Why use carburetors on aircraft engines... at all?

Discussion in 'General Experimental Aviation Questions' started by choppergirl, Sep 16, 2016.

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  1. Sep 16, 2016 #1

    choppergirl

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    Why use carburetors on airplanes, if fuel injection has been around since WW2?

    It seems a Very Bad Idea (TM) to use a carburetor with a float bowl on a vehicle that could be a multitude of orientations rather than always flat and level like a car, with various potential +/-G forces acting on it.... :-/
     
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  2. Sep 16, 2016 #2

    VFR-on-top

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    Occam's Razor says carbs over fuel injection.
     
  3. Sep 16, 2016 #3

    mcrae0104

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    If you find a fuel injection system for the engine on your airplane, by all means, use it.

    Carburetors are comparatively inexpensive, and although some can be relatively more complex than others, they remain pretty simple devices to maintain and clean.

    Not too many of us are tangling with Messerschmitts around here, and besides, with the exception of dedicated aerobatic pilots, positive is more pleasant than negative.
     
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  4. Sep 16, 2016 #4

    StarJar

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    There's some really inexpensive bolt on FI systems out there, but one thing is, they require an electrical system - an electrical system that never fails/or a backup battery.
    That tilts the scale pretty far for a lot of us. I still think of it though, for half and full VW's. http://www.ecotrons.com/products/small_engine_fuel_injection_kit/
     
  5. Sep 16, 2016 #5

    choppergirl

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    I dont see that they could be any less reliable than say your spark system. Basically doing the same thing.... sending an electrical signal to a device to do something...whether to collapse a magnetic field or open a little valve. The first generation fuel injection system on my old car turned out to be even more reliable than the ignition system.

    I suppose you have a bit more complexity with a computer box, mass airflow meter, and O2 exhaust gas sensor... but then you also get some efficiency through constant ongoing sensor readings, calculation, and tweaking ability (if your computer fetures reprogramming) in return.
     
  6. Sep 16, 2016 #6

    Swampyankee

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    Fuel injection has become de rigueur on autos because of the need to very closely control the engine fuel flow and air:fuel ratio to meet, primarily, emissions and, in the US, fuel economy regulations. Aircraft engines do not need to meet those requirements; without both demands, EFI would be rare in cars. Put emissions requirements in place, and EFI will become standard for certified engines, in general, and probably non-certified production engines, simply because a carburetted engine won't have acceptable performance while having sighnificant air pollution emission limitations.
     
  7. Sep 16, 2016 #7

    Swampyankee

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    Carburetors without floats do exist, and, I believe were in use before WWII on some aircraft.
     
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  8. Sep 16, 2016 #8

    Himat

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    When it comes to cost, I am not sure if not a “cheap” injection system cost less to manufacture than a decent carburettor. That is when the volumes get high and the vehicle where the engine is to be used already has an electrical system. I base this assumption on how many precise machined parts a carburettor need compared to a throttle body injection.
     
  9. Sep 16, 2016 #9

    Dana

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    For certificated engines, carburetors rule because of the cost of developing and certifying any new hardware.

    For most planes, varying g-loadings aren't any more that they are for a car going over hills, bumps, and around turns. When I had my Jeep, I had all kinds of carburetor problems when crawling over rocks at steep angles.

    There are many carburetors without floats. There is the Aeroinjector, which is about a simple as you can get, and the Walbro/Tillotson type with integral pump commonly used on small engines (and many PPGs).

    Besides the emissions and fuel economy reasons mentioned above, FI is also excellent for supplying an engine with the optimum amount of fuel under widely varying speeds and loads. An aircraft engine spends most of its time at just two power settings and rpm: full power for climb, and cruise power. It's not difficult to tune a carburetor to run efficiently at those two points.

    Dana
     
  10. Sep 16, 2016 #10

    rv6ejguy

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    Cost is the big driver on engines in the sub 100hp range. In the 150+hp range, we see a very large percentage of people building new aircraft, switching choices from carbs and Bendix style FI over to EFI now, especially integrated EFI/EI which gets rid of the periodic maintenance inspection and overhauls of magnetos.

    For new Lycoming engines, the price of 2 mags and a carb is not exactly cheap so many will spend an extra couple $K to switch to EFI/EI.

    The efficiency gains are well documented which eventually pays back the initial investment.

    Modern aircraft EFI can help fix mixture distribution problems many aircraft engines like Jabiru, Rotax, Lycoming and Continental engines have and allow them to operate smoothly well lean of peak.

