Full rich mixture

Discussion in 'Engines' started by greybeard, Dec 11, 2011.

  1. greybeard

    greybeard Member

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    Please, when pilot set mixture as rich as possible and why?

    Cheers,
    GB
     
  2. vanir

    vanir Banned

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    #2 vanir, Dec 11, 2011
    Last edited: Dec 11, 2011
    At sea level in high ambient temperature, under heavy loading, and for overboost at take off/emergency.

    Varying mixture keeps the spark ahead of preignition. If it pings, move the mixture if you need that power setting under present conditions. Single seat pilots are trained to perform the role of flight engineer, they're taught a bit of engine stuff.

    There's a popular little trick you can do in Spits and Warhawks. Open full cold carb air and do a dip, as you get best momentum and airspeed keep going ballistic on the throttle past the gate and go full rich. Gives you thousands of extra feet in full throttle altitude at maximum war emergency power. It's like a 150hp energy bonus or so. Haven't done this or anything, just trying to put it together from various pilot descriptions of what is referred to as "ram air"
     
  3. greybeard

    greybeard Member

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    Thanks for your reply.

    GB
     
  4. engguy

    engguy Member

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    The term preignition means before ignition. So saying keeping spark ahead of, does not make sense.
    Fat fuel mixtures "cool" the charge and help prevent detonation. Preignition is generally known to be caused by hot spots like glowing carbon deposites etc.
    Also extreamly lean mixtures will cool as well, because of an excess of air.
     
  5. vanir

    vanir Banned

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    I know less, just a hobby racer, I'm thinking in terms that make sense to me but it would be much better to know industry nomenclature for what I'm trying to say. I have to set spark curve manually between two points, then advance by turning the whole thing, and I really have to use the plugs and exhaust to adjust the carbs because they feed different cylinders and lean at the front of the bank, richen at the back carbs, so I have to use different needles for each (they're SU), that means different mixtures for each.

    So I tune the thing by ear so I don't pay $150/wk for a shop to do it, and I found what works best on the track is keep the spark just ahead of the ping. How does that not make sense? I mean it might sound childish to a professional mechanic, but I'm not making it up or anything.
     
  6. Trilisser

    Trilisser Member

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    As far as wartime aircraft are concerned, the "official" mixture lever position "full rich" was to be used only when the automatic richh position was suspect to malfunction. In many Merlin engined aircraft even the auto rich position was locked off. With Merlins and Griffon using the S.U. or the R-R injection pump, there was no mixtyure control lever at all, only ON/OFF fuel flow control. And that is interesting for many US engines of the same era (when German and British engines had this feature fully automatic) still had 4-position mixture controls...
     
  7. GregP

    GregP Well-Known Member

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    All engines have a design temperature range ... that's cylinder head temperature. Today, we can run very lean mixture (relative to 70 years ago) because we have fuel injection and each cylinder gets the same mixture via computers of mechanical fuel injection ... albeit, very well-designed injection. In WWII we mostly flew carburetors except for the German DB series. Their injection was mechanical and worked well. Great engine!

    With carbs, the mixture was right when it left the carburetor, but the intake plumbing that runs from the carb to the cylinders always introduced variation, and you had to run rich enough so the leanest cylinder didn't suffer damage. So ,everyone ran a bit rich. What you do is to set the mixture to auto lean or auto rich, depending ion what youa re doing, set the rpm and gradually lean or rich the mixture until you get to the desired cylidner head temperature. If the temp is too high, go richer; if the temp is too low, go leaner. In WWII, you started from the rich side (because your engine was your life) and leaned gradually until you got to the correct temperature.

    As you climb, you lean because the air gets thinner. As you descend, you go richer becasue the air gets thicker. In combat, you went to auto rich and left it there unless you were at 20,000 + feet, in which case you probably were already at the right mixture and went lean or rich depending on climb or dive. If you were diving and the prop went all the way to the limits of constant speed, you had to pay attention to rpm at near the dive limits or blow up the engine.

    After some training, it is not difficult and comes naturally.

    In RC models with 2-stroke, loop-scavenged glow fuel engines, setting the mixture is EASY. Once running you lean until max rpm and then raise the nose. If the rpm drops, you go a click or two rich ubntil the rpm does not drop when the nose is raised. At idel if it is rich, the engine will gradually slow down ans stop. If it is lean, the engine will speed up and suddenly stop. All this sounds complicated, but it very simple when someone shows you and you DO it several times. Then it cecomes automatic and easy, and you can hear other people's engine sounds and immediately tell if they know what they are doing or not.

    If you switch to a 4-stroke, the procedure is different, but also becomes easy once done a few times.
     
  8. Trilisser

    Trilisser Member

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    Hmm, if we are talking about US WW2 engines, all combat operations (regardless of altitude), take-off etc. was usually done on auto-rich. There was no necessity to richen the mixture unless the automatic compensation failed. Positions between auto-rich and auto -lean were not used, at least as per any period manual.

    As for correct temperature, for a B-series R-2800 the recommended CHT range for continuous operation was 120 deg C to 230 deg C.

    As for current GA aircraft, it is quite insignificant whether one has a carburetted or injected version as in most GA aircraft the engine installation is poorly designed so the variation of CHT due cooling air flow differences is alone very significant. In fact, when operating on 100/130 fuel, you can probably get better cruising fuel economy from an R-2800 than from a Lycoming or Continental air-cooled GA engine of today (about 190-200 g/hp/h).
     
