Side-valved Two strokes for angels.

Discussion in '2-Stroke Aircaft Engines' started by Niels, Aug 18, 2019.

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  1. Aug 18, 2019 #1

    Niels

    Niels

    Niels

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    Two stroke engines without valves are dying fast.
    Two strokes with exhaust valves sell very well for ships.
    It is great fun for an old man to see if exhaust-valved two strokes can be useful in aircrafts.
    I made a PPL forty years ago and cancelled it when money went low 36 years ago.
    It was always a feeling of there must be a better way of turning propellers without making so much noise and pollution.
    Let me try to argue using two plant sizes:

    80 horsepower at 2400 rpm/1,7meter prop.Direct coupled
    40 horsepower at 3000 rpm/1,4meter prop.Ditto
    The difference in vibrations beween a six- and a four-cylindered C-172 was real.
    A friend sold aircrafts with two cylinder ,boxer HKS engines and that was unpleasant.
    Mass balance is almost perfect so the difference comes from the unsteady torque between engine and propeller.
    A two cylinder boxer two-stroke is a beast as well.
    A 90 degree V2 two stroke can be shown to be between a four and a six cylindered fourstroke in torque variation so that is my dream concept.
    Pistons are not really suited to control exhaust and normal valves has been formidable for this job.
     
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  2. Aug 18, 2019 #2

    Niels

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    The Rotax 912 run 12 m per sec piston mean speed at max rpm and so will I.
    Many WW2 engines was more.
    The 80 hp shall have a stroke of 150mm and the 40hp 120mm.
    My easy formula for cylinder volume,power and MEP is

    (MEP in bar)*(Cylinder volume in litres)*(working beats per second)=Horsepower*75
    Lycoming,Continentals etc run at MEP ca 10 wide open throttle but without supercharge.
    My beloved MZ motorbike of East German ancestry ran almost 80000km at 90 km/h at a MEP of 6.5 using same formula.Same piston, sparkplug changed once and modern two stroke oil.
    Let us say that MEP of 7.5 and 12 m/sec piston speed sounds healthy.
    A two litre ,two stroke will make 80 horsepower at 2400 rpm
    and a 0.8 litre will make 40 at 3000.
    The two cylinders of the 80 hp engine shall thus be 92mm bore and 150mm stroke.
    the 40 hp engine shall have cylinders of 65mm bore and 120mm stroke.
     
    Last edited: Aug 18, 2019
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  3. Aug 18, 2019 #3

    John Bass

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    Two stroke diesel engines with a blower/supercharger seem a better fit to me. Been thinking about this for a few years now. As a 4 cyl boxer design with the second pair of cylinders 90deg offset, there is a power burst every 90deg and much less torque ripple in the output. No carb icing, higher efficiency, higher torque at lower rpms, and doesn't de-rate with altitude
     
  4. Aug 18, 2019 #4

    John Bass

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    There are enough small diesel engines coming out of Asia these days that there are plenty of parts of prototype wiith, starting with a VW block, pistons, sleves, and some modest machine work
     
  5. Aug 18, 2019 #5

    Niels

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    The Four stroke DB 601 inverted V12 from Messerscmidt 109 E engine was close to the technology I have in mind.
    Aluminium crancase and roller bearing crankshaft and was ca 20 kg per litre and ran at MEP 13 with a compression ratio of 7.2 and had propeller gear that I do not use.
    I do not have camshaft either so runnig at MEP of 7.5 can only make for lower specific mass.
    Pessimistic around15 kg per litre that is 30 kg for a direct coupled V2 two-stroke 80 horsepower low noise , leadfree.
    Beat that Rotax.
    A sidevalve two stroke model aircraft engine
    https://i.imgur.com/sOCfRuu.jpg
     
    Last edited: Aug 19, 2019
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  6. Aug 18, 2019 #6

    Vigilant1

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    Niels,
    Are you mulling over the idea of a clean-sheet engine, or are you thinking of adapting something/parts from something that is already in production? For experimental (homebuilt) aircraft, cost is an issue and mass-produced parts (for mass produced products00crs, motorcycles, industrial equipment) that can be adapted have a lot of appeal.
     
  7. Aug 19, 2019 #7

    pictsidhe

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    Niels, interesting project with sensible targets. Have you heard of the Rolls Royce Crecy? It was a WWII experimental 2 stroke V12. There's a good book on the Rolls Royce Crecy at a reasonable price. You'll probably find it informative even if you don't use sleeve valves. I bought mine direct from RR heritage via ebay. The book that you must read before building any two stroke is Harry Ricardo's "high speed Internal combustion engine". Sadly, not easy to find at a low price. C.F. Taylor's two books are well worth reading, too.

    You had much better luck with your MZ than I had with mine. Mine would eat the little end bosses of the expensive Mahle piston every 15,000km, big ends every 30k. It was the ETZ300. I frequently rode it at 120+. I heard that the 301 was much improved, but never ran one myself. Other than its appetite for engine parts, it was a great bike.

    My dream two stroke would be a V4 Crecy type engine. Even fire, perfect primary balance.
     
