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Re: Auxiliary Braking Unit or Brake Slug
Author: SP5103
Date: 02-11-2019 - 12:22
Max Wyss Wrote:
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> Last point first: Because Caltrain is running on
> industry frequency, there is no need for a
> converter, and not even for a phase adjustment
> device, because they would run in phase with the
> grid. So there is always enough capacity to take
> up a couple of MW of regenerative braking energy.
It will be interesting to see how well the grid holds up. I just don't know its capacity or resilience to be able to absorb the sudden load of an EMU set (as I recall their intended use) or several accelerating at the same time. I know the San Diego Trolley got caught with their pants down during the Super Bowl in San Diego years ago when they quickly discovered they didn't have the substation capacity (trolley 600vdc supply) to handle the number of trolley sets they were running to the stadium.
> Because regenerative braking is not the only
> system (you state correctly, it is not fail-safe),
> there is always a backup system around
> (conventional air brakes).
The issue raised in various accident reports has been too much reliance being placed on dynamic braking for train control, the inability to determine if trailing units are properly functioning, and if the dynamics fail to provide the necessary retarding force the ability for the engineer to recognize that fact and correct it with air braking in a timely manner.
If you recall, the SP runaway down Cajon that piled up at Duffy St. had numerous factors involved. The train's weight was grossly under reported, the road and helper engineer had not communicated with each other the condition of their dynamic brakes, and they were already going too fast down the grade based on assumptions of incorrect information regarding the tons per operative brake and available dynamic braking effort. One of the recommendations from that accident was the ability to monitor the dynamic braking effort on trailing units of the consist. So far, I have only been on one locomotive (a newer CP GE) that gave me the TE of the trailing unit, though the DPU system will give you a readout of the amps/TE of the responding DPU.
> Do I assume correctly, that those 250'000 pound is
> the maximum braking force? If this is also the
> maximum tractive force, there is no issue, even
> with the best electronics, you won't get beyond
> that force when braking. And, as stated elsewhere
> in this discussion, when you are electrified, you
> do not run underpowered overweight trains; you run
> faster, lighter and more frequent trains. Keep in
> mind that modern locomotives can maintain the
> maximum tractive force up into the 50 to 80 km/h
> range.
The 250,000 pound limit is based on generally accepted railroad physics - creating more than that buff force can, especially on curves or through switches, allow the flanges to climb over the rail. The coupler/draft gear strength on cars was also 250,000 pounds, but most cars now with HT grade steel are designed for a maximum of 360,000 pounds. Two six motor AC units can easily develop close to the 360,000 pound mark in tractive effort, so if two six-axle AC units are pushing the rear of a train (DPU or manned helper) they either have to be in CTE mode (DPU) or manually limited by the engineer to stay with the 250,000 pound limit.
In typical North American practice, a diesel-electric locomotive has almost always been able to create more tractive effort than it can in maximum dynamic braking. The EMD SD70ACe-T4 brochure shows 175,000 continuous tractive effort, up to 200,000 pounds starting tractive effort, and only 105,000 pounds dynamic braking effort. The GE ET44AC brochure shows 145,000 or 166,000 continuous tractive effort, up to 180,000 or 200,000 pounds starting tractive effort, and only 98,000 pounds dynamic braking effort.
Your comment regarding faster, lighter trains with electric locomotives may very well be European practice, but - North American railroads have no electrified general freight lines and rely on diesel-electrics to handle as big a train as they can. It seems to me that 30-40 years ago, trains running 50 cars were considered more average, and a 10,000 ton train (short ton of 2,000 pounds) was considered large. Nowadays, 112 car trains at 16,000 tons and 8,000 feet long is fairly common, if not about the approximate standard for unit trains. To keep investors happy, US and Canadian railroads are continuing to run longer and heavier trains in an effort to reduce costs and increase profits. Doubling trains together using DPU is common, and trains primarily of empties can approach 3 miles in length. There is not that much commonality between North American versus European railroad practice - and in some cases I actually prefer the European model. I agree you can throw enough horsepower at a train to move it faster, but you also increase your operating costs to do so.
Going back to the electronic brake control issue - it is going to be outrageously expensive and difficult to integrate into North America. The proposed plan was to start with dedicated train sets (oil, then unit and intermodal) finally expanding it to the general freight pool. The system should work better in dedicated service, but in general freight service the question becomes can they maintain the connector/jumper cable continuity? (One patent incorporates it into the air hose gladhands, but I believe they are aiming towards manually installed jumpers now.) This is one of those proposals in the name of "safety" where the implementation and increase maintenance costs far exceed any potential cost benefit. In order to justify the cost, they are only proposing a stand alone system, as a dual electronic/conventional air brake control system is far more expensive. There was a proposed requirement (now suspended) for electronic brakes on any oil trains, or they would be restricted to 30 mph after a certain date for conventional air brakes. An argument I heard was that with DPU, the difference in an emergency brake application rate is insignificant.
Throughout the history of air braked trains, the common issue has always been how to deal with trains getting longer and heavier. The use of DPU has not only allowed larger trains as far as drawbar/buff limits, but it has also allowed for shorter air brake reaction times and multiple recharging points within the train. I imagine that CP is using their DPU enabled air repeaters on their flat prairie routes where they can run all the power on the head end but need the extra air brake control, particularly in the winter to stay within the brake pipe gradient limits.
