Principles of Air Quality Management - Chapter 7 docx

184 Principles of Air Quality Management, Second EditionBlowby, which is the flow of fuel vapors past the cylinder walls into the crankcase,may be a significant source of uncontrolled hy

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The significance of these mobile sources is that they are powered primarily bygasoline-burning combustion systems Gasoline-powered vehicles are responsiblefor 78% and 92%, respectively, of the gallons of fuel consumed and miles driven.Diesel fuel oil accounts for the balance Individual emissions from each source aresmall; however, because of the large number of sources involved, the aggregateemissions are significant

As a consequence, we need to understand how the major types of engines thatdrive our mobile sources work, how they can be modified, and the most efficientways available of controlling the emissions that are ultimately released Equallyimportant is the effect of the fuels they burn on their respective criteria and toxic/haz-ardous air emissions Finally, a word needs to be said about alternatives to traditionalengines and fuels and some new approaches

ENGINES AND AIR POLLUTANT EMISSIONS

On a pollutant-specific basis, mobile sources account for varying percentages of aircontaminant emission Figure 7.1 indicates that these percentages vary from 77%

of the total national CO emissions to less than 30% of the particulate matter sions However, on a specific geographic basis, such as in California, mobile-sourceemissions are significantly higher The internal combustion engine (ICE) is the basicpower plant for these vehicles, whether spark ignited or compression ignited

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180 Principles of Air Quality Management, Second Edition

There are also a large number of nonvehicular ICEs that may play a significantpart in air quality management strategies It is estimated that between 7 and 8 millionoutboard engines are in use in the United States at the present time, as well as 8–12million engines in lawn mowers, leaf blowers, chain saws, and similar applications.The emissions from these devices have been largely uncontrolled up to the 21stcentury, and therefore their contribution, in addition to that from aircraft, mayrepresent a significant local effect on air quality The principles involved in under-standing air pollutant emissions from ICEs are the same as for mobile vehicularsources

Likewise, stationary-source ICEs have the same pollutant formation patterns;however, they are not subject to the changes in operating modes typical of a mobileICE While stationary gasoline- or diesel-poweredreciprocating ICEs have the sameemission patterns they are usually operated in a “cruise” mode rather than the cyclicpattern of mobile sources Mobile sources, for instance, change from idle to

TABLE 7.1 U.S On-Road Motor Vehicles — 2003

Vehicle Type

Number, millions

Gallons of Fuel, billions

VMT, billions

VMT = vehicle miles traveled

FIGURE 7.1 Mobile source emissions (US data from the 1999 EPA National Air Quality and Emissions Trends Southern California data from the 2004 California Air Resources Board Emission Inventory.)

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Mobile Sources and Control Approaches 181

acceleration to cruise to deceleration to stop and back again Thus, emissions utable to changes of operating mode are not significant for stationary ICEs but areimportant for mobile sources

attrib-The most significant difference between mobile sources driven by piston enginesand those driven by combustion turbines (Figure 4.6 seen earlier) is that the latterexperience continuous combustion The processes occurring in piston engines areessentially a series of explosions internal to the cylinder so that there are tremendousdifferences in temperature, pressure, gas composition, and volume occurring through-out its cycle

POLLUTANT FORMATION IN SPARK-IGNITED ENGINES

Formation of air pollutants in spark-ignited ICEs occurs in two regions: the bulkgas region and the boundary layer, or surface region Each region has uniqueproperties; therefore, the relative amounts of criteria pollutants and their formationmechanisms differ in each region These regions are illustrated in Figure 7.2

B ULK G AS P OLLUTANT –F ORMATION R EGION

The bulk gas reactions for a spark-ignited engine produce both fuel hydrocarbonsand CO and are generally formed by similar mechanisms Oxides of nitrogen for-mation occurs solely in the bulk gas reactions and is a function of many variables.Particulate matter is a significant contaminant for diesel ICEs and is addressedprimarily through back-end controls

Hydrocarbons and CO form through two mechanisms in the cylinder, depending

on whether they are in a fuel-rich or the fuel-lean region of the bulk gas at any point

in the thermodynamic cycle In a fuel-rich region, hydrocarbon fuel fragments and

CO will be formed as a result of a deficiency of oxygen to support completecombustion Such fuel-rich regions occur during start-up, deceleration, and warm-

up operating modes In the bulk gas in which the fuel/air mixture is in an extremely

FIGURE 7.2 Regions of ICE pollutant formation.

