{"data":{"id":"us/40-cfr-1066.1005","jurisdiction":"us","citation":"40 CFR 1066.1005","heading":"Symbols, abbreviations, acronyms, and units of measure.","body":"The procedures in this part generally follow either the International System of Units (SI) or the United States customary units, as detailed in NIST Special Publication 811, which we incorporate by reference in § 1066.1010. See 40 CFR 1065.20 for specific provisions related to these conventions. This section summarizes the way we use symbols, units of measure, and other abbreviations.\n(a) Symbols for quantities. This part uses the following symbols and units of measure for various quantities:\nTable 1 of § 1066.1005—Symbols for Quantities\nSymbol Quantity Unit Unit symbol Unit in terms of SI base units\nα atomic hydrogen to carbon ratio mole per mole mol/mol 1.\nA area square meter m2 m2.\nA vehicle frictional load pound force or newton lbf or N m·kg·s− 2.\nag acceleration of Earth's gravity meters per second squared m/s2 m·s− 2.\nAm calculated vehicle frictional load pound force or newton lbf or N m·kg·s− 2.\na0 intercept of least squares regression.\na1 slope of least squares regression.\na acceleration feet per second squared or meters per second squared ft/s2 or m/s2 m·s- 2.\nB vehicle load from drag and rolling resistance pound force per mile per hour or newton second per meter lbf/(mi/hr) or N·s/m kg·s- 1.\nβ ratio of diameters meter per meter m/m 1.\nβ atomic oxygen to carbon ratio mole per mole mol/mol 1.\nc conversion factor.\nC vehicle-specific aerodynamic effects pound force per mile per hour squared or newton-second squared per meter squared lbf/(mi/hr)2 or N·s2/m2 m- 1·kg.\nC# number of carbon atoms in a molecule C# number of carbon atoms in a molecule C#.\nCd discharge coefficient.\nCd A drag area meter squared m2 m2.\nCf flow coefficient.\nCp heat capacity at constant pressure joule per kelvin J/K m2·kg·s- 2·K- 1.\nCv heat capacity at constant volume joule per kelvin J/K m2·kg·s- 2·K- 1.\nd diameter meters m m.\nD distance miles or meters mi or m m.\nD slope correlation pound force per mile per hour squared or newton second squared per meter squared lbf/(mi/hr)2 or N·s2/m2 m− 2·kg.\nDF dilution factor. 1.\ne mass weighted emission result grams/mile g/mi.\nF force pound force or newton lbf or N kg·s− 2.\nƒ frequency hertz Hz s− 1.\nƒn angular speed (shaft) revolutions per minute r/min π·30·s− 1.\nFC friction compensation error horsepower or watt W m2·kg·s− 3.\nFR road-load force pound force or newton lbf or N kg·s− 2.\nγ ratio of specific heats (joule per kilogram kelvin) per (joule per kilogram kelvin) (J/(kg·K))/(J/(kg·K)) 1.\nH ambient humidity grams water vapor per kilogram dry air g H2 O vapor/kg dry air g H2 O vapor/kg dry air.\nΔh change in height meters m m.\nI inertia pound mass or kilogram lbm or kg kg.\nI current ampere A A.\ni indexing variable.\nIR inertia work rating.\nK correction factor 1.\nKv calibration coefficient m4·s·K0.5/kg m4·kg− 1·s·K0.5.\nµ viscosity, dynamic pascal second Pa·s m− 1·kg·s− 1.\nM molar mass gram per mole g/mol 10-3·kg·mol− 1.\nMe effective mass kilogram kg kg.\nm mass pound mass or kilogram lbm or kg kg.\nN total number in series.\nn total number of pulses in a series.\np pressure pascal Pa m− 1·kg·s− 2.\nΔp differential static pressure pascal Pa m− 1·kg·s− 2.\npd saturated vapor pressure at ambient dry bulb temperature kilopascal kPa m− 1·kg·s− 1.\nPF penetration fraction.\nρ mass density kilogram per cubic meter kg/m3 m− 3·kg.\nR dynamometer roll revolutions revolutions per minute rpm π·30− 1·s− 1.