A quesiton came up about what actually causes the RPMs to drop when the magneto check is performed. What actually is happening with the engine that causes the performance degradation such that the RPMs drop? I am curious how others would explain this. Would you please post your comments here?
Thanks...Herb
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I am not sure that this is totally 100% accurate but this is my explanation after some research and after talking with some of the maintenance folks at WVFC and with Kevin/Carrie.
ReplyDeleteAn aircraft engine has two spark plugs per cylinder. These spark plugs are energized by two independent magnetos that provide an electrical current at a specific time that ignites the fuel/air mixture. The timing of the spark is controlled/set by timing mechanisms that determine the optimum point for the magnetos to fire based on the timing of the piston rise/fall in the cylinder. The goal being that the combustion occurs such that the pressure created from the explosion has the biggest force on the piston in addition to other factors such as cylinder temperature. As I understand, the point at which the spark occurs (or when the electrical current is sent to the spark plus) is specified in terms of degrees before TDC. TDC is known as Top Dead Center and is the point at which the piston is at the top of the cylinder. The timing of the spark I believes is between 25 and 26 degrees before TDC. This is based on a 360 degree revolution of the crank shaft. This point is determined such that the expansive force of the combusted fuel/air mixture will have the greatest impact on the piston and produce the greatest force. As the piston travels down the cylinder, the resulting force of the combusted fuel/air mixture decreases since the volume is increased and thus the pressure starts to drop. With only one magneto, there will be only one spark from the associated spark plug. This spark ignites the fuel/air mixture in a suboptimal pattern causing a slight longer burn. The reason for the suboptimal pattern is due to the placement of the two spark plugs, i.e. the two spark plugs are optimally located based on both of them working. With Only one spark plug, its placement is suboptimal since it is not centered. So even though the spark occurs at the optimal timing, the burn occurs over a longer period time and the force is reduced since the piston had traveled a longer period of time during which the pressure is reduced as the volume increases and the pressure from the combustion of the fuel/air mixtures as a slightly smaller affect.