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2 Answers

RPM drop on a magneto check

Asked by: 4656 views Aircraft Systems

Why do the rpms drop when you turn off one mag? is it just because your running on half the spark plugs ?

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2 Answers

  1. Best Answer

    John D. Collins on Sep 26, 2012

    Each spark plug ignites the fuel nearby it. The fuel burns and a flame front moves away from the plug. The pressure of the combusted fuel mixture builds until it reaches a maximum pressure that pushes the top of the piston down inside the cylinder. The timing of a typical spark in an aircraft piston engine is approximately 22 degrees. This timing allows for the pressure to be at its maximum by a little time past the piston arriving at top dead center so that the maximum mechanical advantage can be used to drive the piston down. With both plugs firing, the two flame fronts burn the available fuel faster, eventually combine into a single flame front, and the peak pressure arrives where you get the maximum energy out of the fuel. With only one plug firing, the fuel load does not complete burning until later in the cycle because the single flame front has to cover more more fuel than it would if it had the help of the other plug to start a second flame front. This results in the peak pressure occurring later in cycle and therefore doesn’t get the best mechanical advantage for pushing the piston down in the cylinder. You notice the difference in two ways, the RPM drops when running on a single plug and the EGT (Exhaust Gas Temperature) increases. Not all airplanes are equipped with an EGT gage, but if the aircraft is equipped, the reason the EGT increases is because when operating on one plug, the slower flame front means that the temperature is higher later in the cycle and has less time to cool before the exhaust valve opens and the piston pushes the exhaust gases out past the EGT sensor.

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  2. TexxassRanger on Sep 26, 2012

    John, what a well-written explanation! You should write user manuals for living! Thx

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