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Diesel
The combustibility of a diesel fuel in diesel engines is characterized by its cetane number, which is a measure of its ability to undergo compression ignition under standard test conditions. Fuels with a higher cetane number more readily ignite in compression ignition engines.

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Diesel fuel is a complex mixture of hydrocarbons composed primarily of paraffin and aromatics, with olefin content amounting to only a few percent by volume. The combustibility of a diesel fuel in diesel engines is characterized by its cetane number, which is a measure of its ability to undergo compression ignition under standard test conditions. Fuels with a higher cetane number more readily ignite in compression ignition engines.

<>The most important pollutants of concern from diesel engines are PM and NOx.

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A number of diesel properties have been linked to emissions of these pollutants from diesel engines.

These properties include the following:

  • Density
  • Cetane number/index
  • Aromatic content (and the particular types of aromatics, e.g., mono-, di- or tri-aromatic compounds)
  • Distillation properties (e.g., T10, T50, T90)
  • Sulfur content

In addition, studies have been conducted examining the effects on diesel engine emissions as a result of adding oxygenated compounds to diesel.

Although a large number of studies of the impact of diesel on emissions have been performed, it has been difficult to identify which fuel properties can be manipulated to reduce emissions because different engines of the same or similar design can and have been observed to react differently to specific changes in fuel properties. Another factor is that, in many studies, individual fuel properties are frequently correlated with other fuel properties thus making it difficult to determine whether it is one fuel property or another, or some combination of both, that is important with respect to diesel engine emissions.

A recent review addressed the impact of changes in fuel composition on emissions from current heavy-duty, direct-injection diesel engines based only on studies where there were no significant correlations among germane fuel properties.

Conclusions from this review are summarized below:

Summarized Influence of Fuel Properties on
Heavy Duty Diesel Emissions

Fuel modification

NOx

Particulates

Reduce sulfur *

No effect

Large Effect [1]

Increase cetane

Small reduction

No effect

Reduce total aromatics

Small reduction [2]

No effect

Reduce density

Small reduction

No effect [3]– Large reduction [4]

Reduce polyaromatics

Small reduction

No effect [3]– Large reduction [4]

Reduce T90/T95

Very small reduction

No effect

Notes:
[1] Reducing S from 0.3% to 0.05% gives relatively large benefits, reducing S from 0.05% to lower levels has minimal direct benefit but it is necessary to enable technologies
[2] Polyaromatics are expected to give a bigger reduction than mono-aromatics
[3] low emission emitting engine
[4] high emission emitting engine

* For engines WITHOUT aftertreatment systems


As shown, compositional properties of at least some importance with respect to emissions are sulfur, aromatic, and oxygenate content; the physical properties identified are density and the T90 or T95 distillation temperature. The cetane number/index was also identified as a factor with respect to emissions.

It is important to note that, based on the data presented in the table, the impact of a given change in fuel composition can be different depending on the relative emissions level of the engines. Emissions from engines with high base emission rates -- generally older designs -- tend to be more sensitive to changes in fuel composition than those from engines with lower base emissions rates -- which tend to be newer designs. Changes in all of the fuel properties have been found to have, at most, small impacts on emissions from engines with low base emission rates.

In addition to studies of fuel composition effects on emissions in heavy-duty diesel engines, there have also been numerous studies performed on engines used in passenger cars and light-duty trucks.

The most recent, comprehensive, study of fuel composition impacts on light-duty Diesel emissions was performed as part of the European Programs on Emissions, Fuels and Engine Technologies (EPEFE).

This study investigated the impact of the density, cetane number, T95, and polycyclic aromatic content of diesel fuels on emissions from light-duty vehicles sold in Europe. The program involved 19 vehicles, 14 of which had indirect-injection engines. Indirect-injection engines currently dominate the market, but are expected to be supplanted over the next several years by direct-injection engines due to the latter's greater fuel efficiency.

