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Re: Auxiliary Braking, DPU is better
Author: SP5103
Date: 02-09-2019 - 23:02
Original post:
>In light of the discussions regarding trains operating in mountain regions and cold climates, I was curious if any railroads have experimented with auxillary braking units?
>I'm thinking along the lines of a slug unit that would have both dynamic braking technology plus an air compressor with an extra large air reservoir that would provide additional air supply capacity to the train brake system.
Subsequent:
>Both SP and the Canadiam road have built 'Air Repeater' boxcars.
>Perhaps you need to reread the air brake in emergency part of the discussion.
>If ever there was an extreme case that highlighted the idea of electric activation of air brakes this would be it.
>Railroads have always scoffed at the cost.
I think there is some confusion here as to various terms/nicknames, technologies and their limitations or advantages.
BRAKE SLEDS - Brake sleds seemed to be more common, often made from old tenders but sometimes cars or locomotives. Typically switching is done without air brakes cut in on cars, so the switch engine brakes must be sufficient to stop whatever cars it might have a hold of. By using a brake sled, a switch engine's braking effort can be doubled. In cases where I was switching large heavy cuts of cars in a yard, the conductor would simply cut the air into a few cars net to the switch engine, allowing their braking effort to be used in addition to the switch engine(s). There would be no real advantage to using a brake sled in a train, only for switching without air cut in to the cars.
SLUGS - are typically converted from retired locomotives, though some have been purposely built new (GE built "tractors" for BA&P electrics, some 45 ton slugs sets and SCL MATES; GMD built many 4 axle yard slugs for CN; others?) A slug has traction motors, but gets their power through jumper cables and are integrated into the control from an attached "mother" unit, or sometimes sandwiched between two mother units. By default, a slug also functions as a brake sled, but has the advantage at low speed of providing large amounts of additional tractive effort at far less of an investment as adding conventional locomotives. Slugs can be equipped with control cabs or dynamic brakes. Generally slugs are used where high tractive effort at low speeds is required such as in a hump yard, heavy switching or transfer service. Some railroads (SP, CSXT, SCL) paired 4 axle road slugs with one or two high horsepower 4 axle mothers, creating the needed tractive effort at reasonable cost while avoiding the extra wear from a six axle unit, using the excess power from high horsepower units and at a lower initial/operating cost less than two or more smaller locomotives. Recent emission reduction programs have supported the slug concept because the slug themselves are considered zero emission locomotives.
AIR SUPPLY - Larger air compressors and additional air reservoirs have always been an option and occasionally specified by various railroads. Modern locomotives have electric motor driven air compressors that can provide almost all the needed air without any additional modifications. The largest limiting factor in modern systems is the 1-1/4" brake pipe itself, you can only cram so much air through it especially considering the friction and length.
DYNAMIC BRAKING - Additional dynamic braking comes with two distinct disadvantages - first is that they are not a fail safe system, and the FRA requires that the railroads must have rules and procedures to be able to control the train with air brakes alone. Second - those pesky rules of physics become involved. At a buff force exceeding 250,000 pounds (either shoving under power or due to the use of dynamics) can be sufficient to cause a wheel flange to lift and climb over a rail, derailing and jackknifing the train. Many modern locomotive consists are operated with some of their dynamic brakes purposely cut out to comply with a railroad's rules to stay within the allowable buff forces.
AIR REPEATER CARS - In extreme cold conditions (below zero F), the increased leakage and continued cooling/contraction of the brake pipe air usually results in creating an unacceptable "brake pipe gradient", or difference between the front and rear of the train. Under such conditions, it is often necessary to reduce the train length until the required minimum pressure can be obtained on the train. To deal with this issue, SP, BN and other railroads would insert an air repeater car into the train. These were typically a box car equipped with an air compressor and special air brake valve arrangement. The basic operating theory was this - The air repeater car would take the brake pipe pressure present at it that originated from the front of the train, and use its own air supply to control the rear of train behind it with a proportionally higher amount. For example, the lead controlling engine provides 90 pounds of pressure to the brake pipe. Further back in the train, the air repeater car is getting only 80 psi due to the brake pipe gradient. In this example, the brake pipe behind the air repeater is provided with 20% higher or 96 psi from the air repeater's own air supply. It is actually more complicated than this, because the air repeater car's air controls must allow for full control of the brake pipe including transmitting an emergency brake application between the front and rear halves regardless of origin.
