Gasoline Vehicles and Fuels

The use of catalyst exhaust gas treatment required the elimination of lead from gasoline. Other gasoline properties that can be adjusted to reduce emissions include, roughly in order of effectiveness, sulfur level, vapor pressure, distillation characteristics, light olefin content, and aromatic content. Of these, sulfur is the most important in terms of the impact on advanced pollution control technology.

Gasoline is a complex mixture of volatile hydrocarbons used as a fuel in internal combustion engines. The pollutants of greatest concern from gasoline-fueled vehicles with regard to urban and regional pollution are CO, HC, NOx, lead and certain toxic hydrocarbons such as benzene.8 Each of these can be influenced by the composition of the gasoline used by the vehicle.

8 PM emissions from gasoline-fueled vehicles have traditionally not been regulated because their emissions are so much lower per mile driven than from diesel vehicles. However, it is now recognized that in many countries and cities where the gasoline vehicle population is much larger than the diesel population, they are a more important source. Also, health studies continue to point to lower and lower levels of ambient PM being acceptable from a public health standpoint. As a result, PM standards from gasoline-fueled vehicles may emerge.

The use of catalyst exhaust gas treatment required the elimination of lead from gasoline. This change, which started in the US and Japan during the 1970s and has now occurred throughout most of the world, has resulted in a dramatic reduction of ambient lead levels. Other gasoline properties that can be adjusted to reduce emissions include, roughly in order of effectiveness, sulfur level, vapor pressure, distillation characteristics, light olefin content, and aromatic content [10].

Modern gasoline engines use computer-controlled intake port fuel injection with feedback control based on an oxygen sensor to meter precisely the quantity and timing of fuel delivered to the engine. Control of in-cylinder mixing and use of high-energy ignition promote nearly complete combustion. The three-way catalyst provides greater than 90% reduction of carbon monoxide, hydrocarbons, and oxides of nitrogen. Designs for rapid warm-up minimize cold-start emissions. On-board diagnostic (OBD) systems sense emissions systems performance and identify component failures. Durability in excess of 160,000 km, with minimal maintenance, is now common in many countries.

Lead additives have been blended with gasoline, primarily to boost octane levels, since the 1920s [11]. Lead is not a natural constituent of gasoline, and is added during the refining process as either tetramethyl lead or tetraethyl lead.

Vehicles using leaded gasoline cannot use a catalytic converter because lead poisons the catalyst, and therefore have much higher levels of CO, HC, and NOX emissions. In addition, lead itself is toxic. Lead has long been recognized as posing a serious health risk. It is absorbed after being inhaled or ingested, and can result in a wide range of biological effects depending on the level and duration of exposure. Children, especially under the age of 4, are more susceptible to the adverse effects of lead exposure than adults.

Almost every country in the world has eliminated the use of leaded gasoline; the latest estimate is that only 17 countries continue to add lead.

6.5.2.2 Sulfur

Sulfur occurs naturally in crude oil. Its level in refined gasoline depends upon the source of the crude oil used and the extent to which the sulfur is removed during the refining process.

Sulfur in gasoline reduces the efficiency of catalysts designed to limit vehicle emissions and adversely affects heated exhaust-gas oxygen sensors. High sulfur gasoline is a barrier to the introduction of new lean burn technologies using DeNOX catalysts, while low sulfur gasoline will enable new and future conventional vehicle technologies to realize their full benefits. If sulfur levels are lowered, existing vehicles equipped with catalysts will generally have improved emissions.

Laboratory testing of catalysts has demonstrated reductions in efficiency resulting from higher sulfur levels across a full range of air/fuel ratios. The effect is greater in percentage for low-emission vehicles than for traditional vehicles. Studies have also shown that sulfur adversely affects heated exhaust-gas oxygen sensors; slows the lean-to-rich transition, thereby introducing an unintended rich bias into the emission calibration; and may affect the durability of advanced on-board diagnostic (OBD) systems.

The European Programme on Emissions, Fuels and Engine Technologies (EPEFE) study demonstrated the relationship between reduced gasoline sulfur levels and reductions in vehicle emissions. It found that reducing sulfur reduced exhaust emissions of HC, CO and NOX (the effects were generally linear at around 8-10% reductions as fuel sulfur is reduced from 382 ppm to 18 ppm)9. The study results confirmed that fuel sulfur affects catalyst efficiency with the greatest effect being in the warmed up mode. In the case of air toxins, benzene and C3-12 alkanes were in line with overall hydrocarbon reductions, with larger reductions (around 18%) for methane and ethane.