Impact of Fuel Composition Changes on Emissions of
Current Light-Duty Diesel Vehicles

Change

Nox indirect injection

Direct injection

PM indirect injection

Direct injection

Increase cetane (50-58)

Very small increase

Very small decrease

No effect

Small increase

Decrease density (0.855 - 0.828 gr/cm3)

No effects

Small increase

Large decrease

Large decrease

Decrease T 95 (700-620)

Very small increase

Small increase

No effect

Small decrease

Decrease polycyclics (8-1 vol. %)

Very small decrease

Very small decrease

Very small decrease

Small decrease

Notes:
"large" - 10% or greater change in emissions
"small" - 5% to 10% change
"very small" - 1% to 5% change or less

As shown, although there are some differences in terms of the magnitude of fuel composition effects on emissions from vehicles with indirect- and direct-injection engines, the directional impact on emissions is usually the same.

A comparison of the heavy duty and light duty tables indicates that there are some instances where changing a given diesel fuel property is expected to have the opposite directional impact on emissions depending on whether the fuel is being used in a heavy-duty engine or light-duty vehicle. The most notable are the increase in NOx emissions from light-duty direct-injection engines in response to a decrease in fuel density and the increases in NOx emissions from both light-duty indirect- and direct-injection engines in response to a decrease in the T95 temperature.

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Sulfate particulate and SOx emissions, both of which are harmful pollutants, are emitted in direct proportion to the amount of sulfur in diesel fuel. Sulfate PM contributes to PM10, and PM2.5 emissions directly with their associated adverse health and environmental effects.

Sulfur dioxide (SO2), one fraction of the SOx, is a criteria pollutant with associated adverse effects. The health and welfare effects of SO2 emissions from diesel vehicles are probably much greater than those of an equivalent quantity emitted from a utility stack or industrial boiler, since diesel exhaust is emitted close to the ground level in the vicinity of roads, buildings, and concentrations of people. Some of the SOx is also transformed in the atmosphere to sulfate PM with the associated adverse effects noted for PM.

Diesel PM, which has been found to be a human carcinogen by the California Air Resources Board, consists of three primary constituents:

  • carbonaceous core
  • soluble organic fraction (SOF) which sits on the surface of this core, and
  • mixture of SOx and water, which also sits on the surface of the core

Lowering the sulfur in the fuel lowers the SOx fraction of PM thus lowering the overall mass of PM emitted. Recent diesel fuel evaluations carried out in Europe, for example, show the benefits of reduced sulfur in diesel fuel for lowering particulate. For example, data released from the auto oil study showed that lowering the diesel fuel sulfur level from 2000 ppm to 500 ppm reduced overall particulate from light duty diesels by 2.4% and from heavy-duty diesels by 13%.

The relationship between particulates and sulfur level was found to be linear; for every 100-PPM reduction in sulfur, there will be a 0.16% reduction in particulate from light duty vehicles and a 0.87% reduction from heavy-duty vehicles.

Sulfur in diesel fuel has a comparable technology enabling effect as lead and sulfur in gasoline, especially in the context of advanced engines that hold the promise of substantially increased fuel economy and, hence, lowered emissions of carbon dioxide. New advanced diesel engines can achieve comparable increases in fuel economy compared to current versions. But in doing this, diesel advanced engines increase emissions of oxides of nitrogen. Catalytic converters or NOx adsorbers can eliminate much of this NOx, but sulfur disables them in much the same way that lead poisons the three-way catalyst. Thus, the presence of sulfur in diesel fuel effectively bars the path to fuel savings and climate protection as well as low emissions of conventional pollutants. As stated by the German government in a recent petition to the European Commission in support of low sulfur fuel, "A sulphur content of 10 ppm compared to 50 ppm increases the performance and durability of oxidizing catalytic converters, DeNOx catalytic converters and particulate filters and therefore decreases fuel consumption. There are also lower particulate emissions (due to lower sulfate emissions) with oxidizing catalytic converters. For certain continuously regenerating particulate filters, a sulphur content of 10 ppm is required for the simple reason that otherwise the sulfate particles alone (without any soot) would overstep the future [European] particulate value of 0.02 g/kWh."

In addition to its role as a technology enabler, low sulfur diesel fuel gives benefits in the form of reduced sulfur induced corrosion and slower acidification of engine lubricating oil, leading to longer maintenance intervals and lower maintenance costs. These benefits can offer significant cost savings to the vehicle owner without the need for purchasing any new technologies.

The individual components of the engine system, which might be expected to realize benefits from the use of low sulfur diesel fuel, are summarized in the Table below.