LOCOTROL/DPU - The original versions of Locotrol from Harris Control allowed for the remote control of a helper unit through a "slave" equipped locomotive or control car from a controlling "master" equipped lead locomotive. As part of the equipment, the slave locomotive also controlled the brake pipe, recharging or making brake applications. Harris Control was subsequently purchased by GE, and subsequent generations of the design switched to digital technology for communications, control and logic. The limitation of only one remote consist expanded to up to four remote consists in today's generation of Locotrol, now more commonly referred to as DPU (Distributed Power Units). The primary purpose of DPU is to stay within the basic physics limitations of trains, not exceeding 360,000 pounds of pulling force or 250,000 of buff force. In addition, each DPU remote consist also follows the lead unit in controlling the brake pipe, allowing a more responsive control as well as multiple air supplies to recharge the brake pipe. I know at least CN has a new generation air repeater car that uses modified DPU technology to control the brake pipe and provide an additional recharge supply.
"ELECTRIC ACTIVATION OF AIR BRAKES" - Not a new concept. This has been considered for over a century almost since the very inception of the air brake. Simply, electrons travel at the speed of light, whereas air travels at the speed of sound. The holy grail of air brakes as far as train handling has always been simultaneous operation (to prevent the application or release of the air brakes from affecting slack) and graduated release (freight trains are direct release, the air brakes may be gradually applied in steps, but must be completely released and time allowed to recharge before reapplying). Electro-pneumatic brakes did see reasonably wide application to early streamliners and commuter operations, and some commuter operations continue to use them. In these various designs, the brake pipe continues to function as before, following the electric brake control application. The biggest problem has always been to maintain electrical continuity throughout the train. The modern proposal for this system is EPC which is very different than previous systems. With EPC, the hard wire system will provide power, control and feedback from the braking system. The present brake pipe will only be used as an air supply and in an emergency. Prototype systems have been tested and there are a few in captive service. The FRA recently backed off its requirement to install EPC on oil trains as part of total implementation. The version being proposed would not have a pneumatic parallel/backup system to reduce cost (because the cost benefits of EPC, like PTC, can't be justified). Should there be a communications or continuity failure with EPC, the system would have no choice but to make a penalty brake application until repaired. This would also mark the first time since the introduction of the automatic air brake to US railroads that the systems would not be backwards compatible. This would require the eventual replacement the brake system on every car and locomotive in use in North America, or the segregation of pneumatic and EPC equipped cars into separate trains.
Under the presumed conditions the recent CP runaway faced, none of the above improvements by themselves would have likely prevented it. While EPC might have resolved some train control issues, it would not be immune to the problems with the extreme cold, has the additional consideration of probable continuity issues, and would still require an intact brake pipe for an air supply.
My opinion ...
>In light of the discussions regarding trains operating in mountain regions and cold climates, I was curious if any railroads have experimented with auxillary braking units?
>I'm thinking along the lines of a slug unit that would have both dynamic braking technology plus an air compressor with an extra large air reservoir that would provide additional air supply capacity to the train brake system.
Subsequent:
>Both SP and the Canadiam road have built 'Air Repeater' boxcars.
>Perhaps you need to reread the air brake in emergency part of the discussion.
>If ever there was an extreme case that highlighted the idea of electric activation of air brakes this would be it.
>Railroads have always scoffed at the cost.
I think there is some confusion here as to various terms/nicknames, technologies and their limitations or advantages.
BRAKE SLEDS - Brake sleds seemed to be more common, often made from old tenders but sometimes cars or locomotives. Typically switching is done without air brakes cut in on cars, so the switch engine brakes must be sufficient to stop whatever cars it might have a hold of. By using a brake sled, a switch engine's braking effort can be doubled. In cases where I was switching large heavy cuts of cars in a yard, the conductor would simply cut the air into a few cars net to the switch engine, allowing their braking effort to be used in addition to the switch engine(s). There would be no real advantage to using a brake sled in a train, only for switching without air cut in to the cars.
SLUGS - are typically converted from retired locomotives, though some have been purposely built new (GE built "tractors" for BA&P electrics, some 45 ton slugs sets and SCL MATES; GMD built many 4 axle yard slugs for CN; others?) A slug has traction motors, but gets their power through jumper cables and are integrated into the control from an attached "mother" unit, or sometimes sandwiched between two mother units. By default, a slug also functions as a brake sled, but has the advantage at low speed of providing large amounts of additional tractive effort at far less of an investment as adding conventional locomotives. Slugs can be equipped with control cabs or dynamic brakes. Generally slugs are used where high tractive effort at low speeds is required such as in a hump yard, heavy switching or transfer service. Some railroads (SP, CSXT, SCL) paired 4 axle road slugs with one or two high horsepower 4 axle mothers, creating the needed tractive effort at reasonable cost while avoiding the extra wear from a six axle unit, using the excess power from high horsepower units and at a lower initial/operating cost less than two or more smaller locomotives. Recent emission reduction programs have supported the slug concept because the slug themselves are considered zero emission locomotives.
