| speed profiles and braking percentages [message #302] |
Thu, 26 April 2012 20:46  |
Dirk Bräuer
Messages: 311
Registered: August 2008
|
Senior Member |
|
|
Hello to all,
Susanne wrote:
> [minimum percentage of brake power]
>
> At some railway infrastructure companies the minimum percentage of
> brake power can't be directly calculated by means of physics. It is
> somehow defined by some legal body.
>
> Therefore we would suggest an additional attribute
> "minimumBrakePercentage" for this value in the <speedProfile> element.
Sorry: It _can_ always be "directly calculated by means of physics" but it
is not done so because of arbitrariness... ;-)
Anyway, I know that there are such rules but it is not so easy at least
from a theoretical point of view.
There is a strong physical relation between
- the braking distance of a train,
- the braking power of the train (brake percentage, deceleration - anyway
in which unit),
- the gradient of the line at the braking distance,
- the current speed of the train.
By setting a "minimumBrakePercentage" to a <speedProfile> you skip the
other two of the above named values.
Therefore, this implies the assumptions
- of the (maximum) braking distance being constant for all the route of
the speed profile (which may be acceptable in many cases),
- of the gradient being constant for all the route of the speed profile
???
At least the last one is improbable and possibly a little bit too rough.
You may have a ruling gradient at a line but surely not a constant one.
This would mean that a train running a short section only (e. g. between
two stations) of a speed profile does need the brake percentage of the
steepest section of all the line even if it does not pass that steepest
section?
A more proper solution would be:
There is a "minimumBrakePercentage" for each section of a speed profile
between two places where trains can start or end (i. e. between two
stations).
However, I am aware that there are such "rough" rules in practice but I
think that this is "not the complete truth". There are also rules which
apply additionally to avoid that a train needs to run unnecessarily slow.
May be these additional rules are not obvious or not shown in the fist
place. To avoid mistakes which we can hardly correct only I would
recommend to think about "sectional minimum brake percentage" rather than
one for all the speed profile (which would lead to many many short speed
profiles).
---
At least, for completeness: If we add a "minimumBrakePercentage" to
<speedProfile> we also have to provide them with a brake type and a brake
switch position (rail:tAirBrakeApplicationPosition). The same brake
percentage can mean totally different braking power depending on the brake
position (G or P,...).
Best regards,
Dirk.
|
|
|
|
Members
Search
Help
Login
Home