The combustion of sulfur produces sulfur dioxide (SO2), an acidic irritant that also leads to acid rain and the formation of sulfate particulate matter.

Certain other additives which are put into gasoline [generally to increase octane] can also affect vehicle emissions. Metallic-based, ash-forming, octane-enhancing additives such as Methylcyclopentadienyl manganese tricarbonyl (MMT) and ferrocene when added to gasoline will increase manganese-oxide and iron oxide emissions respectively from all categories of vehicles. Because of health concerns, participants in a workshop convened by the Scientific Committees on Neurotoxicology and Psychophysiology and Toxicology of Metals of the International Commission on Occupational Health recently published their conclusion that, "The addition of organic manganese compounds to gasoline should be halted immediately in all nations" [12]. The Health Effects Institute noted, "There is a large body of evidence that [13] under certain circumstances, manganese can accumulate in the brain [14-16], chronic exposure can cause irreversible neurotoxic damage over a lifetime of exposure [17], manganese may cause neurobehavioral effects at relatively low doses [5, 18], and these effects follow inhalation of manganese-containing particles."

Vehicle manufacturers have expressed concerns regarding catalyst plugging and oxygen sensor damage with the use of these additives which could lead to higher in-use vehicle emissions especially at higher mileage. The impact seems greatest with vehicles meeting tight emissions standards and using high cell density catalyst substrates.

A brief summary of the impact of various gasoline parameters on vehicle emissions is provided in Table 6.4.

9 The study found that the effects tended to be larger over higher speed driving than in low speed driving.

Table 6.4 Impact of gasoline composition on emissions from light duty vehicles

Gasoline No catalyst Early three way catalysts More advanced catalysts Comments

Table 6.4 Impact of gasoline composition on emissions from light duty vehicles

Gasoline No catalyst Early three way catalysts More advanced catalysts Comments

Leadf

Pb. HQ

CO. HC. NOx all increase dramatically as catalyst destroyed

Sulfur t

so2t

CO. HC. NOx all increase -15-20% SO, and SO, increase

MIL light may come on

(50-450 ppm)

incorrectly

Olefins t

Increased 1. 3 butadiene, increased HC reactivity for 03 formation. NOx. small increases in HC

Potential deposit buildup

for Euro 3 and cleaner

Aromatics f

Increased benzene in exhaust

Deposits on intake valves

Potential increases

HC t. NOxJ,. CO t HC. NOx. CO t

and combustion

in HC. NOx

chamber tend to

increase

Benzene f

Increased benzene exhaust and

evaporative emissions

Ethanol f up to

Lower CO. HC. slight NOx

Minimal effect with new vehicles equipped with oxygen sensors.

Increased evaporative

3.5% O,

increase(when above

adaptive learning systems

emissions unless RVP

2% oxygen content).

adjusted, potential

higher aldehydes.

effects on fuel system

especially acetaldehyde

components, potential

deposit issues, small

fuel economy penalty

MTBE t up to

Lower CO. HC. higher

Minimal effect with new vehicles equipped with oxygen sensors, adaptive

Concerns over water

2.7% O,

aldehydes, especially

learning systems

contamination

formaldehyde

Distillation

Probably HQ

HQ

characteristics

T50. T90f

(continued)

Table 6.4 (continued) Gasoline No catalyst

MMT t

Early three way catalysts

More advanced catalysts

Comments

Increased manganese emissions

RVP t

Deposit control additives f

Increased evaporative HC emissions

Possible

Likely catalyst

Potential HC, NOx emissions benefits

O, sensor and OBD may

Possible

Likely catalyst

O, sensor and OBD may

catalyst

plugging [PM

be damaged, MIL

plugging

and other

light may come on

[PM and

pollutants can

incorrectly

other

also increase.

pollutants

especially

can also

with catalyst

increase.

problems]

especially

with catalyst

problems]

Critical parameter for countries with high ambient temperatures Help to reduce deposits on fuel injectors, carburetors, intake valves, combustion chamber

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