Ozone-Forming Potential of Reformulated Gasoline (1999) / Chapter Skim
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4 Motor Vehicles as a Source of Ozone Precursors
4 Motor Vehicles as a Source of Ozone Precursors
Pages 73-100
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From page 73... ...
Moreover, these mobile sources distribute ozone precursors more broadly than stationary sources. This chapter reviews motor-vehicle emissions from light-duty vehicles (LDVs)
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From page 74... ...
. After adding the 5 repeat cycles following the hot soak, the entire FTP urban driving schedule covers Il.]
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From page 75... ...
FIGURE 11 The Federal Test Procedure urban driving schedule covers 11.1 miles of driving in 31 min. excluding the 10-min hot soak.
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From page 76... ...
Thus running-Ioss emissions depend on the driving schedule, ambient conditions, fuel volatility, type of vehicle, and condition of the control system. Resting-Ioss emissions include escape of fuel vapor by means of permeation of nonmetallic components of the fuel system while the vehicle is inoperative.
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From page 77... ...
(However, even in this case, use of the vehicle does in fact result in ozone precursor emissions whenever fossil fuels are burned to generate the electricity required for battery charging.) California allows manufacturers to mix conventional vehicles and members of the low-emissions family, within certain constraints, in a manner that forces a gradual reduction in fleet-average emissions in successive years.
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78 ._ ._ o In E o o to to Q In a)
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79 c~ c~ c~ c~ o o oo oo c~ c~ o o · .
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From page 80... ...
This should have a substantial air-quality benefit because these heavier vehicles accounted for nearly half of new-vehicle sales to the general public in 1998. MAGN ITU DES AN D TREN DS OF LIGHT DUTY VEHICULAR EMISSIONS To gauge the potential air-quality benefit from the use of reformulated gasolines, in general, and specific oxygenates within these gasolines, it is useful to review the magnitudes and trends in the emissions of VOC, CO, and NOx from motor vehicles and other sources The 55-year trends illustrated in Figures 4-2, 4-3, and 4-4 for VOC, NOx, and CO, respec TABLE 12 Automobile Emissions Standards, Tier I and Beyond (standards for 50,000 miles or 5 years (100,000 miles or 10 years)
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From page 81... ...
By 1995, this share had declined to 2~3% of the anthropogenic total, by which time industrial processes were estimated to account for 47% of 12 10 _ _ 8 0 6 4t 2,~ it- A i I ~ / / Or/ _ ~ ~7_, ~v _ o 1940 1 950 1 1 /~ 1 1 HIGHWAY VEHICL// ~ INDUSTRIAL _ r WASTE/RECYCLING OFF-HIGHWAY VEHICLES //r STATIONARY COMBUSTION in__ ~ -- -<~> -- c ~ -- - ~ -- to 1 1960 1 970 YEAR 1 1 1 1980 1990 2000 FIGURE 42 Estimated trends in VOC emissions from various types of sources in the United States. Emissions are presented in units ofteragrams (Tg)
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From page 82... ...
12 10 8 o in ~4 - i' ~ L' I' ., - CK , / Y_ O -O STATIONARY COMBUSTION ~ ~0 /HIGHWAY VEHICLES ~0~ OFF-HIGHWAY VEHICLES _ ~ INDUSTRIAL PROCESSES n ~O-n 1950 1 960 1980 1990 2000 YEAR FIGURE 43 Estimated trends in NOx emissions from various types of sources in the United States. Emissions are presented in units of teragrams (Tg)
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From page 83... ...
In contrast, the federal inventory for 1995 ascribes 2~3% of VOC emissions and 31% of NOx emissions to on-road vehicles (Davis 19983. The differences in the on-road-vehicle contribution (especially for NOx)
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From page 84... ...
the advent of more stringent standards for vehicles that gradually replace old vehicles built to more lenient emissions standards than current models; (2) maturation of new-vehicle emissions control hardware and software as field experience accumulated; and (3)
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From page 85... ...
, VOC emissions (B) , NOx emissions (C)
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From page 86... ...
