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Re: Recent derailments - Metro North
Author: SP5103
Date: 12-04-2013 - 11:16
> "Blended braking" refers to the combined use of air and dynamics to control train speed,
> regardless of how it's accomplished. You can go into blended braking manually with either
> passenger or freight simply by using the appropriate throttle handles on the control
> stand.
I have never seen the term "Blended Braking" applied to the combined use of train air and dynamic brakes, though it might be used by some. For this discussion, I assumed Blended Braking to refer to a system that combines dynamics and brake shoe forces on the same vehicle. It sems to me some LRVs also use a similar system. In typical freight service, the combined use of the engine independent brakes and dynamic brakes is specifically prohibited by rule, except just prior to stopping when dynamcis are inefective.
>However from what I've seen and heard of management's new way
> of running trains, controlling slack isn't a very high on their agenda these days.
Agreed - Fuel saving methods seem to have taken priority over slack control in managements' often tiny little minds. I don't recall burning a little extra fuel to control slack as being the cause of derailing a train.
> Dynamics are NOT a "primary brake" because they don't work when the train is not moving. This is
> the FRA's definition of "primary brake".
True, but in a properly working passenger engine blended brake system, it is the dynamic brake that is primarily used and is supplemented by the engine independent brake pressure as required. This in turn results in the common issue that dynamics are not a fail safe system, and the unexpected or unrealized partial or full failure can quickly get you in trouble, especially if your expected stopping distance or grade speed balance is based on the dynamics working with no or little room for compromise.
> >When dynamic brake is at capacity, the system allows brake cylinder pressure to build up to the
> appropriate level.
>
> No, this is when it bleeds it off to the 10 psi or so which you mentioned. The idea is to have the
> dynamics do as much of the work as possible until they fade (at around 11 mph)
I stand by my statement. The Blended Brake system controls the amount of engine brake cylinder pressure when an automatic brake pipe application is in effect on the engine(s). When the dynamic brakes are generating the retarding force being called for, the brake cylinder pressure is reduced to a minimum amount. On a typical freight engine, maximum dynamic braking effort is obtained around 21 mph, but this can vary depending on the gear ratio and dynamic grid resistance. On passenger engines with higher gear ratios, the maximum braking effort is reached at a higher speed - the operator's manual shows 27 mph for a F40PH-2D with 58:19 gearing for 100 mph maximum speed. The dynamic baking effort is directly proportional to speed between the maximum dynamic peak and stop, since the amount of retarding force depends on the power being developed by the wheel speed. With extended range, a series of shorting contactors bypass part of the resistor grids, decreasing resistance, and allowing more current to flow. The majority of dynamic braking effort is then available to somewhere around 6 miles an hour when it drops off.
Above the peak dynamic braking effort, the limiting factor becomes the current rating of the dynamic brake grides. The dynamic brake retarding force is reduced as speed increases since their amperage output is limited. Most, if not all, of MPI's passenger lcomotives (MP36 and MP40 designs) delivered over the last few years have had a "High Speed" dynamic brake arrangement. These have more grids and two fans, similar to a SD arrangement. At higher speeds, these provide more resistance grids and braking capacity compared to the typical four axle system. As speed slows, a shorting contactor bypasses the extra grids, and the engine then has the typical standard dynamics. Same theory as extended range for lower speeds, but used in the opposite manner for higher speeds.
When Blended Braking is in effect, the system controls the engine's brake cylinder pressure so that the total retarding force developed by the engine is in proportion to the amount being requested by the brake pipe application. When dynamics are providing sufficent braking effort to meet the requested retarding force, brake cylinder pressure is at a minimum. At either side of the maximum dynamic braking peak, as speed slows or increases and dynamic brake alone is insufficent to meet demand, the system will adjust the engine brake cylinder pressure so that the resulting combined (or blended) braking effort of both the dynamic brakes and engine brake cylinder pressure is fairly constant.
> regardless of how it's accomplished. You can go into blended braking manually with either
> passenger or freight simply by using the appropriate throttle handles on the control
> stand.
I have never seen the term "Blended Braking" applied to the combined use of train air and dynamic brakes, though it might be used by some. For this discussion, I assumed Blended Braking to refer to a system that combines dynamics and brake shoe forces on the same vehicle. It sems to me some LRVs also use a similar system. In typical freight service, the combined use of the engine independent brakes and dynamic brakes is specifically prohibited by rule, except just prior to stopping when dynamcis are inefective.
>However from what I've seen and heard of management's new way
> of running trains, controlling slack isn't a very high on their agenda these days.
Agreed - Fuel saving methods seem to have taken priority over slack control in managements' often tiny little minds. I don't recall burning a little extra fuel to control slack as being the cause of derailing a train.
> Dynamics are NOT a "primary brake" because they don't work when the train is not moving. This is
> the FRA's definition of "primary brake".
True, but in a properly working passenger engine blended brake system, it is the dynamic brake that is primarily used and is supplemented by the engine independent brake pressure as required. This in turn results in the common issue that dynamics are not a fail safe system, and the unexpected or unrealized partial or full failure can quickly get you in trouble, especially if your expected stopping distance or grade speed balance is based on the dynamics working with no or little room for compromise.
> >When dynamic brake is at capacity, the system allows brake cylinder pressure to build up to the
> appropriate level.
>
> No, this is when it bleeds it off to the 10 psi or so which you mentioned. The idea is to have the
> dynamics do as much of the work as possible until they fade (at around 11 mph)
I stand by my statement. The Blended Brake system controls the amount of engine brake cylinder pressure when an automatic brake pipe application is in effect on the engine(s). When the dynamic brakes are generating the retarding force being called for, the brake cylinder pressure is reduced to a minimum amount. On a typical freight engine, maximum dynamic braking effort is obtained around 21 mph, but this can vary depending on the gear ratio and dynamic grid resistance. On passenger engines with higher gear ratios, the maximum braking effort is reached at a higher speed - the operator's manual shows 27 mph for a F40PH-2D with 58:19 gearing for 100 mph maximum speed. The dynamic baking effort is directly proportional to speed between the maximum dynamic peak and stop, since the amount of retarding force depends on the power being developed by the wheel speed. With extended range, a series of shorting contactors bypass part of the resistor grids, decreasing resistance, and allowing more current to flow. The majority of dynamic braking effort is then available to somewhere around 6 miles an hour when it drops off.
Above the peak dynamic braking effort, the limiting factor becomes the current rating of the dynamic brake grides. The dynamic brake retarding force is reduced as speed increases since their amperage output is limited. Most, if not all, of MPI's passenger lcomotives (MP36 and MP40 designs) delivered over the last few years have had a "High Speed" dynamic brake arrangement. These have more grids and two fans, similar to a SD arrangement. At higher speeds, these provide more resistance grids and braking capacity compared to the typical four axle system. As speed slows, a shorting contactor bypasses the extra grids, and the engine then has the typical standard dynamics. Same theory as extended range for lower speeds, but used in the opposite manner for higher speeds.
When Blended Braking is in effect, the system controls the engine's brake cylinder pressure so that the total retarding force developed by the engine is in proportion to the amount being requested by the brake pipe application. When dynamics are providing sufficent braking effort to meet the requested retarding force, brake cylinder pressure is at a minimum. At either side of the maximum dynamic braking peak, as speed slows or increases and dynamic brake alone is insufficent to meet demand, the system will adjust the engine brake cylinder pressure so that the resulting combined (or blended) braking effort of both the dynamic brakes and engine brake cylinder pressure is fairly constant.
