Environmental Engineering Reference
In-Depth Information
To see how this vehicle performs under cruise conditions of 55mph in fourth
gear (24.6 m/s) and for the consequent load imposed on the engine, we start with
the road load equation:
2
F
r
¼
R
0
m
v
g
cos
ð
a
Þþ
0
:
5
r
C
d
A
ð
V
V
0
Þ
þ
m
v
g
sin
ð
a
s
Þ
ð
3
:
4
Þ
a ¼
arctan
%
grade
100
Following this, the resultant loading at the driven wheel axle is reflected back
to the engine's crankshaft, or engine plus hybrid M/G, output shaft. In (3.4), grade
is converted to an angle in radians, wind speed (headwind or tailwind) in m/s (mph
in (3.1.5)), and the remaining parameters are listed in Table 3.3. Equation (3.5)
explains the procedure:
F
r
r
w
h
dl
z
4
z
FD
T
e
¼
ð
3
:
5
Þ
0
:
447
z
4
z
FD
r
w
w
e
¼
V
Taking the cruise conditions as 55mph, driving the Focus vehicle reflects a
load of
T
e
= 38.4 Nm and
w
e
= 244 rad/s (2,330 rpm) at the crankshaft. Figure 3.5
shows the road load for 0% and 7.2% grades. At 33% grade, the vehicle can sustain
23mph by completely loading its 85 kW ICE. These data are shown scaled in
Figure 3.5 to illustrate that it would take nearly 600 kW of engine power to sustain
top speed over a 33% grade.
Road load, grade, no headwind
5.862618
6
4
F
e
33
(
k
) × 10
-5
F
r
7
(
k
) × 10
-3
2
10
-3
F
r
0
(
k
)
×
0
0
0
20
40
60
80
100
120
0
k
120
Vehicle speed (mph)
Figure 3.5 Vehicle cruise performance on level terrain and on 7.2% grade
Grade climbing presents a particular challenge to hybrid vehicles because
the main source of sustainable propulsion power is the ICE and not the hybrid
battery.
