Shop Notes - Locomotive Blended Braking
Blended braking is a feature widely used on passenger type locomotives since the late seventies. On non-blended brake locomotives the locomotive engineer has two different air brake systems, the same as for freight locomotives, the independent and the automatic. Use of the independent brake results in brake application in the locomotive consist only. The automatic brake system is used to apply and release the brakes on the entire train including locomotives. Both the independent and the automatic can be used simultaneously.
Locomotive blended braking is the braking of a locomotive through simultaneous use of both the dynamic brake and the air brake. On locomotives equipped with the Blended Brake system, the setup of the dynamic brake is obtained simply through the use of the automatic brake valve by the engineer. Any brake pipe pressure reduction of more than approximately 8 PSI results in the kick in of the dynamic brakes if all other control conditions are met. This reduction of brake pipe pressure is normally obtained at or near the minimum reduction position of the automatic brake valve handle.
Blended brake control components
On EMD Dash 2 locomotives an electronic module has been developed to provide an interface between the air brake and the dynamic brake systems. This module is known as the DA module. The following components are also required over standard 26L type air brake system for blended brake control (one of each):
- PTCA pressure transducer (call)
- PTBC pressure transducer (brake effort)
- MVH magnet valve -- Hold
- MVR magnet valve - Release
- MVSR magnet valve - wheel slip relay
- EPS pressure switch - emergency
- H-5 Relayair® valve - 25 PSI
When the engineer reduces the brake pipe pressure, an air brake signal is developed by the 26-C/F control valve at pipe #16 in response to the brake pipe pressure drop. The pressure of this pneumatic signal builds up in proportion to the brake pipe reduction and is used as the brake cylinder control pressure. The greater the brake pipe reduction the higher the #16 pipe pressure. As the #16 pipe is used as the brake cylinder pilot air pressure to the J relay, brake cylinder pressure is directly proportional to #16 pipe pressure during automatic brake application.
A pressure transducer reads the #16 pipe during brake application and converts this pressure signal into an electric value to the electronic DA module. This transducer is known as the PTCA (or "call" transducer). With all control conditions encountered, when the pressure exceeds 10 PSI (nominal) at the PTCA the dynamic brake is kicked in by the DA module. About 3 seconds are required to complete the switching loop as to have both the DB and the air brake acting simultaneously.
As #16 pressure keeps increasing, a second pressure transducer (PTBC) sends a signal to the DA module as the brake effort reference.
As both the air brake effort and the dynamic braking force are taken by the DA module for braking calculation, the maximum braking capability of the locomotive will be obtained from maximum dynamic brake retarding force in combination or "blending" with a portion of the brake cylinder pressure as required.
To avoid wheel sliding during blended brake the air brake pilot pressure is exhausted at the MVR through the MVH. When the desired pressure is obtained, MVR is released while MVH is maintained energized. As the dynamic brake retarding force is proportional to the locomotive speed, the brake cylinder pressure is increase through the action of the DA module and magnet valves to maintain maximum locomotive braking efficiency. In such case the MVH is triggered by the DA module to increase the #16 pipe pressure thus brake cylinder pressure. When the locomotive speed is reduced down to 5 MPH the dynamic brake is kick out and maximum air brake pilot signal is restored.
The MVSR magnet valve is energized if wheel sliding occurs providing the exhaust of the pneumatic pilot pressure while DB is maintained during wheel sliding correction.
As the independent can be used at any time during blended brake the DA module provides wheel slide protection if the maximum braking capability is exceeded.
If for any reason the brake pipe pressure drops below 25 PSI, the #16 pilot pressure bypasses the blended brake control system and is directly ported to the J relay valve for maximum brake application, picking up the EPS pressure switch. Actuation of the EPS switch results in a limitation of dynamic brake output to 250 amps over air brake.
Even though today's locomotives are equipped with full electronic air brake systems, these extra components are not required to provide blended braking because of smart software capabilities--the logic of the system is very similar.