2 stroke engine @ 55% or 4 stroke at 80%, which is better?

Discussion in 'General Experimental Aviation Questions' started by JayKoit, Feb 6, 2013.

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  1. Feb 6, 2013 #1

    JayKoit

    JayKoit

    JayKoit

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    Hi all,

    The title pretty much sums up my post, but here's more specifically what I'm pondering:

    I read in one of the many 2 stroke vs. 4 stroke debate threads that it's better to run ANY engine at 55% than 75-85%, regardless of 2 stroke or 4 stroke.

    I'm interested in doing a budget build of either a Zenith 701 or a Savannah Bingo with either a Simonini Victor 2 (92-110 HP), Hirth 3503 (70 hp and fuel injected), or HKS 700e (60 HP). (Let's assume for a moment that these are the only engines on earth that will fit these two airframes, so we don't stray from this topic into the world of "you should really consider _____ engine instead of these engines for your build..."):)

    Both airframes have FWF components for all these engines, so all these options are easily installable as well.

    From what I've read and seen on the ICP/Savannah website, the HKS has the weakest performance/climb/cruise, obviously because it's the lowest HP, BUT I also find more pics/videos of the HKS Savannah Bingo online than any other model, so it appears to be the most popular. Makes sense, because this engine gets pretty amazing reviews and is a 4 stroke, which are generally lower maintenance.

    BUT, in order to really properly fly either of these two airframes, especially with two people and fuel, 60 HP is pretty low, and you'd pretty much have to run the HKS to the max all the time in order to get even average performance, which brings me back to the top:

    Would it be better to get a higher hp two stroke (like the 102-110 hp Simonini) and run it less intensively than the HKS? Even the 70 hp Hirth? It does share the same 1000 hr TBO as the HKS, and I could fly it like a 60 HP engine (except when extra power is needed) and put less strain on it, heck, if I went with the Simonini I could run the 110 HP 2S model at half power and go REALLY easy on that. Would I end up with a more reliable two stroke over the four stroke in this kind of situation?

    Would love to hear your thoughts. Thanks!

    Jaykoit
     
  2. Feb 6, 2013 #2

    TFF

    TFF

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    I think before you can really pick, you need to look at the recommended props these engines are to turn, and what you want to turn. RPM an size.
     
  3. Feb 6, 2013 #3

    JayKoit

    JayKoit

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    Good point, I forgot to include that. Here's some real world numbers of thrust tests. It seems the engine/prop combos listed are the most ideal for maximum thrust, as the numbers all seem to float around the same point for each engine/prop:

    Hirth 3503 2.29 Ivo 60" 355Lb


    Rotax 582 2.68 Warp 60" 370Lb


    Rotax 582 2.58 Ivo 60" 342Lb


    Rotax 582 2.58 Powerfin 60" 360Lb


    HKS 700e 2.58 GSC Tech III 305 lb (at DA 3200')


    I don't know the DA of the other pulls listed above the HKS, but I will say that at 3200'DA the Rotax 582 that went head-to-head with the HKS only pulled a couple more pounds, they we're nearly identical in performance. The head to head test was performed by Green Sky Adventures, the other tests by some pilots on the Rotary Wing Forum. I would assume the Rotary Forum guys were probably a little lower in altitude.
     
  4. Feb 6, 2013 #4

    WurlyBird

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    I believe that one of the big differences you will find between the 2 stroke and 4 stroke engines is the HP curve. The curve will be much flatter on a 4 stroke then a 2. I am switching from a 582 to a 700E which I have unfortunately not flown yet. But the 582, all two strokes really, are tuned to operate at a pretty specific power output and RPM. 2 strokes have a very define HP peak and operating on either side of it the power drops off pretty quickly. Think of the other applications that 2 strokes strive in; dirt bikes, jet skis, lawn equipment, etc. All these things operate and either idle or full throttle most of the time. A 100 HP 2 stroke is obviously better then a weed eater but it will still be tuned. Check out the power curves very carefully.
     
  5. Feb 6, 2013 #5

    Dan Thomas

    Dan Thomas

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    Those thrust figures are probably for static thrust, and that can be really misleading. Even a small engine can produce significant static thrust if the pitch is low enough to let it reach redline, but as soon as the airplane starts rolling the RPM goes past redline and its forward speed will be low if you throttle back to keep it within redline.

