A jet engine can be made more efficient by the addition of a large fan surrounded by a metal cowling at the front of the engine. The fan is somewhat similar to a propeller except that it has many more blades than a simple propeller. The fan is attached to a shaft that is also powered by the turbine at the rear of the engine. When exhaust gases from the compression chamber cause the turbine to spin, rotational energy is transmitted not only to the compressor, as described above, but also to the fan at the front of the engine.
The spinning fan draws more air into the engine, where some of it follows the pathway described above. Some of the air, however, bypasses the compressor and flows directly to the back of the engine. There, it joins with the exhaust gases from the combustion chamber to add to the engine's total thrust.
The turbofan jet engine has the advantage of operating more efficiently and more quietly than turbojet engines. However, they are heavier and more expensive than are turbojets. As a consequence, turbofan engines are usually found only on larger commercial and military aircraft (such as bombers), while turbojets are the preferred engine of choice on smaller planes, such as smaller commercial aircraft and military fighters.