AIR SUPPLY - Larger air compressors and additional air reservoirs have always been an option and occasionally specified by various railroads. Modern locomotives have electric motor driven air compressors that can provide almost all the needed air without any additional modifications. The largest limiting factor in modern systems is the 1-1/4" brake pipe itself, you can only cram so much air through it especially considering the friction and length.
DYNAMIC BRAKING - Additional dynamic braking comes with two distinct disadvantages - first is that they are not a fail safe system, and the FRA requires that the railroads must have rules and procedures to be able to control the train with air brakes alone. Second - those pesky rules of physics become involved. At a buff force exceeding 250,000 pounds (either shoving under power or due to the use of dynamics) can be sufficient to cause a wheel flange to lift and climb over a rail, derailing and jackknifing the train. Many modern locomotive consists are operated with some of their dynamic brakes purposely cut out to comply with a railroad's rules to stay within the allowable buff forces.
AIR REPEATER CARS - In extreme cold conditions (below zero F), the increased leakage and continued cooling/contraction of the brake pipe air usually results in creating an unacceptable "brake pipe gradient", or difference between the front and rear of the train. Under such conditions, it is often necessary to reduce the train length until the required minimum pressure can be obtained on the train. To deal with this issue, SP, BN and other railroads would insert an air repeater car into the train. These were typically a box car equipped with an air compressor and special air brake valve arrangement. The basic operating theory was this - The air repeater car would take the brake pipe pressure present at it that originated from the front of the train, and use its own air supply to control the rear of train behind it with a proportionally higher amount. For example, the lead controlling engine provides 90 pounds of pressure to the brake pipe. Further back in the train, the air repeater car is getting only 80 psi due to the brake pipe gradient. In this example, the brake pipe behind the air repeater is provided with 20% higher or 96 psi from the air repeater's own air supply. It is actually more complicated than this, because the air repeater car's air controls must allow for full control of the brake pipe including transmitting an emergency brake application between the front and rear halves regardless of origin.
LOCOTROL/DPU - The original versions of Locotrol from Harris Control allowed for the remote control of a helper unit through a "slave" equipped locomotive or control car from a controlling "master" equipped lead locomotive. As part of the equipment, the slave locomotive also controlled the brake pipe, recharging or making brake applications. Harris Control was subsequently purchased by GE, and subsequent generations of the design switched to digital technology for communications, control and logic. The limitation of only one remote consist expanded to up to four remote consists in today's generation of Locotrol, now more commonly referred to as DPU (Distributed Power Units). The primary purpose of DPU is to stay within the basic physics limitations of trains, not exceeding 360,000 pounds of pulling force or 250,000 of buff force. In addition, each DPU remote consist also follows the lead unit in controlling the brake pipe, allowing a more responsive control as well as multiple air supplies to recharge the brake pipe. I know at least CN has a new generation air repeater car that uses modified DPU technology to control the brake pipe and provide an additional recharge supply.
"ELECTRIC ACTIVATION OF AIR BRAKES" - Not a new concept. This has been considered for over a century almost since the very inception of the air brake. Simply, electrons travel at the speed of light, whereas air travels at the speed of sound. The holy grail of air brakes as far as train handling has always been simultaneous operation (to prevent the application or release of the air brakes from affecting slack) and graduated release (freight trains are direct release, the air brakes may be gradually applied in steps, but must be completely released and time allowed to recharge before reapplying). Electro-pneumatic brakes did see reasonably wide application to early streamliners and commuter operations, and some commuter operations continue to use them. In these various designs, the brake pipe continues to function as before, following the electric brake control application. The biggest problem has always been to maintain electrical continuity throughout the train. The modern proposal for this system is EPC which is very different than previous systems. With EPC, the hard wire system will provide power, control and feedback from the braking system. The present brake pipe will only be used as an air supply and in an emergency. Prototype systems have been tested and there are a few in captive service. The FRA recently backed off its requirement to install EPC on oil trains as part of total implementation. The version being proposed would not have a pneumatic parallel/backup system to reduce cost (because the cost benefits of EPC, like PTC, can't be justified). Should there be a communications or continuity failure with EPC, the system would have no choice but to make a penalty brake application until repaired. This would also mark the first time since the introduction of the automatic air brake to US railroads that the systems would not be backwards compatible. This would require the eventual replacement the brake system on every car and locomotive in use in North America, or the segregation of pneumatic and EPC equipped cars into separate trains.
Under the presumed conditions the recent CP runaway faced, none of the above improvements by themselves would have likely prevented it. While EPC might have resolved some train control issues, it would not be immune to the problems with the extreme cold, has the additional consideration of probable continuity issues, and would still require an intact brake pipe for an air supply.
My opinion ...
