5a' Motor Vehicle Emission Control Technology

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5a. Motor Vehicle Emission Control Technology
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VEHICLE EMISSIONS

• Control technology is aimed at reducing the
  second term fuels, engines, vehicles etc.
• Urban and transportation planning addresses the
  first term housing density, location,
  transportation infrastructure
• the second term is relatively insensitive to the
  number of passengers in the vehicle
• Increasing vehicle occupancy helps reduce
  emissions mass transit, car pooling etc.

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Vehicle Emission Control Technology

• Technology forcing regulations
• Technologies
• Engine Emission Controls
• Evaporative Emission Controls
• On-board diagnostic systems

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Vehicle Emissions Control Technology
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Vehicle Emissions Control Technology
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Vehicle Emissions Control Technology
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Vehicle Emissions Control Technology
Projections for Southern California
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• Crankcase Ventilation
• First control system
• Prevents HCs from venting from cylinder and
  crankcase
• Takes escaping vapours and recycles them into
  engine

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CONTROL TECHNOLOGY SI EXHAUST EMISSIONS

• Air/Fuel ratio. CO and HC emissions increase as
  mixture gets richer in fuel (start and high power
  conditions), NOx emissions peak near
  stoichiometric ratio
• Fuel metering systems carburetors and fuel
  injectors (throttle body TBI, multi-port PFI,
  simultaneous or sequential)
• Electronic Control Systems adjust the air/fuel
  ratio based on the signal from an oxygen sensor
  in the exhaust

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EXHAUST GAS RECIRCULATION (EGR) - SI AND CI
ENGINES

• Dilutes Air/Fuel mixture with exhaust gases
  thereby reducing peak combustion temperatures and
  NOx formation
• There are limits to how lean an air-fuel-exhaust
  gas mixture can be for ignition
• Ignition systems (spark plugs etc.) and
  combustion chambers can be designed to improve
  performance with these lean mixtures

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Exhaust Gas Recirculation

• Returns 5 of Exhaust to Intake Charge
• Displaces Air/Fuel Charge Without Affecting Ratio
• Reduces Peak Temperature
• Reduces NOx Emissions

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OTHER COMBUSTION SYSTEM MODIFICATIONS - SI
ENGINES

• Optimal combustion timing combustion occurs
  rapidly, near and shortly after TDC
• Combustion timing - initial spark, ignition
  delay, flame speed
• Geometry and turbulence level in the combustion
  chamber
• Spark timing of 20-40 degrees crank angle before
  top ded center (BTDC) - function of engine speed
• High speeds and lean mixtures require higher
  spark advance

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OTHER COMBUSTION SYSTEM MODIFICATIONS - SI
ENGINES

• Cold-Start Emission Control - rich mixture and
  poor combustion, inactive catalyst (when present)
  result in high proportion of CO and HC emissions
  to take place during initial start of cold engine
  
• Automatic choke and inlet air heaters
• Idling emission control and fuel cut-off systems
  - CO and HC emissions are high at idle and
  deceleration
• reduce idle speed (e..g. from 900 to 600 rpm and
  adjust ignition timing to achieve stable
  conditions, cut off fuel supply during
  deceleration

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EXHAUST AFTERTREATMENT SI ENGINES

• Air injection - thermal oxidation of residual CO
  and HC with excess air introduced after the
  engine into the exhaust system, very temperature
  sensitive Minimum 600 C for HC, 700 C for CO
• Catalytic convertors can achieve conversion at
  lower temperatures 350 C
• Oxidation (two-way) catalyst - for HC and CO
• Oxidation-reduction (three-way) catalyst (TWC)
  for HC, CO, and NOx according to

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Function of Supplementary Air
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Typical Closed Loop Oxygen Sensor Voltage Signal
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EVAPORATIVE EMISSION CONTROL SI ENGINES

• Blowby and Crankcase emissions - fuel and partial
  combustion product molecules pass by the piston
  into the crankcase - recycled back to air intake
  manifold by Positive Crankcase Ventilation (PCV)
• Charcoal canister for capturing fuel tank,
  carburetor and miscellaneous evaporative
  emissions. Adsorption during hot-soak, diurnal
  heat build (breathing), refuelling periods,
  desorption into the air intake during engine
  operation (regeneration)

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Control of Refuelling emissions
Here, the onus is on the fuel dispensing pump
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Control of Refuelling emissions

• On-board refuelling vapour recovery (ORVR)
  systems put the onus on the vehicle.
• The carbon canister that was previously used to
  control the running and diurnal emissions now has
  to be bigger to deal with refuelling

