Environmental Engineering Reference
In-Depth Information
fresh charge through the cycle past BDC, after which point the intake valve closes
(IC). The air-fuel mixture is then compressed as the piston moves towards TDC.
Ignition timing is engine speed and load dependant, so that spark application will be
at some point ahead of TDC. The ignition point (IP) lights off the fuel-air mixture,
which then burns throughout the expansion stroke when both valves are closed. At
some point before BDC, the exhaust valve opens (EO), venting the combustion gases
to atmosphere. As the piston continues to move past BDC towards TDC the
remaining spent gases are forced out of the cylinder until the exhaust valve closes
(EC). Depending on engine strategy, particularly exhaust gas recirculation (EGR),
some amount of internal EGR is provided by the intake opening and exhaust valve
closing event timing. A consequence of fixed valve timing and stoke (camshaft
design) is that the engine is optimized at one particular operating point. Introduction
of variable valve timing and lift has added a new dimension to engine optimization.
Variable valve timing means that intake valve events are controllable, so that throt-
tling losses at low rpm and induction efficiency at high rpm are now more efficient.
1.7.1 What is brake mean effective pressure (BMEP)?
The four modes (strokes) of the internal combustion engine are more clearly
defined in a pressure-volume (
P
-
V
) diagram. Figure 1.32 illustrates the four
strokes in detail for a cold motoring engine (dotted traces) and a firing engine (solid
traces). The
P
-
V
diagram is taken at the standard mapping point of 1,500 rpm, 2.62
bar BMEP and A/F ratio of 14.6:1 (i.e. worldwide mapping point). These terms will
be defined shortly.
Mean effective pressure (MEP) is the value of constant pressure that would need
to be applied to the piston during the expansion stroke that would result in the same
work output of the engine cycle. Equation (1.35) illustrates the definition of MEP:
p
4
BS
Work
¼
cylinder pressure
cylinder volume
ð
V
d
Þ
Work
=
rev
¼
2
p
torque
p
4
bore
stroke
¼
V
d
¼
work
=
cycle
V
d
work
=
rev
2rev
=
cycle
V
d
ð
1
:
35
Þ
MEP
¼
¼
4
p
T
e
V
d
ð
N
=
m
2
MEP
¼
Þ
4
p
100
T
e
V
d
MEP
¼
ð
bar
Þ
Torque and volume are in Nm and litres, respectively.
To understand BMEP, as used in Figure 1.29, it is necessary to first understand
indicated mean effective pressure (IMEP); mechanical efficiency,
h
m
; indicated
specific fuel consumption (ISFC); indicated specific air consumption (ISAC) and
volumetric efficiency,
h
v
. 'Indicated' refers to the net process such as work or
power performed by the working mixture in the cylinder acting on the piston over
