fuel injection vs carburetors

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by yankeeclipper, Jun 1, 2009.

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  1. Jun 1, 2009 #1

    yankeeclipper

    yankeeclipper

    yankeeclipper

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    Anyone know why so many powerplants are using carburetors rather than fuel injection?

    It's hard to imagine it's for the sake of reliability or weight when I consider the simplicity and durability of many F.I. systems available today. Perhaps it's not a good comparison, but the F.I. on my BMW motorcycle is now 12 years and 100k miles old and is entirely original--hardly "babied" either. It is very lightweight, and has the obvious advantage of adjusting to altitude and other environmental conditions, to say nothing of the vastly superior efficiency over carburetors.
     
  2. Jun 1, 2009 #2

    bmcj

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    I've been told by some reliable engine shops that carbed engines use less fuel. I'm not sure, though, how they compare in peak or steady state power output, But it seems to me like I may have heard that the carb wins out there too.

    Bruce :)
     
  3. Jun 1, 2009 #3

    PTAirco

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    For certified airplanes the reason is probably still the cost of testing and certifying a system. Also you would need redundant electrical systems to run modern F.I. Dual alternators, dual batteries, dual black boxes. I guess you could make a case for not needing dual injectors, since the engine would not quie outright iof one failed, but still.
    It cost more. Most A&P mechanics would have to learn about them before they can work on them. I guess it boils down to the momentum of proven technology - it takes a long while to change direction.

    As for vastly superior efficiency? I doubt the term vastly could be used. Airplanes operate at constant power settings most of their lives and a carefully tuned carb will probably have pretty much the same efficiency under those conditions. In a car it may be a different matter, but we're talking a few percent here.

    I still think reliability is the main problem; you're relying on electronics and an electrical system (unless you're talking about purely mechanical fuel injection which is rare) and far more parts, all of which can go wrong. The typical airplane carb is a marvel of simplicity (some might say crudity), but simplicity means reliability. I have just as simple a carb on my old 63 GMC truck and as long as you clean it every few years it is utterly reliable.
     
  4. Jun 1, 2009 #4

    wally

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    Most aircraft engines today (lycoming, Contentinental) are continuations of very old designs. They have a lot in common with old farm tractor engines. They are designed to be simple and reliable.

    The FAA rules for production aircraft engines are very specific and involved as to testing required for any change done.

    Carbureters work, so don't change unless you need to. Most of homebuilt airplanes are also designed to be simple and a gravity feed fuel system to an old design "tractor" carb is about as bullet proof as you can get!
    Wally
     
  5. Jun 1, 2009 #5

    yankeeclipper

    yankeeclipper

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    Thanks for the replies.

    I see your points about adding electric power supply and fuel pressure to the list of POF. The fairly constant range of power makes sense of it as well.
     
  6. Jun 2, 2009 #6

    Dan Thomas

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    I don't know of any new airplane using a carburetor. Aircraft fuel injection systems are common enough, but are unrelated to automotive systems. They're completely mechanical and are little more complicated than a carb, and deliver much more uniform fuel distribution than any aircraft carb. Most aircraft induction systems are really short and are full of elbows and other restrictions that result in fuel puddling and uneven mixtures and the subsequent rough running. We have one new 172S with injection and there's no comparison with the older 172M's with their carbs. It's smoother and much more responsive, and the smoothness shows up when leaning. It will die smoothly, instead of starting to shudder as carbed engines do as the leanest cylinders will quit firing first. There are calibrated injectors available (GAMI) that will tweak injection distribution even further, but with a carb you're stuck with whatever it will give you. On the old O-200s we had in 150s, for instance, the some cylinders ran way richer than others due to the way the fuel nozzle sprayed and the throttle plate deflected that spray.

    But injection systems do cost a bit more to make. They should cost a LOT more, considering the age of carburetor designs and their crudity. Aircraft have been using fuel injection systems for 40 or 50 years, at least. It's not a new idea, and has been around at least as long as automotive fuel injection. Electronic fuel injection is possible using FADEC systems but right now the reliability is still a bit suspect. Diesels are using FADEC but there have been failures when the electrical systems quit. The mechanical injection systems are far better, still.

