Appendix I

Symbols and abreviation
Symbol	Name	Unit
A	Ampere (unit for current)
A	area	m², mm²
Ag	cross-sectional area of gas supply pipe	m², mm²
Av	cross-sectional area of venturi contractor ("bottleneck")	m², mm²
a	angle, crank angle	°
ai	crank angle at ignition	° before TDC
AC	alternating current
bar	pressure unit
BDC	bottom dead center, piston's lowest position
BG, bg	biogas
c	velocity	m/s
ci	intake velocity	m/s
cv	velocity at venturi bottleneck	m/s
cg	velocity through gas nozzle	m/s
°C	degree Celsius (temperature unit)
cp	specific heat at constant pressure	kJ/(kg K)
cv	specific heat at constant volume	kJ/(kg K)
D, d	diameter	m, mm
Deng	diameter of engine pulley	m, mm
Dmach	diameter of machine pulley	m, mm
d	day (time unit)
d;	inlet diameter	mm
dg	gas nozzle diameter	mm
dv	venturi contraction diameter	mm
Ap	pressure difference	mbar
DV	volume to be added	cm³
DC	direct current
E	energy	J, kJ, kWh
E	energy flow	flow J/s, kJ/s, kW
Ef	fuel energy	J, kJ, kWh
Ef	fuel energy flow	J/s, kJ/s, kW
h	efficiency
hb	boiler efficiency
hc	cogeneration efficiency
heng	engine efficiency
hg	generator efficiency
hmach	driven machine's efficiency
hmech	mechanical efficiency
hp	pump efficiency
ht	transmission efficiency
f	frequency	l/s, Hz
fc	fuel consumption	1/h, m³/s, m³/h
fcd	Diesel fuel consumption	l/h
g	gram (unit for mass)
g	gravity constant	9.81 m/s2
g	isentropic exponent (= cp/cv)
h	hour (time unit)
H	height, water head	m
HCV	hand-controlled valve
I	current	A
i	number of cylinders
J	Joule (energy unit)
k	heat transfer coefficient	kJ/(m² · h · K)
K	Kelvin (temperature unit)
kg	kilogram (unit for mass)
kJ	kilojoule (energy unit)
kW	kilowatt (power unit, energy flow unit)
I	liter (volume unit)
l	excess air ratio in air/fuel mixture
m, mm	meter, millimeter (length unit)
m	mass	kg
m	mass flow	kg/s, kg/in
mf	mass flow of fuel	kg/s, kg/in
m³ n	cubic meter at standard conditions
mbar	milibar (pressure unit)
min	minute (time unit)
n	polytropic exponent
n	shaft (rotational) speed	min-1
nr	rated shaft speed	min-1
neng	speed of engine	min-1
nmach	speed of driven machine	min-1
P	power	kW
Pel	electric power	kW, kVA
Pgen	power of generator	kW
Pmach	power demand of driven machine	kW
Pmech	mechanical power	kW
Pp	number of pole pairs
p	pressure	bar, Pa, mm WH
Pa	ambient pressure	bar, mbar
Pc	pressure after compression	bar
pp	biogas plant pressure	bar; mbar
ps	section pressure	bar
Pa	Pascal (pressure unit)
ppm	parts per million (volume unit)
Dp	pressure difference	bar
Q	heat	kJ
Q	heat flow, heat transferred	kW, kJ/s
Q	pump capacity (volume flow rate)	m³/s, m³/h
R	specific gas constant	kJ/(kg K)
r	"rated" (design conditions)
r	density	kg/m³, kg/1
rw	density of water	1000 kg/m³
s	second (time unit)
sfc	specific fuel consumption m³/kWh,I/kWh,kWh/kWh
sgp	specific gas production	m³ga/(d* Vp)
t	Celsius temperature	°C
D t	temperature difference	°C, K
Dtm	mean temperature difference	°C, K
T	absolute temperature	- K
Tc	absolute temperature after compression	K
Ts	suction temperature	K
TDC	top dead center, highest position of piston
to	time of operation	h
U	voltage	V
V	Volt (unit for "tension", voltage)
V	volume	m³, 1, cm³
Vc	compression volume	1 cm³
Vdc	displaced volume of one cylinder	1 cm³
Vde	displaced volume of engine	1, cm³
Vp	volume of biogas plant	m³
Vprev	previous volume of compression chamber	cm³
Vnew	new volume of compression chamber	cm³
Vs	storage volume for biogas	m³
Vtot	total volume of cylinder	l, cm³
V	volume flow rate	m³/s, m³/h, 1/h
Vbg	biogas volume flow rate	m³/s, m³/h
Vw	water volume flow rate	m³/s, m³/h
DV	volume to be added	cm³
WH	water head	m