Environmental Engineering Reference
In-Depth Information
Table 11.4 SUV with covered trailer coast down test data
Results
High speed: 100-90 kph
Low speed: 20-15 kph
Time (s)
t
1
= 4.53
t
2
= 8.81
Average velocity (m/s)
V
1
= 26.4
V
2
= 4.86
Average acceleration (m/s
2
)
a
1
= 0.618
a
2
= 0.158
curious that the drag coefficient is in the range of the sum of the SUV alone plus the
trailer alone. The following calculation supports this contention:
2
:
56
2
:
476
C
d
-
SUV
C
d
-
combo
¼
0
:
4
þ
1
:
034
ð
0
:
4
Þ¼
0
:
814
C
d
-
combo
¼
C
d
-
SUV
þ
ð
11
:
11
Þ
It is well known from studies of vehicle platooning that closely spaced vehicles
(
1/2 vehicle length) permit substantial fuel savings for the drafting vehicles
because of this [2,3]. Passenger trains rely on this by covering the car-to-car space
with articulated cowling or fairings. Figure 11.5 illustrates the impact of closely
spaced vehicles in terms of total drag effect.
<
1.1
Lead vehicle
1.0
2-vehicle formation
0.9
Trailing vehicle
0.8
0.7
0.6
0.5
0
0.5
1.0
1.5
2.0
2.5
Spacing (% of vehicle length)
Figure 11.5 Influence on vehicle aerodynamic drag of 2-vehicle formation
As Figure 11.5 shows, the aerodynamic drag coefficient of a two-vehicle pla-
toon normalized by the drag coefficient of a single vehicle is substantially lower
than the drag coefficient of a single vehicle [4]. A rather unusual reversal of drag
coefficient occurs when vehicle-vehicle spacing reduces below 0.4 vehicle lengths.
When this happens the drag of the drafting vehicle actually increases above the
drag of the lead vehicle. This illustration may help understand why the drag
