40 CFR § 1066.1005  Symbols, abbreviations, acronyms, and units of measure.
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.
(a) Symbols for quantities. This part uses the following symbols and units of measure for various quantities:
Table 1 of § 1066.1005  Symbols for Quantities
Symbol  Quantity  Unit  Unit symbol  Unit in terms of SI base units 

α  atomic hydrogen to carbon ratio  mole per mole  mol/mol  1. 

area  square meter  m 
m 

vehicle frictional load  pound force or newton  lbf or N  m·kg·s−


acceleration of Earth's gravity  meters per second squared  m/s 
m·s−


calculated vehicle frictional load  pound force or newton  lbf or N  m·kg·s−


intercept of least squares regression.  

slope of least squares regression.  

acceleration  feet per second squared or meters per second squared  ft/s 
m·s


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

β  ratio of diameters  meter per meter  m/m  1. 
β  atomic oxygen to carbon ratio  mole per mole  mol/mol  1. 

conversion factor.  

vehiclespecific aerodynamic effects  pound force per mile per hour squared or newtonsecond squared per meter squared  lbf/(mi/hr) 
m


number of carbon atoms in a molecule 

number of carbon atoms in a molecule 


discharge coefficient.  

drag area  meter squared  m 
m 

flow coefficient.  

heat capacity at constant pressure  joule per kelvin  J/K  m 

heat capacity at constant volume  joule per kelvin  J/K  m 

diameter  meters  m  m. 

distance  miles or meters  mi or m  m. 

slope correlation  pound force per mile per hour squared or newton second squared per meter squared  lbf/(mi/hr) 
m 

dilution factor.  1.  

mass weighted emission result  grams/mile  g/mi.  

force  pound force or newton  lbf or N  kg·s 
ƒ  frequency  hertz  Hz  s 
ƒn  angular speed (shaft)  revolutions per minute  r/min  π·30·s−


friction compensation error  horsepower or watt  W  m 

roadload force  pound force or newton  lbf or N  kg·s−

γ  ratio of specific heats  (joule per kilogram kelvin) per (joule per kilogram kelvin)  (J/(kg·K))/(J/(kg·K))  1. 

ambient humidity  grams water vapor per kilogram dry air  g H 
g H 
Δ 
change in height  meters  m  m. 

inertia  pound mass or kilogram  lbm or kg  kg. 

current  ampere  A  A. 

indexing variable.  

inertia work rating.  

correction factor  1.  

calibration coefficient  m 
m 

µ  viscosity, dynamic  pascal second  Pa·s  m−

M  molar mass  gram per mole  g/mol  103·kg·mol 

effective mass  kilogram  kg  kg. 

mass  pound mass or kilogram  lbm or kg  kg. 

total number in series.  

total number of pulses in a series.  

pressure  pascal  Pa  m 
Δ 
differential static pressure  pascal  Pa  m 

saturated vapor pressure at ambient dry bulb temperature  kilopascal  kPa  m 

penetration fraction.  
ρ  mass density  kilogram per cubic meter  kg/m 
m 

dynamometer roll revolutions  revolutions per minute  rpm  π·30 

ratio of pressures  pascal per pascal  Pa/Pa  1. 
r 
coefficient of determination.  

Reynolds number.  

response factor.  

relative humidity.  

Sutherland constant  kelvin  K  K. 

standard error of the estimate.  

specific gravity.  
Δ 
distance traveled during measurement interval  meters  m  m. 

absolute temperature  kelvin  K  K. 

Celsius temperature  degree Celsius  °C  K−273.15. 

torque (moment of force)  newton meter  N·m  m 

time  hour or second  hr or s  s. 
Δ 
time interval, period, 1/frequency  second  s  s. 

voltage  volt  V  m 

speed  miles per hour or meters per second  mi/hr or m/s  m·s 

volume  cubic meter  m 
m 

flow volume rate  cubic feet per minute or cubic meter per second  ft 
m 

volume percent.  

concentration of emission over a test interval  part per million  ppm.  

generic variable.  

compressibility factor. 
(b) Symbols for chemical species. This part uses the following symbols for chemical species and exhaust constituents:
Symbol  Species 