Spark plug Cylinder wall

Bulk gas reactions

Flame front Surface eﬀects Piston head

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fuel-lean (excess air) condition, oxidized carbon gases will be formed and remain

as a result of incomplete flame propagation In these regions, carbon monoxide levelsare high

Carbon monoxide, once formed, is fixed by the chemical kinetics of reactions

in the bulk gases Carbon monoxide is difficult to oxidize without high temperatures;therefore, its formation occurs as a result of thermal quenching. This effect is rapid

at high air-to-fuel ratio mixtures Hydrocarbon levels depend on the amount ofoxygen present The effect of thermal quenching for hydrocarbons is much moresevere for a given temperature gradient than it is for CO

Oxides of nitrogen formation is a function of many variables, including the gastemperature (for thermal NOx), the residence time at high temperature, and theavailability of excess oxygen The latter is a function of the air-to-fuel ratio.Most oxides of nitrogen are formed in the hot, turbulent gas regions of the flame.The thermal NOx formation process is termed the Zeldovich mechanism In thesehigh-temperature regions, molecular oxygen is dissociated into oxygen free radicals,which react very quickly with nitrogen to yield one NO molecule plus a nitrogenfree radical The nitrogen free radical then attacks an oxygen molecule to yield one

NO plus an oxygen free radical, and so on Equations (7.1–7.3) illustrate these steps

in the formation of NO by the Zeldovich mechanism

S URFACE P OLLUTANT –F ORMATION R EGION

The other major region of air pollutant formation is on the walls and surfaces of thecylinder Within the cylinder, a boundary layer of fuel and air will form along thesurface of the piston head and cylinder walls, which significantly influences emissionformation The cylinder of a spark-ignited ICE also serves as a very largeheat sink,

as well as providing high surface areas for physical or chemical reactions Thus, the walls and head of the cylinder and piston are a major source ofhydrocarbons, carbon monoxide, aldehydes, and other products of incomplete com-bustion (PICs) These result from the quenching of combustion resulting from heatsink temperature losses It has been estimated that approximately 1% of the entirefuel charge is not burned as a result of these wall effects

Deposits, as well as cracks and crevices in and on the surfaces of the cylinder,will enhance the trapping of fuel hydrocarbons in such deposits or crevices Depositsare formed in localized hot spots that cause metal corrosion Localized cold spotsmay condense out fuel fragments as tar These carbonaceous deposits act like a fuel

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vapor “sponge.” During the varying temperature regimes of the cycle, these spongesact to adsorb and desorb fuel components and products of incomplete combustion

As the piston moves up and down in the cylinder, a film of oil forms on thewalls, yielding a “wet” effect This wetted wall serves as an additional location forabsorption or desorption of fuel fragments These factors contribute to hydrocarbonand PIC emissions during operation

F OUR -S TROKE P OLLUTANT M ECHANISMS

An illustration of one cylinder typical of a gasoline-powered ICE during the four

“strokes” of normal operation is seen in Figure 7.3

During the compression stroke, when the fuel and air are in the chamber, oiland deposit layers absorb hydrocarbons Fuel and PICs (from the previous cycle)are forced into cracks and crevices in the cylinder surfaces as the pressure increases.During the combustion stroke, the pressure is still rising as the spark from thespark plug ignites the entire mixture As the flame front moves through the mixture,

NO forms in the high-temperature burning gas CO, if the mixture is fuel rich, will

be present in the high-temperature gases As a result of the increasing pressure atthis point in the cycle, unburned fuel will be further forced into crevices on thesurfaces of the piston head and exposed cylinder walls

During the “power” expansion stroke, the piston is forced downward, and thevolume begins increasing in the chamber The temperature begins dropping, and NOformation is frozen as the burned gases cool This is followed by a freezing of the

