5.5 Why control tailpipe emissions?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
5.5 Why control tailpipe emissions?
Tailpipe emissions were responsible for the majority of pollutants in the
late 1960s after the crankcase emissions had been controlled. Ozone levels
in the Los Angeles basin reached 450-500ppb in the early 1970s, well above
the typical background of 30-50ppb [74].
Tuning a carburetted engine can only have a marginal effect on pollutant
levels, and there still had to be some frequent, but long-term, assessment
of the state of tuning. Exhaust catalysts offered a post-engine solution
that could ensure pollutants were converted to more benign compounds. As
engine management systems and fuel injection systems have developed, the
volatility properties of the gasoline have been tuned to minimise
evaporative emissions, and yet maintain low exhaust emissions.
The design of the engine can have very significant effects on the type and
quantity of pollutants, eg unburned hydrocarbons in the exhaust originate
mainly from combustion chamber crevices, such as the gap between the piston
and cylinder wall, where the combustion flame can not completely use the HCs.
The type and amount of unburnt hydrocarbon emissions are related to the fuel
composition (volatility, olefins, aromatics, final boiling point), as well
as state of tune, engine condition, and condition of the engine
lubricating oil [75]. Particulate emissions, especially the size fraction
smaller than ten micrometres, are a serious health concern. The current
major source is from compression ignition ( diesel ) engines, and the
modern SI engine system has no problem meeting regulatory requirements.
The ability of reformulated gasolines to actually reduce smog has not yet
been confirmed. The composition changes will reduce some compounds, and
increase others, making predictions of environmental consequences extremely
difficult. Planned future changes, such as the CAA 1/1/1998 Complex model
specifications, that are based on several major ongoing government/industry
gasoline and emission research programmes, are more likely to provide
unambiguous environmental improvements. One of the major problems is the
nature of the ozone-forming reactions, which require several components
( VOC, NOx, UV ) to be present. Vehicles can produce the first two, but the
their ratio is important, and can be affected by production from other
natural ( VOC = terpenes from conifers ) or manmade ( NOx from power
stations ) sources [62,63]. The regulations for tailpipe emissions
will continue to become more stringent as countries try to minimise local
problems ( smog, toxins etc.) and global problems ( CO2 ). Reformulation
does not always lower all emissions, as evidenced by the following aldehydes
from an engine with an adaptive learning management system [55].
FTP-weighted emission rates (mg/mi)
Gasoline Reformulated
Formaldehyde 4.87 8.43
Acetaldehyde 3.07 4.71
The type of exhaust catalyst and management system can have significant
effects on the emissions [55].
FTP-weighted emission rates. (mg/mi)
Total Aromatics Total Carbonyls
Gasoline Reformulated Gasoline Reformulated
Noncatalyst 1292.45 1141.82 174.50 198.73
Oxidation Catalyst 168.60 150.79 67.08 76.94
3-way Catalyst 132.70 93.37 23.93 23.07
Adaptive Learning 111.69 105.96 17.31 22.35
If we take some compounds listed as toxics under the Clean Air Act, then the
beneficial effects of catalysts are obvious. Note that hexane and iso-octane
are the only alkanes listed as toxics, but benzene, toluene, ethyl benzene,
o-xylene, m-xylene, and p-xylene are aromatics that are listed. The latter
four are combined as C8 Aromatics below [55].