CH4  methane. 
CH3OH  methanol. 
CH2O  formaldehyde. 
C2H4O  acetaldehyde. 
C2H5OH  ethanol. 
C2H6  ethane. 
C3H7OH  propanol. 
C3H8  propane. 
C4H10  butane. 
C5H12  pentane. 
CO  carbon monoxide. 
CO2  carbon dioxide. 
H2O  water. 
HC  hydrocarbon. 
N2  molecular nitrogen. 
NMHC  nonmethane hydrocarbon. 
NMHCE  nonmethane hydrocarbon equivalent. 
NMOG  nonmethane organic gas. 
NO  nitric oxide. 
NO2  nitrogen dioxide. 
NOX  oxides of nitrogen. 
N2O  nitrous oxide. 
O2  molecular oxygen. 
OHC  oxygenated hydrocarbon. 
PM  particulate matter. 
THC  total hydrocarbon. 
THCE  total hydrocarbon equivalent. 
(c) Superscripts. This part uses the following superscripts for modifying quantity symbols:
Table 3 of § 1066.1005  Superscripts
Superscript  Meaning 

overbar (such as 
arithmetic mean. 
overdot (such as 
quantity per unit time. 
(d) Subscripts. This part uses the following subscripts for modifying quantity symbols:
Table 4 of § 1066.1005  Subscripts
Subscript  Meaning 

0  reference. 
abs  absolute quantity. 
AC17  air conditioning 2017 test interval. 
act  actual or measured condition. 
actint  actual or measured condition over the speed interval. 
adj  adjusted. 
air  air, dry. 
atmos  atmospheric. 
b  base. 
bkgnd  background. 
c  cold. 
comp  composite. 
cor  corrected. 
cs  cold stabilized. 
ct  cold transient. 
cUDDS  coldstart UDDS. 
D  driven. 
dew  dewpoint. 
dexh  dilute exhaust quantity. 
dil  dilute. 
e  effective. 
emission  emission specie. 
error  error. 
EtOH  ethanol. 
exh  raw exhaust quantity. 
exp  expected quantity. 
fil  filter. 
final  final. 
flow  flow measurement device type. 
gas  gaseous. 
h  hot. 
HFET  highway fuel economy test. 
hs  hot stabilized. 
ht  hot transient. 
hUDDS  hotstart UDDS. 
i  an individual of a series. 
ID  driven inertia. 
in  inlet. 
int  intake. 
init  initial quantity, typically before an emission test. 
IT  target inertia. 
liq  liquid. 
max  the maximum ( 
meas  measured quantity. 
mix  dilute exhaust gas mixture. 
out  outlet. 
PM  particulate matter. 
record  record. 
ref  reference quantity. 
rev  revolution. 
roll  dynamometer roll. 
s  settling. 
s  slip. 
s  stabilized. 
sat  saturated condition. 
SC03  air conditioning driving schedule. 
span  span quantity. 
sda  secondary dilution air. 
std  standard conditions. 
T  target. 
t  throat. 
test  test quantity. 
uncor  uncorrected quantity. 
w  weighted. 
zero  zero quantity. 
(d) Subscripts. This part uses the following subscripts to define a quantity:
Subscript  Quantity 

0  reference. 
abs  absolute quantity. 
AC17  air conditioning 2017 test interval. 
act  actual or measured condition. 
actint  actual or measured condition over the speed interval. 
adj  adjusted. 
air  air, dry. 
atmos  atmospheric. 
b  base. 
bkgnd  background. 
c  cold. 
comp  composite. 
cor  corrected. 
cs  cold stabilized. 
ct  cold transient. 
cUDDS  coldstart UDDS. 
D  driven. 
dew  dewpoint. 
dexh  dilute exhaust quantity. 
dil  dilute. 
e  effective. 
emission  emission specie. 
error  error. 
EtOH  ethanol. 
exh  raw exhaust quantity. 
exp  expected quantity. 
fil  filter. 
final  final. 
flow  flow measurement device type. 
gas  gaseous. 
h  hot. 
HFET  highway fuel economy test. 
hs  hot stabilized. 
ht  hot transient. 
hUDDS  hotstart UDDS. 
i  an individual of a series. 
ID  driven inertia. 
in  inlet. 
int  intake. 
init  initial quantity, typically before an emission test. 
IT  target inertia. 
liq  liquid. 
max  the maximum (i.e. peak) value expected at the standard over a test interval; not the maximum of an instrument range. 
meas  measured quantity. 
mix  dilute exhaust gas mixture. 
out  outlet. 
PM  particulate matter. 
record  record. 
ref  reference quantity. 
rev  revolution. 
roll  dynamometer roll. 
s  settling. 
s  slip. 
s  stabilized. 
sat  saturated condition. 
SC03  air conditioning driving schedule. 
span  span quantity. 
sda  secondary dilution air. 
std  standard conditions. 
T  target. 
t  throat. 
test  test quantity. 
uncor  uncorrected quantity. 
w  weighted. 
zero  zero quantity. 
(e) Other acronyms and abbreviations. This part uses the following additional abbreviations and acronyms:
Table 5 of § 1066.1005  Other Acronyms and Abbreviations
Acronym  Meaning 

