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RECIP TECHNOLOGY explain the difference between a naturally aspirated engine, and a mechanically aspirated engine. In addition, the students learn that constant speed propellers and supercharged or turbocharged engines require additional instrumentation. These designs require a manifold absolute pressure (MAP) gauge. During these lessons the students learn about density altitude (DA), both high- and low-density altitude. Low DA enables the engine to produce more power, and then the airfoil surfaces can generate more lift. The opposite occurs during operations that have high DA. Crucial to understanding DA are the differences between pounds per square inch gauge pressure (psig), and pounds per square inch absolute pressure (psia). One example of a differential pressure indication is an oil pressure gauge. The pressure of the oil system is that which is greater than atmospheric pressure. The MAP instrument is not a differential pressure gauge and provides the pilot a means of determining the actual density of the air in the engine's induction system. An engine equipped with an exhaust gas temperature (EGT) instrument, enables the operator a more accurate means for adjusting the fuel/air mixture ratio when leaning the mixture using the mixture control system. The A&P; student is also introduced to the various mixture ratios used for engine operation. They learn that a 1:8 ratio is the richest mixture that will burn in a cylinder, and that a 1:18 ratio is the leaned mixture that can sus- Redstone students receive a detailed study about the intake, compression, power, and exhaust strokes used in reciprocating engines. tain flame propagation. Neither of these ratios is used for normal operation. Because an air-cooled engine has limitations at high power settings, the students learn that a 1:10 ratio — defined as takeoff mixture — is used for maximum power settings to provide additional internal cooling. Once the airplane achieves cruise speed, they learn that a 1:16 best economy ratio is used. A 1:12 ratio, best power, is used when the engine is equipped with an anti-detonation injection system. A stoichiometric mixture ratio is defined as chemically correct. This fuel/air mixture ratio, 1:15, is not commonly used due to the high temperatures created because all of the fuel and air is consumed during flame propagation in the cylinders assemblies. Also included in the lessons are the reasons an aircraft reciprocating uses two spark plugs. They learn that using two magnetos for ignition provides a margin of safety should one system fail. Using two spark plugs ensures a more effective burning of the fuel/air change for increased power production. Crucial to understanding the theory of operation for a gas piston engine centers upon the four strokes with an ignition event. The lessons in this area detail the Otto Cycle, used in the discussion of reciprocating engines. Students learn that the engines have a constant volume in the cylinders. The students receive a detailed study about the intake, compression, power, and exhaust strokes used in reciprocating engines. Understanding the Otto Cycle and the events of a four-cycle engine