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 #21

    StarJar

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  2. Sep 16, 2016 #22

    Dana

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  3. Sep 16, 2016 #23

    nerobro

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    It's a bit of a red herring. But there are kickstart dirtbikes, without a battery, that have EFI.

    Carbs have their place, if you want the lightest possible airplane, and you can do gravity feed, a carb will save you a few (at least) pounds.
     
  4. Sep 16, 2016 #24

    TFF

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    Nothing is foolproof, but carbs are pretty close. As long as the engine is turning, it draws and mixes fuel. Everything has a place; the more simple you need an engine, the more it has to operate without outside help. Mags are in the same club.
     
  5. Sep 16, 2016 #25

    TFF

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    Although not a normal part of RC engines. Small generators have been used. Adding it is doable if you could ever find one of those OS engines. They are collectors items now. Carbs still were hard to replace. And the cost differencial was too great great. $500 for the normal engine; $1000 for efi.
     
  6. Sep 17, 2016 #26

    VFR-on-top

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    I know what Occam's Razor is. KISS just didn't seem quite right, although I agree that this is what homebuilders ought to keep in mind when building and/or designing components. I took license with "Occam's Razor" to respond to the OP's question the way it was written. Your definition of it is correct.
     
  7. Sep 18, 2016 #27

    SVSUSteve

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    Except for the fact that the fuel system failure rate of carb equipped engines is a couple to several times that of fuel injection simply because of icing. Yeah, carb heat is a partial solution but if you can easily get something off the shelf that more or less eliminates the problem instead of having a power robbing, human factors dependent Band-Aid of a fix to the issue....why chance it? It might be simpler but it sure as **** isn't anywhere near as reliable.

    The old "...but then you need an electric system" just makes me laugh because of how it's hilarious that we are in the 21st century and people still act like their Amish with regards to electrical systems in aircraft.
     
  8. Sep 18, 2016 #28

    nerobro

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    If we count "cant restart" or "can't start" as failures, I bet the number get to be pretty similar. It's also worth noting that aircraft injection and what people think fuel injection is, aren't really the same thing. Most of the installed airplane systems (If I understand right..) are continious flow systems. Which are closer to carbs than they are EFI.

    Are there even enough EFI equipped planes out there to seriously consider their number, or what their failure modes are?
     
  9. Sep 18, 2016 #29

    VFR-on-top

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    Not that I entirely disagree with you here, but carb icing is a known and relatively easy-to-understand phenomena. If a FI engine starts running rough or not at all, it's a 'wtf' moment as opposed to a 'hmm, what's this?' moment. I'm not an A&P and can't afford A&P labor. Aviation carb systems I understand completely and that's what my plane will have.

    If there's a link to info re: carb vs FI reliablity, I'd like a look-see.
     
  10. Sep 18, 2016 #30

    Arthur Brown

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    Electronic engine management has many advantages, BUT it usually needs the "chip" programming for each purpose. A friend has a road and track Subaru, on road it's tame and legal and economical (34MPG) on the track chip it is a real race car with speed and acceleration but does about 5MPG.

    Someone has to develop the chip for each individual engine and aircraft, and doing a full engine model and chip for each individual installation isn't cheap/viable/affordable. Now if someone can make a FADEC that suits any engine any fuel and any altitude and costs under £/$50 then...

    Oh! I can buy a gasket set for a carb for a lot less than the diagnostic software.

    Actually I have recently seen a diesel engined C172, (in the UK they are authorised to run on JetA-1 ) -skip the ignition system all together. All our London Ambulance fast response vehicles are diesel so I see no reason not to go diesel.
     
  11. Sep 18, 2016 #31

    gtae07

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    Umm, take a look at the EFI system Ross (rv6ejguy) sells. You don't need to develop a special chip for the engine; the system comes with a programmer and a default table (in the manual) you can tweak to your liking. You can even change the programming on the fly, as it were (get it? :gig:)

    You can sell your carb and magnetos, get an EFI system, and make up the cost difference in fuel savings, spark plugs, and magneto overhauls.

    Those diesel 172s and those ambulances are FADEC-controlled. There's a little box runs the engine. Yeah, you do away with ignition but there's plenty of other things to deal with.

    The problems with going diesel in airplanes are:
    - There are few options on the market right now
    - The options that are on the market are very expensive to buy and some have inconvenient maintenance requirements
    - In the US (largest market for light airplanes) gasoline is much cheaper than in most of the rest of the world, so the fuel cost savings often don't work out over the lifetime of the engine
    - The performance tradeoffs (weight, power)
     
  12. Sep 18, 2016 #32

    AdrianS

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    Getting back to CGs original post, there have been millions of mechanical injected diesels sold, and one of the claimed advantages was that they needed no electrical systems apart from starting.
    I had a Petters single cyl diesel in a boat once - I beleive the engine was originally designed for lifeboat use.
    No carby, no spark plug, it hand started every time without fail.

    Having said that, mechanical fuel injection requires precision metering pumps, and is less flexible and probably more expensive than EFI these days.

