Hybrid spark plug. A fine-wire spark plug that has a platinum center electrode and iridium ground electrodes. 

Hydraulic lock. A condition in which oil drains into the lower cylinders of a reciprocating engine and leaks past the piston rings to fill the combustion chamber. If the oil is not removed before the engine is started, it can cause serious damage. 

Hydromechanical. Any device that combines fluid pressures with mechanical actions to achieve a desired result. In a hydromechanical fuel control used for a turbine engine, hydraulic servos are used in conjunction with the mechanical linkages. 

Ice bridging. A spark plug failure that occurs when starting a reciprocating engine in extremely cold weather. When a cylinder fires, the fuel-air mixture is converted into carbon dioxide and water vapor. The water vapor condenses on the spark plug electrodes and forms ice that bridges the electrode gap and prevents the plug firing until the ice is melted. This normally requires removing the spark plugs from the engine. 

IDG.  Integrated drive generator. An AC generator installed on turbine engines. An IDG incorporates a brushless, three- phase AC generator and a constant-speed drive in a single component. 

Igniter.  The component in a turbine-engine ignition system that provides a high-energy spark for igniting the fuel-air mixture in the combustion chamber for starting. 

IHP.  Indicated horsepower. The theoretical horse-power a reciprocating engine develops. 

IMEP.  Indicated mean effective pressure. The average pressure existing inside the cylinder of a reciprocating engine during its power stroke. 

Impulse coupling.  A spring-loaded coupling between a magneto shaft and the drive gear inside the engine. When the engine is rotated for starting, the impulse coupling locks the magnet so it cannot turn. The spring in the coupling winds up as the crankshaft continues to turn, and when the piston is near top center, the coupling releases and spins the magnet, producing a hot and retarded spark. 

In-line engine. A reciprocating engine with all of the cylinders arranged in a straight line. 

Incandescent. Glowing because of intense heat. 

Inconel. The registered trade name for an alloy of chromium, iron, and nickel. Inconel is similar to stainless steel, but cannot be hardened by heat treatment. 

Inductive reactance. An opposition to the flow of AC or changing DC caused by inductance in the circuit. Inductive reactance, whose symbol is XL, causes a voltage drop, but it does not use power nor produce heat. 

Inertia. The tendency of a body to resist acceleration. A body at rest will remain at rest or a body in motion will stay in motion in a straight line unless acted on by an outside force. 

Inertia starter.  A starter for a large reciprocating engine that uses energy stored in a rapidly spinning flywheel to turn the crankshaft. 

Inlet guide vanes.  A set of stator vanes in front of the first stage of compression in a gas turbine engine. The inlet guide vanes deflect the air entering the compressor in the correct direction for optimum operation. Inlet guide vanes may be fixed, or their angle may be controlled hydraulically by fuel from the fuel control. 

Integral fuel tank. An aircraft fuel tank made by sealing off part of the structure so fuel can be carried in the structure itself. 

Intercooler. An air-to-air heat exchanger installed between a turbosupercharger and the carburetor. Intercoolers decrease the temperature of compressed air to prevent detonation. 

Interference angle (poppet valve dimension). The difference between the valve seat and the valve face angles. Normally, the valve seats are ground with between 0.5º and 1º greater angle than the valve face. This allows the face to touch the seat with a line contact that provides the best sealing. 

Interference fit. A type of fit used when assembling certain mechanical devices. The hole is made smaller than the part that fits into it. The material containing the hole is heated to expand the hole, and the part that fits into the hole is chilled to shrink it. The parts are assembled, and when they reach the same temperature their fit is so tight they will not loosen in service. 

Internal-combustion engine. A form of heat engine in which the fuel and air mixture is burned inside the engine to heat and expand the air so it can perform useful work. 

Internal timing. The adjustment of the breaker points of a magneto so they will begin to open at the time the magnet is in its E-gap position. 

