Friday, 12 May 2017

Braking Curve

In railway engineering, you will sometimes hear talk of a "braking curve", often in connection with train performance, platform re-occupation times or signalling. We use a braking curve to calculate how long it will take a train to stop from a given speed. It can be used to determine both service and emergency braking distances. It can also give braking times, if needed. Here we look briefly at the braking curve and what it means.

The braking curve is the shape formed on a speed/distance chart by a train as it slows down from normal speed to a stop.  A typical curve looks like this:

The curve begins when the driver applies the brake. The brake system take a few seconds to build up to the required braking rate (the "feed up" time) and then the train begins to slow down.

With a constant brake demand, as selected by the driver, the train slows down more rapidly as the speed falls. This is because, at the lower speed, the train has less energy to dispose of. If the brake is left on at the same level all through the stop, eventually the curve will get steeper and steeper until it ends vertically at the stop. If this is allowed to happen, the train will stop with a sharp bump and a lot of coffee will be spilt. To prevent it, a skilled driver will ease off the brake as the speed falls and this will allow him to stop the train gently. The effect of this can be seen on our curve as it nears the stopping point.

Our drawing also shows an "equivalent straight line" curve. This is a simple way of showing the stopping distance that we can expect a train to cover, given an equivalent deceleration rate. It can be used to calculate stopping distances for approximate signalling calculations, for example, although today, computer programs make accurate and detailed calculations simple. The straight line braking curve can used, for example, for outline planning or to demonstrate principles.

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