CO combustion chemistry Along the walls and from crevices in the cylinder, anoutflow of hydrocarbon fuel fragments from those crevices begins Some portions

of those hydrocarbons will form CO and products of incomplete combustion.During the exhaust portion of the cycle, the pressure in the cylinder drops toslightly above atmospheric, and wall effects begin to dominate Deposits, cracks,and crevices desorb additional hydrocarbons, fuel fragments, and PICs Desorption

of fuel fragments from the oily layers along the walls of the cylinder occur at thispoint The cylinder head scrapes more fuel from the wall layers and desorbs thoseinto the exhaust gases before the closure of the exhaust valve

From this we may understand some of the basics of air pollutant formation in

an ICE These steps in the process are a function of the complex interactions ofpressure, temperature, volume, combustion kinetics, and mechanical effects in aspark-ignited gasoline-powered engine

L ESSER S OURCES OF C ARBON G AS P OLLUTANT E MISSIONS

The effects of wear and aging on engines may contribute significantly to hydrocarbonand CO emissions These are partly because of the formation of surface deposits orcorrosion building up over the course of time Poorly seated valves and rings mayalso cause leaks of fuel or fuel fragments into the exhaust Likewise, poor or faultyignition generates pure hydrocarbon emissions during cranking, and scoring andcrevice formation on aging engine surfaces lead to high emissions

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Blowby, which is the flow of fuel vapors past the cylinder walls into the crankcase,may be a significant source of uncontrolled hydrocarbon emissions In older, uncon-trolled vehicles, these fuel vapors may account for 20%–25% of the total hydrocarbonemissions Newer vehicles recirculate crankcase gases through positive ventilationsystems back into the combustion air intake for reburning

Scavenging losses occur when both intake and exhaust valves are open at thesame time In a two-stroke engine, where both valves must be open for the engine

FIGURE 7.3 Combustion in an automobile engine (one cylinder of a typical automobile engine shown).

Air Carburetor Fuel-air mixture Fuel

Cylinder (combustion chamber)

(2) Compression stroke

(3) Power stroke (4) Exhaust stroke

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to operate, scavenging losses are a major source of hydrocarbons, as the seating ofthe valves and the design of the combustion chamber require both to be open duringportions of the cycle For a four-stroke engine, the scavenging losses occur when asupercharged or turbocharged system is in operation This causes portions of thefuel–air mixture to pass directly from the intake to the exhaust

F UEL C OMPOSITION AND E XHAUST E MISSIONS

Figure 7.4 illustrates the variety of air pollutants (organic compounds and fuelfragments) in the exhaust of a spark-ignited gasoline-powered four-stroke engine.The four categories are the paraffins (saturated hydrocarbons), the aromatics orunsaturated ring structures, the olefins or double-bonded carbon systems, and theoxygenates or fuel fragments containing oxygen These categories are charted byspecies percentage for each carbon number represented The actual number ofindividual compounds in gasoline and its exhaust ranges into the hundreds of discretechemical species This figure also illustrates the typical average gasoline composi-tion, listed by species Oxygenates are the partially burned fragments of fuel left inthe exhaust Interestingly enough, the largest single oxygenate is the single-carbonatom species formaldehyde The highest two-carbon atom compound is acetaldehyde

A comparison of the fuel composition with exhaust hydrocarbon compositiondemonstrates the strong correlation between the exhaust species distribution and thefuel species For paraffins and olefins, there is also a “downshift” to lower carbonnumbers, representing fuel fragments This indicates that, except for the oxygenates,the exhaust hydrocarbon emissions are essentially components of the original gasoline

DIESEL IGNITION EMISSION CHARACTERISTICS

The significant differences between a diesel-ignited system and a spark-ignitedsystem are that the diesel system operates at extremely high pressures (approximately

100 atmospheres) and high (lean) air-to-fuel ratios, producing high excess air in thechamber The bulk gas temperature range is about the same

