What is the magic about two pistons pulling about so many carreges in steam rail engines?

Question

for that matter of pulling a rail carreges only two pistons are put to work,while so many carreges are linked ,i am wondering what will be the horse power that is employed to engine and carreges to run in leverage

Answer 1

the "magic" is the mechanisms, bringing back the nostalgia of a by gone era.

they are much more evisceral and exciting to watch, it is not anything people can give you a quantitive answer for.

You can actually see things happening as they go by and just seems exciting.

as for the techincal aspects of your question, like any modern locomotive the horsepower aspect will very from model to model.

The key to just a few pistons being able to do that work is pressure and leverage, the pistons are huge compared to amount of cubic inches displacement of a deisel engine. If you look at the attatchement points on the wheel of the connecting rod from the piston you will see it is out near the edge of the wheel, taht increases the leverage or turning force.

Answer 2

It is a mistake to think that only two pistons are always used. Those are the ones you see, as they are on the outside, but many locomotives have four - two inside the frame, two outside. Some have just three - Sir Nigel Gresley, the designer of the fastest steam loco ever, favoured this configuration on the basis that the third middle cylinder balanced out the hammer blow on the track caused by two or four cylinders. Other locos were compounds - that meant they had high and low pressure cylinders. Beyond that, I cannot help with the theoretical calculation of the horse power (usually drawbar horse power) of a steam locomotive.

Answer 3

The pistons are supplied by steam from the boiler which is kept under pressure at anything from 150 to 280 pounds per square inch, depending on the size of the locomotive.

If that sounds bafflingly technical, imagine the inside of the boiler divided into squares of an inch and the weight of a grown man bearing on each of those squares. That is how much pressure is contained in the boiler.

If this pressure were released instantly, the result would be a massive explosion, with the destructive capacity of a fairly large conventional bomb, but since the release is controlled the steam is able to do useful work powering the engine.

When the steam is admitted into the cylinder, first by opening the regulator (throttle, US/Canada) and second by the opening of the inlet port through the valve gear, its pressure is sufficient to overcome the inertia of the train, and the engine and its carriages start to roll. As the piston reaches the end of its stroke, the inlet port furthest from the piston closes while the inlet port nearest the piston opens. This forces the piston back along the cylinder; at the same time the exhaust port furthest away from the piston opens and the used steam is expelled.

In a 'simple' engine the exhaust steam goes straight up through the blast pipe and the chimney while in a compound engine it goes into another cylinder before being exhausted.

As the steam passes through the blast pipe and the chimney it draws the air out of the smokebox (the hollow chamber at the front of the boiler) and this in turn is replaced by air passing through the fire grate, the fire and the firebox at the rear of the boiler and then through tubes which run from the rear of the boiler to the front, thus spreading the heat from the fire and helping to generate steam. This is known as 'draughting' and the first locomotive to do this was George Stephenson's 'Rocket'.

The oscillating motion of the piston is converted to the circular motion of the driving wheel through the mechanism of connecting rod and cranks (known as 'reciprocation').

As the train gathers speed it also gathers momentum and does not need so much steam in the cylinder to overcome the inertia because it is already moving, so the valve gear can be adjusted to close the inlet valve sooner along the stroke. This is done by adjusting the valve gear and is known as 'notching up' and the point along the stroke at which the inlet valve is closed is known as the 'cut off'.

As another poster pointed out, a steam engine may have three or four cylinders and may be a simple or compound type.

Such is the power of the steam locomotive that, according to the work they are designed for, they are able to pull trains at speeds of 100mph+ or haul thousands of tons of freight.

The normal way of calculating the power of a locomotive (in the UK) is through tractive effort, expressed in pounds. This can be around 15,000lbs for a small tank engine compared with 40,000lbs or more for a large, heavy duty engine.

Steam locomotives are extremely robust and reliable but their main drawback is that they are extremely heavy on water and fuel, and are also labour intensive which is why they have largely been replaced by the more energy efficient and less labour intensive diesel and electric locomotives.