Diesel Vehicles and Fuels

Diesel engines emit more nitrogen oxides (NOx) and particulate matter (PM) than equivalent gasoline engines per mile driven. Reducing PM emissions tends to be the higher priority because ambient PM levels are often above WHO recommended levels and are responsible for hundreds of thousands of premature deaths each year. Diesel particulate (soot) is thought to be particularly hazardous and has been characterized as toxic or potentially toxic by the California Air Resources Board, EPA, the International Agency for Research on Cancer (IARC) the National Institute for Occupational Safety and Health (NIOSH) and others. NOx emissions are also important, however, since they cause or contribute to ambient nitrogen dioxide, ozone, and secondary PM (nitrates).5

5 Certain pollutants which are emitted from vehicles as gases undergo transformation in the atmosphere and are converted into particles. For example, some of the gaseous nitrogen oxides (NOx) emitted from vehicles chemically react with other gases and are converted into nitrates which contribute to urban PM air quality levels. Nitrates can account for as much as 20-30% of ambient PM in the US (although that fraction varies regionally).

Modest to significant NOX control from diesel engines can be achieved by delaying fuel injection timing and adding exhaust gas recirculation (EGR). Very high pressure, computer controlled fuel injection can also be timed to reduce PM emissions. (Modifying engine parameters to simultaneously reduce both NOX and PM is difficult and limited since the optimal settings for one pollutant frequently increases emissions of the other.) To attain very low levels of NOX and PM therefore requires exhaust treatment. Lean NOX catalysts, selective catalytic reduction, NOX storage traps with periodic reduction, PM filter traps with periodic burn-off, and oxidation catalysts with continuous burn-off are technologies that are being phased in at differing rates in various parts of the world. Japan for example, is tending to lead the world in the widespread use of PM filters on new diesel vehicles whereas Europe is tending to lag.6 A new type of diesel, the homogeneous charge compression ignition engine, provides another approach to reducing NOX and particulates that is receiving significant attention and may be introduced on some engines for at least portions of the engine map within a few years.

Diesel fuel is a complex mixture of hydrocarbons with the main groups being paraffins, naphthenes and aromatics. Organic sulfur is also naturally present at varying levels depending on the source of the crude oil. Additives are generally used to influence properties such as the flow, storage, and combustion characteristics of diesel fuel. The actual properties of commercial motor vehicle diesel depend on the refining practices employed and the nature of the crude oils from which the fuel is produced. The quality and composition of diesel fuel can significantly influence emissions from diesel engines.

To reduce PM and NOX emissions from a diesel engine, the most important fuel characteristic is sulfur because sulfur contributes directly to PM emissions and high sulfur levels precludes the use of or impairs the performance of the most effective PM and NOX control technologies. For the control of PM, most new vehicles in Japan and the US and a growing fraction in Europe are equipped with filters or traps which reduce over 90% of the particles. NOX adsorbers and Selective Catalytic Reduction systems are also starting to be introduced; NOX adsorbers are especially sensitive to sulfur levels in the fuel.

Sulfur occurs naturally in crude oil, and the sulfur content of diesel fuel depends on both the source of the crude oil and the refining process.

The contribution of the sulfur content of diesel fuel to exhaust particulate emissions has been well established with a general linear relationship between fuel sulfur levels and this regulated emission. Shown below (Fig. 6.3) is one estimate of this relationship provided by the U.S. EPA. (This figure shows only the sulfur-related PM and not the total PM emitted from a diesel engine.) An indirect relationship also exists as some emissions of sulfur dioxide will eventually be converted in the atmosphere to sulfate PM.7 Only a small fraction of the diesel

6 Some European countries are using tax incentives to accelerate the introduction of PM filters beyond the rate required by the Euro new vehicle standards.

7 Similar to the secondary transformation of NOX to nitrate discussed earlier.

Tons Per Million Gallons

Tons Per Million Gallons

Sulfur in Fuel (PPM)

Fig. 6.3 Tons of directly emitted PM from diesel fuels sulfur (Notes: PPM = parts per million. only particulate matter [PM] related to sulfur and not the total PM emitted from a diesel engine are reflected in this figure. Source: United States Environmental Protection Agency [US EPA])

Sulfur in Fuel (PPM)

Fig. 6.3 Tons of directly emitted PM from diesel fuels sulfur (Notes: PPM = parts per million. only particulate matter [PM] related to sulfur and not the total PM emitted from a diesel engine are reflected in this figure. Source: United States Environmental Protection Agency [US EPA])

fuel sulfur (1-2%) is converted to sulfate emissions in the exhaust with the remaining 98-99% emitted as gaseous SO2; a substantial fraction of the SO2 is lost to deposition with the remainder gradually converted in the atmosphere to sulfate PM.

Light duty diesel engines (<3.5 tons gross vehicle weight (GVW)) generally require oxidation catalysts to comply with Euro 2 or more stringent vehicle emission standards. Oxidation catalysts lower hydrocarbons, carbon monoxide, and particle emissions, typically removing around 30% of total particle mass emissions through oxidation of a large proportion of the soluble organic fraction. The conversion of sulfur in the catalyst reduces the availability of active sites on the catalyst surface and therefore reduces catalyst effectiveness. This catalyst deactivation is reversible through high temperature exposure - the sulfur compounds decompose and are released from the catalyst wash coat. However, due to generally low diesel exhaust temperatures, in many diesel engine applications the conditions needed for full catalyst regeneration may rarely be reached. High sulfur content in the fuel can also lead to the formation of sulfates in the converter which are then emitted as additional particles. Therefore, it is important to match fuel sulfur levels to the after-treatment technology present in the vehicle fleet.

