Environmental Engineering Reference
In-Depth Information
its density may range from 0.72 to 0.74 g/cm
3
, yielding a gravimetric heat value
of 11,939-12,270 Wh/kg. In the descriptions to follow we use a heat value of
12 kWh/kg or 43.2 MJ/kg.
1.6.1 Emissions
Emissions of CO
2
are a function of the fuel heat rate,
Q
f
, in MJ/kg, the engine brake
thermal efficiency,
h
e
, the driveline efficiency,
h
d
, the average tractive effort over
the drive cycle,
F
tr
in MJ, the total distance travelled on the drive cycle,
S
in km,
and the fuel specific CO
2
content, ESCO
2
as a fraction. The emissions in g/km or g/
mi can then be calculated from
ESCO
2
h
e
h
d
Q
f
F
tr
S
E
CO
2
¼
ð
1
:
27
Þ
Testing laboratories monitor emissions using the following methodology. The
exhaust gas flow must match the intake of air and fuel into the engine to realize a
mass flow balance. Fuel mass is metred by the injectors and induction mass airflow
by an MAF sensor. Exhaust gas oxygen content is monitored by an exhaust gas
oxygen sensor, EGO. Airflow,
m
a
, is used by the engine controller to metre fuel
mass,
m
f
, according to the desired lambda control parameter. Lambda equals the
inverse ratio of air to fuel, A/F. Emission pollutant mass is then:
m
p
¼ð
m
a
þ
m
f
Þ
C
p
mw
p
h
mw
p
i
ð
1
:
28
Þ
where
C
p
is the volumetric concentration of pollutants in the exhaust, and
mw
p
is
the molecular mass of the pollutant with
h
mw
p
i
the average pollutant molecular
mass. The engine controller relies on the Lambda sensor to maintain a neutral
engine, that is one with a stoichiometric balance of air to fuel of 14.7 g air to 1 g
fuel for gasoline. In narrow band sensors the transition from lean to rich mixtures
occurs over a very narrow A/F band so the controller dithers about this value.
1.6.2 Brake specific fuel consumption
Fuel economy is dependant on engine operating point and total road load given by
(1.24). Fuel economy is calculated from knowledge of the engine power at the
crankshaft, its resultant brake specific fuel consumption (BSFC) and specific
gravity of the fuel at the prevailing conditions. In general, the fuel specific gravity
is assumed constant for the drive cycle. Engine power is determined by extra-
polating operating points on an onion plot of engine power output versus speed for
constant BSFC contours. Contours of constant BSFC are determined empirically by
laboratory characterization of the given engine at various torque-speed points.
Engine power given crankshaft torque (Nm) and speed in either rad/s or rpm is
2
p
60
T
e
n
e
ð
W
Þ
P
eng
¼
T
e
w
e
¼
ð
1
:
29
Þ
