Electrically Controlled Pneumatic Brakes

Introduction

A description, specially written by Randy Buchter of the Electronically Controlled Pneumatic brakes being used on various railroads in the US.

More information on train braking systems in the Brake GlossaryAir BrakesE-P BrakesVacuum BrakesNorth American Freight Train Brakes and the Modern Railway Glossary.


ECP Brakes - Background

A new form of electrical control of air braking is currently being tested by a number of railroads in the US. It is known as ECP and uses modern electronic techniques to overcome the problems of air braking on long freight trains.

The pure air control brake system invented by George Westinghouse in the 1860s and still used by almost all freight trains in the US and in many other parts of the world suffers from two main problems. It takes a long time for the air messages to travel along the train and there is no graduated release. For example, the delay for a reduction in train line pressure to travel from the leading locomotive to the rear of a 150 car consist can be 150 seconds. Also, you have to fully release the brake and wait for the supply reservoirs to recharge before you can reapply. Electrical control can overcome these difficulties.

ECP refers to Electronically Controlled Pneumatic brakes, key word being "Electronically" as opposed to "electrically". Older systems fitted to passenger trains, (E-P brakes) use several train wires to operate individual valves or variations in switching of the wires to control brakes. Most of these systems use a second train line for main reservoir air supplies and they do not have the built-in two-way communications that ECP systems have. A car in an ECP brake train can do a self-diagnosis and report the information to the engineer and it only requires the standard train line pipe.

Operation

There is a control box on top of the engineer's console. When he wants to apply brakes the engineer pushes the button until the readout shows the amount of brake cylinder pressure (or percentage of braking effort) he wants. He releases the button; the control unit then codes and sends the signal to all cars. They in turn receive and interpret the message. They then begin allowing compressed air from their reservoirs to go to the brake cylinder until the desired cylinder pressure is achieved. The microprocessors on the cars will continuously monitor brake cylinder pressure against leakage and maintain the desired pressure.

If the engineer wants to reduce brake cylinder pressure he simply pushes the release button until the desired level is indicated, either partial or full release. Again a signal is coded and transmitted to the cars. The cars in turn do as commanded. If the engineer asks for only a partial reduction of braking effort, he can increase the effort again as needed without doing a full release first. The processor on the car is constantly monitoring brake pipe, reservoir tank and brake cylinder pressures.

When braking commands are not being transmitted, the head end (control) unit is sending out status messages. The last car in the train (which knows it is last due to the head end doing a train query and initialisation at start-up) will respond to each status message from the head end. All cars in the consist will monitor these messages, and if a car fails to receive three status messages in a row from either the head end or the rear end, it will assume that the train is broken in two or that the electrical line is broken. It will then initiate an emergency stop, while trying to tell the other cars and loco that it is doing so.

Power Sources

Each car has a rechargeable battery to provide the high power requirements when solenoids need to be activated. When the high power is not being used, the batteries will trickle recharge from the communications/power cable. (If the train uses radio communication the batteries will recharge while the car is in motion via an onboard generator creating power from the motion of the car, either an axle generator, or natural frequency vibration generator or some other type of device.)

The hardwired system uses roughly 25% of its signal capacity for brake commands and status messages. Distributed power, controlled via the same cable uses another 10-15%, leaving 60-65% of the signal capacity for special monitors on the car, such as bearing sensors, temperature sensors for reefers on tankers, pressure sensors for tankers, etc.

Manufacturers Systems

TSM, which was a subsidiary of Rockwell International, developed the first working ECP brake units. They are now owned by WABCO. In addition, Westinghouse Air Brake, New York Air Brake (a subsidiary of Knorr Corp.), GE/Harris and a small company called Zeftron, are developing ECP units.

TSM's first units worked in an "overlay" mode, where a module was placed between the air pilots and the actual valves, so that the system could work both ways. Zeftron started out working on an "emulator" brake valve, which totally eliminates the air pilots. The system, which must always be powered, looks for ECP commands. If it finds none, it monitors brake pipe pressure and behaves just like a standard air brake. If ECP command signals are present, the units behave like an ECP brake.

Because of the sequential operations of standard brakes, there is a flow control which limits how fast the air can flow into the brake cylinder. On ECP systems, because there is instantaneous reaction from all cars at once, these flow controls are not used. The lack of sequential activation and flow controls combined is what makes ECP brakes so responsive.

TSM is now introducing an emulator system. This enables cars fitted with it to work in ECP trains and non-ECP trains.  New York Air Brake has a system available for sale in the very near future. Westinghouse Air Brake is playing it cool, waiting for all of the specs to be written and all of the bugs worked out before they commit to anything.

Benefits

Some of the benefits of ECP braking have already been mentioned; instantaneous response to the engineer's commands on all vehicles, graduated release of brakes and continuous replenishment of reservoirs. But there are other and more significant benefits for the industry as a whole.

With the new responsiveness of ECP braking, braking distances will be reduced. A range of 30 - 70% reduction has been quoted. This will allow shorter stopping distances and will, in turn, allow higher speeds. The improved train handling will reduce slack action, breakaways and derailments and will lead to a reduction in draft gear maintenance.

There may be a price to pay. Although the current view is that brake shoe and wheel wear will be reduced, it is easy to see that engineers will develop their handling skills with the new system and this will lead to higher speeds needing more and heavier brake applications. A wise railway management will recognise this and will review its speed limit zones to ensure the maximum benefits are obtained without excessive brake usage.

Developments

There was much discussion amongst experts regarding the need for an end-of-train (EOT) device or letting the last car act as the end-of-train beacon. It seems that the last word on EOT beacons was that there will be one!

There are committees that are developing specs right now to permit the addition of monitors onto cars. The monitors will have their own microprocessors and will only send a signal to the head end when something on the car is going out of specified limits. This keeps the communications line open for brake commands, loco commands, and emergency messages.

A further development will be the use of the electronic train line for diagnostics, where the head end position can be informed of hot boxes, car load temperatures, tanker pressures, wagon doors not closed, parking brake off/on and the like.

ECP Record

There was a record-breaking, 600 km round trip by a train fitted with ECP braking in Australia.  On 28 June 1999, a train comprising 240 wagons, five GE Dash 8 diesel-electric locomotives and weighing 37,500 tonnes was equipped with the GE Harris EPx radio-based, electronic brake control system.  It was the longest and heaviest train ever to be fitted with an ECP brake system.  The locomotives were fitted with the same company's Locotrol remote locomotive control system.  The train operated over the BHP Iron Ore line between Port Headland and Yandi Mine.  Source IRJ.

See also the Brake GlossaryAir BrakesE-P BrakesVacuum BrakesNorth American Freight Train Brakes and the Modern Railway Glossary.