MOBILE6 A Revised Model for Estimation of Highway Vehicle Emissions

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MOBILE6A Revised Model for Estimation of Highway
Vehicle Emissions

• David J. Brzezinski and Terry P. Newell
• U.S. EPA Office of Mobile Sources
• Assessment and Modeling Division

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Background

• MOBILE5 originally released in 1992.
• MOBILE5 last updated in 1997.

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Changes for MOBILE6

• Emissions, fleet and activity data
• Structural changes
• Input and output formats

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Basic Exhaust Emissions

• Update in-use deterioration estimate
• New emission standards
• Effects on in-use program benefits

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Off-Cycle Effects

• Air conditioning effects
• Non-FTP driving behavior
• Effects of the Supplemental FTP rule

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Effects of Fuel Composition

• Updated effects of oxygenated fuels on CO
  emissions
• Explicit effects of sulfur on exhaust emissions

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Effects of Driving Behavior

• Facility-based speed corrections
• New in-use driving cycles
• Separate estimates for freeway ramps

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Real-Time Diurnal Emissions

• Old one-hour diurnal data not used
• New data collected in real time (24 hours)
• Multiple day measurements
• Hourly measurements

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Hot Soak Emissions

• Old laboratory measurements still used
• New data for fuel RVP less than 9 psi
• New data based on real world conditions
• Evaluation of hot soak time distributions

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Heavy Duty Emissions

• New emission standards
• New conversion factors
• Light heavy-duty engine start effects

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Fleet Characterization

• New national average mileage accumulation
  estimates
• New national average registration (age)
  distributions
• New national vehicle class counts

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Vehicle Activity

• New trip length estimates
• Engine start soak time distributions
• Diurnal soak time distributions
• Trip start and trip ends
• VMT by time of day (temporal)

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Engine Start and Running

• Effect of engine start in grams per start
• Hot running emissions in grams per mile
• Composite emissions using activity data

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Expanded Vehicle Classes

• Four light-duty truck subcategories
• Heavy-duty truck subcategories
• Same eight MOBILE5 composite categories

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Compressed Natural Gas Vehicles

• Data from the Natural Gas Vehicle Coalition
• Assumes non-exhaust emissions are zero
• Default number of vehicles is zero

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Hourly Emission Factors

• All emissions calculated hourly
• Composite daily output as in MOBILE5
• Detailed hourly output available

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Labeled Input

• Eliminates the need for control flags
• Reduces the amount of required input
• Expanded use of user-supplied comments
• Less dependent on record order

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External Data Files

• Registration and mileage accumulation
• In-use program descriptions
• Local activity data

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Modified Output Formats

• Eliminate most existing options
• Descriptive (120 column) output
• Electronic (database) output

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Stakeholder and Peer Review

• Extensive documentation
• Expanded formal peer review
• Stakeholder review
• MOBILE Workshops
• EPA list server

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EPA Website
http//www.epa.gov/oms/m6.htm
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Data - FTP

• EPA Emission Factors Database (n3,208) - mostly
  pre-1990
• Manufacturers (n2,256) - 1990-93 model years
• API (n157) - high mileage 1981-93
• Representativeness questioned due to voluntary
  sampling

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Dayton, Ohio IM 240

• 181,018 from model years 1981-93
• 142,044 cars
• 38,974 trucks
• Considered more representative than FTP samples
• Unreliable odometer

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Wisconsin IM 240

• Random selection of vehicles for full 240 second
  test
• 3,206 cars and 1,193 trucks
• Used to predict full 240 second emissions from
  fast-pass

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Other Data

• Hammond/Phoenix
• 938 tests of lane and lab IM240 matched with lab
  FTP
• used to convert IM240 to LA4
• Hot Running 505
• 77-car program to measure no-start condition
• used to estimate running and start emissions from
  FTP bags

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General I/M Model
High
Waiver
Repaired
Emissions (g/mi)
Average
Normal
Mileage
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General I/M Model (Continued)
Before I/M (Average)
Emissions (g/mi)
After I/M
Mileage
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I/M Parameters

• Normal Emitters (Tier0)
• Generated from EPA / AAMA FTP Dataset
• Vehicles less than 2x standards for HC NOx 3x
  for CO
• Analysis - Linear Regression of the Data versus
  Mileage
• High Emitters (Tier0)
• Generated from EPA / AAMA FTP Dataset
• Vehicles greater than 2x standards for HC NOx
  3x for CO.
• Analysis - Mean Emission Value Used - No
  Regression
• Fraction of Highs in the Fleet.
• Average Emission Level (Tier0)
• Based on FTP Data and Ohio High Emitter
  Adjustment Factor
• After Repairs (Tier0 and Tier1 Non OBD)
• Generated from Arizona IM240 Test Lane Data

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I/M Parameters (Cont)

• Waivers
• Assumed 20 percent Emission reduction from
  Repairs
• I/M Identification Rate (IDR) (Tier0)
• I/M Tests Ability to Identify Excess Emissions
• Based on Data and Assumptions
• Non Compliant Vehicles
• Represents Vehicles which Disappear Prior to
  Completion of the I/M Process.
• Full Loss of Credit
• I/M Participation Rate
• User Input Ranging from 0 - 100
• I/M Credit is Directly Proportional to the
  Participation Rate.

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New I/M Features for MOBILE6

• Internal Calculation of I/M Effects - No external
  I/M Credit files to attach to the main program.
• I/M Reductions given for IM240, ASM, Idle and OBD
  programs
• Custom User supplied IM240 cutpoints can be
  entered directly into the program. Only 207(b)
  Idle and Two Speed Idle cutpoints are available.
• Annual, Biennial and change of ownership I/M
  testing frequency can be modeled.
• MOBILE6 will have the ability to model up to five
  different I/M programs simultaneously.

