Why does a piston-powered wheel keep turning in one direction?

[danihel]

Hi,
First I'd like to apologize, for this question probably sounds devastatingly stupid to people on the forum.

Why does the wheel powered by a typical locomotive steam engine rotate only in one direction?

I don't even understand why doesn't it sometimes just rotate half-way around and then turn backwards. Would that be only because of inertia?

I guess the same could be asked about a more complicated modern type engine with pistons.

 

[Bystander]

Google "valve gear."

 

[Torbert]

A two stroke diesel will run whichever way it is spun to start it. The force on the piston cares not whether the crankshaft throw is to the left or right.
Scale model airplane engines often run backwards when started by hand.

 

[hutchphd]

I am reminded of a vignette from E.B White:

My boy loved our rented outboard, and his great desire was to achieve single-handed mastery over it, and authority, and he soon learned the trick of choking it a little (but not too much), and the adjustment of the needle valve. Watching him I would remember the things you could do with the old one-cylinder engine with the heavy flywheel, how you could have it eating out of your hand if you got really close to it spiritually. Motor boats in those days didn't have clutches, and you would make a landing by shutting off the motor at the proper time and coasting in with a dead rudder. But there was a way of reversing them, if you learned the trick, by cutting the switch and putting it on again exactly on the final dying revolution of the flywheel, so that it would kick back against compression and begin reversing.Approaching a dock in a strong following breeze, it was difficult to slow up sufficiently by the ordinary coasting method, and if a boy felt he had complete mastery over his motor, he was tempted to keep it running beyond its time and then reverse it a few feet from the dock. It took a cool nerve, because if you threw the switch a twentieth of a second too soon you would catch the flywheel when it still had speed enough to go up past center, and the boat would leap ahead, charging bull-fashion at the dock.We had a good week at the camp.

From perhaps my favorite essay from a craftsman of the language:
http://fd.valenciacollege.edu/file/jcarpen1/Week_3_EBWhiteLakeEssay.pdf

 

[gmax137]

Two stroke dirt bikes would run backwards, too. I had a friend who let out the clutch, while unloading a bike from a pickup - backing down the ramp. He did it on purpose for a laugh, scooting away from the bottom of the ramp "in reverse."

 

[anorlunda]

I don't even understand why doesn't it sometimes just rotate half-way around and then turn backwards. Would that be only because of inertia?

Yes inertia. In many engines, they add a flywheel to increase the inertia.

 

[Baluncore]

It depends on the steam engine.

Single cylinder steam engines will often need to be turned to a starting position.

On the other hand, a railway engine has double acting cylinder(s) on each side, with a solid axle. The crank pins on either side are in quadrature phase, 90°, sine and cosine. So the torque and direction of rotation is under full control and there is no dead position.

A compound steam engine also employs a phase difference between the high and low pressure cylinders, so it is both efficient, generates a more constant torque, and can be started from any position.

 

[danihel]

@anorlunda Thanks! Never heard of flywheel, interesting concept.

@Baluncore Thank you! ..*facepalm!.. Of course, when the piston is at the very end on one side, its in the middle on the other side of the train and the crank attachment is positioned either at the top or bottom from the wheel-axis, clearly pulling/pushing it in one direction.

 

[cosmik debris]

Two stroke dirt bikes would run backwards, too. I had a friend who let out the clutch, while unloading a bike from a pickup - backing down the ramp. He did it on purpose for a laugh, scooting away from the bottom of the ramp "in reverse."

Yes, in Motorbike Gym-kanas some of the old guys would slow their (often 197 Villiers) engines until they nearly stalled and with a flick of the throttle reverse the engine and back through a slalom course. As youngsters we were pretty impressed. My own motocross bike (Bultaco Pursang) would sometimes run backwards after a kick start and after roaring backwards of of the start line at one race I learned to always push start.

Cheers