    Dan
     
  7. Jun 2, 2009 #7

    Canuck Bob

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    My Jeep wrangler has an excellent old design straight 6 cylinder with a good computerized fuel injection system. It is a pushrod OHV engine with its design roots in the old AMC lineup.

    I mention this because it is an excellent example of older engine design updated with computer designed heads, exhausts, etc. as well as fuel injection. My mileage is terrible by modern standards. However my performance compared to earlier versions with carbs is outstanding.

    A dependible FI system far outperforms any carb engine. As far as lighter that does not always apply. My A65 uses gravity feed and 2 mags. FI aircraft should have top quality electrical systems, probably 2 batteries, 2 fuel pumps and such.

    The new variable valve timing systems (the real secret to our new engines incredible performance), detailed and programmable electronic timing curves (no hot start problems, imagine a start curve of ATDC jumping to a timing map based on real time engine readings with a learning loop!), blazing hot spark, and dead on fuel mixture (based on accurate O2 sensor readings) is why modern vehicles do so well. A person can drive a car specifically tuned for stop and go traffic jams then take off with a monster torque, tiny, high performance Formula 1 inspired engine to 7500 rpm.

    FI primarily replaced carbs because carb engines could never meet fuel and emmisions standards. A worthy goal for airplane engines.

    Also, REALLY good carb men where hard to find 30 years ago, today they are a dying breed of mechanic. Todays best mechanics are masters of the laptop, and fine mechanics indeed. I recently watched the JCB team from the UK on a TV show set the diesel land speed record at Bonneville, impressive indeed.

    I suggest that based on the number of carb ice incidents reported or suspected every year that computerized engine control and FI is already far more reliable than mags and carbs even with the complexity based on automotive performance and reliability. If all the cars on the highways for a busy long weekend were powered by Lycomings how many more dead cars on the side of the road would we see? In todays world it is amazing just how few failures one sees in with computerized vehicle control.
     
  8. Jun 2, 2009 #8

    Dana

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    Hah! I had an older Jeep, not exactly the same engine but similar... with the *^$@&$#!! Carter carburetor... flooded going up the hill, starved going down. But I digress...

    I'm with PTAirco... an aircraft engine that runs mostly at a constant high rpm won't see nearly as much advantage from fuel injection as a car which needs low end torque, high end power, mid range fuel efficiency and low emissions.

    -Dana

    A goverment that fears arms in the hands of it people should also fear ROPE!
     
  9. Jun 2, 2009 #9

    Canuck Bob

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    I actually agree as well except for the carb icing issue. Designing out this source of risk is worth careful thought. However proper pilot technique reduces the risk to very acceptable levels. Considering that one of the most common fuel system malfunctions is running out of gas!

    Upgrading the static timing curve is the one thing that I would say gives the best bang for the buck for traditional aero engines. The reports for the new electronic mags and systems like the Lightspeed would be my choice if my engine had electrics.

    I've actually daydreamed of a twin small permanent magnet alternator system driven from the mag drives with CDI waste spark for my A65. Handprop at 1 or 2 degrees ATDC so kickback is eliminated and a timing curve tailored from idle to WOT with manifold pressure sensors. A well tuned Marvel with proper mixture control and a better intake manifold design ... .

    I just wished my government feared the vote, apathy and influence peddling is the enemy of democracy.

    As far as rope is concerned Wall Street and Bay Street could be a great place to experiment with a more aggresive statement.
     
  10. Jun 2, 2009 #10

    Dana

    Dana

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    A guy at my home field has a Thorp with an 0-320 with the standard magneto on one side and an electronic ignition replacing the other mag. We were talking about it the other day, he said it runs a lot better... the electronic ignition can go from zero to 40° BTDC. Standard aircraft shielded plugs on the bottom, automotive plugs on the top.

    -Dana

    When I was young I was told that anyone could be President. I'm beginning to believe it.
     
  11. Jun 2, 2009 #11

    Dan Thomas

    Dan Thomas

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    Carburetor ice is the biggest single cause of engine failures, by a wide margin over other factors such as running out of fuel or mismanaging the fuel supply, contaminated fuel, or oil starvation. It is often only "suspected" in accidents because the ice is long gone by the time investigators get there. And it's a big factor because fewer students these days know anything at all about engines or their systems and have no clue as to what's going on in the cowling and how environmental factors such as temp and dewpoint affect those systems.