One of the more significant differences, though, between diesel- and ignited systems is that current diesel engines operate by injecting a measured amount

spark-of oil into the cylinder at high compression With oil injection, the mixing andevaporation of fuel components into the gas phase is significantly different from acarbureted system, which uses gasoline or other low–molecular weight fuels Thesignificance of the liquid fuel spray cannot be overestimated, as it strongly affectsthe pattern of air pollutants formed in the diesel system

Another difference is the air-to-fuel ratio in the diesel combustion chamber Thisair-to-fuel ratio varies spatially throughout the combustion zone as a result of thefuel spray Air swirl will also influence the geometry of the flame pattern Liquid fuelspray, air-to-fuel ratio, and air swirl interact under the high-pressure regimes toinfluence the combustion contaminants associated with diesel emissions As the jet

of fuel is injected into the combustion chamber, the high temperature and air swirlcause the formation of a fan-shaped pattern of evaporating fuel droplets and vapors

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FIGURE 7.4 Exhaust gas distributions versus fuel composition.

20

10 15

5 0

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

C6

14 12 10 8 6 4 2 0

1.0

14 12 10 8 6 4 2 0

0.5 0.4 0.3 0.2 0.1 0.0

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As a consequence, four major regions have been identified in the ignited system Air contaminant generation varies significantly in each zone Theseregions are the fuel spray edge, the flame zone, the core, and the droplet or impinge-ment zone These four zones are illustrated in Figure 7.5

compression-At the edge of the spray, the air-to-fuel ratio is too lean for flame propagationand good combustion because of the high excess air This is a zone of formation forcarbon monoxide and other PICs, as well as gaseous hydrocarbon fuel fragments

At low or idle conditions, this zone is relatively large, and therefore emissions ofhydrocarbons are greater However, as the pressure and temperature increase withincreasing load, this zone decreases, and the overall emission of hydrocarbons, CO,and PICs will decrease

The flame zone is operating at near stoichiometric conditions at the highest flametemperatures Therefore, this area tends to form high quantities of oxides of nitrogenbut very little CO or hydrocarbons Also, this zone is relatively long lasting, andtherefore more NOx is generated as a result of the relatively longer time that theburning parcels exist at those high temperatures

The spray core is that zone in which droplet evaporation is the predominantmechanism The combustion in this zone is limited because of the relatively slowdiffusion of combustible vapors from droplets into the available surrounding airmass As a result of the diffusion-controlled nature of this combustion, the amount

of swirl air interacting in this zone will significantly affect the pollutant mix

At low loads, where a relatively smaller fuel is available, some oxides of nitrogenwill be formed here because of the amount of excess air available to the diffusionflame At higher loads, however, this zone will be responsible for CO, hydrocarbons,and PICs, as well assoot or carbonaceous particles Diesel particulates, once formed,are not easily oxidized and will therefore be emitted Soot particulate is a significantproblem with diesel fuel and is listed in California as a toxic air contaminant.The fourth zone is the location at which large fuel droplets are responsible formost of the pollutant generation This occurs in two different portions of the com-bustor The first portion is where the relatively larger droplets occur at the end ofthe fuel injection, close to the injection port These large drops form as a result ofreduced pressure at the end of the injection and the higher combustion chamberpressure

FIGURE 7.5 Diesel fuel liquid spray pattern.

Injector

3 Core 4 Impingement area

Piston head

2 Flame zone

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These relatively large drops are also responsible for diffusion-controlled bustion and, likewise, form soot particles on the evaporation of volatile components.Hydrocarbon and PICs are also found in the emission Finally, some of these largedrops may impinge on the cylinder head and are responsible for additional hydro-carbon and PIC emissions as well as soot formation and carbonaceous deposits

com-P OLLUTANT P ATTERNS

A comparison of the pollutant patterns of gasoline (Otto cycle) engines and dieselengines is shown in Table 7.2 for two operating modes In this table, we see thedifferences in emission patterns under idle conditions and under normal cruiseconditions for the two engine types

For oxides of nitrogen under cruise conditions, a diesel cycle system producessignificantly higher overall NOx emissions than a spark-ignited or Otto cycle engine