A/C  air conditioning. 
AC17  air conditioning 2017 test interval. 
ALVW  adjusted loaded vehicle weight. 
ASME  American Society of Mechanical Engineers. 
CFR  Code of Federal Regulations. 
CFV  criticalflow venturi. 
CNG  compressed natural gas. 
CVS  constantvolume sampler. 
EPA  Environmental Protection Agency. 
ETW  equivalent test weight. 
EV  electric vehicle. 
FID  flameionization detector. 
FTP  Federal test procedure. 
GC  gas chromatograph. 
GEM  greenhouse gas emissions model. 
GHG  greenhouse gas (including CO 
GPS  global positioning system. 
GVWR  gross vehicle weight rating. 
HEV  hybrid electric vehicle, including plugin hybrid electric vehicles. 
HFET  highway fuel economy test. 
HLDT  heavy lightduty truck. 
HPLC  high pressure liquid chromatography. 
IBR  incorporated by reference. 
LA92  Los Angeles 1992 driving schedule. 
MDPV  mediumduty passenger vehicle. 
NIST  National Institute for Standards and Technology. 
NMC  nonmethane cutter. 
PDP  positivedisplacement pump. 
PHEV  plugin hybrid electric vehicle. 
PM  particulate matter. 
RESS  rechargeable energy storage system. 
ppm  parts per million. 
SAE  Society of Automotive Engineers. 
SC03  air conditioning driving schedule. 
SEA  selective enforcement audit. 
SFTP  Supplemental Federal Test Procedure. 
SI  International System of Units. 
SSV  subsonic venturi. 
UDDS  urban dynamometer driving schedule. 
US06  aggressive driving schedule. 
U.S.C.  United States Code. 
WWV  NIST radio station call sign. 
(f) Densities of chemical species. This part uses the following densities of chemical species:
Table 6 of § 1066.1005  Densities of Chemical Species
Symbol  Quantity 
g/m 
g/ft 


density of methane  666.905  18.8847 

density of methanol  1332.02  37.7185 

C 
957.559  27.1151 

C 
915.658  25.9285 

density of propane  611.035  17.3026 

C 
832.74  23.5806 

density of carbon monoxide  1164.41  32.9725 

density of carbon dioxide  1829.53  51.8064 

effective density of hydrocarbon  gaseous fuel 
(see 3)  (see 3) 

density of formaldehyde  1248.21  35.3455 

effective density of hydrocarbon  liquid fuel 
576.816  16.3336 

effective density of nonmethane hydrocarbon  gaseous fuel 
(see 3)  (see 3) 

effective density of nonmethane hydrocarbon  liquid fuel 
576.816  16.3336 

effective density of nonmethane equivalent hydrocarbon  gaseous fuel 
(see 3)  (see 3) 

effective density of nonmethane equivalent hydrocarbon  liquid fuel 
576.816  16.3336 

effective density of oxides of nitrogen 
1912.5  54.156 

density of nitrous oxide  1829.66  51.8103 

effective density of total hydrocarbon  liquid fuel 
576.816  16.3336 

effective density of total equivalent hydrocarbon  liquid fuel 
576.816  16.3336 
a Densities are given at 20 °C and 101.325 kPa.
b Densities for all hydrocarbon containing quantities are given in g/m3carbon atom and g/ft3carbon atom.
c The effective density for natural gas fuel and liquefied petroleum gas fuel are defined by an atomic hydrogentocarbon ratio, α, of the hydrocarbon components of the test fuel. ρHCgas = 41.57·(12.011 + (α·1.008)).
d The effective density for gasoline and diesel fuel are defined by an atomic hydrogentocarbon ratio, α, of 1.85.
e The effective density of NOX is defined by the molar mass of nitrogen dioxide, NO2.
(g) Constants.
(1) This part uses the following constants for the composition of dry air:
Symbol  Quantity  mol/mol 


amount of argon in dry air  0.00934 

amount of carbon dioxide in dry air  0.000375 

amount of nitrogen in dry air  0.78084 

amount of oxygen in dry air  0.209445 
(2) This part uses the following molar masses or effective molar masses of chemical species:
Symbol  Quantity  g/mol
(10−3 · kg · mol−1) 


molar mass of dry air ^{1}  28.96559 

molar mass of water  18.01528 
1 See paragraph (g)(1) of this section for the composition of dry air.
(3) This part uses the following molar gas constant for ideal gases:
Symbol  Quantity  J/(mol · K)
(m ^{2} · kg · s−2 · mol−1 · K−1) 


molar gas constant  8.314472 
(h) Prefixes. This part uses the following prefixes to define a quantity:
Symbol  Quantity  Value 

n  nano  10−9 
µ  micro  10−6 
m  milli  10−3 
c  centi  10−2 
k  kilo  10 ^{3} 
M  mega  10 ^{6} 