How does a double acting steam engine work?

[thegr81]

hello

i'm a first year student of mech eng...although its embarassing to admit, but i actually do not understand how a double acting steam engine functions. more specifically, i don't understand the inner-mechanism of the return-stroke. how does the piston come back to the top dead centre? is the steam re-routed to the other side? or does the flywheel's inertia cause the return?

can anybody please show me a diagram/animation of what happens inside the valve-system and cylinder during a complete revolution of the flywheel?

thanks

 

[brewnog]

http://www.howstuffworks.com/steam.htm