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
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 (Ford)
• air-pumps to promote catalytic HC oxidation and
  light-off
• electrically heated catalyst beds

7
Catalytic Reactions 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.
• Oxidation reactions
• CmHn 2(mn/4) O2 ? n/2 H2O m CO2
• CO 1/2 O2 ? CO2
• CO H2O ? CO2 H2
• 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
8
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 requires O2
• To avoid a reducing reactor and an oxidizing
  reactor in series, we require both oxidation and
  reduction reactions in the same space. For this
  to occur, a catalyst must react all potential
  reducing agents (CO, H2 and hydrocarbons) with
  all oxidizing agents (O2 and NO).
• TWC reactions at stoichiometric A/F mixtures
• CO NO ? 1/2 N2 CO2
• CmHn 2(mn/4) NO ? (m n/4) N2 n/2 H2O m
  CO2
• H2 NO ? 1/2 N2 H2O
• CO H2O ? CO2 H2 (water-gas shift
  rn)
• CmHn m H2O ? m CO (m n/2) H2 (steam
  reforming)
• Other NO-reduction reactions
• 2 NO 5 H2 ? 2 NH3 2 H2O
• NO hydrocarbons ? N2 H2O CO2 CO NH3

9
TWC Catalysts

• The operating window for efficient TWC
• catalyst operation is narrow and the fuel mixture
  must be carefully controlled.
• Rhodium and Iridium are capable of catalyzing the
  reaction of CO, H2 or hydrocarbons selectively
  with NO as opposed to O2, which is important
  under lean 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 and hydrocarbon oxidation through the
  water-gas shift reaction and steam reforming
  reaction, respectively.
• North American TWC systems use approx 101 Pt to
  Rh, with about 0.05 troy oz/converter of Pt.

10
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.
• The TWC catalyst sees an exhaust composition
  that fluctuates between rich and lean, and must
  be capable of dealing with a range.

11
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.

12
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. The application procedures highly
  evolved and are proprietary.