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CONTROL TECHNOLOGY - CI

• PM and NOx more important in diesel exhaust than
  CO and HC, relative to gasoline exhaust
• A general trade-off between PM and NOx exists
  although reductions in absolute levels of both
  emissions have been achieved
• Emissions more strongly dependent on engine
  design - most emission reductions so far have
  been achieved through combustion modifications
  rather than exhaust aftertreatment in contrast
  to gasoline engine emissions

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DIESEL NOx FORMATION CHARACTERISTICS

• Most NOx formed during the high T and P premixed
  combustion phase
• NOx formation can be reduced effectively by
  reducing flame temperature
• delay combustion into the expansion phase
• cool the air charge going into the cylinder
• exhaust gas recirculation (EGR)

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DIESEL PM FORMATION CHARACTERISTICS

• Particulate Matter forms in fuel rich zones
  primarily during the mixing controlled combustion
  phase
• mostly an aggregate chain carbon core (soot)
• adsorbed hydrocarbons (aliphatic and
  polyaromatic) soluble organic fraction (SOF)
• significant fraction of SOF may come from
  lubricating oil
• Most of the PM formed during combustion is
  subsequently burned during the expansion stroke,
  the unburned part forms the emissions
• Sulfur in the fuel forms sulfuric acid which is
  later sampled as PM

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PARAMETERS AFFECTING DIESEL PM AND HC EMISSIONS

• Air/Fuel ratio, generally lean overall, to allow
  for complete combustion within limited time
  available for mixing
• Minimum ?? 1.5 for smoke point, smoke
  increases dramatically below this limit
• Rate of air-fuel mixing, can be enhanced by
  imparting a swirl to the injected fuel
• fuel injection timing
• compression ratio
• temperature and composition of charge in the
  cylinder

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TURBOCHARGING AND INTERCOOLING OF COMBUSTION AIR

• Turbocharging The mechanical energy in the
  exhaust can be used to compress the air prior to
  introduction to the cylinder
• Intercooling and aftercooling Turbocharging
  raises peak temperatures in the cylinder, cooling
  of compressed air before the cylinder controls
  this rise
• This can be realised by air-to-air or
  air-to-water heat exchangers

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DIESEL PM AND HC EMISSIONS

• Naturally aspirated engines (no turbocharging)
• air in cylinder independent of power output
• power is smoke limited, i.e. fuel can be
  increased until smoke point limit is reached
• Maximum fuel setting represents a compromise
  between smoke emissions and power output
• Tampering the maximum fuel setting can result in
  excessive PM emissions at high loads
  (accelerations and grade climbing)

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DIESEL PM AND HC EMISSIONS

• Turbocharged engines
• turbocharger increases air in cylinder with
  amount of fuel injected
• Power is usually limited not by smoke point but
  by turbochargher speed, mechanical, and thermal
  loading of engine components
• Low air/fuel ratios are normally not experienced
  under steady state conditions thus low steady
  state smoke even at full power
• Transient conditions may result in air/fuel
  ratios below smoke point and a puff of smoke
  because the turbocharger cannot respond fast
  enough to a change in fuel flow

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DIESEL PM AND HC EMISSIONS

• Turbocharged engines
• Acceleration smoke limiter (instead of the
  maximum power limiter on naturally aspirated
  engines)
• Setting on smoke limiter compromises between
  acceleration performance (drivability) and low
  smoke emissions

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Diesel aftertreatment

• Diesel oxidation catalyst (DOC)
• Diesel Particulate Filter (DPF, CRDPF)
• Selective Catalytic Reduction (SCR)
• Lean NOx Trap (LNT)

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DIESEL EXHAUST AFTERTREATMENT

• Flow through oxidation catalyst (two-way
  catalytic converter) for reduction of CO and VOC
  (80), and PM SOF (20-30), does not retain PM
• Trap oxidizer (Diesel particulate filter), reduce
  PM by 95, filter oxidation (regeneration)
  functions
• active and passive regeneration types
• Passive regeneration catalyst coated onto trap
  or added to fuel bring regeneration temperature
  down to 400-450 C which can be achieved in diesel
  exhaust
• Active regeneration monitors PM build-up on the
  trap and triggers regeneration by diesel fuel
  burning, electric heating, catalyst injection

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Diesel Particulate Matter (schematic
representation, after dilution with air)
Semi-Volatile Condensed Aerosol (VOCsulfateH2O
trace metal compounds)
Adsorbed Semi-Volatile Compounds (VOCsulfateH2O
trace metal compounds)
0.1 ?m
Elemental Carbon Agglomerate
Source EPA
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Lean NOx traps (LNTs, also known as NOx adsorber
catalysts).
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ULSD Standards, Walsh