At idle conditions, as noted above, the NOx emissions are much lower than for thespark-ignited systems

With respect to hydrocarbons and carbon monoxide, spark-ignited emissions aresignificantly higher — in some cases by an order of magnitude — than those fromdiesels because of the excess air and compression conditions noted earlier for diesels.Carbonaceous particulate formation for diesels is significantly greater than for spark-ignited systems because of the higher molecular weight and oily nature of diesel fuels

H YDROCARBON E MISSIONS FROM T RIP C YCLES

Quite apart from the comparisons of the two engine types is the influence of “coldstart” and “hot soak” hydrocarbon emissions as opposed to those from cruise or

TABLE 7.2 Typical Internal Combustion Engine Cylinder Exhaust Concentrations

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running exhaust In Table 7.3, the influence of trip length on uncontrolled bon emissions is summarized Cold start emissions are fairly constant at about 9 g

hydrocar-of fuel hydrocarbons per start The end-hydrocar-of-trip period or hot soak generates roughly

2 g of hydrocarbons for the average car equipped with a catalyst Cold start and hotsoak emissions are termed standing emissions The cruise emissions are approxi-mately 0.55 g per mile and are thus a function of the total miles traveled

The significance of these differences in running versus standing emissions is inthe control approach taken Implementing emission reductions in the first 2–3 min-utes of operation, as well as during the hot soak or cool-down at the end of the trip,would significantly lower overall air quality effects Thus, air quality managementstrategies will have to identify techniques for controlling evaporative hydrocarbonemissions as a function of the number of individual trips

These standing versus running emissions appear to explain some studies thatshow greater frequencies of elevated ozone levels on weekends when, presumably,there are a greater number of short trips but a lesser number of total miles traveled

in areas such as Los Angeles

E NGINE T HERMODYNAMIC C YCLES

The purpose of any mobile source of emissions is to provide useful work to drive

a vehicle, whether automobile, truck or airplane, to a different location To plish this, useful work must be extracted from the engine

accom-Useful work from a system is described by its thermodynamic cycle The threecycles representing mobile sources are illustrated in Figures 7.6,7.7, and 7.8 by theirrespective pressure–volume diagrams In piston engines, the cycles are represented

by what happens to the fuel, air, and combustion byproducts mixture in the cylinder.The four steps of the cycle are compression, ignition, expansion or work-producing(step), and exhaust These four steps are the same whether it is a two-stroke or afour-stroke engine (The number of strokes refers to the number of times the pistontraverses the length of the cylinder for each power step.)

Figure 7.6 refers to the spark ignition cycle, commonly called the Otto cycleafter the German engineer who built the first successful operating spark-ignited

TABLE 7.3 Influence of Trip Cycles on HC Emissions*

* Grams total hydrocarbons.

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gasoline engine This figure illustrates each of those steps on an arbitrary

pres-sure–volume diagram Beginning in the lower right-hand corner, the fuel and air are

compressed to much smaller volume with a slight increase in pressure, at which

point ignition occurs From that point, combustion begins, which dramatically

increases the pressure in the system with only slight variations in volume As the

combustion proceeds, expansion of the hot combustion gases increases the volume

and decreases the pressure as useful work is accomplished The last step of the cycle

is when the combustion gases are exhausted from the cylinder This brings us to the

point of return at which the cylinder is ready for a fresh charge of fuel and air

Figure 7.7 illustrates the diesel cycle in which much higher compression of the

air in the cylinder occurs At a point near the maximum pressure, liquid fuel is

FIGURE 7.6 Otto cycle, compression ratio of 9:1.

