EMISSION FACTORS AND EMISSION MODELLING

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EMISSION FACTORS AND EMISSION MODELLING

• Emission modelling from motor vehicles involves
  the consideration of different types of vehicles
  and their driving conditions to arrive at a grand
  total
• The km travelled comes from Transportation
  Demand Models
• The nature of the km travelled (e.g. speed and
  type of road) also has a bearing on the
  emissions/km
• Emission models attempt to estimate average g/km
  emissions from the in-use fleet of vehicles

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TRANSPORTATION DEMAND MODELLING

• Questions of interest Why, Where, How
• Purpose of a trip (school, work, shopping,
  recreation etc.)
• Destination
• How to get there (walk, ride a bike, drive car,
  take bus, etc.)
• How to get there (what route to take)
• The aggregation of answers to these questions by
  all the residents of a region can predict the
  volumes of traffic on the various elements of a
  an existing transportation system

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TRANSPORTATION DEMAND MODELLING

• Given
• a particular transportation network
• the volumes of traffic between zones
• We can estimate
• average trip lengths and distribution
• average speed and distribution
• This information can be used
• to estimate emission factors
• to develop an emission inventory

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TRANSPORTATION DEMAND MODELS

• EMME/2
• MINUTP
• MICROTRIPS
• TMODEL2
• TRANPLAN

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POLLUTANTS AND PROCESSES

• Exhaust
• CO, HC, NOx, PM g/km,
• cold transient, hot transient, hot stabilized,
  composite
• Evaporative, HC
• Diurnal g/test
• Hot soak g/test
• Running g/km
• Resting g/hr
• Refuelling g/L
• Reconciliation requires info/assumptions about
  number of trips, distance travelled etc.

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EMISSION MODELLING PRINCIPLES - EXHAUST EMISSIONS

• For a single vehicle, instantaneous exhaust
  emissions are governed essentially by the
  Air/Fuel ratio.
• Air/Fuel ratio dependent on mode of operation
• idle
• cruise (speed)
• acceleration
• deceleration
• A driving cycle attempts to mimic variations
  during a typical trip by combining these modes.
• 3 phase FTP, 11 miles travelled at an average
  speed of 21.2 mph (1874 seconds, 30 min)

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PROBLEM

• The same 11.04 miles and modal mix can be
  travelled at a different average speed by scaling
  instantaneous speeds.
• A driving style of faster acceleration/deceleratio
  n and longer idle and/or cruise times can give
  the same average speed but obviously very
  different emissions.
• Solution introduce Speed correction factors and
  cycle correction factors to apply to emission
  rates from the FTP
• These will require further chassis dynamometer
  testing.

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PROBLEM

• The emissions measured over the 11 miles of the
  FTP are from a particular combination of times
  spent in the cold start (505 s) - hot transient
  (864 s) - hot start (505 s) states of the
  engine/catalyst.
• For trips of different lengths the ratio of these
  three phases will be different.
• Solution Use individual emission factors from
  each phase of the test if we can determine the
  fraction of trips spent in these states in the
  real world.

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EXHAUST EMISSIONS ALSO AFFECTED BY

• Fuel
• Combustion and emission control technology on the
  vehicle
• Vehicle age (odometer reading) and maintenance
  condition
• Ambient conditions, temperature, humidity,
  elevation (pressure)

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EMISSION MODELLING PRINCIPLES - EVAPORATIVE
EMISSIONS

• Evaporative emissions are governed essentially by
  the temperature and volatility of the fuel.
• Fuel temperature is affected by ambient
  temperature (refuelling and diurnal losses) and
  engine operation (hot soak and running losses)

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PROBLEM

• Diurnal and hot soak evaporative emissions are
  not directly related to distance travelled. In
  fact diurnal emissions increase the more days
  that a vehicle stays idle.
• Running evaporative emissions are related to
  distance travelled, but not directly
  proportional. The fuel gets warmer in longer
  trips but reaches some steady state value after a
  while.
• Solution estimate evaporative emissions per
  distance travelled on the basis of expected
  travel behaviour (number of trips, length of
  trips, number of days idle etc.)

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• For a fleet of n vehicles, multiply the above
  problems by n!
• n 106
• Solution divide the fleet of vehicles into
  categories of similar vehicles. Essentially
  governed by the regulations that have been
  imposed at production time.
• Assume vehicle classes behave similarly.

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CLASSES OF VEHICLES

• Light duty
• LDGV, LDGT1, LDGT2, LDDV, LDDT
• Heavy Duty
• HDGV, HDDV
• Motorcycles
• MC
• Vehicle classes defined by
• Gross Vehicle Weight Rating GVWR
• (Loaded Vehicle Weight LVW)
• Curb Weight
• Adjusted Loaded Vehicle Weight, ALVW

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POLLUTANT CHARACTERIZATION

• PM
• PM10, PM2.5, chemical nature (C, SOF, PAH etc.)
• HC
• THC, Total hydrocarbons
• NMHC, non-methane hydrocarbons
• VOC, (NMHC - ethane alcohols carbonyls)
• NMOG, non-methane organic gas
• (NMHC alcohols carbonyls)
• NMHCE, NMHC equivalent, representing all the
  carbons but with a H/C ratio equal to that of the
  HC vapour.

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INCREASE OF EMISSIONS WITH AGE
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AVERAGE EMISSION FACTORS BY VEHICLE CLASS

• Vehicles in one class may be of different ages
  (by model year), odometer readings, and operating
  at different modes (cold start, hot stabilized,
  hot start)
• To arrive at an average emission factor, we need
  to know (estimate, model) information relating
  to
• combustion and emission control technology (hence
  regulated emission levels) penetration rate by
  year
• Vehicle age distribution and VKT distribution by
  age
• (hence the fraction of total VKT attributed to
  each age group)

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EMISSION CONTROL TECHNOLOGY PENETRATION
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VEHICLE AGE DISTRIBUTION
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MILEAGE ACCUMULATION RATES
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EFFECT OF I/M PROGRAMS

• I/M programs help to identify and repair vehicles
  emitting at rates higher than the fleet average
• With an effective I/M program fleet average
  emission factors should be less, compared to a
  fleet without an I/M program

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EMISSION FACTOR MODELS

• Incorporate FTP test data from in-use vehicles
  (as opposed to new vehicles which are known to be
  below emission regulations)
• Incorporate all of the above mentioned
  corrections to FTP derived emission factors.
• Incorporate fleet composition data (types of
  vehicles, age distribution, annual VKT by age)
• Incorporate estimations of fuel and temperature
  effect on emissions
• Incorporate the estimated effect of I/M programs
  on fleet average emissions

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EMISSION FACTOR MODELS

• MOBILE (4.1, 5, 5a, 6) U.S. EPA (CO, HC, NOx)
• PART5 , Particulate emission model, complementary
  to MOBILE
• MOBILE5c, Environment Canada
• EMFAC, California Air Resources Board
• (part of MVEI - Motor Vehicle Emission
  Inventory)
• Others in Europe and Japan

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PART5 Model for Motor Vehicles Particulate
Emissions

• Lead exhausted lead
• SOF soluble organic fraction
• RCP remaining carbon portion
• Direct and Indirect SO4
• Brake wear emissions
• Tire wear emissions
• Total PM Exhaust PM brake tire indirect
  SO4
• Road dust from paved and unpaved roads

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EMISSION FACTOR MODELS Continuing work

• To
• improve estimates,
• verify against other observations (tunnel
  studies, long term trends in ambient
  concentrations)
• integrate better with transportation demand models