Advantages of pressure carburetors

Discussion in 'Aviation' started by Jenisch, Mar 1, 2012.

  1. Jenisch

    Jenisch Active Member

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    #1 Jenisch, Mar 1, 2012
    Last edited: Mar 1, 2012
    Hello,

    Many aircraft (I think most American) used pressure carburetors in WWII. I was wondering what they provided of good since direct injection was a superior design. I think that easy manufacture convertion from the conventional carburetor design can be a reason.
     
  2. wuzak

    wuzak Well-Known Member

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    Charge cooling.

    Maybe better fuel atomisation.
     
  3. Siegfried

    Siegfried Banned

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    By injecting the fuel into the eye of the supercharger it precools the charge and reduces its volume, thereby reducing the amount of work the supercharger has to do or rather increasing the mass of air it can process.

    Some German engines, the later variants of the BMW 801 had "C3 einspritzung" whereby the fuel was injected into the supercharger to get "sonder noteleistung" i.e. "special war emergency power" which is a sort of special WEP. This despite the BMW 801 already having multipoint in cylinder fuel injection. In other words the BMW almost converted to a pressure carburator system to get more WEP. Injecting water (MW50) tended to crack the cyclinder heads so this method was preferred. Latter a MW50 was however added succesfully.

    The Jumo 213A, essentially and 1770hp bomber engine was also given a similar system, this injected B4 and thereby increased power to 1900hp.

    The advantage direct in cylinder injection is that better fuel distribution is achieved (given airflow is even!) and that the valves can have a large open overlap thereby allowing better scavenging, including resonance scavenging as less fuel is lost in the exhaust. This allows more space for air/fuel but also purges the mimxture of end gas species from the previous exaust stroke that are notorious for initiating pre-ignition.

    The better vaporisation possible with even throttle body fuel injection would have been important for German (and Russian) engines as it was harder for them to control this characteristic in their fuels; the Germans being rather focused on synthesising the fuel. However AFAIKT most German engines used multipoint injection. This became a technological advantage in most areas, however one reason they used this method is that patents had been tightly controlled by allied companies and there was no other way of getting around them.
     
  4. Kryten

    Kryten Member

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    Carbs hold one huge advantage over fuel injection (the crude fuel injectiojn available back then) and that is they work relative to air density, as you climb the carb self adjusts its fuelling, mechanical injection has to be "mapped" and have this compensation adjustment built in , this tends to be rather crude in comparison, it took many years and complex electronics to perfect fuel injection systems!
     
  5. Jenisch

    Jenisch Active Member

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    Thanks for the excellent answers guys!
     
  6. fastmongrel

    fastmongrel Well-Known Member

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    I used to race a bike with pre electronic fuel injection and boy did we have fun with seizures. Sorting a bike with a carb takes minutes when you have the practice do a plug chop go up or down a main needle jet size and do a plug chop again if plug is right colour and its pulling fine and your done. Thats being simplistic but its a hell of a lot better than playing with even a mechanical single point injector. Never mind a common rail or multi point injector, nowadays plug in a laptop and remap the ECU and your good to go. With the old fashioned pumps you could end up trying to retime the damn thing with a protractor, not something I would like to do on a forward airstrip.

    Its only in the last 10 years or so that injection with the aid of cheap rugged electronics and emissions laws has won the intake war but even now a good carb can still match an injector for pure raw power.
     
  7. Elmas

    Elmas Active Member

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    #7 Elmas, Mar 3, 2012
    Last edited: Mar 3, 2012
    Listen to a Jaguar engine of, say, 1965 and a Jaguar engine today: they are not anymore the same engines they used to be.......
     

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