[Traindude]

I have been on a spree of retrofitting all my locomotives and rolling stock with flange squeal and brake cylinder-controlled brake squeal lately. Since I alternate between American and British trains, this means that I am using equipment equipped with both air and vacuum brakes.

When I first started retrofitting my vacuum-braked equipment with the brake squeal sound, I soon found out that the way vacuum brake cylinders worked was the opposite of air brake cylinders--in an air-brake system, the cylinder pressure increases upon application and decreases upon release, but with vacuum brakes the cylinder pressure decreases upon application and increases upon release.

Aside from having to invert the variable triggers and any brake-cylinder-related volume curves, there are some odd side effects:

1. Differing pipe and cylinder pressures for different equipment.
In the UK, most vacuum brakes release at 21 Inches of Mercury. Converted to PSI, this amounts to 10.31 PSI. However, the brakes usually don't begin to apply until the brake cylinder pressure drops below 18 Inches of Mercury (8.84 PSI). With that in mind, here's how that'd be configured:

Stream (
		Skip ( Brake squeal )
		Priority ( 6 )
		Triggers ( 2
			Variable_Trigger ( BrakeCyl_Dec_Past 8.84	
					   StartLoop ( 1
					        File ( "brakesqueal.wav" -1 )
					        SelectionMethod ( RandomSelection )	
                                                 )	
                                         )
			Variable_Trigger ( BrakeCyl_Dec_Past 8.84	ReleaseLoopRelease ()	)
				         )
			FrequencyCurve(
				SpeedControlled
				CurvePoints ( 5
					-30		43800
					-1		44100
					 0		49000
					 1		44100
					30		43800
					)
				Granularity ( 0.001 )
				)
				VolumeCurve(
					BrakeCylControlled
					CurvePoints ( 4
					    0		1.0
                                            8.83        0.14
                                            8.84        0.0
					   10.31	0.0
					)
					Granularity ( 0.01 )
				)
				VolumeCurve(
					SpeedControlled
					CurvePoints ( 5
						-.4		1.0
						-.1		0.05
						  0		0.0
						 .1		0.05
						 .4		1.0
						)
					Granularity ( 0.001 )
				)
				VolumeCurve(
					DistanceControlled
					CurvePoints ( 3
						10		1.0000
						40		0.2500
						200		0.0000
					)
					Granularity ( .001 )
				)
			)

However, on the Great Western Railway (GWR) the brakes release at 25 Inches of Mercury (12.27 PSI) instead. So then the code would be:

Stream (
	        Skip ( Brake squeal )
		Priority ( 6 )
		Triggers ( 2
			Variable_Trigger ( BrakeCyl_Dec_Past 10.81	
					  StartLoop ( 1
						File ( "brakesqueal.wav" -1 )
						SelectionMethod ( RandomSelection )	
                                               )	
                                 )
			Variable_Trigger ( BrakeCyl_Dec_Past 10.81	ReleaseLoopRelease ()	)
				)
				FrequencyCurve(
					SpeedControlled
					CurvePoints ( 5
						-30		43800
						 -1		44100
						  0		49000
						  1		44100
						 30		43800
						)
					Granularity ( 0.001 )
				)
				VolumeCurve(
					BrakeCylControlled
					CurvePoints ( 4
						 0		1.0
                                                 10.80         0.14
                                                 10.81          0.0
					         12.27		0.0
					)
					Granularity ( 0.01 )
				)
				VolumeCurve(
					SpeedControlled
					CurvePoints ( 5
						-.4		1.0
						-.1		0.05
						  0		0.0
						 .1		0.05
						 .4		1.0
						)
					Granularity ( 0.001 )
				)
				VolumeCurve(
					DistanceControlled
					CurvePoints ( 3
						10		1.0000
						40		0.2500
						200		0.0000
					)
					Granularity ( .001 )
				)
			)

However, when one runs a mix of GWR and non-GWR equipment, there are some false side effects. When a non-GWR loco is hauling a mix of GWR and non-GWR equipment, then the result is that the GWR equipment sounds like their brakes are still applied even though the brakes are technically fully released. Likewise, if a GWR loco is hauling a similar mix of equipment, then the non-GWR equipment sounds like their brakes are fully released even though (according to the extended HUD's brake and forces information section) the brakes are starting to apply.

2. What do you do when the locomotive has both a vacuum and a steam brake?
When I modified the CTN Merchant Navy to use the BrakeCyl-controlled brake squeal code, I overlooked the fact that it also has a steam brake for its independent (engine) brake. Since a steam brake acts like an air brake (and thus is the opposite of a vacuum brake) it caused the loco to sound as if the brakes were applied in full-strength when, in fact, BOTH the vacuum and steam brakes were released. In the end I decided the only way around this was disabling the engine brake in the loco *.eng file.

If any of you guys (especially the folks across the pond) could give me some advice (and also let the ORTS developers know about this so they can come up with a workaround), that'd be great! Thanks in advance!

