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

In summary, a locomotive's wheel is powered by steam that rotates only in one direction due to inertia. A modern engine with pistons will run in whichever direction it is spun to start. Scale model airplane engines often run backwards when started by hand.
  • #1
danihel
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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.
 
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  • #2
Google "valve gear."
 
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  • #3
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.
 
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  • #4
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
 
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  • #5
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."
 
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  • #6
danihel said:
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.
 
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  • #7
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.
 
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  • #8
@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.
 
  • #9
gmax137 said:
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
 

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

A piston-powered wheel keeps turning in one direction due to the principles of Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. In the case of a piston-powered wheel, the action is the combustion of fuel in the engine, which creates a force that pushes the piston downwards. This downward force is then transferred to the crankshaft, causing it to rotate. The rotation of the crankshaft, in turn, moves the pistons back to their original position, creating a continuous cycle that keeps the wheel turning in one direction.

2. How does the direction of the piston movement affect the rotation of the wheel?

The direction of the piston movement is crucial in determining the rotation of the wheel. In a piston engine, the pistons move in a reciprocating motion, which means they move back and forth. The direction of this movement is controlled by the timing of the engine's valves. If the valves are timed correctly, the pistons will move in a sequence that creates a continuous rotation of the crankshaft, resulting in the wheel turning in one direction.

3. What role do the gears play in keeping the wheel turning in one direction?

The gears in a piston-powered wheel act as a mechanical advantage, amplifying the force created by the engine and transferring it to the wheels. The gears also help to maintain a consistent speed and torque, which is essential for the wheel to keep turning in one direction. The size and ratio of the gears are carefully designed to ensure efficient power transfer and smooth operation of the wheel.

4. Can the direction of the wheel's rotation be reversed in a piston-powered engine?

Yes, the direction of the wheel's rotation can be reversed in a piston-powered engine by changing the timing of the valves. This can be achieved by adjusting the camshaft or using a different camshaft profile. By changing the timing, the sequence of piston movement is altered, resulting in a reversal of the wheel's rotation direction.

5. What happens if the pistons in a piston-powered engine move in the wrong direction?

If the pistons in a piston-powered engine move in the wrong direction, it can cause severe damage to the engine. This is because the timing of the valves and the sequence of piston movement are designed to work together to create a continuous rotation of the wheel. If the pistons move in the wrong direction, the timing is disrupted, and the engine may not function correctly, leading to potential engine failure.

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