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Why do pistons rotate in only one direction?

  1. Dec 30, 2012 #1
    Why is it that pistons always, say rotate clockwise, and not counterclockwise?
    Is there some sort of imbalance on the piston that causes it to only move in one direction?
  2. jcsd
  3. Dec 30, 2012 #2


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    Could you qualify your question somewhat?

    Perhaps you mean a rotating crankshaft attached to a piston through a connecting rod.
  4. Dec 30, 2012 #3
    The types of engines used in automobiles have pistons connected in such a fashion mentioned above, able to pivot relative to the connecting rod, but do not rotate around in the cylinder. Lifters that operate the intake and exhaust valves do rotate I believe, not sure if the direction of rotation is specific to a single direction or random though.
  5. Dec 30, 2012 #4


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    Doesn't clockwise/counterclockwise depend entirely on what side you are looking at them from?
  6. Dec 30, 2012 #5
    Sorry, I meant to say a rotating crankshaft attached to a piston through a connecting rod. Of course clockwise/counterclockwise depends on the side you're looking at, but I was just giving an example of one viewpoint of the crankshaft. Sorry for the mix up!

    Let me clarify:

    Why do pistons always turn the crankshaft in only one direction? Why is it that the crankshaft always turns in the same direction? What causes this to happen?
  7. Dec 30, 2012 #6


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    4-stroke gasoline engines and diesel engine are designed to rotate one way only because of the times the valves open and close relative to the piston movement.

    The reason the crankshaft always starts turning the same way is because the engine's starter motor always turns it that way!

    Some 2-stroke gasoline engines will run "backwards" if you start them rotating the opposite way, but this isn't a good idea. On the power stroke when the piston is moving down, the crank shaft is offset in different directions (mirror images of each other) depending which way the engine is turning. The parts are designed to work better turning the "right" way than the "wrong" way.

    Some steam pston engines with fixed transmission to the vehicle's wheels (e.g. railway engines) are designed to run in either direction, with "reversing valve gear" to send the steam to the right place at the right time .
  8. Dec 30, 2012 #7
    I understand this, but how does it cause the motor to turn that way?

    Say there is a 1 stroke engine, what causes the crankshaft to rotate in the same direction every time?


    In the diagrams on this site, there is some sort of counterweight (looks like the head of an axe) that is opposite of the piston. The shape of this "counterweight" is asymmetrical. Could imbalance cause the crankshaft to rotate in the same direction every time?
  9. Dec 30, 2012 #8


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  10. Dec 31, 2012 #9

    Ranger Mike

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    alpha zero is correct. you can make an automotive engine run " backwards ' so that the lifting action of the left of the engine is now made to push down under acceleration. This would assist a race cars handling by canceling out the left to right weight transfer but it is very expensive to set up the starter, cam shaft , ignition gear, and assorted components. One more thing..just about all production pistons are made with offset of wrist pin location to piston diameter. This is done to reduce piston slap so you would need custom pistons for reverse rotation engine as well.
    finally, you would need all new transmission gears and differential gears as well...not cheap
  11. Dec 31, 2012 #10


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    About 10 years ago, when Valentino Rossi switched from Honda to Yamaha, he took some members of the engineering team with him. And from what I remember the chief engineer, Jeremy Burgess ordered to change the way the engine rotated as he believed the bike will handle better after that (for sure there were plenty of other changes as well, I just remember this one being reported). So my understanding is when money is not a concern, it is doable.
  12. Dec 31, 2012 #11


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    Of course you can build an engine that is the "mirror image" of another one, and run it turning the opposite way. For bike engines, the gyroscopic forces on the engine can be significant, so that would affect the performance of a race bike. It is also significant for aircraft engines. For example WWII fighter plames handled differently in tight left and right turns. With any type of aircraft, you need to be aware whether the gyro forces will push the nose to the left or the right when you lift the nose wheel off the runway and the aircraft rotates from "horizontal" to "nose pointing upwards". Different designs of aircraft engines rotate in different directions (but all the engines of one type rotate in the same direction).

    But I don't think that is what the OP was asking about.

    With some old two-stroke bike engines, you could start the engine the "wrong way" by pushing the bike off its stand backwards with the bike in gear, and pulling the clutch as the engine fired. This could be "interesting" if you then let a friend have a ride, and he didn't expect to set off travelling backwards.....
  13. Dec 31, 2012 #12

    jim hardy

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    Who said they all rotate one way?

    Many boats are equipped with twin V8 engines having opposite rotation.
    It's a simple matter of making a camshaft that opens the valves in right sequence, fitting an oil pump, water pump, distributor and alternator that're made for "reverse" rotation.

    Aircraft engines generally rotate backward from automobile engines.
    (Aircraft usually CW viewed from rear, automobiles CCW.)

    Most two stroke engines will run either way. Many Mercury outboards of the 1960's had no gearshift - to go into reverse you retard the spark past TDC and start the engine backward with a reversible electric starter.
    In the days of two stroke motoecycles, many a rider got a surprise when he let out the clutch and took off backward because his cantankerous engine was running backward.

    oops - i see somebody else answered. Oh well -

    EDIT perhaps i misunderstood the question.
    Were you asking why the crankshaft does not reverese rotation, perhaps oscillating?
    Angular momentum.
    The crankshaft system is always built with a moment of inertia that guarantees enough momentum at lowest operating speed to carry a piston through its next compression stroke. Usually it's done with a substantial flywheel , but on lawnmowers and airplanes the flywheel is undersized and the blade or propeller provides part of the necessary momentum.

    old jim
    Last edited: Dec 31, 2012
  14. Dec 31, 2012 #13
    thanks everyone. i understand the breadth of the topic now. i have one more question though.
    how do you control the direction of the rotation of a crankshaft?
  15. Dec 31, 2012 #14
    How do you control which direction the bike moves if you start it from a complete standstill and no other force is applied to it (besides the force of the pistons on the crankshaft)?
  16. Jan 1, 2013 #15
    You appear to be under the impression that internal combustion engines begin running from a dead stop -- that the ignition is turned on, causing combustion that starts the rotation from zero rpm. Engines are started with the assistance of an external force (such as an electric motor) that initiates the rotation. Therefore, it is that external starter which facilitates the "correct" direction of crankshaft rotation.
  17. Jan 4, 2013 #16
    Thank you!!! That's exactly what I was thinking, but now I understand.
  18. Jan 13, 2013 #17
    BRP has electronic reverse on my of there skidoo snowmobiles. It shuts the engine down and starts it up in the reverse direction to allow the snowmobile to back up. Great invention. Lightweight and reliable!
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