    AOPA has a searchable database for carb ice accidents, and some good articles on the problem. Here's an older NTSB article:

    http://www.ntsb.gov/recs/letters/1989/A89_140_142.pdf

    We could maybe come up with a computerized system to control carb ice so that the pilot didn't have to worry about it, but we'd be adding complexity and weight and more failure points. It would require a computer and not just a simple thermostat like your 1968 Chevy used in its air cleaner. The old thermostatted systems heated the incoming air to around 70°F all the time, even when there was no risk of ice, and in severe icing conditions 70 wasn't high enough. Carb ice can form at temps up to 100°F. Since heated air has much less density, power output suffers in a thermostatted system and there are many times when flying that we need all the power the engine can give us. So we'd need a computer taking temp readings in the carb, figuring in outside temp and dewpoint, and an MP sensor to calculate icing risk with some means of overriding the whole system when full throttle is applied in an emergency. There are many variables, and the airplane builders found that a manual heat control and some training did the trick with much less hassle. It's not a big deal to understand carb ice, how to detect it, and how to control it.

    As far as aircraft injection systems, the Continental C-85 had injection a long time ago. See Harry Fenton on Continental Engines

    Dan
     
  12. Jun 3, 2009 #12

    bmcj

    bmcj

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    Sorry to tell you this, but carb icing can be a problem on fuel injected engines too, but I guess it wouldn't be called "carb" icing. Granted, the butterfly valve in the venturi is a prime candidate for icing, but any intake structure can ice up under the proper conditions.

    Bruce :)
     
  13. Jun 3, 2009 #13

    Dan Thomas

    Dan Thomas

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    Yup. But the injection system has a very minor venturi or none at all so that the temp drop is minimal, and there's no fuel evaporating in the carb so no temp drop there. There's a temp drop at the throttle plate when it's closed, though, that can cause a hassle.

    Dan
     
  14. Jun 3, 2009 #14

    bmcj

    bmcj

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    Yep, carbed setups are far more likely to experience carb ice, but FI pilots need to be openminded about the possibility of intake icing, even though it is not very likely.
     
  15. Jun 5, 2009 #15

    handprop

    handprop

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    Just for kicks here is a Pietenpol with a homebuilt FI system on a model A. It works great and is real simple. The owner designed it himself and it's been working great for years.

    Mike
     

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  16. Jun 7, 2009 #16

    Canuck Bob

    Canuck Bob

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    That is spectacular, any details?
     
  17. Jun 7, 2009 #17

    handprop

    handprop

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    I can get details. The fella is an engineer and lives real close. He has built several Pietenpols including one with a Warner and H. Ribblet airfoil, a real performer to say the least.

    Mike
     
  18. Jun 7, 2009 #18

    wsimpso1

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    Thread drift.

    What follows is a brief summary of fuelling methods...

    Carbs are simple and were standard since the beginning of IC. Some of the most sophisticated and highest specific output engines ever produced were carburated, with the turbochargers downstream of the carb. In those airplanes, the mixture was well distributed. The absolute highest efficiency aircraft engines was a diesel made by Napier.

    Mechanical fuel injection was in airplanes and high performance cars where they wanted more power. The venturi that allows carburaters to work also causes some energy losses and the variability in mixture distribution both reduce output. An O-360 is 180 hp, and IO360 is 200 hp, and the only real difference between them is the fueling method.

    Electronic fuel injection became standard in cars when emissions required it. To run it well, it requires sensors for top water, induction air temp, air mass rate, knock, engine crank speed and position, throttle position, exhaust gas oxygen, and others. When it does work, it is fine, but the fuss that is gone through to make them work is impressive. I have seen vehicles sit for six weeks because the programming to make it start or run under some condition was not working yet. Lose one sensor in a car, and it can limp home. Lose a sensor in the air, and your engine output is too small for it to do much more than extend your glide a bit. So you end up with redundant sensors and computers...

    Billski
     

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