Environmental Engineering Reference
In-Depth Information
25
Standard mapping point
1.8 L,
I
4
, 2 V engine
20
Firing
15
Ignition
10
IP
BDC
5
TDC
Exhaust
Motoring
0
Intake
0
50
100
150
200
250
300
350
400
450
500
550
Cylinder volume (
V
d
) in cc
Figure 1.32 P-V diagram of ICE at standard mapping point
the compression and expansion strokes. 'Brake' means the torque or power at the
engine crankshaft measured at the flywheel by a dynamometer [17]. 'Friction'
refers to the work required to overcome engine mechanical friction and pumping
losses (work is necessary to induct the air-fuel mixture into the cylinder and to
expel the excess spent charge). These terms are defined as follows:
Indicated torque
friction torque
IMEP
¼
BMEP
þ
FMEP
¼
brake torque
þ
ð
1
:
36
Þ
BMEP
IMEP
¼
BMEP
BMEP
þ
FMEP
h
m
¼
ð
1
:
37
Þ
ISFC
h
m
BSFC
¼
ð
1
:
38
Þ
Equation (1.37) defines the mechanical efficiency of the engine in terms of its
brake and friction MEPs. Fuel consumption, BSFC, is then defined as indicated
specific fuel consumption, ISFC (1.43), divided by mechanical efficiency. Engine
volumetric efficiency is a measure of how close the engine is to a positive displace-
ment air pump. Volumetric efficiency is defined as the ratio of actual airflow through
the engine to its ideal airflow, where 'ideal' is defined as the displacement volume
filled with a fresh charge at standard temperature and pressure. Volumetric efficiency
is dependant on valve number and sizes (e.g. 4 V is more efficient than 2 V), valve
