Feasibility of small 4-cycles in practical usage?

Discussion in 'General Engine Discussion' started by KennyMcCormick315, Oct 26, 2019.

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  1. Nov 24, 2019 #21

    Jennifer Edwards

    Jennifer Edwards

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    Peter,

    My next project is a 1/4 scale Anzani three cylinder radial engine. The type in the aircraft Bleriot used in his channel crossing. Quite a bit larger than he is seeking here.

    However I must agree that you could go with a reed valve on the intake port reducing head area taken up by valving considerably. It would most likely also reduce power wasted on operating the valve train considerably.

    One thing I am not at all sure about is the efficiency of a reed valve that small.

    I wonder if it would be possible to employ some kind of atmospheric operated ball valve with a weak spring to keep it seated?

    It seems that the Air/Fuel mixture would flow around a ball much more freely than a flat reed.

    Jenny
     
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  2. Nov 24, 2019 #22

    Peter Twissell

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    Hi Jennifer,
    The advantage of a reed valve is that the moving part is very light.
    A ball with a light spring will have a low resonant frequency, causing issues with slow opening and closing. It will also be prone to bouncing.
    It's worth having a look at the tiny reed valves used in Cox 2 strokes and small compressors, like those used in tyre inflators.
    The Cox engines use Mylar, but stainless steel is used in many similar applications, including automotive dampers (shock absorbers) in which they are required to operate at high frequency for long periods over a wide range of temperatures.
    Many small motorcycle 2 strokes use reed valve induction. The shim is usually fixed at one edge and the intake port is shaped to flow gas tangential to the open valve.
     
  3. Nov 24, 2019 #23

    kadora

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    I have machined small four stroke engine 1.2 ccm but I am not able to start it on glow plug
    probably low compression . I will try spark plug and petrol. 2018-06-28-2882.jpg 2019-11-16-3189.jpg
     
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  4. Nov 25, 2019 #24

    Jennifer Edwards

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    Thanks Peter

    I think you are right about a ball valve on a very small engine.

    Yah I remember the old Cox “Thimble Drome” .049’s had them.

    My mind has trouble with tiny displacement engine physics. Forces are much lower and they need to run so much faster.
     
  5. Nov 25, 2019 #25

    Niels Abildgaard

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  6. Nov 26, 2019 #26

    josodl1953

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    Hi Jenny,
    If you consider the use of a reed valve for a 4-stroke you must bear in mind that, unlike in in 2-stroke engines , they have to withstand high temperatures and pressures. To my best knowledge are atmospheric operated valves always of the poppet type.

    Jos
     
  7. Dec 14, 2019 #27

    Peter Twissell

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    Hi Jos,
    It is only in small sizes that a reed valve could be practical for an atmospheric inlet 4 stroke engine.
    The stiffness of the reed material against pressure changes as the inverse cube of the diameter exposed to pressure and it's ability to transfer heat away to the surrounding material changes as the inverse square of the same diameter.
     
  8. Dec 15, 2019 #28

    lohring

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    Intake reed valves in two strokes run in the crankcase while in a four stroke they would run in the combustion chamber. They would be totally unsuitable. There a lot of rotary valves that, while leaky, would be simple and suitable.

    Lohring Miller
     
  9. Dec 15, 2019 #29

    Peter Twissell

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    It is true that 2 stroke reed valves run in the crankcase, however they are also used in other applications, including compressors where they are used in the head.
    I would assess whether a reed valve is suitable in a particular application by calculating the mechanical and thermal stress on the parts, rather than by simply citing other applications.
     
  10. Dec 16, 2019 #30

    lohring

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    I don't believe there are any cases of reed valves being used to seal combustion chambers. The crankcase of a two stroke is a cold, low stress area by comparison.

    Lohring Miller
     
  11. Dec 16, 2019 #31

    TSutrina

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    Heat of combustion is not that much different in a two or four stroke engine. The reed valve is closed when it is exposed on one side by the ~2000+F temperature gas, but on the other side of the thin sheet of metal it is pressed hard against a metal surface often aluminum. Aluminum is a puddle at 2000 F. Aluminum is seldom used for applications above 800 F. The conductivity of the thin reed is very small as is the gap resistance under combustion pressure. So the reed temperature is at most 1000 F. All other types of springs will have much poorer capacity to conduct the heat away. Make sure that you let the reed valve contact a lot of metal. So as the reed gets longer it will cool better and open more but will also move slower and may bend more.
     
  12. Dec 16, 2019 #32

    lohring

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    I forgot about pulse jet engines where the reeds are actually in the combustion chamber, but that is still a relatively low pressure and temperature application. Reeds have also been used in four strokes to back up intake valves with a lot of overlap to prevent blow back at low rpm. I still think they are a bad idea in the combustion chamber of a four stroke. If you know an actual engine that uses reeds, I would be interested.

    Lohring Miller
     
  13. Dec 18, 2019 #33

    bluejets

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    Built this maybe 20 years ago and it's done a lot of running at local pond.
    Runs just fine on 4:1 methanol/castor(castrol M)
    On-board glow (D-cell Nicad) for starting and ultra-low idle.
    Original casting and plan from the Master...
    Mr George Punter.(this year's Duke of Edinburgh Award winner)
     

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  14. Dec 19, 2019 #34

    kadora

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    bluejets is your engine 4 stroke
     
  15. Dec 19, 2019 #35

    bluejets

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    Yes. The 2 single cams run on the rear of the engine.
    You can see the back end of one cam where it exits the rear casing as a points cam for spark ignition in the "engine" photo above.
    Valve springs etc. are hidden by the exhaust pipe unfortunately.
     
  16. Dec 20, 2019 #36

    ALEX1952

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    Cox produced a Peewee and a Teedee at .020 & .010 not sure which is which, we used to convert rubber powered models into glow power in my youth (to long ago to admit to), surprising the height you can get on a short motor run, invariably resulting in running after the model whilst looking up trying to keep sight of the model. We thought it great but in reality was 30 seconds of excitement followed by risking life and limb legging it. Those were the days or perhaps not!
     
  17. Dec 21, 2019 #37

    Peter Twissell

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    I did the same in the 1970s, with a Cox .020 pee wee. The pee wee was 0.327cc, while the tee Dee was half that!
    Both were 2 strokes and this thread is looking for a small 4 stroke.
    I am now very tempted to modify my lawn mower engine with a reed inlet, just to see how it holds up!
     
  18. Dec 21, 2019 #38

    kadora

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    Bluejets your engine is evidence that small engines are not utterly useless.
     

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