Warbird automixture

Discussion in 'Engines' started by KevinB, Jul 1, 2008.

  1. KevinB

    KevinB New Member

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    Can anyone give me the "cliffs notes" version of how the auto-mixture system works on a typical warbird?
    For example P-51d:
    Are there limits of pressure altitudes where it works/ doesn’t work? And does one have to revert to manual leaning by recommended procedure by use of egt or cht possibly?? outside the limits? Is there a rich and lean of peak setting within the automixture auto setting?

    Thanks,
     
  2. GADGET

    GADGET Member

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    It is not a hard system to do.

    Optimal fuel combustion depends on the amount of air/fuel quantity, called stequiometric when no fuel nor oxigen are wasted and their proportion is optimal.

    To obtain the stequiometric proportion it is required to know the amount of oxygen available and add enough fuel to keep the balance. The amount of oxygen can be obtained be knowing the static presure of the air that sorounds the aircraft, via a simple pressure metering device (anaerobic capsule for instance).

    This way a mechanical computer can be built and tuned to give optimal performance at any altitude, allowing only the right amount of fuel to reach the cilinders.

    Hope this simple explanation might be useful to you.
     
  3. hvengel

    hvengel New Member

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    Old post but the OP didn't really get his question answered.

    In the Packard Merlin powered P-51 aircraft there were two different "injector carburetors" used. The use of the term carburetor is misleading since these were really machanical throttle body fuel injection systems and were fully automatic if functioning properly.

    With the V-1650-3 engines (P-51B/C and very early P-51D/K) the mixture lever had 4 positions. Idle cutoff, auto lean, auto rich and emergency full rich. On the V-1650-7 engines (most P-51D/Ks) it had 3 positions. Idle cutoff/auto lean, run and emergency full rich.

    To cut off the fuel to shut down the engine on the V-1650-7 the pilot would put the mixture lever into cutoff/auto lean with the throttle closed otherwise this position was like the auto lean position on the V-1650-3. The other positions are fairly self explanatory. The run/auto rich position was used most of the time. The auto lean position was only used at lower manifold pressure/RPM cruise settings. Emergency full rich was for when the auto mixture feature failed.

    Also these units had no icing issues since the fuel was injected behind the butterfly. But other parts of the induction system like the air intake below the spinner could ice up.

    These injection units with individual variations for each aircraft/engine type were used on virtually all US fighter aircraft during WWII.

    In addition the Merlin engines had an automatic system that controlled the manifold pressure with changing altitude. On the V-1650-3 engines this would function at any MP over 40 inHg but on the V-1650-7 engines this worked at any MP over 21 inHg. Also the coolant and oil cooler flaps were also automatic. Thus P-51D pilots had set it and forget it engine controls which allowed them to focus on other things.
     
  4. tommayer

    tommayer New Member

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    Haven't much experience with injector system on Merlins. Do on P&Ws. I think the injector carbs were Simmonds or, more often, Bendix units. It is possible to for those injector carbs to ice. The P&W systems I am familiar with all had provision for carb heat as well as alternate air. There is an interesting segment on this in Robert Buck's autobioghraphy. I'm away from my library and cannot remember the title. Buck flew Connies with injector carbs for TWA. They iced. The management didn't want to spend the money to install carb heat. He had a maintenance honcho on board one trans-Atlantic flight when they got carb ice. The engines went to snorting and shaking over the cold black ocean. TWA installed carb heat. There was a story with interesting graphics on this in a recent Sport Aviation. I'm sorry I can't remember which issue. But the point was same. Injector carbs can ice. Maybe there was something different about the set up Merlins, but I believe the correct assessment would run something like "carb ice is unlikely and unusual on injected engine but possible in certain conditions."
     
  5. Johnny .45

    Johnny .45 Member

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    Maybe it's a language thing, but the term is "stoichiometric" here in the US. And I've only ever heard of "pressure carburetors", although I'd certainly believe "injector carb" just as well.

    And, of course, it's "cylinder" not "cilinder", but I'll assume that was just a typo! =)
     
  6. Colin1

    Colin1 Active Member

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    Here too
    not seen that other term (or spelling) used before
     
  7. hvengel

    hvengel New Member

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    The injector units on the on the Packard Merlins were from Bendix.

    And tommayor is correct about icing. It was possible for there to be induction system icing with these injection systems but it tended to happen in places like the air duct leading to the injection unit or at the mouth of the induction system intake rather than in the actual injection unit. But it was possible for the injection unit to ice up under the correct conditions. This is the reason that the "carburetor heat" system on the P-51 introduced the heated air near the front of the induction air duct.
     
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