  8. Aug 19, 2019 #8

    Sockmonkey

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    The V4 would be air cooled yes? Because for liquid cooled you would want an inline 4.
     
  9. Aug 19, 2019 #9

    pictsidhe

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    liquid cooled is less picky about cylinder arrangement. A V4 could be either. Air cooling tends to be limited to lower power levels than air cooling.
     
  10. Aug 19, 2019 #10

    Niels

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    It is a pure CAD project with no binding from existing parts whatsoever.
    A non-flying demonstration model can surely be made with parts from here or there,but Ricardo warns against that in his book that I have.
    He says that You run into problems with holes in wrong places etc in no time
    The Crecy and all that sleve valve thing was a blind alley.
    Normal valves are better.
    Ricardos love of sleeve valves was their nock resistance and that is not so important today with 95 octane everywhere.
    Higher compression ratio means higher maximum pressure means higher specific mass.
    A V4 has perfect first order balance but not second order.
    Second order unbalance is the reason that classy car makers fool around with balance shafts running twice engine speed in fourcylinder in line engines over a certain size.
    An earlier concept for an OHV study with roughly same bore/stroke.
    Prop diameter ca 1.6meter and distance between valve stem ends 0.5 meter
     
    Last edited: Aug 19, 2019
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  11. Aug 19, 2019 #11

    Sockmonkey

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    Yeah, that's what I'm saying. Inline means fewer parts, one cylinder head, etc etc, so I'd only go V4 if it was going to be air cooled. Maybe inline 3 so the crank is lighter.
     
    Last edited: Aug 19, 2019
  12. Aug 19, 2019 #12

    blane.c

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    I like it especially with liquid cooled heads.

    To keep size and weight down more cylinders means more rpm's, which depending on your goals can be self defeating. I like a two cylinder as a reasonable compromise to keep size small, rpm's low and vibration minimal.
     
  13. Aug 19, 2019 #13

    Niels

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    Mercedes made a pushrod OHV engine that won Indy1994.
    It had 53mm inlet valves that was opened 90 times per second.
    My cylinders needs around 45mm exhaust valves to pass 40 horsepower exhaust per cylinder.
    As they open and close 40 times per second it is possible both as pushrod OHV,OHC, and crankshaft operated SW.
    The twostroke SW is novel,very compact and interresting so thats the one to go for.
    Some belgians are making flathead fourstrokes that are less than 20 kg per litre and looks very nice on computerscreen.

    https://www.d-motor.eu/lf-39

    Does any here know max dynoed power for Harley flatheads and how big exhaust valves?
     
  14. Aug 19, 2019 #14

    pictsidhe

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  15. Aug 19, 2019 #15

    Niels

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  16. Aug 19, 2019 #16

    pictsidhe

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    Are you familiar with the concept of 'time-area'?
     
  17. Aug 20, 2019 #17

    Niels

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    Yes and no.
    That is why I ask what max power Harley flat head racers made and how big the exhaust valves were.
    If 40 horsepower needs 50mm valve,it is no show stop I think
    Even better would information on exhaust valve diameter on the Dmotor be.
     
  18. Aug 20, 2019 #18

    pictsidhe

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    To get a 2 stroke to run well, you need to have a certain time-area for intake, transfer and exhaust ports. I've written programs using simpsons rule for both a calculator when I was a teenager and a spreadsheet now I have something a tiny bit faster ;)

    Attached is a good tuning book that has a description of the method. There is some difference in the flow coefficient between piston ports and poppet valves that you'll need to account for. I didn't like his way to approximate time-area, preferring to use simpsons rule. It's not hard with a spreadsheet.

    With a poppet valve, you can use assymetric timing. The Crecy book has some information on what worked for Rolls Royce. I will look at some point.

    Your high stroke to bore ratio and moderate piston speed mean you'll be able to use quite short timing, which is good for BMEP. You may be surprised at how large an exhaust valve will be optimum.
     

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    Last edited: Aug 20, 2019
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  19. Aug 21, 2019 #19

    Niels

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    I have the Crecy book as well as the Ricardo one from mid sixties I think.
    Playing motorbike tuner during a war.
    We all know that Aprillia made 55 horsepower from a 125 ccm engine and there is no 110 horsepower 250 ccm for aircrafts around the corner.
    Motorbike engines are cubic optimized and aircrafts engines for low mass,thermal efficiency and reliability.
    One cannot bring much expirience from one to the other apart from oil and sparkplug technology.
    The concept I have in mind has no expansion chamber pipes but uses a turbo to assist crancase pumping.
    This is nessecary because the two conrods sit side by side on the single crankthrow.This means that direct piston underside pumping action is only half of the conventional.
    Hakkrank.JPG
     
    Last edited: Aug 21, 2019
  20. Aug 21, 2019 #20

    pictsidhe

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    The time-area method holds for cox RC engines up to ship engines. You don't have to be spinning an engine at crazy piston speeds for it to be applicable. I'd be concerned about uneven cylinder charging if using crankcase compression as well as a turbo on a v-twin. A centrifugal pump geared to the crank actually provides the rght pressure ratio from starting to max revs...
     
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