To enable compliance with tighter particle emission standards for diesel vehicles, tighter limits on the maximum sulfur content of diesel fuel have been, or are being, introduced in many countries. While substantial reductions in particle emissions can be obtained without reducing sulfur levels, compliance with Euro 2 or tighter vehicle emission standards is generally not possible when fuel sulfur levels are greater than 500 ppm because of the relatively greater proportion of sulfates in the total mass of particle emissions.

In the case of Euro 3 and Euro 4 vehicle emission standards, even lower sulfur levels (350 ppm and 50 ppm, respectively) in diesel fuel will be required to ensure compliance with the standards. Complying with Euro 5 and 6 requirements or US Tier 2 standards will require maximum sulfur levels as low as 10-15 ppm. Apart from contributing to the effective operation of catalysts and reducing particle emissions, these further reductions in sulfur levels will enable tighter emission standards to be met by the use of next generation "de-NOX" catalysts, especially NOx adsorber systems. These are currently extremely sensitive to sulfur. An alternative emission control technology for Euro 5 or cleaner diesel vehicles is Selective Catalytic Reduction (SCR). These systems are not particularly sensitive to sulfur levels in fuel.

Sulfur content is also known to have an effect on engine wear and deposits, particularly under low temperature, intermittent operating conditions. Under these conditions there is more moisture condensation, which combines with sulfur compounds to form acids and results in corrosion and excessive engine wear. Generally lower sulfur levels lessen engine wear. With Euro 4+ or equivalent emission standards, the role of engine oil will be equally critical in ensuring sustained performance of engines/tail pipe devices. Low sulfur levels also allow the use of extended oil-change intervals reducing operating costs.

Diesel fuel has natural lubricity properties from compounds including the heavier hydrocarbons and organo-sulfur. Diesel fuel pumps (especially rotary injection pumps in light duty vehicles), without an external lubrication system, rely on the lubricating properties of the fuel to ensure proper operation. Refining processes to remove sulfur and aromatics from diesel fuel tend to also reduce the components that provide natural lubricity. In addition to excessive pump wear and, in some cases, engine failure, certain modes of deterioration in the injection system could also affect the combustion process, and hence emissions. Additives are available to improve lubricity with very low sulfur fuels and should be used with any fuels with 500-ppm sulfur or less.

A brief summary of the impact of various diesel fuel parameters on diesel vehicle emissions is provided in Tables 6.2 and 6.3.

In summary, from the standpoint of emission control technology, the most important diesel parameter is the sulfur content of the fuel, mainly since it allows for better after-treatment control technologies. Once standards sufficiently stringent to require oxidation catalysts are introduced, the sulfur content should be reduced to a maximum of 500 ppm; for the most advanced NOx and PM controls, the maximum should be 10-15 ppm sulfur. If sulfur levels are higher than these levels, the optimal performance of the pollution control systems will not be achieved and the in-use emissions will likely exceed standards. For cleaner vehicles, depending on the technology selected by the vehicle manufacturer, permanent damage could occur from the use of higher sulfur fuels.

Table 6.2 Impact of diesel fuel characteristics on light duty diesel vehicles

Diesel fuel Modest Strong Advanced characteristic controls controls controls Comments

Diesel fuel Modest Strong Advanced characteristic controls controls controls Comments

Sulfurt

S02, PMt If ox cat, If Filter,

If NOX adsorber used

S03, S02, 50 ppm

requires near zero

PMt maximum,

sulfur (<10 ppm)

10-15 ppm

With low S, use

better

lubricity additives

Cetanet

Lower CO, HC, benzene, 1,3 butadiene,

Higher white smoke

formaldehyde and acetaldehyde

with low cetane fuels

Density J,

PM, HC, CO, formaldehyde, acetaldehyde

and benzene J,, NOXt

Volatility

NOX, HC increase, PM, CO decrease

(T95 from

370 to 325 C)

PolyaromaticsJ

NOX, PM, formaldehyde and acetaldehydeJ

Some studies show that

but HC, benzene and COt

total aromatics are

important

Table 6.3 Impact of diesel fuel characteristics on heavy duty diesel vehicles Modest

Diesel controls Strong controls Advanced controls Comments

Sulfurt S02, PMt If ox cat, S03, If Filter, 50 ppm If NOx adsorber used

S02, PMt maximum, requires near zero

10-15 ppm better sulfur (<10 ppm)

With low S, use lubricity additives

Table 6.3 Impact of diesel fuel characteristics on heavy duty diesel vehicles Modest

Diesel controls Strong controls Advanced controls Comments

Sulfurt S02, PMt If ox cat, S03, If Filter, 50 ppm If NOx adsorber used

S02, PMt maximum, requires near zero

10-15 ppm better sulfur (<10 ppm)

With low S, use lubricity additives

Was this article helpful?

0 0
Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Post a comment