jet engine

jet engine
Name	Jet engine
Caption	A simplified diagram of a turbojet engine.

jet engine. A jet engine is a reaction engine that discharges a fast-moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad class of propulsion systems encompasses a variety of designs, all of which function by taking in air, increasing its pressure and temperature through combustion, and expelling it at high velocity. The development of practical jet engines in the mid-20th century revolutionized aviation, enabling unprecedented speeds and altitudes for military and civilian aircraft. Today, they are the dominant powerplant for everything from commercial airliners to high-performance fighter aircraft.

Principles of operation

The fundamental principle relies on Newton's laws of motion, specifically the third law, where the engine's forward thrust is the reaction to the rearward expulsion of mass. Most designs follow the Brayton thermodynamic cycle, involving intake, compression, combustion, and exhaust. Air is drawn in through an inlet, compressed by a compressor, mixed with fuel and ignited in a combustion chamber, dramatically increasing its energy. The hot, high-pressure gas then expands through a turbine—which powers the compressor—before being accelerated through a nozzle to produce thrust. The efficiency of this process is heavily influenced by the pressure ratio achieved by the compressor and the turbine inlet temperature.

Types of jet engines

The primary categories include the turbojet, the simplest form, where all thrust comes from the exhaust jet. The turbofan engine, the most common type on modern airliners, features a large fan at the front that bypasses a portion of the air around the core, improving efficiency and reducing noise. The turboprop and turboshaft engines convert most of the engine's power to shaft power to drive a propeller or rotor, as seen on aircraft like the Lockheed C-130 Hercules and helicopters like the Sikorsky UH-60 Black Hawk. Other types include the ramjet and scramjet, which have no moving compressors and operate efficiently at supersonic speeds, and the pulsejet, a simple, intermittent combustion engine.

History and development

Early theoretical work was conducted by inventors like Henri Coandă and Frank Whittle, who patented a design for a turbojet in 1930. Independent development in Nazi Germany led by Hans von Ohain resulted in the first operational jet aircraft, the Heinkel He 178, which flew in 1939. Whittle's engine powered the Gloster E.28/39 on its first flight in 1941. Wartime development accelerated, producing aircraft like the Messerschmitt Me 262 and the Gloster Meteor. Post-war, companies like General Electric, Pratt & Whitney, and Rolls-Royce advanced the technology, leading to the commercial jet age inaugurated by the de Havilland Comet and the Boeing 707.

Major components

The core components are arranged along a central shaft. The intake or inlet duct is designed to deliver air to the compressor at the optimal velocity and pressure; advanced designs for aircraft like the SR-71 Blackbird are highly complex. The compressor, typically an axial or centrifugal design, raises the air's pressure. The combustion chamber, or combustor, where fuel is injected and burned, is a high-stress environment requiring advanced materials from firms like Chromalloy. The turbine extracts energy from the hot gas to drive the compressor and, in some engines, external accessories. Finally, the exhaust nozzle shapes the exiting jet to maximize thrust.

Performance and efficiency

Key metrics include thrust, specific fuel consumption, and thermal efficiency. Performance is critically dependent on the overall pressure ratio and the turbine inlet temperature, which has been pushed higher through innovations like single-crystal turbine blades and intricate cooling channels. Propulsive efficiency improves with higher bypass ratios, as seen in modern engines from CFM International and International Aero Engines. Thrust-to-weight ratio is a paramount concern for military engines powering aircraft like the F-22 Raptor, achieved through lightweight composites and advanced alloys. Operational efficiency is also affected by factors like altitude and Mach number.

Applications

The primary application is in aviation. Commercial aviation is dominated by high-bypass turbofans on aircraft from Airbus and Boeing. Military aviation utilizes low-bypass turbofans and turbojets for fighters like the Eurofighter Typhoon and Sukhoi Su-57, and turbofans for bombers like the Northrop Grumman B-2 Spirit. Turboshaft engines power rotary-wing aircraft such as those from Bell Helicopter and Airbus Helicopters. Beyond aviation, jet engines are used in industrial gas turbines for power generation by companies like Siemens Energy, and in marine propulsion for fast naval vessels. They also serve as the basis for auxiliary power units on aircraft and as propulsion for cruise missiles like the Tomahawk.

Category:Aircraft engines Category:Gas turbines