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Reversible & non reversible internal combustion engines

  1. Jul 11, 2010 #1
    Please can someone explain clearly in perhaps lay man terms the difference between REVERSIBLE & NON REVERSIBLE INTERNAL COMBUSTION ENGINES. Thanks
     
  2. jcsd
  3. Jul 11, 2010 #2

    S_Happens

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    It depends on whether you are talking about a reversible thermodynamic process or a real world engine.

    If you are talking about something that could actually exist (my assumption of your question) then a reversible engine is one that will operate in either direction of rotation. It is more common for 2 stroke engines since no real valvetrain modification is neccessary (when only done for short periods). In this case, only the starter mechanism must be reversed.

    I've heard that reversible 4 stroke engines exist, but these would have to have valvetrains that are actuated by some other method than simply being driven mechanically off of the crankshaft. Typically 4 stroke engines are only able to function in one direction of rotation because the valvetrain is mechanically linked to the crankshaft rotation. This means that if you spin the crankshaft backwards, you will reverse the action of the intake and exhaust valves. The entire intake and exhaust systems are designed to function in one direction. Reversing that would be poorly efficient at best and most likely render the engine inoperable.

    Basic answer- In the real world a reversible engine is one that can function in both directions of rotation. A nonreversible can only operate in one direction of rotation.

    If you were asking about a reversible thermodynamic process, then that is another discussion, the simple answer being one is an ideal process and the other is what actually exists in the real world.
     
  4. Jul 11, 2010 #3
    There are no reversible internal combustion engines! To prove this, try turning the crankshaft and wait for the exhaust gases to get sucked in and become fine fuel droplets mixed with air agian.
     
  5. Jul 11, 2010 #4

    S_Happens

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    Certainly not true of two strokes.

    As for four strokes, although mostly a good point, this could be remedied by direct injection or valvetrain actuation that is not mechanically linked to the crankshaft.

    To concede, I have never seen any evidence of a reversible four stroke. All I have seen/heard are vauge rumors.
     
  6. Jul 11, 2010 #5
    What the hell are you talking about? The very process of combustion is irreversible. You cannot create fuel - air mixtures by compressing smoke.
     
  7. Jul 11, 2010 #6

    S_Happens

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    Nothing in my posts is strictly prohibitive of combustion. In fact I explicitly state what would have to be done in order to reverse the rotation of a 4 stroke and still achieve combustion (nothing specific prohibits reverse operation in a 2 stroke for short duractions). I never even come close to pretending to combust spent air/fuel.

    To be blunt, why do you think that if valvetrain actuation is independant of crankshaft rotation you would be "compressing smoke"? I think you are confusing reversing the rotation of the crankshaft/rotating assembly with completely reversing the fuel/air/exhaust flow through the entire engine.

    You seem to ignore 2 stroke engines. There are many cases of 2 stroke engines being reversed (almost exclusively in very short durations). In most cases this is used in lieu of a reverse gear.
     
  8. Jul 11, 2010 #7

    S_Happens

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    For clarity, I think the crux of the problem is that your example is to take a conventional four stroke engine and try to run it backwards without any modification. I am talking about what it would take to reverse the crankshaft rotation in a four stroke engine and still achieve combustion. The valvetrain actuation would have to be reversed as well (meaning it would need to function in the same order it did before reversing crankshaft rotation), which I have mentioned in every post of mine so far. This is exactly what the rumored engines do.
     
  9. Jul 11, 2010 #8
    Why don't you use a gearbox like any normal person would do then? Also, this is not what I meant by reversible heat engine.
     
  10. Jul 12, 2010 #9
    I appreciate the responses.
     
  11. Jul 13, 2010 #10
    When you only need to run at a set speed in either direction a reversible two stroke is more suited to the task as you don't have the losses associated with a transmission.

    S_Happens also made the clear distinction that he was not talking about thermodynamically reversible systems.
     
  12. Jul 13, 2010 #11
    First time I saw a two stroke run backwards was when my buddy momentarily popped the clutch on his dirt bike, while rolling it (backwards) down the ramp off his pickup... I will never forget the look on his face when he then revved the motor and let the clutch out ...

    Though I suspect the OP here is talking about reversible in the thermo sense
     
  13. Jul 13, 2010 #12

    Danger

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    I'm not sure what the official engineering take on this is, but I can say from experience that you can make a 4-stroke run backwards by using a reversed-grind cam and bass-ackwards starter. (It might be a tad more complicated now; that was when the distributor was driven from the camshaft.) You would then, of course, have to also build a custom bell-housing so you could flip your tranny upside-down. Elsewise, you would have 4 reverse gears and one forward.
     
  14. Jul 13, 2010 #13
    That would do the trick, a mirrored cam was what I used to get a straight 4 to run backwards as one bank of a V8. Spark timing is the easy bit.
     
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