Otto cycle.  The constant-volume cycle of energy  transformation used by reciprocating engines. A mixture of  fuel and air is drawn into the cylinder as the piston moves  to the bottom of its stroke. The mixture is compressed as the  piston moves upward in the cylinder, and when the piston is  near the top of its stroke, the mixture is electrically ignited  and burns. The burning mixture heats and expands the air  inside the cylinder and forces the piston down, performing  useful work. The piston then moves back up, forcing the  burned gases out of the cylinder. Overboost.  A condition of excessive manifold pressure  in a reciprocating engine. Overboosting occurs when the  supercharger is operated at too high a speed. Overrunning clutch.  A type of clutch that couples an input  shaft with an output shaft. When the input shaft is driven, the  output shaft rotates with it. When the output shaft is driven,  the output shaft rotates with it. But when the output shaft is  driven, the input shaft does not turn. Overspeed condition.  A speed condition in which the engine  is turning at an rpm higher than that for which the propeller  governor is set. P-lead.  Primary lead. The wire that connects the primary  winding of a magneto to the ignition switch. The magneto  is turned off by grounding its P-lead. Pascal.  The unit of pressure produced when one newton  of force acts uniformly over an area of one square meter.  One pascal is equal to 14.503 ∙ 10-5 (0.00014503) psi. The  kilopascal (kPa) is easier to manipulate. 1 kPa = 1,000 Pa  = 0.14503 psi. PCB.  Plenum chamber burning. A method of thrust  augmentation used on engines with vectored nozzles. Fuel  injected into the fan-discharge air is burned to increase thrust. Peak voltage.  The voltage of AC electricity that is measured  from zero voltage to the peak of either alternation. Penetrant dwell time.  The length of time a part is left in the  penetrant when preparing it for inspection by the fluorescent  or dye penetrant method. The hotter the part and the longer  the penetrant dwell time, the smaller the fault that will be  detected. Performance number.  The rating of antidetonation  characteristics of a reciprocating engine fuel that is better  than the high rating reference fuel, iso-octane. Performance  numbers are greater than 100. Permanent magnet.  A piece of hardened steel that has been  exposed to a strong magnetizing force which has aligned the  spin axes of the electrons surrounding its atoms. The high  retentivity of the material causes the electrons to retain their  magnetic orientation. Permanent-mold casting.  A casting made in a reusable  metal mold. The walls of permanent-mold castings can be  made thinner than similar walls made by sand casting. Permeability.  A measure of the ease with which lines of  magnetic flux can pass through a material. Phase sequence, or phase rotation.  The sequence with  which the output phases of a three-phase generator are  connected to the load. Reversing the phase sequence of a  generator from A-B-C to A-C-B prevents the generator from  being synchronized with the others on the bus. Pi (π) filter.  An electronic filter used to prevent radio  frequency energy produced in the ignition exciter from  feeding back into the aircraft electrical system. The filter is  made of an inductor with a capacitor on its input and output.  The name is derived from the resemblance of the three  components on a schematic diagram to the Greek letter pi (π). Pinion.  A small gear that meshes with and drives a larger  gear. Piston (reciprocating engine component).  The movable  plug inside the cylinder of a reciprocating engine. The piston  moves in and out to compress the fuel-air mixture and to  transmit the force from the expanding gas in the cylinder to  the crankshaft. Piston pin.  See wrist pin. Pitch angle.  The angle between the chord line of a propeller  blade and the plane of rotation. See blade angle. Pitch distribution.  The gradual change in pitch angle of a  propeller blade from the root to the tip. Plane of rotation.  The plane in which a propeller blade  rotates. The plane of rotation is perpendicular to the propeller  shaft. Planetary gears.  A type of large-ratio reduction gearing.  A series of small planetary gears are mounted on a spider  attached to the output shaft. The planetary gears rotate  between a fixed sun gear and a driven ring gear.  G-20