  9. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    Agree with almost everything you said. What type of GA engine are we talking about? An IO-540 is getting about 25 GPH tops depending on the aircraft it's installed in.
     
  10. Trilisser

    Trilisser Member

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    I recall it being the AEIO-360 as installed in the Vinka (Finnish trainer). The fellow I talked these topics with served as a FinnAF mechanics school instructor.
     
  11. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    I can tell you an AEIO-360 burns no where near 190 - 200 gph even doing aerobatics. I've worked on them as installed on Super Decathalons and Pitts and the most I think you'll see out of them regardless of the aircraft is maybe 15-20 GPH at full rich, 100%. Under normal cruise you're looking at 9-11 GPH.
     
  12. Trilisser

    Trilisser Member

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    FLYBOYJ, I used "g/hp/h"=grams per horsepower per hour, not gph=gallons per hour...:) BTW, a F8F flying at low level at its best range speed burns less than 40 gph...
     
  13. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    OK, that makes more sense. Normall GA fuel burns are calculated as Gallons Per Hour based on throttle settings. In the end I think an R2800 will still burn a heck of a lot more fuel than an AEIO-360 at cruise, not factoring what type of airframe each engine is installed in.
     
  14. Trilisser

    Trilisser Member

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    Yep, GPH is a convenient way to hide the actual engine efficiency. I think the most damning indictment of these "modern" GA engines is that using fuel injection and 100LL fuel they fail to have better efficiency than the old carburetted Argus As 10 running on 87 octane fuel...
     
  15. GregP

    GregP Well-Known Member

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    If you think you can get better fuel economy from an R-2800 than from a Lycomming IO-540, you have obviously never flown or, more correctly PAID FOR flying, an R-2800.

    The 2800 is 2800 cubic inches. The 540 is 540 cubic inches. That's over 5 times the displacement!

    What were you thinking?

    The R-2800, at full power, drinks 485 - 550 GPH, depending on the installation. The IO-540, at full power, drinks, maybe, 22 - 25 GPH. I'd say that if money is important to you, fly the IO-540! Whether grams or gallons, 500 is WAY more the 25, say 20 times the fuel consumption.
     
  16. Trilisser

    Trilisser Member

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    Greg, please try to understand that I am talking about specific fuel consumption, not total fuel consumption. By your definition, a chainsaw is more efficient than a large marine diesel despite having at least 100% higher SFC.

    And your R-2800 fuel flow figure is also way wrong. E.g. a B-series R-2800 drinks about 270 US gph at 52"/2700 rpm (about 2000 hp). A water-injected C-series burns about 300 gph at WER.
     
  17. donkeyking

    donkeyking Member

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    Hi Greg

    In WWII, was German injection engine better for prevention ping than allies' carburetor engine?

    Thanks
     
  18. engguy

    engguy Member

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    Well at .5 BSFC and 3K HP and 6.5 lbs per gallon that is 230 gallons, gosh an R-4360 cruises at about 150 gallons per hour. And to use a ridiculous rich number of
    .6 BSFC and 3600 hp that is 332 gallons per hourish. Your a bit way high on your GPH estimate.
     
  19. tommayer

    tommayer New Member

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    I believe careful study of the POH for a supercharged--not turbo charged as in P-47 or B-24--WWII P&W radial will show that the BSFC varied enormously for a given horsepower from low to high altitude. An R-985 in in a GB-2, to pick the one I just finished flying a half hour ago, will deliver 300 hp to the prop at a BSFC of about .41 at 1000 msl. The same 300 hp will require a BSFC of .5 at 8300 msl. RPM and MP setting almost identical.
    That's for an engine equipped with a float carb. A Bendix injector, which modern types would probably describe as a pressure carb, on the same engine will improve BSFC by almost 10%.
    Point being those old engines, even with float carbs, had very good specific fuel consumption numbers as long as manifold pressure did not exceed ambient atmospheric pressure.
    It wasn't a WWII variant, but the turbo compound versions of the Wright 3350 got BSFCs down in the .375 range. I don't think anybody's done much better than that since.
    For what it's worth, the BSFC for an injected 985 at, say, 65% at is better than I've ever been able to get from a Lyc IO-360 angle head with very carefully matched injectors running LOP, just as long as MP on the 985 does not have to exceed ambient. The best the Lyc will do is about .4.
    Anyway, be careful about assigning one BSFC figure to the old engines. The didn't work that way.
     
  20. greybeard

    greybeard Member

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    OMG! :shock:

    I couldn't figure my question would have caused all this flood of comments. I would just add some more personal one, hopefully for clarity.

    First, efficiency means what I get for what I spend and is a technical tool to assess how much is reached best result in terms of highest power for lowest consumption. So it is independent from engine class of power: you may have a big efficient one and a little hog...

    Second, main advantage of fuel injection over carburetor is efficiency, since, as someone mentioned, carburetor mixture must be richer to compensate for vapour condensation into manifold. Be aware there are TWO kind of fuel injection: direct (into cylinder) and indirect (into manifold, close to inlet valve); the former is also said high pressure one, since its injector must resist to cylinder internal pressure during its entire cycle (otherwise exhaust gases would enter into injector), the latter is low pressure for obvious reasons. Direct injection also allows to scavenge cylinder with air instead of mixture and greatly reduce risk of ice formation. That was not featured only by german WWII engine, but also by Japanese ones like Homare.

    Cheers,
    GB
     
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