\nr ratio of pressures pascal per pascal Pa/Pa 1.\nr2 coefficient of determination.\nRe# Reynolds number.\nRF response factor.\nRH relative humidity.\nS Sutherland constant kelvin K K.\nSEE standard error of the estimate.\nSG specific gravity.\nΔs distance traveled during measurement interval meters m m.\nT absolute temperature kelvin K K.\nT Celsius temperature degree Celsius °C K−273.15.\nT torque (moment of force) newton meter N·m m2·kg·s- 2.\nt time hour or second hr or s s.\nΔt time interval, period, 1/frequency second s s.\nU voltage volt V m2·kg·s- 3·A- 1.\nv speed miles per hour or meters per second mi/hr or m/s m·s- 1.\nV volume cubic meter m3 m3.\nV flow volume rate cubic feet per minute or cubic meter per second ft3/min or m3/s m3·/s− 1.\nVP volume percent.\nx concentration of emission over a test interval part per million ppm.\ny generic variable.\nZ compressibility factor.\n(b) Symbols for chemical species. This part uses the following symbols for chemical species and exhaust constituents:\nTable 2 to Paragraph (b) of § 1066.1005—Symbols for Chemical Species and Exhaust Constituents\nSymbol Species\nCH4 methane.\nCH3 OH methanol.\nCH2 O formaldehyde.\nC2 H4 O acetaldehyde.\nC2 H5 OH ethanol.\nC2 H6 ethane.\nC3 H7 OH propanol.\nC3 H8 propane.\nC4 H10 butane.\nC5 H12 pentane.\nCO carbon monoxide.\nCO2 carbon dioxide.\nH2 O water.\nHC hydrocarbon.\nN2 molecular nitrogen.\nNMHC nonmethane hydrocarbon.\nNMHCE nonmethane hydrocarbon equivalent.\nNMOG nonmethane organic gas.\nNO nitric oxide.\nNO2 nitrogen dioxide.\nNOX oxides of nitrogen.\nN2 O nitrous oxide.\nO2 molecular oxygen.\nOHC oxygenated hydrocarbon.\nPM particulate matter.\nTHC total hydrocarbon.\nTHCE total hydrocarbon equivalent.\n(c) Superscripts. This part uses the following superscripts for modifying quantity symbols:\nTable 3 of § 1066.1005—Superscripts\nSuperscript Meaning\noverbar (such as y ) arithmetic mean.\noverdot (such as y ) quantity per unit time.\n(d) Subscripts. This part uses the following subscripts for modifying quantity symbols:\nTable 4 of § 1066.1005—Subscripts\nSubscript Meaning\n0 reference.\nabs absolute quantity.\nAC17 air conditioning 2017 test interval.\nact actual or measured condition.\nactint actual or measured condition over the speed interval.\nadj adjusted.\nair air, dry.\natmos atmospheric.\nb base.\nbkgnd background.\nc cold.\ncomp composite.\ncor corrected.\ncs cold stabilized.\nct cold transient.\ncUDDS cold-start UDDS.\nD driven.\ndew dewpoint.\ndexh dilute exhaust quantity.\ndil dilute.\ne effective.\nemission emission specie.\nerror error.\nEtOH ethanol.\nexh raw exhaust quantity.\nexp expected quantity.\nfil filter.\nfinal final.\nflow flow measurement device type.\ngas gaseous.\nh hot.\nHFET highway fuel economy test.\nhs hot stabilized.\nht hot transient.\nhUDDS hot-start UDDS.\ni an individual of a series.\nID driven inertia.\nin inlet.\nint intake.\ninit initial quantity, typically before an emission test.\nIT target inertia.\nliq liquid.\nmax the maximum (i.e., peak) value expected at the standard over a test interval; not the maximum of an instrument range.\nmeas measured quantity.\nmix dilute exhaust gas mixture.\nout outlet.\nPM particulate matter.\nrecord record.\nref reference quantity.\nrev revolution.\nroll dynamometer roll.\ns settling.\ns slip.\ns stabilized.\nsat saturated condition.\nSC03 air conditioning driving schedule.\nspan span quantity.\nsda secondary dilution air.\nstd standard conditions.\nT target.\nt throat.\ntest test quantity.\nuncor uncorrected quantity.\nw weighted.