A 1	total number of timeliness microstates
A 2	total number of quality microstates
cp	specific heat capacity, kJ·kg−1·K−1
g	gravitational acceleration, m·s−2
G	mass flow rate, kg·s−1
h	flight altitude, km
H 1	timeliness entropy
H 2	quality entropy
H 1m	maximum timeliness entropy
H 2m	maximum quality entropy
k	adiabatic index
ki	node degree
Lij	path length
Ma	mach number
N	number of working conditions
p	probability
P	pressure, kPa
Ph	atmospheric pressure, kPa
Ph 0	atmospheric pressure at sea level, kPa
QC	heat load, kW
R	gas constant, J kg−1·K−1
s	entropy, J·K−1
S	order degree of the system
S 1	timeliness of the system
S 2	quality of the system
T	temperature, K
Th	atmospheric temperature, K
Th 0	atmospheric temperature at sea level, K
v	volume, m3·kg−1
WN	power required to drive ACS, kW
Y 1	coefficient in the analytical solutions of the outlet temperature
Y 2	coefficient in the analytical solutions of the outlet temperature
cal	calculation data
exp	experimental data
Greek Symbols
$\alpha$	weight coefficients of timeliness
$\beta$	weight coefficients of quality
$\gamma$	the lapse rate of temperature, K·m−1
$\xi$	ratio between the mass flow rates of the ram air and fresh air
$\epsilon$	error
$\overline{\epsilon }$	average error
$\theta$	excess temperature, K
$\phi$	excess temperature, K
${\eta }_{HX1}$	effectiveness of primary heat exchanger
${\eta }_{HX2}$	effectiveness of secondary heat exchanger
${\eta }_{\mathrm{c}}$	efficiency of compressor
${\eta }_{\mathrm{f}}$	efficiency of fan
${\eta }_{\mathrm{t}}$	efficiency of turbine
${\eta }_{\mathrm{s}}$	mechanical efficiency of the shaft
${\pi }_{\mathrm{c}}$	pressure ratio of compressor
${\pi }_{\mathrm{f}}$	pressure ratio of fan
${\pi }_{\mathrm{t}}$	expansion ratio of turbine
$\Omega$	customized variable
Subscripts
b	bleed air
c	compressor
e	cabin exhaust air
f	fan
HX	heat exchanger
max	maximum
o	outlet of ACS
R	ram air
t	turbine