1. Automobile Emission Control
Government regulations require that automobile manufacturers control the amount of carbon monoxide, hydrocarbons, and nitric oxide in the exhaust of vehicles.
Unburnt hydrocarbons from crevices and/or “cold” walls
NO from atmospheric N2 reaction with O at high temp
CO from incomplete combustion at fuel rich conditions
These issues are not as much fuel quality related, as inherent difficulties in combustion engine design.
“end-of-pipe” treatment is currently the best solution
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2. Exhaust Gas Composition
Spark-ignited gasoline engine emissions of CO, NO and hydrocarbons (expressed as hexane) as a function of intake air-to-fuel ratio in grams of air per gram of fuel.
HC and CO emissions decline with increasing O2 injection.
Conditions that maximize the flame temperature will generate high NO levels.
Need to consider fuel economy as well as pollution abatement.
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3. Automobile Emission Standards (U.S.)
Exhaust emission standards for automobiles and trucks were established in 1970 and amended in Below are shown the standards for automobiles.
Emission Standards (g/km)
Year Hydrocarbons CO NOx
Uncontrolled
Compliance is now required for 10 years or 160,000 km, with relaxed standards for the second 80,000 km of vehicle life. Note that testing is conducted throughout this mileage, and the vehicle must meet the standard at the end of the period.
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4. Emissions Testing
The 1975 Federal Test Procedure (FTP) is a driving cycle through Los Angeles over which total pollutant emissions are measured.
Cycle Length: miles
Cycle Duration: sec plus 600-second pause
Bag I (cold start) sec
Bag ,370 sec
Hot soak (run idle) sec
Bag 3 (hot start) sec
Average Speed: km/hr
Maximum Speed: km/hr
Number of hills:
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5. Example of an Emissions Test
CO and hydrocarbon tailpipe
emissions from a test vehicle
during a test cycle. Also shown
is the catalyst temperature and
speed during the cycle.
Catalyst mounted 1.2 m from
exhaust manifold.
As can be seen, the principal CO
and hydrocarbon emissions
occur catalyst warmup. Data for
NO production is not reported.
When hot, the catalyst is very
effective. In practice, one can expect
between 60 and 90% of the engine CO and hydrocarbon emissions, as measured over the whole test cycle, to be removed by the catalyst after 50,000 miles of use.
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6. Catalytic Converters for Emission Control
Up until about 1980, catalytic converters were used to control only CO and hydrocarbon emissions. The engine was run lean for performance reasons, and air was mixed with exhaust into the oxidizing converter.
Dispersed Pt and Pd in a 5:2 ratio on alumina was used for reasons of durability and activity (size of unit).
Comparison of relative activities of precious
and base metal catalysts
Reactant % CO % C2H % C2H6 Pd Pt Co
Au <0.2
MnO
CuO Fe
Reaction in Oxidizing Atmosphere at 300°C
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7. Three-Way Conversion (TWC) Catalysts
NOx emission standards created real design problems:
NOx reduction is most effective in the absence of O2
CO and HC abatement generally requires O2
To avoid a reducing reactor and an oxidizing reactor in series, effluent oxidations and reductions must be conducted in the same space.
Available reducing agents (CO, H2 and hydrocarbons) must react with available oxidizing agents (O2 and NO).
Fuel mixtures must be controlled to stay within a narrow TWC catalyst operating window
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8. TWC Catalysts
Rhodium and Iridium will catalyze the reaction of CO, H2 and hydrocarbons selectively with NO as opposed to O2, which is important when an engine is run under lean (fuel rich) conditions. Pt reduces NO to NH3, which is ineffective.
Oxidation in the presence of O2 is relatively simple, but in a rich condition Pt is found to catalyze CO oxidation through the water-gas shift reaction, and hydrocarbon oxidation by the steam reforming reaction.
North American TWC systems use approx 10:1 Pt to Rh, with about 0.05 troy oz/converter of Pt.
Stories regarding cat converter theft have been appearing in the recent news.
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9. Closed-Loop Fuel Metering System
TWC systems require the
air-to-fuel mixture charged
to the engine to be controlled
precisely if they are to
function effectively.
This is accomplished by
positioning an oxygen sensor
in the exhaust manifold to record
the discharge O2 content. Air flow to the engine is monitored as it responds to variations in throttle position and load.
Computer control regulates the fuel metered to the engine to control the reaction stoichiometry.
Nevertheless, a TWC catalyst “sees” an exhaust composition that fluctuates between rich and lean.
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10. TWC Catalyst Design: Monoliths
Design of the catalyst support is as important as fuel mixture control and catalytic chemistry. From the perspective of plug flow reactor design, key issues/design parameters are:
space velocities from 10,000 to 100,000 l/hr depending on engine size and mode of driving
minimal pressure drop for improved engine output
low thermal inertia for quick heat up
Materials design issues include:
stability at to temperatures up to 800°C
ability to withstand rapid heating
surface area, metal dispersion and resistance to sintering
mechanical properties sufficient to last 160,000 km of use.
Most catalytic converters are constructed from ceramic monolithic supports of a magnesium aluminum silicate.
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11. TWC Catalyst Design: Monoliths
Monolithic honeycombs are used
in the place of pellets. The most
common cell structure used has
62 cells/cm2 with a mm
thick wall to give a bulk
density of 0.4 g/cm3.
The ceramic has a relatively
low surface area for catalysis,
and a “washcoat” in the form of an
aqueous suspension of alumina and
other components is applied and fixed by calination.
The washcoat provides a means of dispersing precious metals to a high degree, while reducing coalescence sintering and acting as a sink for poisons. These proprietary application procedures are highly evolved.
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12. TWC Catalyst Design: The Start-up Problem
A poorly adjusted vehicle can fail
a modern emissions test within
the first 100 seconds of operation,
especially if the choke is needed
for starting.
In region I shown at right, the
catalyst is at too low a temperature
to be effective. The “light-off” temp
of today’s catalysts is 250 to 300°C,
shown here as the end of region II
where kinetic control is observed.
Various technologies are being developed to deal with this problem
exhaust gas igniter in the exhaust line
air-pumps to promote catalytic HC oxidation and light-off
electrically heated catalyst beds
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13. TWC Catalyst Design: Reaction Engineering
Design an oxidation catalytic converter for a four-stroke lawn mower engine.
What technical issues can you identify?
What technical information is needed to quantify and address these issues?
What established scientific and engineering principles apply to the problem, and how can solutions be generated?
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