In reality, the ozone reduction would be significantly smaller since the response of ozone concentrations to VOC reductions are generally less than linear. As discussed in more detail in Chapter 6, this shrinking contribution to ozone precursors from gasoline-powered motor vehicles makes it very difficult to discern the impact of reformulated gasoline on ambient ozone concentrations, let alone distinguish between the effects of different reformulated gasoline blends.
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From page 87... ...
87 3 ._ ._ V ._ O m E ~O 0 2 O Z ~ 2 ~ a ~ ~° ~ ~_ ~_ lo, E]
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From page 88... ...
However, it should be noted in this regard that projections of future mobile source emissions depend upon assumptions concerning trends in technology, economics, and human behavior and, as a result, are highly uncertain. INFLUENCE OF DRIVING PATTERNS ON EMISSIONS VARIABILITY As discussed above, regulation of LDV exhaust emissions is based on the FTP which is, in turn, built around the LA-4 driving schedule.
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From page 90... ...
, driving schedule US06, illustrated in Figure 4-7, will be run immediately following the current FTP to contribute exhaust to a fourth collection bag. Emissions from this bag will be weighted in a calculation together with those from the first three to arrive at the exhaust certification emission rate (grams per mile)
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From page 91... ...
and thus are not integrated with tailpipe emissions. Hot soaks and other important aspects of modern driving practices might currently be misrepresented in the overall certification procedure.
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From page 92... ...
Improved traffic management schemes, including variable message signing and increased highway automation, are expected eventually to improve this situation by smoothing traffic flow, but the implementation of such systems will be gradual. Meanwhile, the differences in power demand and the random distribution of individual vehicles operating in these "sawtooth" driving conditions render estimation of their aggregate · · .
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From page 93... ...
This ensures that a three-way catalyst maintains a high conversion efficiency for VOCs, CO and NOX concurrently, as illustrated in Figure 4-~. Typically, the on-board computer closes the mixture-ratio control loop using signals from an inlet airflow sensor, the fuel injectors, and an exhaust-gas oxygen sensor that indicates whether the air-to-fuel ratio is being maintained within the desired range.
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From page 94... ...
Similarly, the exhaustgas oxygen sensor must undergo a warm-up period before it becomes functional, although that problem is now typically minimized by electrically heating the sensor during starting. Unfortunately, the ineffectiveness of the cold catalyst coincides with the need for a rich air-to-fuel ratio to ensure engine starting, because the spark plug cannot ignite the air-to-fuel mixture unless the ratio of air to fuel vapor is within the flammability limits of the fuel.
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From page 95... ...
catalyst, there is strong motivation to decrease emissions during the cold start by shortening catalyst warm-up time. Conserving heat by insulating the pipe or pipes connecting the engine to the catalytic converter is now common practice.
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From page 96... ...
Previously, this technique was able to identify two common causes of malfunction in control systems: misfueling and tampering. Misfueling a catalyst-equipped car with leaded gasoline led to poisoning of the catalytic converter.
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From page 97... ...
Although not yet uniformly adopted, IM240 procedures are now employed in areas such as parts of Indiana, Arizona, and Colorado. Studies of on-road emissions performance have been conducted using a remote sensing technique that involves measuring the absorption of infrared light beamed across a single traffic lane behind a passing vehicle in normal traffic (Stephens et al.
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From page 98... ...
Comparing signals from oxygen sensors located upstream and downstream of the catalytic converter monitors its effectiveness. EPA expects
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From page 99... ...
Moreover, once the catalyst becomes operational, it most readily catalyzes oxidation of the VOC species with the highest reactivity, thus reducing the reactivity of the exhaust stream as well as the total mass of emissions. Oxygenated fuel makes additional oxygen available to the combustion process and thus, under appropriate circumstances, has the ability to decrease CO emissions.
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From page 100... ...
Evidence indicates that the proportion of driving time spent in transient driving maneuvers that depart significantly from those accounted for by the current FTP is small, but those departures can contribute a disproportionate share of tailpipe emissions. Of particular concern are emissions arising from cold starts and trips with multiple stopovers.
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