Environmental Engineering Reference
In-Depth Information
Figure 11.8 Class-8 semi-tractor-trailer rig (from Reference 4)
Table 11.9 Full rig coast down test measured times
Results
High speed: 60-57.5 mph
Low speed: 10-7.5 mph
Time
t
1
= 4.63 s
t
2
= 11.0 s
Average velocity (m/s)
V
1
= 26.26
V
2
= 3.912
Average acceleration (m/s
2
)
a
1
= 0.241
a
2
= 0.102
When the specification data for the combined mass are used in the coast down
simulation, the results for drag coefficient are now lower than the tractor-only drag
coefficient, and, interestingly, with an empty trailer the rolling resistance remains
unchanged. In fact, the aerodynamic drag for the full rig is only 88% of the tractor-
only drag coefficient. It should be pointed out that in the simulation the frontal area
is adjusted to the trailer frontal area.
Figure 11.9 is the simulation result for the semi-tractor-trailer coast down test in
which the rolling resistance was found to remain unchanged, or very nearly so, but the
coefficient of aerodynamic drag is reduced. The decelerations are curiously slower for
the full rig versus the tractor only, as shown by comparing Tables 11.9 and 11.6.
The most obvious difference between Figures 11.9 and 11.7 is that the total
coast down time of the full rig is longer than for the tractor-only case. This is a
result of the full rig having a somewhat lower drag coefficient but more kinetic
energy to bleed off. What is not conveyed in this discussion so far is what effect a
fully loaded trailer will have on the coast down tests. For that, a trailer loaded with
approximately 35,000-40,000 lb is necessary. The loaded trailer mass will then
distribute itself amongst the eight trailer tyres and the rear eight tractor tyres during
steady state.
