Steam engine question?

Question

When a steam engine comes to a standstill, what happens if one of the two cylinders stops at top dead centre, and the other at bottom dead centre. Seems to me the engine could not travel in either direction, as the piston rod and crankshaft are in a straight line on both sides.

Answer 1

I do not believe that scenario ispossible because nither is at tdc or bdc at the same time.

Answer 2

That is something that never happens because when one piston is at the top the other is half way down. Just think of piston one going down under pressure, by the time it gets to the bottom piston two has taken over and is half way down under pressure, The next stage is piston one starting to take over again but going up under pressure and by the time it has got half way up the piston two has started to rise under pressure. A steam engine is different to a petrol engine. Petrol engines work by pressurising only the top of the pistons, where as steam engines work on a double action of pressure being applied to both side of the piston pushing up and pushing down.The connecting cams are not 180 degrees to each other but 90 degrees. ie one at 12 o'clock and one at 3 o'clock

Answer 3

I think you will find that the linkage arrangement obviates that situation.  

Answer 4

I think a steam engine has the ability to isolate one piston from the pressure through use of a valve to overcome this. This is so when one piston is returning, there is no pressure behind it. Otherwise, it wouldn't matter what position the pistons were in, the pressure behind them would always be the same.  

Answer 5

If a 2 cylinded steam engine stops in the position you describe - when the piston is travelling on it's stroke towards TDC, it operates a shuttle which redirects steam to the other side of the piston, hence perpetuating the movement.

This is how a double-acting cylinder works - the act of the piston moving forwards forces steam to the other side, from where this steam will forced the piston back again.

I can't see how the situation you describe, could occur.

DREW R - even if there were equal pressure either side of a piston, it would still move - this is because the steam in the piston rod side of the cylinder acts on a smaller area than in the steam on the side without the piston rod. The equation is force x area......one side of the cylinder has a bigger area because of the area of the piston rod.

Answer 6

i go for the wikipedia answer  

Answer 7

From wikipedia:

"If a single-cylinder steam engine stops in either of the dead centre positions it must be moved off the dead centre before it will re-start. In small engines this is done by turning the flywheel by hand. In large engines the flywheel is moved with a lever or "turning bar". Both operations must be done with care to avoid the operator becoming entangled in the machinery.

Steam locomotives normally have two cylinders with the cranks set at right angles so one piston will always be off the dead centre and no starting assistance is required..."

Answer 8

there is a compression relief valve that kicks it past TDC