Components Potentially Affected by Lower
Sulfur Levels in Diesel Fuel

Affected components

Effect of lower sulfur

Potential impact on engine system

Piston rings

Reduce corrosion wear

Longer engine life, less frequent rebuilds

Cylinder liners

Reduce corrosion wear

Longer engine life, less frequent rebuilds

Oil quality

Reduce deposits, less need for alkaline additives

Reduced wear on piston ring – Cylinder liner, less frequent oil changes

Exhaust (tailpipe)

Reduce corrosion wear

Less frequent parts replacement

EGR

Reduce corrosion wear

Less frequent parts replacement

The actual value of these benefits over the life of the vehicle will depend upon the length of time that the vehicle operates on low-sulfur diesel fuel. For a vehicle near the end of its life when the low sulfur fuel is introduced the benefits would be quite small.

However, for vehicles produced in the years immediately preceding the introduction of low-sulfur fuel the savings would be substantial.

These savings, due to the use of low sulfur diesel fuel, can be expressed in terms of a savings in cents per gallon of low sulfur diesel fuel. The average savings were estimated by EPA to be approximately 1.4 cents/gallon for light heavy-duty diesels, 1 cent/gallon for medium heavy-duty diesel engines and 0.7 cents/gallon for heavy heavy-duty diesel engines.

The average savings estimated across all weight classes is therefore approximately one cent per gallon. These benefits result in an estimated savings of $153 for light heavy-duty vehicles, $249 for medium heavy-duty vehicles, and $610 for heavy heavy-duty vehicles and urban buses.

Diesel fuel consists of a mixture of hydrocarbons having different molecular weights and boiling points. As some of it boils away on heating, the boiling point of the remainder increases. This fact is used to characterize the range of hydrocarbons in the fuel in the form of a "distillation curve" specifying the temperature at which 10%, 20%, etc. of the hydrocarbons have boiled away.

A low 10% boiling point is associated with a significant content of relatively volatile hydrocarbons. Fuels with this characteristic tend to exhibit somewhat higher HC emissions than others. Formerly, a relatively high 90% boiling point was considered to be associated with higher particulate emissions.

More recent studies have shown that this effect is spurious -- the apparent statistical linkage was due to the higher sulfur content of these high-boiling fuels. In a Dutch study, however, the test fuels were composed of two sets at clearly different 85 or 90 per cent boiling points, among which sulfur content varied independently.

A highly significant effect of 85% or 90% boiling point temperatures was found, in addition to a significant effect of sulfur and a probably significant effect of aromatics contents. A typical effect of a 20degC change in 85% boiling point is 0.05 g/kWh at present particulate levels. This may be related to generally higher 85 or 90 per cent points, which in the test fuels went up to 350 or 360degC. Commercial diesel fuels in Europe show values up to about 370degC.

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Aromatic hydrocarbons are hydrocarbon compounds containing one or more "benzene-like" ring structures. They are distinguished from paraffins and napthenes, the other major hydrocarbon constituents of diesel fuel, which lack such structures.

Compared to these other components, aromatic hydrocarbons are denser, have poorer self-ignition qualities, and produce more soot in burning. Ordinarily, "straight run" diesel fuel produced by simple distillation of crude oil is fairly low in aromatic hydrocarbons.

Catalytic cracking of residual oil to increase gasoline and diesel production results in increased aromatic content.

A typical straight run diesel might contain 20 to 25% aromatics by volume, while a diesel blended from catalytically cracked stocks could have 40-50% aromatics.

Aromatic hydrocarbons have poor self-ignition qualities, so that diesel fuels containing a high fraction of aromatics tend to have low cetane numbers. Typical cetane values for straight run diesel are in the range of 50-55; those for highly aromatic diesel fuels are typically 40 to 45, and may be even lower. This produces more difficulty in cold starting, and increased combustion noise, HC, and NOx due to the increased ignition delay.

Increased aromatic content is also correlated with higher particulate emissions. Aromatic hydrocarbons have a greater tendency to form soot in burning, and the poorer combustion quality also appears to increase particulate SOF emissions. Increased aromatic content may also be correlated with increased SOF mutagenicity, possibly due to increased PNA and nitro-PNA emissions.