    The better starting, smoother, more efficient running with no chance of carb ice are the deciding factors for many.
     
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  11. Sep 16, 2016 #11

    blane.c

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    Carburetors need only gravity fed supply. Some devices call themselves injectors but are really just float-less carburetors. Injectors "inject" by name and design so therefore demand pressure to operate. There are mechanical injectors and also pressure carburetors but both have limited practicality and are not common so therefore relatively expensive.
     
  12. Sep 16, 2016 #12

    gtae07

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    That's not what Occam's Razor is*. I think you're trying to apply "Keep It Simple", but sometimes it would do us well to remember that to some extent, simplicity is in the eye of the beholder. I find Ross's EFI system simpler in terms of understanding the theory, and in terms of operation, than a mechanical fuel injection system or a carb. Mechanical and/or manual don't necessarily equal simple.

    A lot of times carbs are preferred due to cost. As Dana notes, if you need certified hardware a carb might be the only option. Even after accounting for inflation, certifying something else would be exponentially more expensive because the FAA requires a lot more paperwork and the standards are higher today. The carb itself might not be certifiable today, but gets grandfathered in.

    Other times, the carb may just cost less, and so people go with it.

    Other people just have incomplete information. I know one guy in particular who keeps trying to convince me to use a carb and magnetos because he believes I'll have hot-start and vapor lock issues with an EFI system, and because (in his eyes) his system is easier to adjust and tune.



    * Occam's Razor says that, when you have two theories that explain something equally well, the one that is simpler or requires fewer assumptions/inferences/causes/entities/logical steps/etc. is preferred. It's a general heuristic, not an iron law.
     
    Last edited: Sep 16, 2016
  13. Sep 16, 2016 #13

    BJC

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    For a float plane, carbs are more likely to start reliably, when you absolutely need it.

    For a bush plane, or a simple sport plane, they can be propped by one person without battery power.

    I have a PS5C on the Pitts, so I can prop it without assistance.


    BJC
     
  14. Sep 16, 2016 #14

    Aerowerx

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    If you have smaller aircraft with no electrical system and no starter, how are you going to have fuel injection? A mechanical fuel injector? And could you then hand-prop?

    Carbs have been around for well over 100 years now, and they work.

    Now, if you are going to be chasing a ME-262 away from your B-17 fleet at 30,000 feet, then turbocharged fuel injection would certainly be an advantage. But going for a $100 hamburger at 3000 feet??

    Also keep in mind, IIRC, that during WW2 the RAF used 100 octane AvGas, courtesy of the USA, while the Luftwaffe was using regular 87 octane.
     
  15. Sep 16, 2016 #15

    rv6ejguy

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    EFI has been around over 60 years and it works too- tens of millions of engines have been equipped with it.

    Of course if you don't have an electrical system, you won't be using EFI but those aircraft are mostly restricted to sub 100hp types and you wouldn't find the cost of EFI attractive at that level anyway as I said before.
     
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  16. Sep 16, 2016 #16

    choppergirl

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    As soon as you spin the prop your going to be generating electricity... the same that is used to generate a spark. I think you could build an EFI system with no battery at all. Plus a capacitor can hold a charge between spins to keep the computer active.

    The benefits cars get from fuel efficiency at whatever rpm would stretch your fuel in a plane as well... some planes are real gas guzzlers.
     
  17. Sep 16, 2016 #17

    StarJar

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    With FI, you've stepped into alternator + battery territory. The sensitive electronics need a steady voltage, and also substantial current is needed when you consider the fuel pump. Much different that a high voltage zap.
     
  18. Sep 16, 2016 #18

    Little Scrapper

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    When you hand prop an airplane there's no way that's enough to create the electrical current needed.

    My harley is carburetor. Last year before they went FI. You can't bump start a FI bike.
     
  19. Sep 16, 2016 #19

    Himat

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    Well, when I see this one I start to wonder.
    OS_35930.jpg

    A 1.9 cubic inch four stroke glow engine with fuel injection.
    http://www.os-engines.co.jp/english/2007spring-summer/fs200sfi/index.htm

    Maybe we will see fuel injection become common on smaller engines too?
     
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  20. Sep 16, 2016 #20

    BJC

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    The L-16 that I flew as a kid had a mechanical fuel injection and no electrical system. Sometimes it started easily, but most of the time it was a PITA to get it wound up.

    Many aerobatic HBA, as well as some certificated versions, use a manual pump to pressurize the fuel in the injector system. It takes magic to prop one without someone in the cockpit operating the throttle and mixture controls.


    BJC
     
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