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Reverse crankshaft movement

  1. Nov 22, 2009 #1
    Hi guys
    Let a single cylinder ic engine. Now when we start the engine , then depending upon the position of crank the last time we switched off the engine, it is possible it may rotate in reverse direction because piston just has to reciprocate so acco to crank position at rest it may move reverse when piston comes down. But we want movement only in forward( or take clockwise )in order to move car forward.
  2. jcsd
  3. Nov 22, 2009 #2
    The starter rotates the crank 1 way (starter will throw it 1 way only so that a spark can make it run). If you tried rotating it the other way the valves would open at the wrong time and the spark would not ignite anything.

    It would be impossible for an engine to run backwards without retiming the spark and reversing the cam profiles.
    Last edited: Nov 22, 2009
  4. Nov 23, 2009 #3
    Is this true on two-cycle engines? I have heard of cars (SAAB?) with two-cycle engines (and manual transmissions) either coast or push-starting with the engine running backwards.
    Bob S
  5. Nov 23, 2009 #4
    Im not sure about 2 strokes really. i've never heard of one running backwards but that doesnt really mean much :)
  6. Nov 23, 2009 #5
    Here is a SAAB with a two-cycle engine that used all forward (manual transmission) gears while backing up.
    http://www.cartalk.com/content/puzzler/transcripts/200850/answer.html [Broken]
    Bob S
    Last edited by a moderator: May 4, 2017
  7. Nov 23, 2009 #6
    Yes engines can run backward either by design or not by design.

    Two cycles can be designed to start either way with a reversible starter. Some snowmobiles have this system because they have no reverse gear.
    See Gasoline engine reversibility at bottom of page:

    Some diesel engines can end up running backwards when you lug it down. In this case the air filter becomes clogged with exhaust and the oil pump does not work, so it is not a good thing.
  8. Nov 23, 2009 #7
    I recall (back when I earned my spending money mowing lawns as a kid) that my old mower would sometimes start in reverse. This happened when the two-cycle engine back-fired as I pulled the rope. Occasionally, this would result in reverse running. It was easy to tell when it happened because the blade would "chew" the grass rather than cutting.
  9. Nov 24, 2009 #8
    As chris said, 4s cycle cant reverse start, due to inappropriate valve timing. Try to imagine the reverse rotation, valve motion & spark motion, motions are not similar.

    A 2 stroke cycle can run in the reverse direction, its not that hard to imagine, there are no valves, so no timing issue. its perfectly similar in either direction.
  10. Nov 27, 2009 #9
    Just hope your connection rod from your piston is not directly vertical when it fires, then your in trouble
  11. Nov 28, 2009 #10
    With any engine the danger you speak of is non existent darkside.

    The engine doesn't start with the spark. It's got to have been moving before hand. Otherwise how is it going to have compressed fuel in the cylinder eh?

    If your statement is a joke (I can't tell) please ignore the above.
  12. Nov 28, 2009 #11


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    2 stroke engines can clearly start 'backwards', if a compressed air starting mechanism (not motor) is used, and the piston in question is before TDC then the engine may run 'backwards'. Indeed for a car engine (as the OP) this isn't really applicable unless the car is in gear and rolls down a hill backwards.
  13. Nov 28, 2009 #12

    Ranger Mike

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    car rolling down hill backwards still won't start because the spark lites off a the piston is on the down stroke, emptying the cylinder not compressing it and ifin it did fire it would run for about two minutes since it would not be oiling properly
  14. Nov 28, 2009 #13
    u say 4 stroke engines can't run backward due to improper valve timing but this doesn't mean it can't happen. It can happen and then a certain shock we can feel or something unwanted or any noise that means improper valve timing or something but it can atleast happen , i know engine wont start! BUT I NEVER FOUND SUCH THING HAPPENING IN MY CAR WHY???
  15. Nov 28, 2009 #14


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    A 2 stroke automotive engine could run backwards (in gear, rolling back down a hill for instance). Even if it sparks after TDC it could run (not very well). Granted other systems may not function (lubrication for one).

    R Power, I don't know what your last question means.
  16. Nov 28, 2009 #15
    In both cases the crankshaft is moving. Either forward or backwards. CW or CCW

    The comment about it firing whilst the rod is vertical is a non issue (I assume he meant that it wount rotate the crank and would snap it), as for it to be an issue the crank would have to be stationary (to transfer all load as bending). That was the point I was making.

    I don't comment on 2 strokes much, becuase I know nothing about them.

    Hang on, lets break your post down.

    You say 4 strokes CAN run backwards.
    You say you would feel a shock if it happens.
    Then you say the engine won't start.
    Then you say you've never seen it happen.

    What on earth are you acutally trying to say?

    If you are asking why they can run backwards? They can't. See above for details why.
    Last edited: Nov 28, 2009
  17. Nov 28, 2009 #16

    If your timing is not right, it can happen.
  18. Nov 29, 2009 #17
    Let me say again: U say 4 stroke engines can't reverse start due to inappropriate timing.
    But, imagine a crank(in a 4 stroke) is in a position that if it is pushed down it will go reverse, now since engine will not reverse start as u say in this case, so what will happen? Will the piston go upward and start enigne. If yes how ? What would force it to go up? If no, will the engine not start? What would happen?
  19. Nov 29, 2009 #18
    Here's an animation


    I believe a general rule a thump is to time it so your connecting rod position is 3 degrees from the vertical for the ignition (power) stroke or spark.

    Keep in mind on most engines you have a flywheel in one direction
    Last edited: Nov 29, 2009
  20. Nov 29, 2009 #19
    A crank can move either way if you disconnected everything from it, so theoretically it could spin either way.

    An engine (4 stroke) will not RUN in reverse. The valved are controlled by the spinning action of the crankshaft, if it spins in revese the air fuel mixute will be drawn in at the incorrect time. I'll try to make a video showing this, to make it easier to visualise.
    A 2 stoke has a pressure activated reed valve and doesnt depend on crank rotation (i've been reading up so if it's wrong someone will correct it). which is why a 2 stroke can run in reverse.

    A 4 stroke engine has a starter motor. When you turn the key the battery spins a starter motor in 1 direction, which rotates the crank in the correct direction. This activates the valves and covers the intake and compression strokes. The spark then fires (the crank is already rotating) which makes the engine run itsself and the starter motor is disconnected. So before you even get he first spark, a 4 stoke engine is already rotating in the correct direction.

    This is why darksides scenario is not a problem, by the time you have a spark the crank already has enough inertia to complete a push to TDC. If you have the timing wrong enough so that it doesnt, the engine simply won't run. The acutal spark event occurs well before TDC. Spark advance can be up to 20 odd degrees before TDC. Peak coombustion forces are timed to accur approx. 16 degrees aTDC.
    Last edited: Nov 29, 2009
  21. Nov 29, 2009 #20
    I got my answer : The starter motor. I forgot it rotates the crank and donot push piston.
    I just wanted a practical answer why 4 strokes never even attempted a reverse start. It was simple though.
    No need to make video. I can visualize why 4 stroke can't start reverse.
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