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4 Answers

What Is Engine Backfire?

Asked by: 3930 views ,
Aircraft Systems, Private Pilot

When you're at a cruise power setting and suddenly pull the throttle all the way back, it's not rare to hear the engine "backfire," which sounds like an explosive pop. If you pull the throttle back gently and smoothly to idle, it doesn't backfire. So, what exactly causes backfiring?

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4 Answers

  1. James c on Oct 17, 2014

    That’s a question I’ve wondered myself, now this is just a guess but I thinks it’s similar to when you use the transmition in your car to slow down, or downshifting,

    Basically the propeller is driving the engine. In big airplanes with a solid shaft from the prop to the turbine like the c-130, or the c90 this action is very bad for the engine so they developed a system called a Negative Torque System, or “NTS”
    What that system does is when it senses negative torque it will increase blade angle to keep the prop from driving the engine,

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  2. Mitchell L Williams on Oct 17, 2014

    What you are experiencing may be termed as “After-fire” instead of back-fire.

    Back-fire is a premature ignition of fuel in the intake manifold that may result from a failed start, or wrong engine timing.

    After-fire results when a combustible mixture is not burned in the cylinder but burns in the exhaust or muffler.

    Aircraft carburetors such as Marvel-Schebler are simple designs and are not intended for fast changes in power settings. Automobile carburetors have more advanced accelerator pumps and other methods to maintain a proper mixture during abrupt power changes.

    When going from low power to high, the operator opens the throttle and the air flow immediately increases. The fuel flow may lag behind as the fuel bowl level changes and the float valve opening takes some time. This results in a lean mixture that may not be combustible. As the fuel flow catches up to airflow, the unburned fuel in the exhaust from the previous lean misfire may combust in the exhaust.

    During a power reduction, the airflow immediately reduces, but fuel flow may persist at higher levels until the float valve re-positions. This may cause an excessive rich mixture. Any leaks in the exhaust system may bring in more air that may allow a combustible mixture in the exhaust that may after-fire.

    A lot of popping and after-firing at low power setting is a indication of bad exhaust gaskets or other exhaust leaks.

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  3. Drew on Oct 18, 2014

    Thanks for the answers.

    Mitchell, I don’t understand the part about the exhaust system leaking? What do you mean by that?

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  4. Mitchell L Williams on Nov 04, 2014

    Sorry for he delay. If you have a bad/worn/blown gasket at the exhaust flange to head connection (or a broken or missing exhaust stud/nut) then at idle or low power setting, air may get sucked into the exhaust (negative pressure following an exhaust pressure pulse). That air provides oxygen that may react with unburned fuel causing after-fire (popping sound in the exhaust).

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