    I know carbs, and have done many competition miles behind a pair of Webbers, but they are not exactly simple. Things that effect mixture include :
    Float level
    Choke (venturi) size
    Main jet size and clip setting
    Idle jets
    Idle bleed screw
    Emulsion tubes
    Accelerator pump

    Not to mention altitude and temperature.
    I know aviation carbs operate over a more limited regime than cars & bikes, but we (oldies) only think they're simple because we're used to them.
    Simple EFI is a pump, one or more injectors, 2 sensors, and a black box, plus electricity.
     
  13. Sep 18, 2016 #33

    Dana

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    Steve, an aircraft without an electrical system would make no sense for you for the kind of flying you do, but many of us fly older planes that never had one and have no good way to install one, or planes (like mine) where the weight of an alternator and battery would significantly impact performance.

    And the Amish may eschew technology for the most part, but they still know how to have fun.

    Dana
     
  14. Sep 18, 2016 #34

    gtae07

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    Carb ice is a latent failure mode that can occur in the heart of the normal operating envelope. Its mitigation produces a reaction opposite the desired and intuitively expected one (applying carb heat in response to the roughening/power reduction from ice further reduces RPM/power at first). Its symptoms normalize something that could be a sign of worse problems. In short, it's a human factors nightmare. If you proposed a new system with a latent failure mode like that, with similar consequences, and a mitigation method of "rely on humans with plenty of other things to worry about to monitor things and manually apply a workaround", you'd be laughed at.

    I prefer to design things so that likely problems don't happen at all, rather than saying "oh, the pilot can deal with it".

    But I'm funny round these parts. I despise constant manual engine management; it's not "part of the experience" to me, but rather an annoying distraction from flying the airplane and looking out the window. My little blue-green card from the FAA says "private pilot", not "flight engineer". But this is the light airplane community, where someone who wants to rig their flaps with a degree less deflection so they can't pop out of their tracks/cove and jam while on the ground is told to "just do a better preflight".

    But don't you know? Electrical isn't mechanical, so it's not reliable and it can fail!:shock:

    Sorry, had to.

    I can see advantages to not having an electrical system in certain circumstances--in an antique aircraft or replica, for example, or a situation where you'd otherwise have to equip with ADS-B etc. and the cost means it's not an option. But I think a lot of peoples' view of things electrical are tainted by experience with subpar design, installation, and/or component selection. If your electrical system has multiple places where a single failure will bring down the whole thing, either don't be surprised that it happens or design the system better. Use high-quality components and install them with the same care you take in fabricating and assembling your structure. If your experience with 60's and 70's light airplane electrical systems leads to believe they are unreliable, don't copy them!
     
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  15. Sep 18, 2016 #35

    Swampyankee

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    FADECs have pretty much completely replaced pneumatic -- the closest analogue to carburetors -- mechanical, and hydromechanical fuel controls in the turbine engine world, and, when I was working in engine test I saw why: two engineers came down from Hamilton with a prototype FADEC and a laptop, and were able to knock together a control program for the FADEC in a morning. The same thing with a hydromechanical control would take about about a year: designing and manufacturing the 3D cams used to program a hydromechanical control was expensive and time consuming.

    They also permit better controls for small engines. The hydromechanical control was the gold standard -- it was used on almost all of the larger engines -- but was too costly for small engines.
     
  16. Sep 18, 2016 #36

    TFF

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    Steve has always had the same agenda. It is what he believes in, which is fine. The problem with it is that it kills love of aviation for the the people who love aviation. It is great for the terrified of small aviation crowd. I have no need for aviation if it does not involve the love part. I have no need for cars if it does not involve love of autos. I might as well accept a socialist existence.
     
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  17. Sep 18, 2016 #37

    TerryM76

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    It absolutely depends on the mission, capabilities of the aircraft and the preference of the owner as to having an electrical system or electronic fuel metering system or anything else related to flying our planes. My current project has an electrical system for engine starting and keeping the battery charged. It may need to be expanded in the future for other applications. My engine has a carburetor and I like having the simplicity of something that is approximately the complexity of a flying Farmall H. I could hand-prop if I wanted to but I don't like having to do that.

    Fast cross-country flying is best with EFI and electronic ignition in order to maximize fuel efficiency but low and slow is not fuel efficient and the lowly little carburetor in all its simplicity is just fine.
     
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  18. Sep 18, 2016 #38

    Himat

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    Yes, an engine with a total loss system for electric power, fuel and lubrication.
    Actually, I do not see the principal difference between using a total loss battery and a total loss fuel for the engine.;)
     
  19. Sep 18, 2016 #39

    Midniteoyl

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    Last edited: Sep 18, 2016
  20. Sep 18, 2016 #40

    Swampyankee

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    Digital electronics are displacing mechanical control systems just about everyplace: watches, large and small appliances, modern engines, thermostats, household furnaces, etc, because they're cheaper for the same level of functionality and reliability.
     

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