FIGURE 7.7 Diesel cycle, compression ratio of 19:1.

l

Ex pa

nsion

Ac tu

Ignition Compression

Expansion Combustion

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injected (sprayed) into the cylinder near the final portion of the compression stroke,

after which gas combustion occurs by auto ignition The hot gases provide work to

the drive shaft by expansion against the piston This increases the volume and lowers

the pressure in the cylinder until the gases are exhausted

The thermodynamic cycle for combustion turbines is seen in Figure 7.8 This is

the Brayton cycle and operates at significantly lower pressures In this system,

compression is accomplished by compressor blades, and combustion occurs via a

standing flame at nearly a constant pressure Exiting the combustor can, the hot

gases expand and drive turbine blades (which power the compressor shaft) and exit

the exhaust, providing thrust for the aircraft Figure 4,7, seen earlier, is a schematic

of a combustion turbine

A summary of the significant differences in ignition source, pressure, air-to-fuel

ratio, peak temperatures, and peak pressure is seen in Table 7.4 These are useful in

evaluating performance-based emission differences

FIGURE 7.8 Brayton cycle, pressure ratio 4:1.

TABLE 7.4

ICE Operating Parameter Comparisons

Compression ratio 6 to 11:1 14 to 22:1 N.A.

Fuel delivery Aspirated or injected Injected Injected

Air-to-fuel ratio Near ideal Lean Very lean

Peak temperature (˚R) 4500–5000 4500–5000 2250–2600

* Reciprocating piston.

** Continuous, combustion gas turbine.

N.A = Not applicable.

Combustion

Ac tu

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HYBRID INTERNAL COMBUSTION ENGINES

Research into hybrid ICEs has continued to find more efficient engines with lower

air-contaminant emissions resulting from operating fossil fuels

In the last few years, a new syncretistic engine design has been developed called

the HCCI, or homogeneous charge compression ignition engine Earlier names

included the active thermo-atmosphere combustion engine and the hot vapor

injec-tion engine

The HCCI is a relatively new combustion technology It is a hybrid of the

traditional spark ignition (Otto engine) and the compression ignition engine (diesel)

Unlike the spark-ignited or diesel engine, however, HCCI combustion takes place

spontaneously and homogeneously, without flame propagation This alternative

com-bustion process uses lean mixtures ignited without a spark or flame front, which

eliminates heterogeneous air/fuel mixture regions In addition, HCCI operates as a

fuel-lean combustion process These conditions translate to a lower local flame

temperature, which lowers the amount of NOx produced in the process

To achieve the homogeneous charge, fuel and air are well mixed throughout

every region of the cylinder before combustion There is theoretically no localized

high-temperature flame front; instead, there are hundreds of evenly distributed points

of ignition, indicating spontaneous combustion of the gas volume in the cylinder

High exhaust-gas dilution and lean mixtures greatly reduce peak temperatures and

heat-transfer losses, which further reduces NOx formation

As a practical matter, HCCI works better in theory than in practice at this time

Achieving sufficient thermal energy late in the compression stroke to trigger the

required auto-ignition consistently is an ongoing challenge Using a compression

ratio beyond 12:1 causes severe knock during full-load operation Other concerns

are excessive rates of pressure rise and the combustion-generated noise associated

with very high compression

A more practical avenue being pursued by researchers is using high levels of

exhaust gas recirculation by various means to supply the desired thermal energy

needed for auto-ignition One technique, called recompression, traps exhaust in the

cylinder by closing the exhaust valve early Another approach (rebreathing) captures

exhaust after it has left the cylinder and draws it back into the chamber

Unfortunately, power density is lower with HCCI combustion engines than with

gasoline engines running with stoichiometric air/fuel ratios A greater problem,

however, is that HCCI operation is not yet possible during engine warm-up, at very

light loads, or at idle because exhaust energy is too low to trigger auto-ignition

during these conditions Another concern is that peak combustion temperatures well

below the threshold of NOx formation result in incomplete oxidation of the fuel and

air As a result, HC and CO emissions may be 50% higher than today’s normal

spark-ignited gasoline engines A third dilemma is that the exhaust temperature is

too low for proper catalytic converter function

While the HCCI approach may represent the next generation of practical

com-bustion engines, it is believed that it will take 5–10 more years of research before

HCCI systems become a practical reality

Tiêu đề	Mobile Sources and Control Approaches
Trường học	Taylor & Francis Group, LLC
Chuyên ngành	Air Quality Management
Thể loại	Chương
Năm xuất bản	2006
Thành phố	London

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Số trang	29
Dung lượng	1,27 MB