[pschlik]

It's a rarely talked about fact that the MSTS/Open Rails method of assuming dynamic brake resistor grid current is directly proportional to dynamic brake force is actually entirely wrong. (This assumption is also somewhat incorrect during powering, but that is a separate issue.) As dynamic braking systems are just large resistors, it can be determined that the resistor grid current is actually proportional to the square root of the dynamic brake power, not the dynamic brake force. From ohm's law, P=I²R, which can be rearranged to say I=(P/R)^1/2.

This means that for a typical locomotive, dynamic brake current will remain constant at high speeds as most dynamic brake systems are limited by constant power at high speeds. Meanwhile, in Open Rails we only see the current decrease as force decreases with speed. The option for a better calculation of resistor grid current, using power as the reference instead of force, should be implemented to resolve this. However, for a good number of systems, it's not enough to just define one value of "ORTSDynamicBrakesResistorPowerLimit" as the resistance (and thus, the relationship between power and amperage) of the system may change with speed.

https://i.imgur.com/PuF48cP.png


In the case of an EMD locomotive with extended range dynamic brakes here, the max resistor current is 700 amps, but the resistance of the resistors changes with speed, such that the power output which leads to 700 amps also changes with speed. While at speeds above 24 mph, dynamic brake current remains at a constant 700 amps and the braking horsepower at around 630 horsepower per motor, from 0 mph to 9 mph, the maximum resistor power is only about 160 horsepower per motor at 700 amps. From 9 to 15 mph, the max power is about 315 horsepower per axle, and from 15 to 21 mph, 475 horsepower per axle leads to 700 amps. We could also calculate the resistance at each of these speeds and use that to determine the current, but the result is messy and less intuitive than just figuring out the power itself.

As such, I propose the ability to define a table (or single value) of dynamic brake resistor power vs speed. This should only affect calculations of the dynamic brake current. This should not control the actual power output of the dynamic brakes, but should rather be compared to the power output of the dynamic brakes as defined by other parameters in the engine file.

Comment ( 6 axle locomotive without extended range dynamic brakes. )
ORTSDynamicBrakesResistorPowerLimit ( 3802hp )

Comment ( 6 axle locomotive with extended range dynamic brakes. )
ORTSDynamicBrakesResistorPowerLimit (
	0mph	950.5hp
	9mph	950.5hp
	9.5mph	1901hp
	15mph	1901hp
	15.5mph	2851.5hp
	21mph	2851.5hp
	21.5mph	3802hp
	70mph	3802hp
)
Comment ( Default units would be meters per second and watts like usual, just using mph and hp here as this is in the context of an American locomotive. )

The functionality of this would be such that instead of calculating dynamic brake current as (dynamic brake current force / dynamic brake max force) * resistor current limit, it could be calculated as sqrt(dynamic brake current power at the current speed / dynamic brake max power at the current speed) * resistor current limit. Introducing power, a square root, and values that change with speed adds a bit of complexity but allows for more physically accurate ammeter readings during dynamic braking. And while this is coming from the perspective of typical American locomotives, having the ability to define a table of dynamic brake powers should allow for representing any sort of system.

And if such code is not included in the engine, the old logic can be used anyway.

Also, I would like to add that part of this should involve making sure that dynamic brake sounds are set to use the amperage as the control variable, not the force. Dynamic brake sounds are generally the sounds of the dynamic brake blowers, which change sound depending on the dynamic brake current, not dynamic brake force. Of course, for all I know, this could already be the case, but right now dynamic brake force and dynamic brake amps are indistinguishable.