A jet engine is a reaction engine that discharges a fast moving jet to generate thrust to propel a jet forward. The result is known as jet propulsion and works in accordance with Newton`s laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets. In general, most jet engines are internal combustion engines but non-combusting forms also exist.

In common parlance, the term jet engine loosely refers to an internal combustion airbreathing jet engine (a duct engine). These typically consist of an engine with a rotary (rotating) air compressor powered by a turbine (“Brayton cycle“), with the leftover power providing thrust via a propelling nozzle. These types of jet engines are primarily used by jet aircraft for long distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines which give high speeds, as well as (over long distances) fuel efficiency that is about as good as piston and propeller aeroengines.

Types

There are a large number of different types of jet engines, all of which achieve forward thrust from the principle of jet propulsion.

Airbreathing

Commonly aircraft are propelled by airbreathing jet engines. Most airbreathing jet engines that are in use are turbofan jet engines which give good efficiency at speeds just below the speed of sound.

Turbine powered

Gas turbines are rotary engines that extract energy from a flow of combustion gas. They have an upstream compressor coupled to a downstream turbine with a combustion chamber in-between. In aircraft engines, those three core components are often called the “gas generator.” There are many different variations of gas turbines, but they all use a gas generator system of some type.

Turbojet

A turbojet engine is a gas turbine engine that works by compressing air with an inlet and a compressor (axial, centrifugal, or both), mixing fuel with the compressed air, burning the mixture in the combustor, and then passing the hot, high pressure air through a turbine and a nozzle. The compressor is powered by the turbine, which extracts energy from the expanding gas passing through it. The engine converts internal energy in the fuel to kinetic energy in the exhaust, producing thrust.

The turbojet is the oldest kind of general-purpose airbreathing jet engine. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.

Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion

Turbojets are quite inefficient if flown below about Mach 2 and very noisy. Most modern aircraft use turbofans instead for economic reasons.

Turbojet engines had a significant impact on commercial aviation. Majority of big airlines as British Airways, Virgin Atlantic and also small aconomic like European Ryanair or Asian Air Asia followed industry toward more efficient types. Aside from being faster than piston engines, turbojets had greater reliability, with some models demonstrating dispatch reliability rating in excess of 99.9%. Pre-jet commercial aircraft were designed with as many as 4 engines in part because of concerns over in-flight failures. Overseas flight paths were plotted to keep planes within an hour of a landing field, lengthening flights. Turbojets` reliability allowed for three and two-engine designs, and more direct long-distance flights.

Although ramjet engines are simpler in design as they have virtually no moving parts, they are incapable of operating at low flight speeds.