Environmental Engineering Reference
In-Depth Information
steps, facilitated by the presence of atomic oxygen in the combustion zone:
N
2
+
O
→
NO
+
N
(8.1)
N
+
O
2
→
NO
+
O
(8.2)
There would be no NO in the exhaust gas reaching the atmosphere if these reactions reversed
their course as the product gas temperature declined during the expansion stroke and the NO
maintained thermochemical equilibrium with the rest of the exhaust products. Unfortunately, this
reverse process ceases during the expansion, leaving a residual amount of NO that can be reduced
by subsequent treatment in the exhaust system outside the cylinder. This residual NO is sensitively
dependent upon the air-fuel ratio and the flame temperature in the combustion process.
8.3
ENGINE POWER AND PERFORMANCE
The burning of fuel in the cylinder of a reciprocating internal combustion engine produces me-
chanical power by exerting a force on the moving piston. At any instant, if
V
is the volume of gas
in the cylinder and
p
is the gas pressure, then the increment
d
in work done by the gas on the
piston when it moves so as to increase the volume by an amount
dV
is
W
d
W
=
pdV
(8.3)
The total work
done by the gas in one cycle of the engine (two or four strokes) is found by
integrating equation (8.3) over a cycle:
W
W
=
pdV
≡
pV
c
(8.4)
where
p
is an effective average gas pressure and
V
c
is the cylinder displacement—that is, the
volume of gas displaced when the piston moves from the innermost to outermost position. If
the crankshaft rotates at a frequency
N
and
n
is the number of crank revolutions per cycle, then the
time for a complete cycle is
n
/
N
and the power input to the piston
P
is
N
n
=
pV
c
N
n
P
=
W
(8.5)
Figure 8.3 depicts the variation of gas pressure in the cylinder of a four-stroke-cycle SI engine.
The uppermost curve traces the pressure during the outward power stroke, which is followed by the
inward exhaust stroke, for which the cylinder pressure is about equal to that in the exhaust manifold.
The pressure in the following intake stroke is lower, and it is equal to the intake manifold pressure
so that the final, compression stroke starts from a low pressure. The work per cycle,
pdV
, can be
seen to be the difference between the area enclosed by the upper loop of Figure 8.3 minus that of
the lower loop.
Only a part of the work done by the gas on the moving pistons of an engine is delivered to the
output shaft of the engine, where it can then be connected to the transmission and ultimately to the
wheels to propel a vehicle. The friction of the pistons moving along the cylinder surfaces, as well as
