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ENGINE designing

  1. Aug 20, 2005 #1
    Hey friends,
    My question is that why only 2-stroke & 4-stroke engines have been
    made?Why not 6-stroke or 8-stroke?Is it possible to have such engine? Any
    useful suggestions are welcome.
     
  2. jcsd
  3. Aug 20, 2005 #2
    well, the question is, do you have an idea for an 8 stroke engine. What would the other 4 strokes be for?

    Regards,

    Nenad
     
  4. Aug 20, 2005 #3

    FredGarvin

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    What would the two other steps be in a 6 stroke? What would differentiate a 4 stroke from an 8 stroke?
     
  5. Aug 20, 2005 #4

    Q_Goest

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    The main reason higher stroke engines are not used is because energy is consumed by the motor in the form of friction when it rotates. If you wait for the engine to rotate 6 or 8 times before you get a power stroke, you waste energy through frictional losses.
     
  6. Aug 20, 2005 #5
    More than 4 stroke engines have already been designed and used. Apparently no one here has heard of a hit/miss engine.
     
  7. Aug 20, 2005 #6

    GENIERE

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    An antique engine club exhibits at our county fair each summer, in fact about two weeks from now. They have quite a few of the hit/miss engines running. I’ve only seen them operate as stationary engines hooked up to old time farm equipment, were talking about circa 1900 as far as age. All have enormous flywheels that turn about 100-300rpm and a centrifugal governor controls the spark. It appears the flywheel turns about 20 revolutions before spark is allowed and than two or three exhaust noises are heard followed by coasting until the speed drops sufficiently to repeat the process. All were single cylinder and had a cast iron bowl atop the cylinder head holding water, which evaporated and cooled the engine. Fun stuff! :biggrin: I watch for hours until my wife drags me away.


    ...
     
  8. Aug 21, 2005 #7

    Q_Goest

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    A hit and miss engine is a conventional 4 stroke with a governor that prevents it from getting a power stroke when the RPM's go up too high, so its really not a 6 or 8 stroke in the sense it isn't designed to operate on that many strokes, it is designed to operate on 4 strokes. There's a good article that describes the operation of the governor here.

    They are pretty cool though, and to think those massive engines were all that was available back then. Not exactly a chain saw motor.
     
    Last edited: Aug 21, 2005
  9. Aug 21, 2005 #8
    Not only does the governor control spark it holds the exhaust valve open as the link above shows. The intake valve is just a check valve. There really is no carburetor in the conventional sense and it is not uncommon to see them with an open crankshaft and connecting rod. When the exhaust valve is held open no fuel air charge is drawn into the cylinder. When the speed drops to a preset level the exhaust valve is allowed to close and the engine goes through the 4 conventional strokes that we all know. After the RPM comes up the exhaust valve stays open until the RPM drops to the preset level and it starts over again. I have one of these engines and when they are loaded heavy enough they will hit every time running as a conventional 4 stroke.


    One other thing I will add is that alot of them did not actually have spark plugs. They had a set of points in the combustion chamber. The magneto built up enough current going through a coil in series with the internal points and when it was time to fire internal points opened up and there was enough of an arc for ignition.

    Maytag also had hit miss engines for washing machines but I believe they operated more as a 2 stroke. I think they wasted the fuel while the exhaust valve was held open. IIRC they used a conventional spark plug and had kick starters on them.
     
    Last edited: Aug 21, 2005
  10. Aug 21, 2005 #9
    Actually, I can see an advantage in a "6-cycle engine", if someone could figure out how to do it (I don't dee how). Remember, the heated gasses after combustion, need more "expansion' volume than did the original intake that was compressed. The normal Otto Cycle has:
    Intake
    Compression
    (Ignition)
    Combustion/Power/Expansion
    Blowdown
    Exhaust
    In the P-V diagram, the vertical line after the "expansion" stroke, represents unburned gasses being blown out of the system (sometimes called "blowdown"). If, somehow someone could figure out how to come up with something like:
    Intake
    Compression
    (Ignition)
    Combustion/Power/Expansion - first half
    First Exhaust
    Combustion/Power/Expansion - Second half
    Blowdown (remainder)
    Second exhaust
    This would eliminate the "Blowdown" phase, but I don't see a way it can be done. If it could it would increase fuel efficiency at the expense of power - much as is done in the Atkinson Cycle.


    KM
     
  11. Aug 21, 2005 #10

    Danger

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    It should be possible with electronically controlled valves and spark. A gas analyzer just outside the exhaust port could trigger the valve to close as soon as raw fuel is detected, thus trapping it and initiating a second ignition sequence. Of course, that would only work if there's also oxygen left over. I can't help thinking that it would make for one incredibly unbalanced motor, since it would have be done on a per-cylinder basis. It might be fun to tinker with some sketches.

    edit: In that system, the second burn would be under almost no compression, so I don't know if enough power could be extracted from it to overcome the mechanical losses of missing a fresh intake charge and burn. :confused:
     
    Last edited: Aug 22, 2005
  12. Aug 23, 2005 #11

    Danger

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    Rats! I got back to late to do another edit. Oh well, consider this a PS instead. Some thoughts occurred to me in the bar tonight, but I'm too tired to post them right now. I'll do up a chart in Illustrator and post it tomorrow.
     
  13. Aug 24, 2005 #12

    Danger

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    Okay, guys... here's what I came up with. The stroke labelled '5' is simply to ensure that there's enough air for the unburned fuel to mix with. '5 1/3' is the actual second power stroke. I think that it's kind of silly, but I had fun playing with it. The thing would still probably have a power curve like a bipolar weasel on a treadmill.

    [​IMG]
     
  14. Aug 24, 2005 #13
    So the hit miss engine is just a four stroke engine with a rev limiter on it?
     
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