Components of a Jet Engine
A jet engine is a type of engine that is propelled by a stream of hot exhaust gases formed when air is forced into the engine by several stages of compression, heated, and subsequently expanded through a nozzle. While jet engines can refer to rockets, waterjet engines, and hybrid propulsions systems, the term most commonly describes air-breathing gas turbine engines.
Jet engines are broken into two sections: cold and hot. The cold section, at the front of the engine, features the air intake or inlet, compressor, bypass ducts, shaft, and diffuser. The air intake is arguably the most important part of any jet engine. In subsonic aircraft, the intake is a duct that ensures the smooth, controlled flow of air into the engine. It has the tough task of regulating airflow despite the air not approaching the intake from straight ahead, either because of cross winds while grounded or aircraft pitch and yaw motion during flight. Air enters the compressor at approximately half the speed of sound, so the airflow will accelerate at flight speeds lower than this and decelerate at speeds higher than this. Because of this, the intake must be able to accomodate all speeds of flow without losses.
After flowing through the intake, air enters the compressor. The compressor comprises different stages, each stage consisting of rotating blades and vanes. As air flows through the compressor, its pressure and temperature increase markedly. Bypass ducts then deliver the air from the compressor with minimal loss to the bypass propelling nozzle. In certain engine configurations, the compressed air may come in contact with turbine exhaust before entering a single propeller nozzle. There are also uncommon arrangements in which an afterburner is installed between the mixer and nozzle. The engine’s shaft has the important role of connecting the compressor and turbine together, running most of the length of the engine. There can be as many as three independent shafts in a given configuration. The final part of the cold section is the diffuser. The diffusers slow the speed of air flowing from the compressor to reduce losses in the combustor. Slower air is also necessary to stabilize the combustion flame and maintain combustion efficiency.
After the diffuser, the engine’s hot section begins. The hot section consists of the combustor, turbine, afterburner, and the exhaust. The first part of the hot section is the combustor, where fuel is burned at a constant after its ignition during engine start. After the combustor mixes air and fuel, it releases hot gases into the turbine. The turbine is a series of bladed discs that act like a windmill, extracting energy from the hot gases leaving the combustor. Since the turbine blades are exposed to such high heats, their temperature must be maintained. This is done through cooling bleed air from the compressor, which cools the turbine blades, vanes, and discs, allowing for higher gas temperatures and therefore, more power.
Following the turbine is the afterburner. The afterburner creates supplementary thrust by burning fuel in the jet pipe. This reheated exhaust gas increases the exhaust velocity, resulting in higher airspeed. The afterburner is constructed to accommodate the higher volume of this gas, enabling the engine to maintain the same airflow with no change in its operation. The final part of the hot section, and the engine as a whole, is the exhaust. Turbine exhaust gases pass through the exhaust nozzle and produce the high-speed jet stream that allows for flight.