurelimits may cause excessive evaporative losses, resultingin further environmental pollution due to hydrocarbon

emissions.Another critical topic is the content of benzene oraromatic substance in the fuel.

4 Influence of fuel on emissions

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Figure 4 Interrelationship between benzene in fuel

and in exhaust emissions.

4 Influence of fuel on emissions

Figure 4 shows an example of the relationship betweenbenzene in exhaust emissions and benzene and toluenecontent in the fuel.

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4.1 Gasoline

In response to the effect of fuel composition on emissions,emissions of four toxic compounds have to be reduced.

 These four toxic compounds are:– Formaldehyde,

– Acetaldehyde,

– Polycyclic organic compounds,– Benzene.

As reformulated gasoline has a different composition (e.g.

lower content of olefins and aromatic substances, reducedvolatility), more energy will be required to produce this typeof fuel.

 This entails higher CO2 emissions at the refinery approx.

18%.

4 Influence of fuel on emissions

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4.2 Diesel fuels

Fuel quality affects diesel engine emissions even morestrongly than emissions of spark ignition engines.

HC, CO and particulate emissions are the mainpollutants affected by the fuel quality.

Similar to gasoline for S.I. engines, diesel engine fuel is

a mixture of hydrocarbons that is liquid at roomtemperature and covers a boiling range of approx. 170 –360 C.

Major quality criteria are cetane rating, density, viscosity,boiling characteristics, aromatics content and sulphurcontent.

4 Influence of fuel on emissions

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4.2 Diesel fuels

 The following criteria are of importance for exhaustemissions:

– Low density,

– Low content of aromatic compounds,

– Low sulphur content,

– High cetane rating.

4 Influence of fuel on emissions

4 I fl f f l i i

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4.2 Diesel fuelsImportant characteristics that are related to the smokingtendency of hydrocarbons used for diesel engine fuelsare shown in table 4.

Table 4 Characteristic of hydrocarbons.

4 Influence of fuel on emissions

4 I fl f f l i i

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4.2 Diesel fuels The effect of the content of aromatic substances onparticulate emission of a swirl chamber diesel engine isexplained in figure 5.

Figure 5 Particulate emissions as a function of aromaticcompounds for a swirl chamber engine.

4 Influence of fuel on emissions

4 Infl ence of f el on emissions

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4.2 Diesel fuels

A high content of aromatic substances in the fuelreduces its ignition quality.

Particulate emissions are lower if the content of aromaticsubstances is reduced.

Particulate emissions also are a function of the cetane

rating.

4 Influence of fuel on emissions

4 Influence of fuel on emissions

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4.2 Diesel fuelsFigure 6 shows the relative exhaust emissions asafunction of cetane rating for the HC, CO, NOx andparticulate components determined in FTP-75 tests.

Figure 6 Trends of relationship between exhaust emissions

and cetane rating of a swirl chamber engine.

4 Influence of fuel on emissions

4 I fl f f l i i

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Figure 7 shows the particulate, soot and HC emissionsas a function of engine load for two cetane ratings usedon a supercharged direct injection diesel engine. Withthe exception of nitrogen oxides, the essential pollutantcomponents can be modified by the cetane rating of the

fuel.

Figure 7 Dependence of specific pollutants on cetanerating of a DI diesel engine.

4 Influence of fuel on emissions

4 I fl f f l i i

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 The sulphur content of diesel fuel has a pronounce effecton particulate emissions. The effect of such reductions interms of particulate emissions is illustrated in figure 8.

Figure 8 Effect of sulphur content on particulate emissions.

4 Influence of fuel on emissions

4 I fl f f l i i

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A reformulated diesel fuel may have a low content of 

sulphur and aromatic substances and should meet thefollowing specifications:

• Improved readiness to ignite (e.g. CR>55),• Reduced content of aromatic compounds (<10% by

volume),

• Reduced density (e.g. 0.8 to 0.82 kg/l),• Optimized addition of additives,

• 90% boiling point <300 C.

• This type of fuel will allow CO and HC emissions to bereduced by approc. 10% whereas particulate emissionscan be reduced by up to 40%.

4 Influence of fuel on emissions

5 AQIRP & EPEFE Fuel Development

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5 AQIRP & EPEFE Fuel Development

 The Air Quality Improvement Research Program

(AQIRP) and the European Programme on Emissions,Fuels and Engine Technologies (EPEFE) were the first

large scale systematic efforts to develop extensive dataon the inter-relationships among fuel compositions,vehicles, emissions and air quality.

A summary of the results from these two programs ispresented in the following sections.

5 1 Air Quality Improvement Research Program

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5.1 Air Quality Improvement Research Program

Program Participants3 U.S. Automotive Manufacturers14 U.S. Oil Companies

Fuels52 Gasoline Fuels13 Fuels Containing Oxygenates6 CNG Fuels8 M85 Fuels

3 E85 Fuels8 Fuels With Variable Sulfur Levels

5 1 Ai Q lit I t R h P

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Vehicle Types & Technologies

59 Gasoline Vehicles

3 CNG Fuels3 E85 Fuels

28 M85 Fuels

Vehicle technology included– an "Older" fleet (1983-1985 MY),– "Current" fleet (1989 MY),

– "Federal Tier I" fleet (1993-1994 MY),– "California Tier 0" fleet (1993 MY), and– "Advanced Prototypes."

5.1 Air Quality Improvement Research Program

1 Ai Q li I R h P

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Results

AQIRP testing showed the following general trends:

- Lowering RVP reduces evaporative emissions and ozone.

- Lowering sulfur decreases exhaust emissions, toxics andozone.

- Lowering T90 reduces HC and ozone.

- Lowering aromatics reduces toxics.

Adding oxygenates reduces CO.

- Lowering olefins reduces ozone.

5.1 Air Quality Improvement Research Program

5.2 European Programme on Emissions, Fuels

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5.2 European Programme on Emissions, Fuels 

and Engine Technologies 

Program Participants

14 European Automotive Manufacturers

18 European Oil CompaniesEuropean Commission Directorates

Fuels

10 Gasoline Fuels

1 Fuel Containing 2.0% by Weight Oxygen as MTBE

12 Diesel Fuels

4 Fuels With Variable Sulfur Levels

Vehicle Types & Technologies

Prototype Stage II 1996 MY

5.2 European Programme on Emissions, Fuels

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ResultsEPEFE testing showed the following general trends:

- Lowering aromatics decreases HC and CO.- Increasing mid-range volatility decreases HC.

- Lowering gasoline sulfur reduces exhaust HC, CO and NOx.

- Decreasing density reduces light-duty diesel HC, CO andparticulate matter.

- Decreasing density reduces heavy-duty diesel NOx and

particulate matter.

5.2 European Programme on Emissions, Fuels 

and Engine Technologies 

5.2 European Programme on Emissions, Fuels 

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Results

EPEFE testing showed the following general trends:

- Decreasing polyaromatics reduces light-duty dieselNOx and particulate matter.

- Decreasing polyaromatics reduces heavy-duty dieselHC, NOx and particulate matter.

- Increasing cetane reduces diesel HC and CO.

- Decreasing T90 reduces light-duty particulate matter.

- Decreasing T90 reduces heavy-duty NOx.

- Reducing aromatics reduce CO2 at each E100 volatilitylevel with no impact on fuel consumption.

p g ,

and Engine Technologies