-------------------------------------------------------
> Last point first: Because Caltrain is running on
> industry frequency, there is no need for a
> converter, and not even for a phase adjustment
> device, because they would run in phase with the
> grid. So there is always enough capacity to take
> up a couple of MW of regenerative braking energy.
It will be interesting to see how well the grid holds up. I just don't know its capacity or resilience to be able to absorb the sudden load of an EMU set (as I recall their intended use) or several accelerating at the same time. I know the San Diego Trolley got caught with their pants down during the Super Bowl in San Diego years ago when they quickly discovered they didn't have the substation capacity (trolley 600vdc supply) to handle the number of trolley sets they were running to the stadium.
> Because regenerative braking is not the only
> system (you state correctly, it is not fail-safe),
> there is always a backup system around
> (conventional air brakes).
The issue raised in various accident reports has been too much reliance being placed on dynamic braking for train control, the inability to determine if trailing units are properly functioning, and if the dynamics fail to provide the necessary retarding force the ability for the engineer to recognize that fact and correct it with air braking in a timely manner.
If you recall, the SP runaway down Cajon that piled up at Duffy St. had numerous factors involved. The train's weight was grossly under reported, the road and helper engineer had not communicated with each other the condition of their dynamic brakes, and they were already going too fast down the grade based on assumptions of incorrect information regarding the tons per operative brake and available dynamic braking effort. One of the recommendations from that accident was the ability to monitor the dynamic braking effort on trailing units of the consist. So far, I have only been on one locomotive (a newer CP GE) that gave me the TE of the trailing unit, though the DPU system will give you a readout of the amps/TE of the responding DPU.
> Do I assume correctly, that those 250'000 pound is
> the maximum braking force? If this is also the
> maximum tractive force, there is no issue, even
> with the best electronics, you won't get beyond
> that force when braking. And, as stated elsewhere
> in this discussion, when you are electrified, you
> do not run underpowered overweight trains; you run
> faster, lighter and more frequent trains. Keep in
> mind that modern locomotives can maintain the
> maximum tractive force up into the 50 to 80 km/h
> range.
The 250,000 pound limit is based on generally accepted railroad physics - creating more than that buff force can, especially on curves or through switches, allow the flanges to climb over the rail. The coupler/draft gear strength on cars was also 250,000 pounds, but most cars now with HT grade steel are designed for a maximum of 360,000 pounds. Two six motor AC units can easily develop close to the 360,000 pound mark in tractive effort, so if two six-axle AC units are pushing the rear of a train (DPU or manned helper) they either have to be in CTE mode (DPU) or manually limited by the engineer to stay with the 250,000 pound limit.
In typical North American practice, a diesel-electric locomotive has almost always been able to create more tractive effort than it can in maximum dynamic braking. The EMD SD70ACe-T4 brochure shows 175,000 continuous tractive effort, up to 200,000 pounds starting tractive effort, and only 105,000 pounds dynamic braking effort. The GE ET44AC brochure shows 145,000 or 166,000 continuous tractive effort, up to 180,000 or 200,000 pounds starting tractive effort, and only 98,000 pounds dynamic braking effort.
Your comment regarding faster, lighter trains with electric locomotives may very well be European practice, but - North American railroads have no electrified general freight lines and rely on diesel-electrics to handle as big a train as they can. It seems to me that 30-40 years ago, trains running 50 cars were considered more average, and a 10,000 ton train (short ton of 2,000 pounds) was considered large. Nowadays, 112 car trains at 16,000 tons and 8,000 feet long is fairly common, if not about the approximate standard for unit trains. To keep investors happy, US and Canadian railroads are continuing to run longer and heavier trains in an effort to reduce costs and increase profits. Doubling trains together using DPU is common, and trains primarily of empties can approach 3 miles in length. There is not that much commonality between North American versus European railroad practice - and in some cases I actually prefer the European model. I agree you can throw enough horsepower at a train to move it faster, but you also increase your operating costs to do so.
Going back to the electronic brake control issue - it is going to be outrageously expensive and difficult to integrate into North America. The proposed plan was to start with dedicated train sets (oil, then unit and intermodal) finally expanding it to the general freight pool. The system should work better in dedicated service, but in general freight service the question becomes can they maintain the connector/jumper cable continuity? (One patent incorporates it into the air hose gladhands, but I believe they are aiming towards manually installed jumpers now.) This is one of those proposals in the name of "safety" where the implementation and increase maintenance costs far exceed any potential cost benefit. In order to justify the cost, they are only proposing a stand alone system, as a dual electronic/conventional air brake control system is far more expensive. There was a proposed requirement (now suspended) for electronic brakes on any oil trains, or they would be restricted to 30 mph after a certain date for conventional air brakes. An argument I heard was that with DPU, the difference in an emergency brake application rate is insignificant.
Throughout the history of air braked trains, the common issue has always been how to deal with trains getting longer and heavier. The use of DPU has not only allowed larger trains as far as drawbar/buff limits, but it has also allowed for shorter air brake reaction times and multiple recharging points within the train. I imagine that CP is using their DPU enabled air repeaters on their flat prairie routes where they can run all the power on the head end but need the extra air brake control, particularly in the winter to stay within the brake pipe gradient limits.