\nzero zero quantity.\n(d) Subscripts. This part uses the following subscripts to define a quantity:\nSubscript Quantity\n0 reference.\nabs absolute quantity.\nAC17 air conditioning 2017 test interval.\nact actual or measured condition.\nactint actual or measured condition over the speed interval.\nadj adjusted.\nair air, dry.\natmos atmospheric.\nb base.\nbkgnd background.\nc cold.\ncomp composite.\ncor corrected.\ncs cold stabilized.\nct cold transient.\ncUDDS cold-start UDDS.\nD driven.\ndew dewpoint.\ndexh dilute exhaust quantity.\ndil dilute.\ne effective.\nemission emission specie.\nerror error.\nEtOH ethanol.\nexh raw exhaust quantity.\nexp expected quantity.\nfil filter.\nfinal final.\nflow flow measurement device type.\ngas gaseous.\nh hot.\nHFET highway fuel economy test.\nhs hot stabilized.\nht hot transient.\nhUDDS hot-start UDDS.\ni an individual of a series.\nID driven inertia.\nin inlet.\nint intake.\ninit initial quantity, typically before an emission test.\nIT target inertia.\nliq liquid.\nmax the maximum (i.e. peak) value expected at the standard over a test interval; not the maximum of an instrument range.\nmeas measured quantity.\nmix dilute exhaust gas mixture.\nout outlet.\nPM particulate matter.\nrecord record.\nref reference quantity.\nrev revolution.\nroll dynamometer roll.\ns settling.\ns slip.\ns stabilized.\nsat saturated condition.\nSC03 air conditioning driving schedule.\nspan span quantity.\nsda secondary dilution air.\nstd standard conditions.\nT target.\nt throat.\ntest test quantity.\nuncor uncorrected quantity.\nw weighted.\nzero zero quantity.\n(e) Other acronyms and abbreviations. This part uses the following additional abbreviations and acronyms:\nTable 5 of § 1066.1005—Other Acronyms and Abbreviations\nAcronym Meaning\nA/C air conditioning.\nAC17 air conditioning 2017 test interval.\nALVW adjusted loaded vehicle weight.\nASME American Society of Mechanical Engineers.\nCFR Code of Federal Regulations.\nCFV critical-flow venturi.\nCNG compressed natural gas.\nCVS constant-volume sampler.\nEPA Environmental Protection Agency.\nETW equivalent test weight.\nEV electric vehicle.\nFID flame-ionization detector.\nFTP Federal test procedure.\nGC gas chromatograph.\nGEM greenhouse gas emissions model.\nGHG greenhouse gas (including CO2, N2 O, and CH4).\nGPS global positioning system.\nGVWR gross vehicle weight rating.\nHEV hybrid electric vehicle, including plug-in hybrid electric vehicles.\nHFET highway fuel economy test.\nHLDT heavy light-duty truck.\nHPLC high pressure liquid chromatography.\nIBR incorporated by reference.\nLA-92 Los Angeles 1992 driving schedule.\nMDPV medium-duty passenger vehicle.\nNIST National Institute for Standards and Technology.\nNMC nonmethane cutter.\nPDP positive-displacement pump.\nPHEV plug-in hybrid electric vehicle.\nPM particulate matter.\nRESS rechargeable energy storage system.\nppm parts per million.\nSAE Society of Automotive Engineers.\nSC03 air conditioning driving schedule.\nSEA selective enforcement audit.\nSFTP Supplemental Federal Test Procedure.\nSI International System of Units.\nSSV subsonic venturi.\nUDDS urban dynamometer driving schedule.\nUS06 aggressive driving schedule.\nU.S.C. United States Code.\nWWV NIST radio station call sign.