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New I/M Features for MOBILE6 (Cont)

• Non Periodic Identification of High Emitters by
  Remote Sensing can now be modeled.
• Ability to model RSD Clean Screening and High
  Emitter Profiling exemptions.
• Ability to model the exemption of the first n
  model years / ages in an I/M program.
• I/M Credits given for cost waivered vehicles.
• I/M Technician Training Effects are available in
  MOBILE6

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What are Off-Cycle Emissions?
Off-Cycle emissions are the portion of actual
in-use emissions which will be affected by the
Supplemental FTP regulations.
Does not include air conditioning effects.
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MOBILE6 Exhaust Emissions

• Basic running exhaust emission rates do not
  include off-cycle emissions.
• MOBILE6 speed adjustments include off-cycle
  emission effects.
• The Supplemental FTP will reduce off-cycle
  emissions from future vehicles.

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National Average Vehicle Miles Traveled
Driving Cycle Weight Driving Cycle Weight Freew
ay 63.2 0.0062 Arterial/Collector
24.8 0.1944 Freeway 59.7 0.1999 Arterial/Collect
or 19.2 0.0175 Freeway 52.9 0.1548 Arterial/Coll
ector 11.6 0.0058 Freeway 30.5 0.2344 Arterial/C
ollector 7.1 0.0135 Freeway 18.6 0.0109 Local
Roadways 12.9 0.1304 Freeway 13.1 0.0025 Freeway
Ramps 34.6 0.0298 All 1.0000
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Tier 1 Vehicle Results
(grams per mile) THC CO NOx NMHC Average
LA4 0.033 0.795 0.180 0.019 Weighted
Cycles 0.048 1.729 0.274 0.035 Off-Cycle
Difference 0.015 0.934 0.094 0.016
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INTRODUCTION

• Most modern gasoline-fueled vehicles use
  catalysts to reduce HC, CO, and NOx emissions
• Sulfur is a catalyst poison. Increased sulfur
  levels in fuels thereby increase emissions
  through catalyst deactivation

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INTRODUCTION

• Current MOBILE model does not have the capability
  to estimate sulfurs impacts on emissions.
  Current basis of MOBILE is results of emissions
  testing using Indolene, a low-sulfur fuel. An
  adjustment factor is used to account for emission
  differences between sulfur levels in Indolene and
  in-use fuels.

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Tier 1 Normal Emitter Composite Emission
Effects Based on Regression Coefficients
HC
NMHC
CO
NOx
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LEV Normal Emitter Composite Emission Effects
Based on Regression Coefficients
NOx
CO
HC
NMHC
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What are Speed Corrections for?

• Adjust for different average speeds
• Adjust for different roadway types
• Adjust for aggressive driving behavior

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Calculation of Speed Correction Factors
Freeway Emissions speed
SCF speed
Freeway Emission 19.6 mph
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Adjustment of the Basic Emission Rate

• Calculate the basic running exhaust emission rate
  (19.6 mph).
• Calculate the freeway emission rate at 19.6 mph.
• Calculate the difference for each
  emission/technology level.

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Major Differences from MOBILE5

• Based on new facility specific cycles
• Include the effects of aggressive driving
  behavior
• Vary by emission level
• Apply to all model year light-duty vehicles and
  trucks

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Facility Specific Driving Cycles

• Freeway (6 from 13.1 to 63.2 mph)
• Arterial/Collector (4 from 7.1 to 24.8 mph)
• New York City Cycle (NYCC)
• Freeway Ramp
• Local Roadway

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Difficulties Comparing MOBILE6 with MOBILE5

• M6 depends on emissions, M5 depends on model
  years
• M6 split by roadway type, M5 is combined roadways
• M6 apply to hot running emissions, M5 include
  starts
• M6 includes aggressive driving effects

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MOBILE6 versus MOBILE5
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MOBILE6 versus MOBILE5
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MOBILE6 versus MOBILE5
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MOBILE5 Hot Soak emission rates

• Based on laboratory testing, one-hour soaks
• Typically at 82F, 9.0 psi RVP fuel, 40 tank
  fill
• Corrections made for other temperatures, RVP
  levels

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Newer data considered

• Auto/Oil program (1993) Almost 300 LDVs and
  LDTs, MY 1983-93
• EPA testing Almost 200 LDVs and LDTs, MY
  1981-94
• Above testing at Mesa AZ (recruited from I/M
  lanes)
• All tested as received (in-use fuel,
  uncontrolled tank fill levels)

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Stratification for Each SourceFor each type of
source, the evaporative emissions were estimated
separately for each of the following four strata

• ALL gross liquid leakers
• NOT gross liquid leakers
• Passing both purge pressure
• Failing the pressure test
• Failing only the purge test

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Weighting FactorsFor each of the
four strata, the incidence of its occurrence
(frequency) is estimated as a logistic growth
function of vehicle age.
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Gross Liquid LeakersThese are vehicles
with substantial leaks of liquid gasoline (as
opposed to vapor leaks) and having

• Resting Loss Emissions gt 2.0 grams/hr, or
• Diurnal Emissions gt 25.0
  grams/day, or
• Hot Soak Test gt 10.0
  grams/test, or
• Running Loss Test gt 7.0 grams/mile

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Gross Liquid Leakers
Mean Emissions Mean emissions from each source
are

• Resting Loss Emissions 9.16 grams /
  hr
• Diurnal Emissions 104.36 grams
  / day
• Running Loss Test 17.65 grams /
  mile
• Hot Soak Test
• Carbureted 16.95 grams /
  test
• TBI 45.00
  grams / test
• PFI 57.14
  grams / test