Aromatics FTP-weighted emission rates. (mg/mi)
Benzene Toluene C8 Aromatics
Gas Reform Gas Reform Gas Reform
Noncatalyst 156.18 138.48 338.36 314.14 425.84 380.44
Oxidation Cat. 27.57 25.01 51.00 44.13 52.27 47.07
3-way Catalyst 19.39 15.69 36.62 26.14 42.38 29.03
Adaptive Learn. 19.77 20.39 29.98 29.67 35.01 32.40
Aldehydes FTP-weighted emission rates. (mg/mi)
Formaldehyde Acrolein Acetaldehyde
Gas Reform Gas Reform Gas Reform
Noncatalyst 73.25 85.24 11.62 13.20 19.74 21.72
Oxidation Cat. 28.50 35.83 3.74 3.75 11.15 11.76
3-way Catalyst 7.27 7.61 1.11 0.74 4.43 3.64
Adaptive Learn. 4.87 8.43 0.81 1.16 3.07 4.71
Others 1,3 Butadiene MTBE
Gas Reform Gas Reform
Noncatalyst 2.96 1.81 10.50 130.30
Oxidation Cat. 0.02 0.33 2.43 11.83
3-way Catalyst 0.07 0.05 1.42 4.59
Adaptive Learn. 0.00 0.14 0.84 3.16
The author reports analytical problems with the 1,3 Butadiene, and only
Noncatalyst values are considered reliable. Other studies from the
Auto/Oil research program indicate that lowering aromatics and olefins
reduce benzene but increase formaldehyde and acetaldehyde [20]
Emission Standards
There are several bodies responsible for establishing standards, and they
promulgate test cycles, analysis procedures, and the % of new vehicles that
must comply each year. The test cycles and procedures do change ( usually
indicated by an anomalous increase in the numbers in the table ), and I
have not listed the percentages of the vehicle fleet that are required to
comply. This table is only intended to convey where we have been, and where
we are going. It does not cover any regulation in detail - readers are
advised to refer to the relevant regulations. Additional limits for other
pollutants, such as formaldehyde (0.015g/mi.) and particulates (0.08g/mi),
are omitted. The 1994 tests signal the federal transition from 50,000 to
100,000 mile compliance testing, and I have not listed the subsequent
50,000 mile limits [28,76,77].
Year Federal California
HCs CO NOx Evap HCs CO NOx Evap
g/mi g/mi g/mi g/test g/mi g/mi g/mi g/test
Before regs 10.6 84.0 4.1 47 10.6 84.0 4.1 47
add crankcase +4.1 +4.1
1966 6.3 51.0 6.0
1968 6.3 51.0 6.0
1970 4.1 34.0 4.1 34.0 6
1971 4.1 34.0 6(CC) 4.1 34.0 4.0 6
1972 3.0 28.0 2 2.9 34.0 3.0 2
1973 3.0 28.0 3.0 2.9 34.0 3.0 2
1974 3.0 28.0 3.0 2.9 34.0 2.0 2
1975 1.5 15.0 3.1 2 0.90 9.0 2.0 2
1977 1.5 15.0 2.0 2 0.41 9.0 1.5 2
1980 0.41 7.0 2.0 6(SHED) 0.41 9.0 1.0 2
1981 0.41 3.4 1.0 2 0.39 7.0 0.7 2
1993 0.41 3.4 1.0 2 0.25 3.4 0.4 2
1994 50,000 0.26 3.4 0.3 2 TLEV 0.13 3.4 0.4 2
1994 100,000 0.31 4.2 0.6 2
1997 LEV 0.08 3.4 0.2
1997 ULEV 0.04 1.7 0.2
1998 ZEV 0.0 0.0 0.0 0
2004 0.125 1.8 0.16 2
It's also worth noting that exhaust catalysts also emit platinum, and the
soluble platinum salts are some of the most potent sensitizers known.
Early research [78] reported the presence of 10% water-soluble platinum in
the emissions, however later work on monolithic catalysts has determined the
quantities of water soluble platinum emissions are negligible [79]. The
particle size of the emissions has also been determined, and the emissions
have been correlated with increasing vehicle speed. Increasing speed also
increases the exhaust gas temperature and velocity, indicating the emissions
are probably a consequence of physical attrition.
Estimated Fuel Median Aerodynamic
Speed Consumption Emissions Particle Diameter
km/h l/100km ng/m-3 um
60 7 3.3 5.1
100 8 11.9 4.2
140 10 39.0 5.6
US Cycle-75 6.4 8.5
Using the estimated fuel consumption, and about 10m3 of exhaust gas per
litre of gasoline, the emissions are 2-40 ng/km. These are 2-3 orders
of magnitude lower than earlier reported work on pelletised catalysts.
These emissions may be controlled directly in the future. They are currently
indirectly controlled by the cost of platinum, and the new requirement for
the catalyst to have an operational life of at least 100,000 miles.
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