Author Topic: Pinning Down Acceleration Curves For in OpenBVE Using Only Weight & Power  (Read 2926 times)

Offline Fan Railer

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No more excuses for out of whack train acceleration physics lol

Video Transcript:
So in the game of OpenBVE, there has been a bit of uncertainty about what is the
proper and accurate acceleration curve for NYC subway rolling stock. Sometimes the
trains are programmed too slow, sometimes they are too fast. Well, no more. Here
and now, I will show you exactly how to calculate a particular train's maximum
acceleration curve JUST by knowing what the rated power (kW) and weight (m) values

Eventually, I will run through all of the Post-Unification stock and nail down their
individual max acceleration curves, but for the sake of this demonstration, I will
only focus on 4 trains (R1, R32, R142A, R160), one from each generation of stock (plus
an IRT NTT train).

Above, I did state that you only need weight and power rating to calculate the max
acceleration curve, but really, those are the constant variables. If you are to do
the calculations, you need to also know the changing variable, which is either
rated acceleration, or rated tractive effort. Since we know that the maximum starting
acceleration of an NYC subway car is 2.5 mph/s (4.0225 km/h/s) for all stock R10 and
newer, and 1.75 mph/s (2.81575 km/h/s) for R1-R9 stock, all our variables are accounted
for. So let's begin with the R32.

For the sake of clarity, I will try to show both
metric and imperial measurements. I will include a conversion table here:

1 Kilogram = 2.2046 lbs

1 Kilowatt = 1.34 hp

1 mph = 1.609 km/h

1 ms^2 = 3.6 km/h/s = 2.237 mph/s

For each of the calculations, I will be using the values from a single car (not a
full train set) since each car is motorized; acceleration curves will be constant.
This is not the case for the R142, R142A, and R188, which do have unpowered trailer
trucks that must be accounted for. Those values will have to be calculated by train.

Now that we have those values for the R32 nailed down, here are the two important
websites that you need to remember... not so much the second one, but the first one.
We will use the first website to figure out what is the starting tractive effort that
is required to achieve an acceleration rate of 2.5 mph/s for the R32. The second site
is simply used to convert between meters/second squared and km/h/s, since the first
site calculates acceleration in meters/second squared.

After inputting the numbers, we have our starting tractive effort for the R32.

Recall in a different tutorial for TS2015, where I show how to modify train physics
for acceleration, I give you the method to determine the relationship between power
and tractive effort. Let's see if I can pull that up real quick... Well, it seems I did
not save that text file; oh well. I'll just type out the formula really quick again:

Tractive Effort (kN) = [Power (kW) * 3.6] / Speed (km/h)
or as an example (ACS-64 TE @ 201.125 km/h given 6400 kW):

114.56 = (6400*3.6)/201.125

114.56 kN of TE is produced by the ACS-64 @ 125 mph given a power rating of 6400 kW.

Now, given that equation, we can use it to determine the maximum speed at which the R32
can sustain 2.5 mph/s acceleration given its power rating of 115 hp per traction motor.
See if you can follow along...
By plugging in the power rating per car and the calculated starting tractive effort,
we can calculate the point where continuous max power is achieved; at any speed point
lower than the calculated point, acceleration will remain @ 2.5 mph/s, while at any
speed point higher than the calculated point, power will be constant, but acceleration
will decrease.

You now have all you need for coding in acceleration physics in openBVE. Let me
demonstrate. I have the R32 already loaded into the train editor...

You will notice that after the constant acceleration range has been set, you still need
to determine how quickly the acceleration drops off. For the sake of this demonstration
we will set balancing speed at 55 mph (88 km/h). You must manipulate the e(2.0) value
so that the right end of the acceleration curve meets the bottom of the graph @ the desired
X-value (in this case, 88 km/h). A higher e value will result in a lower balancing speed,
and vice versa. That is basically what you need to do. I will now perform the calculations
for the remaining stock I have listed below using the same process. Just follow along
if you need more clarification.

In actuality, it is possible to calculate the speed point where near 0 TE is generated,
which would indicate the actual balancing speed, minus the train resistance. In order to
properly determine balancing speed, you must calculate the train resistance value (kN).
But that is for another time.

Now, GRANTED, these calculated values are for the MAXIMUM *POTENTIAL* acceleration curve.
The actual acceleration curve in real life can be programmed at lower values by
the Transit Authority (to conserve power / enhance operational safety margins). This is
especially true of the NTT rolling stock.

Thanks for watching, hope this was informative and that you guys learned something
new today =)
« Last Edit: May 26, 2015, 07:37:13 am by Fan Railer »

Offline Fan Railer

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Showcasing the difference between restricted acceleration and unrestricted acceleration on the R143:

Restricted (manual operation):

Unrestricted (CBTC / ATO):

L Train CBTC / ATO acceleration and 26 TPH headways: