Environmental Engineering Reference
In-Depth Information
Vehicle acceleration
263.35658
400
300
1
0.447
V
n
,1
200
100
0
0
0
10
20
30
40
50
60
70
80
0
V
n
,0
80
(c)
Time (s)
Figure 3.3 Continued
adequate acceleration and has sufficient power to sustain high speed cruise. How-
ever, even a large engine can be stalled out from standstill without proper gearing
and clutch. Consider a Top Fueler dragster with a 6,000 hp engine: it requires a
5-speed transmission. With large displacement engines, the transmission ratios
were forced to fall within specific bounds due to tyre adhesion limits for the normal
production tyres and maximum engine rpm at cruise. It was shown that as engine
capability increased, the demand for wide gear shift ratio coverage diminished
dramatically because either the tractive force would be too great for the tyre to road
friction or the ratio would be too great for the engine red line limit. The Cadillac
V16 turned out to require virtually no gear shifting whatsoever due to its extreme
torque. Of course, changing to higher road adhesion tyres will change this scenario,
and those familiar with the dragster class of vehicles know that 5-speed shifting
transmissions are needed in a vehicle having a 6,000 hp engine.
More economical engines such as in-line V4s, V6s and even V8s have crank-
shaft torque ratings in the range of less than 100 Nm to perhaps 350 Nm. Because
the vehicle performance targets are not set differently for smaller passenger cars,
the demand for wider gear shift ratio coverage increases as engine torque rating
decreases. This is necessary in order to meet vehicle performance targets, particu-
larly the 0-60 mph acceleration time and the 50-70mph passing times. The fol-
lowing sections will elaborate more on these points.
Example 1:
A Top Fueler dragster can accelerate through the quarter-mile in 3.1 s
reaching a top speed of 310mph. Suppose the dragster is controlled near peak tyre
slip. How much acceleration does the driver experience in the 3 s?
Solution:
The driver is accelerated along with the vehicle and so experiences the
same acceleration
1
4
ð
1
;
609
Þ
m
;
402
:
25m
3
:
1
2
d
¼
V
0
t
þ
at
2
¼
41
:
86m
=
s
2
¼
a
¼
The driver experiences an acceleration of 41.86 m/s
2
or 4.27
g
.
