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Longitudinal Train Dynamics 257
Traction Force, kN
500
400
300
200
100
0
0 50 100 150 200
FIGURE 9.27 Tractive effort thermal derating curve.
Time, minutes
control —slowing the rate at which the throttle can be applied reduces smoke emissions. At least
one fleet of locomotives in Australian service have the application of full power limited to aperiod
not shorter than 80 sec, rate limit being 1.25%/sec. The applicationofpower limited to this rate has
asignificant effect on the train dynamics and the way trains like this are driven. It therefore must be
superimposed on the traction force model.
Akey parameter in any discussion about tractive effort is rail–wheel adhesion or the coefficient
of friction. Prior toenhancement of motor torque control, arail–wheel adhesion level of , 0.20
could be expected. With modern locomotive traction control, higher values of adhesion reaching
, 0.35 are obtained with manufacturers claiming up to 0.46 in published performance curves.
It needs to be remembered that asmooth control system can only deliver an adhesion level up to
the maximum set by the coefficient of friction for the wheel–rail conditions. Wheel–rail conditions
in frost and snow could reduce adhesion to as low as 0.1. Superimposing adhesion levels on
Figure 9.26, asshown in Figure 9.28, shows how adhesion is significant as alocomotive performance
parameter.
The use of dynamic brakes asameans of train deceleration has continued to increase as
dynamic brake systems have been improved. Early systems, as shown in Figure 9.29, gave only a
variable retardation force and were not well received bydrivers. As the effectiveness was so
Traction Force, kN
450
400
350
300
250
200
150
100
50
0
Adhesion =0.35
Adhesion =0.2
0 20 40 60 80 100
Velocity, kph
Notch =2
Notch =4
Notch =6
Notch =8
FIGURE 9.28 Tractive effort performance curves —showing effect of adhesion levels.
© 2006 by Taylor & Francis Group, LLC