\n(f) Densities of chemical species. This part uses the following densities of chemical species:\nTable 6 of § 1066.1005—Densities of Chemical Species\nSymbol Quantity a b g/m3 g/ft3\nρCH4 density of methane 666.905 18.8847\nρCH3OH density of methanol 1332.02 37.7185\nρC2H5OH C1-equivalent density of ethanol 957.559 27.1151\nρC2H4O C1-equivalent density of acetaldehyde 915.658 25.9285\nρC3H8 density of propane 611.035 17.3026\nρC3H7OH C1-equivalent density of propanol 832.74 23.5806\nρCO density of carbon monoxide 1164.41 32.9725\nρCO2 density of carbon dioxide 1829.53 51.8064\nρHC-gas effective density of hydrocarbon—gaseous fuel c (see 3) (see 3)\nρCH2O density of formaldehyde 1248.21 35.3455\nρHC-liq effective density of hydrocarbon—liquid fueld 576.816 16.3336\nρNMHC-gas effective density of nonmethane hydrocarbon—gaseous fuel c (see 3) (see 3)\nρNMHC-liq effective density of nonmethane hydrocarbon—liquid fuel d 576.816 16.3336\nρNMHCE-gas effective density of nonmethane equivalent hydrocarbon—gaseous fuel c (see 3) (see 3)\nρNMHCE-liq effective density of nonmethane equivalent hydrocarbon—liquid fuel d 576.816 16.3336\nρNOx effective density of oxides of nitrogen e 1912.5 54.156\nρN2O density of nitrous oxide 1829.66 51.8103\nρTHC-liq effective density of total hydrocarbon—liquid fuel d 576.816 16.3336\nρTHCE-liq effective density of total equivalent hydrocarbon—liquid fuel d 576.816 16.3336\na Densities are given at 20 °C and 101.325 kPa.\nb Densities for all hydrocarbon containing quantities are given in g/m3-carbon atom and g/ft3-carbon atom.\nc The effective density for natural gas fuel and liquefied petroleum gas fuel are defined by an atomic hydrogen-to-carbon ratio, α, of the hydrocarbon components of the test fuel. ρHCgas = 41.57·(12.011 + (α·1.008)).\nd The effective density for gasoline and diesel fuel are defined by an atomic hydrogen-to-carbon ratio, α, of 1.85.\ne The effective density of NOX is defined by the molar mass of nitrogen dioxide, NO2.\n(g) Constants. (1) This part uses the following constants for the composition of dry air:\nTable 7 to Paragraph (g)(1) of § 1066.1005—Constants for the Composition of Dry Air\nSymbol Quantity mol/mol\nxArair amount of argon in dry air 0.00934\nxCO2air amount of carbon dioxide in dry air 0.000375\nxN2air amount of nitrogen in dry air 0.78084\nxO2air amount of oxygen in dry air 0.209445\n(2) This part uses the following molar masses or effective molar masses of chemical species:\nTable 8 to Paragraph (g)(2) of § 1066.1005—Molar Masses or Effective Molar Masses of Chemical Species\nSymbol Quantity g/mol (10− 3·kg·mol− 1)\nMair molar mass of dry air 1 28.96559\nMH2O molar mass of water 18.01528\n1 See paragraph (g)(1) of this section for the composition of dry air.\n(3) This part uses the following molar gas constant for ideal gases:\nTable 9 to Paragraph (g)(3) of § 1066.1005—Molar Gas Constant for Ideal Gases\nSymbol Quantity J/(mol·K) (m2·kg·s− 2·mol− 1·K− 1)\nR molar gas constant 8.314472\n(h) Prefixes. This part uses the following prefixes to define a quantity:\nTable 10 to Paragraph (h) of § 1066.1005—Prefixes to Define a Quantity\nSymbol Quantity Value\nn nano 10− 9\nµ micro 10− 6\nm milli 10− 3\nc centi 10− 2\nk kilo 103\nM mega 106","path":["Title 40—Protection of Environment","CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY","SUBCHAPTER U—AIR POLLUTION CONTROLS","PART 1066—VEHICLE-TESTING PROCEDURES","Subpart K—Definitions and Other Reference Material"],"source_url":"https://www.ecfr.gov/api/versioner/v1/full/2026-08-25/title-40.xml","current_through":"2026-08-25","vintage":"","retrieved_at":"2026-08-27T02:26:04Z","sha256":"cd4b23c9d659885ff553cc6b67e7944b2e7b9de752d7f753255e4263a09b8a42","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-1066.1001","next":"us/40-cfr-1066.1010"},"notice":"GroundRules: Original legal text. Not legal advice."}