    And that brings us to another problem: Running any engine with a fixed-pitch prop means reducing its RPM, which results in low cruise speed. A good example is the Subaru conversion I put in a Glastar, an airplane designed for a Lycoming O-235 or better. Both the Soob and the O-235 are in the neighborhood of 130 HP, so the comparison is fair. The Lyc runs up to 2700 RPM and can be cruised at any RPM up to that, but typically at 2500 or maybe 2600. The Glastar would cruise at 130 mph at 2500. With the Soob, I couldn't cruise it anywhere near its redline of 5600, since it ate far too much fuel there and the wear rate and temps went up, so we were usually cruising at 4700 or so and the cruise speed was around 105 mph.

    If one has a constant-speed prop, or some other means of adjusting prop pitch in flight, one can run an engine slower and still maintain better cruise speeds. A fixed-pitch prop is like having your car's transmission stuck in second gear.

    Dan
     
  6. Feb 6, 2013 #6

    dino

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    Another thing to consider is fuel burn which will more than compensate for the weight penalty of a 4 stroke. Less maintenance too.

    When I had to make the choice between 2T or 4T though I went with the 582 because of less cost, good factory back up, well documented operation, failure modes and installation not to mention hp/weight in a high drag application like a rotorcraft at 4:1 L/D with a 5 gallon fuel capacity and annual utilization of < 50 hours.

    Dino
     
  7. Feb 6, 2013 #7

    Dave Prizio

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    A couple of points. It is usually best to run an engine closer to 75% power, or at least 65% power. This provide sufficient pressure to properly push the rings against the cylinder walls without unduly straining the engine and producing too much heat.

    The overriding consideration in an airplane engine is, in my opinion, reliability. This heavily weights your consideration towards the 4-stroke options. It just doesn't matter how it performs if it stops running.

    Dave Prizio
     
  8. Feb 6, 2013 #8

    JayKoit

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    Thanks guys, all good points here. Dan, never realized that about static thrust. Too bad I can't fly a constant speed/in flight adj. prop on an LSA...

    Sounds to me like it's pretty hard to beat the HKS 4 stroke option, regardless of slightly lower performance. It's only a couple pounds heavier (all up) than a 582 or Simonini since it' doesn't have coolant and the necessary plumbing. And I still haven't found any really negative reviews on it's reliability, plus the 1000 hr TBO is great. It's true, the power and torque band is much smoother on the HKS, and I also read that due to it's heavy flywheel, it's a smoother running engine over 2 strokes, and you don't lose as much HP if a cylinder quits.
     
  9. Feb 7, 2013 #9

    Lemans

    Lemans

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    Before getting deeper into the subject, what does it mean, running an engine at 55%?
    55% throttle settings?
    55% fuel burn?
    55% of maximum rpm?
    55% of torque?
    55% power output?
    55% of maximum air-plane speed?
     
  10. Feb 8, 2013 #10

    Dan Thomas

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    55% of rated power output. It can be hard to measure without a dynamometer to measure the torque accurately. You need both torque and RPM to get HP.

    At 55% the speed won't be 55%; probably over 55%, since drag builds with the square of speed.

    At 55% RPM the power will be much less than 55%.

    Dan
     
  11. Feb 8, 2013 #11

    cweinacker

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    I have been flying a Quicksilver two seat with Rotax 582 for 15 years...never used Hirth, but I can truly say as to dependability...service/1,500hr TBO and maintenance couldnt be easier...I wouldnt even consider a Hirth..but Im biased.
    ...and that is 2.8gal/hr fuel consumption.
    Tweetie
     
  12. Feb 8, 2013 #12

    cweinacker

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    What are you guys talking about???...every pilot knows flying at 55% is FLYING....and that is all that matters, not hp, not TAS/CAS/IAS...just flying...like beer is one beer 55% or four beers?
    Tweetie
     
  13. Feb 8, 2013 #13

    Jan Carlsson

    Jan Carlsson

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    If the engine is propped properly, so you get 100% rated rpm flying low straight and level.
    then throttle back.
    75% power of the aviable (what ever that is) is at 90,85% rpm
    65% power of the aviable is at 86,62% rpm
    55% power of the aviable is at 81,93% rpm
    one beer out of four is 25%
    but one beer out of 6 is 16,66%
    Don't drink and calculate
     
  14. Feb 8, 2013 #14

    WurlyBird

    WurlyBird

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    I am curious about this. Are you suggesting a 582 has a 1500 hr TBO? The one I took off my plane had a 300 hr TBO with 150 hr inspection if you went by the books. And it averaged about 4 gal/hr flying around rather leisurely.
     
  15. Feb 8, 2013 #15

    Lemans

    Lemans

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    So it should be considered as **% power output. This I can understand.

    Next statement is a bit harder to follow.

