Engine Types
During the development of jet engine technology, several types of engines have been invented. Most of these types perform differently at varying airspeeds, which means that each type of engine has varying characteristics. The turbojet is the earliest jet engine and formed the base for the engines we use today. Besides the turbojet, turboprop engines were widely used and still powers many aircraft today. Nowadays, the turbofan engine is used most common by commercial aviation and makes use of a fan which drives air around bypass ducts. We'll discuss these types in more detail below.

The turbojet is the simplest jet engine in terms of construction. It is still widely used in military aviation since it allows aircraft to fly at speeds exceeding Mach 1. Although there are many types its basic design consists of an air intake, a gas generator unit and an exhaust nozzle. The gas generator in turn consists of the compressor, combustion chamber and turbine. In this design, the turbine drives only the compressor and various accessory systems; this leaves more energy to provide actual thrust than in other types. As said before, the turbojet is commonly used to provide power in order to fly supersonic where it fuel efficiency increases.

The turboprop is similar to the turbojet, except that the turbine drives a propeller, as well, in a two-spool configuration. Contrary to the turbojet, the turboprop accelerates a large amount of air to a relatively low exhaust velocity. Because of this, the turboprop reaches a very high fuel-efficiency at the expense of airspeed. Because of this, the turboprop engine performs best in the 250 to 450 mph speed range. Additionally, the turboprop generates more noise than the turbojet.

Examples of aircraft that are powered by turboprop engines are the: Fokker 50, Dash-8, Saab 200 and ATP.
Turboprop engine powering a Fokker 50

Above 450 mph both the turbofan and turbojet is most widely used on commercial aircraft and business jets. The turbofan engine was designed in order to permit higher turbine temperatures without increasing gas velocity dramatically because this would decrease efficiency in subsonic flight. The turbofan makes use of a huge fan driven by the turbine commonly known as the N1. This fan directs air through bypass ducts, which lead air around the core engine to a separate exhaust nozzle. This method has proven to be very efficient at transonic airspeeds, making the turbofan ideal for modern commercial aircraft. The turbofan can be categorized by the ratio at which air moves through and around the core engine: Low bypass-ratio turbofans and high bypass-ratio turbofans.

Almost every modern transport aircraft is powered by turbofan engines which contribute to high efficiency.
Example of a turbofan engine

Low bypass ratio turbofan
A turbofan is classified as low bypass-ratio if the amount of airflow directed through the bypass ducts is from 20 to 100% of the core airflow. In terms of air mass, these percentages are 5 to 50%. These engines were more fuel-efficient than the turbojet, but because of the relatively high exhaust velocities it generated too much noise to be used by commercial aircraft. The military still makes use of this type of engines.

Aircraft powered by low bypass engines are the McDonnell Douglas MD-80 Series and early Boeing 737 variants.
Turbojet powered MD-80

High bypass-ratio turbofan
At a bypass-ratio of 5:1, the bypass airflow is five times greater than the core airflow. Turbofans using this or an even higher bypass-ratio are classified as high bypass-ratio turbofans. Two main advantages over the previous class of turbofan make it ideal for commercial aviation today: it generates less noise because of the relatively low exhaust velocities, while accelerating a great amount of air mass to achieve unmatched thrust levels.
Turbofan engine powered A330