§1037.805 Symbols, abbreviations, and acronyms.

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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 (incorporated by reference, see §1037.810). 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 chemical species. This part uses the following symbols for chemical species and exhaust constituents:

Symbol	Species
C	carbon.
CH ₄	methane.
CO	carbon monoxide.
CO ₂	carbon dioxide.
H ₂ O	water.
HC	hydrocarbon.
NMHC	nonmethane hydrocarbon.
NMHCE	nonmethane hydrocarbon equivalent.
NO	nitric oxide.
NO ₂	nitrogen dioxide.
NO _X	oxides of nitrogen.
N ₂ O	nitrous oxide.
PM	particulate matter.
THC	total hydrocarbon.
THCE	total hydrocarbon equivalent.

Symbol	Quantity	Unit	Unit symbol	Unit in terms of SI base units
A	vehicle frictional load	pound force or newton	lbf or N	kg·m·s ⁻² .
a	axle position regression coefficient
α	atomic hydrogen-to-carbon ratio	mole per mole	mol/mol	1.
α	axle position regression coefficient
α₀	intercept of air speed correction
α₁	slope of air speed correction
a_g	acceleration of Earth's gravity	meters per second squared	m/s ²	m·s ⁻² .
a₀	intercept of least squares regression
a₁	slope of least squares regression
B	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 .
b	axle position regression coefficient
β	atomic oxygen-to-carbon ratio	mole per mole	mol/mol	1.
β	axle position regression coefficient
β₀	intercept of air direction correction
β₁	slope of air direction correction
C	vehicle-specific aerodynamic effects	pound force per mile per hour squared or newton-second squared per meter squared	lbf/mph ² or N·s ² /m ²	kg·m ⁻¹ .
c	axle position regression coefficient
c_i	axle test regression coefficients
C_i	constant
ΔC_dA	differential drag area	meter squared	m ²	m ² .
C_dA	drag area	meter squared	m ²	m ² .
C_d	drag coefficient
CF	correction factor
C_rr	coefficient of rolling resistance	newton per kilonewton	N/kN	10 ⁻³ .
D	distance	miles or meters	mi or m	m.
e	mass-weighted emission result	grams per ton-mile	g/ton-mi	g/kg-km.
Eff	efficiency
F	adjustment factor
F	force	pound force or newton	lbf or N	kg·m·s ⁻² .
f_n	angular speed (shaft)	revolutions per minute	r/min	π·30·s ⁻¹ .
G	road grade	percent	%	10 ⁻² .
g	gravitational acceleration	meters per second squared	m/s ²	m·s ⁻² .
h	elevation or height	meters	m	m.
i	indexing variable
k_a	drive axle ratio			1.
k_d	transmission gear ratio
k_topgear	highest available transmission gear
L	load over axle	pound force or newton	lbf or N	kg·m·s ⁻² .
m	mass	pound mass or kilogram	lbm or kg	kg.
M	molar mass	gram per mole	g/mol	10 ⁻³ ·kg·mol ⁻¹ .
M	vehicle mass	kilogram	kg	kg.
M_e	vehicle effective mass	kilogram	kg	kg.
M_rotating	inertial mass of rotating components	kilogram	kg	kg.
N	total number in series
n	number of tires
n	amount of substance rate	mole per second	mol/s	mol·s ⁻¹ .
P	power	kilowatt	kW	10 ³ ·m ² ·kg·s ⁻³ .
p	pressure	pascal	Pa	kg·m ⁻¹ ·s ⁻² .
ρ	mass density	kilogram per cubic meter	kg/m ³	kg·m ⁻³ .
PL	payload	tons	ton	kg.
φ	direction	degrees	°	°.
ψ	direction	degrees	°	°.
r	tire radius	meter	m	m.
r²	coefficient of determination
Re^#	Reynolds number
SEE	standard error of the estimate
σ	standard deviation
TRPM	tire revolutions per mile	revolutions per mile	r/mi
TRRL	tire rolling resistance level	newton per kilonewton	N/kN	10 ⁻³ .
T	absolute temperature	kelvin	K	K.
T	Celsius temperature	degree Celsius	°C	K−273.15.
T	torque (moment of force)	newton meter	N·m	m ² ·kg·s ⁻² .
t	time	hour or second	hr or s	s.
Δt	time interval, period, 1/frequency	second	s	s.
UF	utility factor
v	speed	miles per hour or meters per second	mi/hr or m/s	m·s ⁻¹ .
w	weighting factor
w	wind speed	miles per hour	mi/hr	m·s ⁻¹ .
W	work	kilowatt-hour	kW·hr	3.6·m ² ·kg·s ⁻¹ .
w_C	carbon mass fraction	gram per gram	g/g	1.
WR	weight reduction	pound mass	lbm	kg.
x	amount of substance mole fraction	mole per mole	mol/mol	1.

