Automobile Emission Control

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

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.

3
Automobile Emission Standards (U.S.)

• Exhaust emission standards for automobiles and
  trucks were established in 1970 and amended in
  1990. Below are shown the standards for
  automobiles.
• Emission Standards (g/km)
• Year Hydrocarbons CO NOx
• Uncontrolled 6.56 52.2 2.6
• 1975 0.94 9.4 1.9
• 1980 0.25 4.4 1.2
• 1990 0.25 2.1 0.6
• 1995 0.19 2.1 0.2
• 2004 0.08 1.1 0.1
• 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.

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 11.115 miles
• Cycle Duration 1877 sec plus 600-second
  pause
• Bag I (cold start) 0-505 sec
• Bag 2 505-1,370 sec
• Hot soak (run idle) 600 sec
• Bag 3 (hot start) 0-505 sec
• Average Speed 34.1 km/hr
• Maximum Speed 91.2 km/hr
• Number of hills 23

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.

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 52 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 1 CO
0.1 C2H4 0.1 C2H6 Pd 500 100
1 Pt 100 12 1 Co203
80 0.6 0.05 Au
15 0.3 lt0.2 MnO2 4.4
0.04 CuO 45 0.6 Fe203
0.4 0.006 Reaction in Oxidizing Atmosphere at
300C
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

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 101 Pt to
  Rh, with about 0.05 troy oz/converter of Pt.
• Stories regarding cat converter theft have been
  appearing in the recent news.

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.

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 800C
• 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.

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 0.152 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.

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 todays catalysts is 250 to 300C,
• 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

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?