PROPULSION

This word [propulsion] is derived from two Latin words; pro meaning before or forward and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. On airplanes, thrust is usually generated through some application on Newton's third low of an action and reaction. A gas, or working fluid. is accelerated by the engine, and the reaction to this acceleration on produces a force on the engine.

A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exist velocity of the gas. Different propulsion systems generate thrust in slightly different ways: the propeller, the turbine[or jet] engine, the ramjet, and the rocket. Some times a distinction is made from propulsion to traction but  leaving aside internal stresses in the system [ compression in the first case and tension in the latter], push or pull motions produces the same effect; making a body to move against natural forces. In other occasions "traction" is restricted propulsion by shear on the solid surfaces. 

Why are there different types of engines?

If we think about Newton's first law of motion, we realized that an airplane propulsion system must serve two purposes. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising and second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In fact, the greater the different between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.

Some aircraft , like  airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is mot as important as high engine efficiency and low fuel usage. since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it more by a small amount. that is why we find high bypass fans and turboprops on cargo plane e and airliners.

Some aircrafts like fighter planes  or experimental high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern military aircraft typically employ afterburners on a low bypass turbofan care. Further hypersonic aircraft will employ some type of ramjet or rocket propulsion.