Superscript	Meaning
overbar (such as ȳ)	arithmetic mean.
Double overbar y	arithmetic mean of arithmetic mean.
overdot (such as ẏ)	quantity per unit time.

Subscript	Meaning
±6	±6° yaw angle sweep.
A	A speed.
air	air.
aero	aerodynamic.
alt	alternative.
act	actual or measured condition.
air	air.
axle	axle.
B	B speed.
brake	brake.
C	C speed.
Ccombdry	carbon from fuel per mole of dry exhaust.
CD	charge-depleting.
circuit	circuit.
CO ₂ DEF	CO ₂ resulting from diesel exhaust fluid decomposition.
CO ₂ PTO	CO ₂ emissions for PTO cycle.
coastdown	coastdown.
comp	composite.
CS	charge-sustaining.
cycle	test cycle.
drive	drive axle.
drive-idle	idle with the transmission in drive.
driver	driver.
dyno	dynamometer.
effective	effective.
end	end.
eng	engine.
event	event.
fuel	fuel.
full	full.
grade	grade.
H ₂ Oexhaustdry	H ₂ O in exhaust per mole of exhaust.
hi	high.
i	an individual of a series.
idle	idle.
in	inlet.
inc	increment.
lo	low.
loss	loss.
max	maximum.
meas	measured quantity.
med	median.
min	minimum.
moving	moving.
out	outlet.
P	power.
pair	pair of speed segments.
parked-idle	idle with the transmission in park.
partial	partial.
ploss	power loss.
plug-in	plug-in hybrid electric vehicle.
powertrain	powertrain.
PTO	power take-off.
rated	rated speed.
record	record.
ref	reference quantity.
RL	road load.
rotating	rotating.
seg	segment.
speed	speed.
spin	axle spin loss.
start	start.
steer	steer axle.
t	tire.
test	test.
th	theoretical.
total	total.
trac	traction.
trac10	traction force at 10 mi/hr.
trailer	trailer axle.
transient	transient.
TRR	tire rolling resistance.
UF	utility factor.
urea	urea.
veh	vehicle.
w	wind.
wa	wind average.
yaw	yaw angle.
ys	yaw sweep.
zero	zero quantity.

Acronym	Meaning
ABT	averaging, banking, and trading.
AECD	auxiliary emission control device.
AES	automatic engine shutdown.
APU	auxiliary power unit.
CD	charge-depleting.
CFD	computational fluid dynamics.
CFR	Code of Federal Regulations.
CITT	curb idle transmission torque.
CS	charge-sustaining.
DOT	Department of Transportation.
EPA	Environmental Protection Agency.
FEL	Family Emission Limit.
GAWR	gross axle weight rating.
GCWR	gross combination weight rating.
GEM	greenhouse gas emission model.
GVWR	gross vehicle weight rating.
Heavy HDE	heavy heavy-duty engine (see 40 CFR 1036.140).
Heavy HDV	heavy heavy-duty vehicle (see §1037.140).
HVAC	heating, ventilating, and air conditioning.
ISO	International Organization for Standardization.
Light HDE	light heavy-duty engine (see 40 CFR 1036.140).
Light HDV	light heavy-duty vehicle (see §1037.140).
Medium HDE	medium heavy-duty engine (see 40 CFR 1036.140).
Medium HDV	medium heavy-duty vehicle (see §1037.140).
NARA	National Archives and Records Administration.
NHTSA	National Highway Transportation Safety Administration.
PTO	power take-off.
RESS	rechargeable energy storage system.
SAE	SAE International.
SEE	standard error of the estimate.
SKU	stock-keeping unit.
Spark-ignition HDE	spark-ignition heavy-duty engine (see 40 CFR 1036.140).
TRPM	tire revolutions per mile.
TRRL	tire rolling resistance level.
U.S.C	United States Code.
VSL	vehicle speed limiter.

Symbol	Quantity	Value
g	gravitational constant	9.80665 m· ⁻² .
R	specific gas constant	287.058 J/(kg·K).

Symbol	Quantity	Value
µ	micro	10 ⁻⁶
m	milli	10 ⁻³
c	centi	10 ⁻²
k	kilo	10 ³
M	mega	10 ⁶