Published on Jan 10, 2016
In Railway wagon as well as in passenger cars the braking system plays a very important role to stop the train, to maintain the speed of the train within specific limit. Brakes are the devices on the trains to bring it to standstill.
A moving train contains energy, known as kinetic energy, which needs to be removed from the train in order to cause it to stop. The simplest way of doing this is to convert the kinetic energy into heat energy. The conversion is usually done by applying a contact material to the rotating wheels or to discs attached to the axles. The material creates friction and converts the kinetic energy into heat energy. The wheels slow down and eventually the train stops. The material used for braking is normally in the form of a block or pad.
The vast majority of the world's trains are equipped with braking systems which uses compressed air as the force to push the blocks on to wheels or pads on to discs. These systems are known as "Air Brakes" or "Pneumatic Brakes". The compressed air is transmitted along the train through a "brake pipe". Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle. The system is in widespread use throughout the world.
An alternative to air brake known as vacuum brake is also used in railway wagon. Like the air brake the vacuum brake system is contolled through a brake pipe conecting a brake valve in the driver’s cab with braking equipment on each vehicle. The operation of the brake equipment on each vehicle depends on the condition of vacuum created in the pipe by an ejector or exhauster.
Another braking system used by electric train is Electric Dynamic Braking System.The basic principle of operation is to convert electric motor into a braking generator dissipating the kinetic energy as heat energy. Regenerative braking is similar to Dynamic Braking. Only difference is that, it transmits generated electricity to overhead wires instead of dissipating it as heat, and is becoming more common due to it’s ability to save energy.
To overcome the problems associated with air braking system, a new braking system called “Electronically Controlled Pneumatic Brakes” (E.C.P.) is generally used in Railway Wagons. Before E.C.P. Electro Pneumatic Brakes are also in use.
This is all about braking systems used in railway wagon as well as in passenger cars. To prevent accidents some Automatic Systems like Automatic Warning Systems are used. They vary the speed or stop the train according to the signal aspects.
A new form of electronically control of air braking is currently being tested by a number of railroads. 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 consists 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". Before E.C.P. Electro-pneumatic brakes were fitted on trains. They 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 driver and it only requires the standard train line pipe
There is a control box on top of the driver's console. When he wants to apply the brakes, he 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 driver 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 driver 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 will respond to each status message from the head end. All cars 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.
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.
Some of the benefits of ECP braking are Instantaneous response to the engineer's commands on all vehicles, graduated release of brakes and continuous replenishment of reservoirs. With the new responsiveness of E.C.P. braking distances will be reduced. This will allow shorter stopping distances and will in turn, allow higher speeds. Another benefits are in train handling improvements, cycle time reduction energy saving, lower wagon maintenance cost
An alternative to the air brake, known as the vacuum brake, was introduced around the early 1870s, the same time as the air brake. Like the air brake, the vacuum brake system is controlled through a brake pipe connecting a brake valve in the driver's cab with braking equipment on every vehicle. The operation of the brake equipment on each vehicle depends on the condition of a vacuum created in the pipe by an ejector or exhauster. The ejector, using steam on a steam locomotive, or an exhauster, using electric power on other types of train, removes atmospheric pressure from the brake pipe to create the vacuum. With a full vacuum, the brake is released. With no vacuum, i.e. normal atmospheric pressure in the brake pipe, the brake is fully applied.
The vacuum in the brake pipe is created and maintained by a motor-driven exhauster. The exhauster has two speeds, high speed and low speed. The high speed is switched in to create a vacuum and thus release the brakes. The slow speed is used to keep the vacuum at the required level to maintain brake release. It maintains the vacuum against small leaks in the brake pipe. The vacuum in the brake pipe is prevented from exceeding its nominated level (normally 21 Hg) by a relief valve, which opens at the setting and let’s air into the brake pipe to prevent further increase.
It is the means by which the driver controls the brake. The brake valve will have (at least) the following positions: "Release", "Running", "Lap" and "Brake On". There may also be a "Neutral" or "Shut Down" position, which locks the valve out of use. The "Release" position connects the exhauster to the brake pipe and switches the exhauster to full speed. This raises the vacuum in the brake pipe as quickly as possible to get a release.
In the "Running" position, the exhauster keeps running but at its slow speed. This ensures that the vacuum is maintained against any small leaks or losses in the brake pipe, connections and hoses.
"Lap" is used to shut off the connection between the exhauster and the brake pipe to close off the connection to atmosphere after a brake application has been made. It can be used to provide a partial release as well as a partial application, something not possible with the original forms of air brake.
"Brake On" closes off the connection to the exhauster and opens the brake pipe to atmosphere. The vacuum is reduced as air rushes in