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2 Answers

Fuel Pump on Injected vs Carb engines

Asked by: 1859 views Aircraft Systems, Flight Instructor

Sorry if this question seems rather "basic".   On Carb engines with low wings, mixture often goes to rich and fuel pump remains "on" during start.  On injected engines, pump is on for a few seconds,  then turned to "off" , mixture brought back to lean and advanced to rich on start.

Why does a carb engine not get flooded (pump remains on during crank)?   And if you prime engine and then squirt a full prime directly into cylinder as you crank, why do we not pull mixture back to lean as we do on injected engines on start to avoid an engine fire on start?

 I understand on injected engines, the injectors could get flooded if left on for too long (but that brings up a follow up as to why fuel does not stop flowing when a certain pressure is reached; I believe autos have that function for you hear pump turn off a few seconds after you turn a car key to accessory position)

 

I think I have 3 questions buried in there haha

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2 Answers



  1. Mark Kolber on Jan 27, 2019

    I may be the worst person to answer this, but since no one who actually understands engines answered here goes. Maybe someone will correct me.

    In a low-wing airplane, the electric pump serves the function of getting fuel into the system. From engine off, unlike in a high wing, it just ain’t getting there any other way. In a carbureted engine, that’s all it is for. But in a fuel-injected engine, it will also serve as a priming pump.

    Beyond that, your question has an incorrect assumption – that all carbureted engines start alike and all fuel-injected engines start alike. They don’t.

    For just one POH-based fuel-injected example, you are describing the way to start *some* fuel-injected engines – “pump is on for a few seconds, then turned to off , mixture brought back to lean and advanced to rich on start.”

    But here’s the way the manual says to start a 33 series Debonair/Bonanza. Mixture full rich, throttle full open, pump on for a few seconds then turned off, throttle pulled back to about 1/4″, and turn the key.

    In both, mixture rich and electric pump on used as a primer, but the similarity in sequence ends there.

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  2. Ticen on Jan 27, 2019

    Thanks for the reply. My biggest question that was kinda hit on is the fuel pump. So on an injected engine, if you leave pump on in full rich with engine off prior to start, it seems to flood if u leave on for more than a few seconds (to prime for start).

    On Carb low wing, when pump is on routing fuel to carb, why does carb not get flooded (since pump APPEARS to be pumping mixture rich fuel). Does fuel somehow stop even though pump is on? And I have found engines rarely start easily unless primer is used while cranking on carb engines.

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