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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.
  22. Dec 2, 2009 #21

    Ranger Mike

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    IF that crank was spinning backwards it would have just finished the "exhaust" cycle and come up at TDC compression...except, there was no air/fuel mix because the last valves that were open were the exhaust valves. Doesn't matter if the coil lite the plugs right then, there's nothing to ignite. It is the valve/cam or port timing to get the intake mix into the cylinders that determines if an engine can actually run backwards.
  23. Dec 2, 2009 #22
    4 cycle engines can run backwards, and be started with a spark alone. an IC engine in diesel mode (ignition off, pre-detonating) can start running backwards until all residual fuel air is burned out of the exhaust/combustion chamber, (and forward indefinately) 70's smog engines with bad dashpots (the ignition powered thing on the carb that holds the throttle open for idle) were bad about it (shut them off, they'd backfire, run for maybe 10 seconds backward, backfire again and die.) model A's and T's that had manual spark advance and hand cranks were regularly started by retarding the timing before shutting it off, shut it off, than advance the timing until it would hit to restart.


    (and model t/a's would sometines start the wrong way)
    Last edited: Dec 2, 2009
  24. Dec 2, 2009 #23
    We have already said diesels can run backwards, especially mechanical injection/ carbed diesels. Modern ones can't as the injectors wont inject fuel running backwards.

    You cannot however, in anyway get a 4 stroke, poppet valve, spark ignition engine to run backwards. (one that si designed to run forwards anyway)

    You can get it to move in reverese, and even weith clever timing get it to spark with fuel in the chamber. But you wouldnt consider such an engine to be running fir for purpose. You would have to specifically design the engine to do it, and who designes an engine to grenade itsself on purpose?
    Last edited: Dec 2, 2009
  25. Dec 2, 2009 #24
    I have to ask, did you read and understand what I posted???

    a gasoline motor, running on residual unspent fuel air mixture, will run backwards for a short period of time, depending upon the amount unburnt mixture. This was the symptom of a bad dashpot, which would cause the initial backfire, and cause the reverse running. the ignition source is pre-ignition caused (USUALLY) by carbon deposits which act as a glow plug.

  26. Dec 2, 2009 #25
    I read it, I understood it, my conclusion is that it's bollocks.

    1. Where are you getting this supply of air fuel mixture from? How are you getting to the cylinder?

    If you are talking about drawing it back through the exhaust, its massively contaminated with combustion products (which dont burn like fuel) making it hightly unlikely that it would detonate. If you are taking about taking it in through the intake valve, think about it. Which way is the piston moving whilst the intake valve is open if its running backwards? An engine running backwards blows fresh mixture OUT the intake valve.

    Even in the model T case, you had to manually prime the fuel for starting. However after that an engine would not 'run' backwards becuase it wouldnt be able to draw fuel in.

    2. If you are talking from starting from a stand still, you would need to physically crank the engine backwards OR have it cranking slowly enough forwards so that it sparked early to get it going.

    Either way then you will get a single bang, a broken arm and nothing much else. It certainly wont 'run' in any sense of the word.

    I gaurantee you 100% (well maybe 99.5%) that you could NOT get a 4 stroke spark ignition engine to even fire backwards without deliberetely trying to.
    Last edited: Dec 2, 2009
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