There is also some evidence that more highly aromatic fuels have a greater tendency to form deposits on fuel injectors and other critical components. Such deposits can interfere with proper fuel/air mixing, greatly increasing PM and HC emissions.

Polycyclic aromatic hydrocarbons (PAH) are included in the great number of compounds present in the group of unregulated pollutants emitted from vehicles. Exhaust emissions of PAH (here defined as three ringed and larger) are distributed between particulate- and semi-volatile phases.

Some of these compounds in the group of PAH are mutagenic in the Ames test and even in some cases causes cancer in animals after skin painting experiments. Because of this, it's important to limit the emissions of PAH from vehicles especially in densely populated high traffic urban areas. An important factor affecting the emissions of PAH from vehicles is selection of fuel and fuel components.

A linear relationship exists between fuel PAH input and emissions of PAH. The PAH emission in the exhaust consists of uncombusted through fuel input PAH and PAH formed in the combustion process. By selecting diesel fuel quality with low PAH contents, the PAH exhaust emissions will be reduced by up to approximately 80% compared to diesel fuel with PAH contents larger than 1 g/liter (sum of PAH). By reducing fuel PAH contents in commercial available diesel fuel the emissions of PAH to the environment will be reduced.

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Other fuel properties may also have an effect on emissions. Fuel density, for instance, may affect the mass of fuel injected into the combustion chamber, and thus the air/fuel ratio. This is because fuel injection pumps meter fuel by volume, not by mass, and the denser fuel contains a greater mass in the same volume. Fuel viscosity can also affect the fuel injection characteristics, and thus the mixing rate.

The corrosiveness, cleanliness, and lubricating properties of the fuel can all affect the service life of the fuel injection equipment -- possibly contributing to excessive in-use emissions if the equipment is worn out prematurely.

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Several generic types of diesel fuel additives can have a significant effect on emissions. These include cetane enhancers, smoke suppressants, and detergent additives. In addition, some additive research has been directed specifically at emissions reduction in recent years.

Cetane enhancers are used to enhance the self-ignition qualities of diesel fuel. These compounds (generally organic nitrates) are generally added to reduce the adverse impact of high aromatic fuels on cold starting and combustion noise. These compounds also appear to reduce the aromatic hydrocarbons' adverse impacts on HC and PM emissions, although PM emissions with the cetane improver are generally still somewhat higher than those from a higher quality fuel able to attain the same cetane rating without the additive.

No significant effect of ashless cetane improving additives could be detected on NOx or particulates.

Smoke suppressing additives are organic compounds of calcium, barium, or (sometimes) magnesium. Added to diesel fuel, these compounds inhibit soot formation during the combustion process, and thus greatly reduce emissions of visible smoke. Their effects on the particulate SOF are not fully documented, but one study has shown a significant increase in the PAH content and mutagenicity of the SOF with a barium additive. Particulate sulfate emissions are greatly increased with these additives, since all of them readily form stable solid metal sulfates, which are emitted in the exhaust.

The overall effect of reducing soot and increasing metal sulfate emissions may be either an increase or decrease in the total particulate mass, depending on the soot emissions level at the beginning and the amount of additive used.

Detergent additives -- often packaged in combination with a Cetane enhancer -- help to prevent and remove coke deposits on fuel injector tips and other vulnerable locations. By thus maintaining new engine injection and mixing characteristics, these deposits can help to decrease in-use PM and HC emissions.

A study for the California Air Resources Board estimated the increase in PM emissions due to fuel injector problems from trucks in use as being more than 50% of new-vehicle emissions levels. A significant fraction of this excess is unquestionably due to fuel injector deposits.

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  • There is a clear worldwide trend toward lower and lower levels of sulfur in diesel fuel. At a minimum, this reduces the particulate emissions from diesel vehicles; recent European studies indicate that for every 100-PPM reduction in sulfur, there will be a .16% reduction in particulate from light duty vehicles and a 0.87% reduction from heavy-duty vehicles.
  • Other diesel fuel properties such as volatility, aromatic content and additives can also have positive or negative effects on diesel vehicle emissions.
  • In addition to the adoption of mandatory limits, it has been shown that tax policies can be very effective in encouraging the introduction and use of low polluting diesel fuels.
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Topics
Conventional fuels > Diesel

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