[copperpen]

The standard vacuum in the UK was 21Hg, but the GWR used a higher level of 25Hg. This is why the brakes seem to drag when using a non GWR engine to haul GWR stock. I tend to standardise on 21Hg for all stock to avoid this problem. There is less GWR stock overall when comparing with the rest of the UK.

From Wiki
This difference in standards could cause problems on long-distance cross-country services when a GWR locomotive was replaced with another company's engine, as the new engine's large ejector would sometimes not be able to fully release the brakes on the train. In this case the release valves on each vehicle in the train would have to be released by hand, before the brake was recharged at 21 inches.

Not sure if this is yet available in OR.

[Paul B]

Hi all;

With regards to the different pressures/vacuums. I've not tested it with vac brakes but for air brakes we can 'bleed off' the brakes through the F9 train ops window, this releases air pressure from the reservoir and cylinder and releases the brakes on the vehicle.

In real life (automatic) vacuum brakes work by means of a pressure [vacuum] differential. The following three statements are true regardless of the actual level of vacuum in the system.

1. If the vacuum on the 'chamber' side is greater then the vacuum on the 'train pipe' side the brakes will apply.
2. When the two vacuums are equal the brakes will release.
3. If the train pipe vacuum is greater than the chamber vacuum the brakes will release.

The greater the differential the harder the brakes will apply. The GWR adopted 25" because this gives a greater pressure differential meaning greater brake force for a given sized brake cylinder.

As previously stated the difficulty comes when a loco that creates 21" in the train pipe attaches to a train that has been previously run at 25". There will be 25" in the chamber side but the loco will only be able to create 21" in the train pipe. There will be a 4" differential and since the train pipe is lower than the chamber (Statement 1 above) the brakes will be slightly applied. To overcome this we 'pull strings' (slang term for operating the release valve/chord) this lets air into the chamber side of each cylinder, reducing the vacuum thus allowing the vacuums to equalize and the brakes to release (Statement 2 above).

So in theory operating the 'bleed off' with vacuum braked stock should simulate the action of pulling strings to reduce the chamber/reservoir vacuum. Not sure if this actually works in the sim.

It is true that if a BR coach designed to give it's full braking force at 21" chamber vacuum is operated at 25" it will have a slightly higher braking force.
Conversely a GWR coach designed to give it's full braking force at 25" chamber vacuum is operated at 21" it will have a slightly reduced braking force.

I'm not really a sound expert (physics is my thing) so not sure if it's possible to link the vac brake sound to the 'state' of the brakes or the pressure difference somehow? Or at least for British rolling stock only play the brake application sounds when the train pipe drops below say 20" to cover stock operating at both 21" and 25" in Hg.

Just some thoughts

Regards;

-Paul

[Traindude]

Paul B, on 28 May 2023 - 07:05 AM, said:

Hi all;

With regards to the different pressures/vacuums. I've not tested it with vac brakes but for air brakes we can 'bleed off' the brakes through the F9 train ops window, this releases air pressure from the reservoir and cylinder and releases the brakes on the vehicle.

In real life (automatic) vacuum brakes work by means of a pressure [vacuum] differential. The following three statements are true regardless of the actual level of vacuum in the system.

1. If the vacuum on the 'chamber' side is greater then the vacuum on the 'train pipe' side the brakes will apply.
2. When the two vacuums are equal the brakes will release.
3. If the train pipe vacuum is greater than the chamber vacuum the brakes will release.

The greater the differential the harder the brakes will apply. The GWR adopted 25" because this gives a greater pressure differential meaning greater brake force for a given sized brake cylinder.

As previously stated the difficulty comes when a loco that creates 21" in the train pipe attaches to a train that has been previously run at 25". There will be 25" in the chamber side but the loco will only be able to create 21" in the train pipe. There will be a 4" differential and since the train pipe is lower than the chamber (Statement 1 above) the brakes will be slightly applied. To overcome this we 'pull strings' (slang term for operating the release valve/chord) this lets air into the chamber side of each cylinder, reducing the vacuum thus allowing the vacuums to equalize and the brakes to release (Statement 2 above).

So in theory operating the 'bleed off' with vacuum braked stock should simulate the action of pulling strings to reduce the chamber/reservoir vacuum. Not sure if this actually works in the sim.

It is true that if a BR coach designed to give it's full braking force at 21" chamber vacuum is operated at 25" it will have a slightly higher braking force.
Conversely a GWR coach designed to give it's full braking force at 25" chamber vacuum is operated at 21" it will have a slightly reduced braking force.

I'm not really a sound expert (physics is my thing) so not sure if it's possible to link the vac brake sound to the 'state' of the brakes or the pressure difference somehow? Or at least for British rolling stock only play the brake application sounds when the train pipe drops below say 20" to cover stock operating at both 21" and 25" in Hg.

Just some thoughts

Regards;

-Paul

Thank you, Paul! AFAIK, there's no sound triggers that are vacuum-brake-exclusive yet. I've pitched my concerns on the "Wishes for Improvements for Braking Systems" thread (see post # 488), but so far, the folks over there have ignored it.