    75% power of the available (what ever that is) is at 90,85% rpm
    65% power of the available is at 86,62% rpm
    55% power of the available is at 81,93% rpm


    Very precise figures. So no difference between 2 or 4-stroke engines?
    Also equal with different torque or power curves?
    Or does this mean that flight speed is the most important factor? – A flight speed / prop revolutions /prop blades angle of attack – sort of equation...


    If fuel consumption seems a bit strange, never forget that running an engine with partially closed
    throttle is very fuel inefficient. Specific fuel consumption can go up to 800gr/hp/hour.
    If you burn 4 gallons an hour your average power output will be between 20 and 25HP.
    If you burn 2,8 gallons an hour it will be between 13 and 17 HP. ( difference due to mixture settings and general condition of the engine)
    This is a general rule and figures may vary a bit but it gives a clear idea how engines reacting at different throttle settings.
     
  16. Feb 8, 2013 #16

    Jan Carlsson

    Jan Carlsson

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    When throttle back (with a fixed pitch prop) it is not the power or engine torque curve that give/tell you the power, it is the propeller demand curve.
    but going slower then max but with WOT, meaning climbing in most cases, then the engine power curve is important.
    power changes with altitude is a different story
     
  17. Feb 9, 2013 #17

    Lemans

    Lemans

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    ... it is the propeller demand curve.....
    Flying at 100% or at 75%is related to the propeller demand curve?
    The engine needs to produce just as much power as the prop demands (or the prop will spin faster or slower). The more power the engine delivers the more fuel he demands.
    So flying at 100% means...let's say, burning 50 liters an hour and flying at 50% means burning 50/2 or 25 liters an hour. Or, is this to easy?


    Here are the figures I found on the website from Simonini , their 48 hp Victor1 plus.
    Simonini USA - US Distributor of Simonini Engines and Replacement Parts
    We may assume that the max thrust is related to the max power output of 48 hp.
    A desent BSFC for 2-strokes is 427gr/pk/hr or 0,59 liter/pk/hr....


    RPM .............Consumption Liter/hr ..............Kg of thrusting
    6150......................9.2....................................134
    6000........................8.....................................126
    5500...................... 6 ......................................104
    5000..................... 4.5 ......................................86
    4500 ......................3 ........................................56


    There is something I don't understand... 9,2 liters/hr equals 15,59 hp, not 48 hp ... (9,2 divided by 0,59)

    If I look at the 4500 rpm figures I'm completely lost. - 56 kg trust so the engine needs todeliver
    48 hp/134*56 = 20 hp (at least) The figures claim a fuel consumption of 3 liters witch gives5 hp, not 20...


    Can somebody explain where I'm wrong ?


    Btw., A decent BSFC for a 4-stroke is 0,32 liter/hp/hr.
     
  18. Feb 9, 2013 #18

    Jan Carlsson

    Jan Carlsson

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    Some, sometimes, somewhere say we get 3 lb ~1,5 kg of static thrust per HP, if we can trust that.
    so 134 kg = 89 HP but sure not at 9 l/h are you sure it is not gallon?

    as a thumb of rule, use half a liter per 2-stroke HP, and 0,3 l per 4-stroke HP. per hour.

    And NO, it is not linear, near max power extra fuel is needed to cool piston and things. like 30% extra
     
  19. Feb 9, 2013 #19

    Lemans

    Lemans

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    Once the plane is in motion, thrust will go up to ...hmmm..... 2,5 to 3 kg per HP?
    134kg = 44 to 53,6 HP so thrust and power figures are most likely correct.
    I just don't understand the fuel consumption figures. I have looked at the site and it's inliters, not gallons. BSFC is best (lowest) on maximum torque rpm withfull open throttle.
    Based on published consumption figures they claim a 2-stroke BSCF well below the best results for 4-stroke diesels in the rest of the world.


    Be careful with the 'cool the engine' with extra fuel statement. Okay, @ max. power you can't use a lean mixture as explosion temperature goes up. A rich mixture burns 'cooler', and you may take thisliterally, in absolute temperature.
     
  20. Feb 9, 2013 #20

    Jan Carlsson

    Jan Carlsson

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    Don't ever think an american can tell the difference of a liter and a gallon, or a cm and a mile, or a lire and a dollar, and never trust an engine or car sales man.
    If the 2-stroke is a long stroke, slow turning, 20000 hp marin diesel running on tar thick oil, it will run on less then 0,3 lb-HP/h
    So 48 HP likely use 24 liter or more, at full power. calculate with 365 gr hp h at full, and 350 at max torque.
     

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