3.3 Different types of jet engines

The jet engine is clearly the choice for aircraft propulsion due to all its advantages over the other engine types as seen in the previous section. The term “jet engine” is often used as a generic name for a variety of engines, including the turbojet, turbofan, turboprop, and ramjet. These engines all operate by the same basic principles, but each has its own distinct advantages and disadvantages. All jet engines operate by forcing incoming air into a tube where the air is compressed, mixed with fuel, burned, and exhausted at high speed to generate thrust. Following we shall compare briefly the differences between the types of jet engines.

Table 3.3.1 below shows some specifications of a few selected engine models. This is a very limited list but nonetheless provides us with figures that will show why the turbofan remains the choice of today.

1.1.1 Turbojet

This is the most basic of the jet engines. Most of today’s turbojets are fitted with axial-flow compressors due to their aerodynamic properties and compact packing. Afterburners can be fitted just before the nozzle to burn additional fuel, providing an extra boost. This however, consumes a lot of fuel and is hence only used for brief periods (eg.: in dog fights). Despite being able to provide higher thrust to weight ratio, the fuel consumption of these engines were very high, making flights more expensive for civilians.

Figure 3.3.1.1 (a) Schematics of a centrifugal flow turbojet; (b) Schematics of an axial flow turbojet (Source: http://en.wikipedia.org)

1.1.2 Turbofan

The turbofan has a similar structure to the turbojet except for a fan section in front of the compressors. The fan is also driven by the turbine, but instead of compressing air like the other rotating blades, its primary purpose is to force more air through outer ducts that go around the engine core. Despite this cooler air travelling at much lower speeds, the sheer mass of air that is accelerated by the fan produces a significant thrust on top of what the turbojet core engine is producing. This additional thrust is obtained without burning extra fuel since the fan is powered by the turbine. Thus, the turbofan is much more fuel efficient than the turbojet. In addition, the low-speed air helps to cushion the noise of the core jet’s exit air making the engine much quieter. This is highly desirable in commercial aircrafts as there are residential areas near the vicinity of most airports.

Figure 3.3.2.1 (a) Schematics of a low bypass turbofan; (b) Schematics of high bypass turbofan (Source: http://en.wikipedia.org)

1.1.3 Turboprop

The turboprop is similar to the turbofan; however, the fan is not ducted and is a completely external propeller. In turboprops, it is not surprising to see more than 80% of the thrust coming from the propeller. This is attractive because of its very high fuel efficiency, even higher than the turbofan’s. But the mechanical structure of the turboprop introduces a lot of noise and vibration, making it unsuitable for commercial aircrafts. The centre of rotation of the propeller is never in the centre of the engine, causing violent and noisy vibrations. It is, however, a clear choice when it comes to engine selection for long-range planes that require high endurance.

Figure 3.3.3 Schematics of a turboprop (Source: http://en.wikipedia.org)

1.1.4 Ramjet

Figure 3.3.4.1 Schematic of a ramjet (Source: www.answers.com)

The ramjet looks very different from all the other jet engines in that it has no more rotating components. It is simply a jet engine with all its interior components removed, for greater simplicity and efficiency. The only drawback is that it needs to be moving at very high speeds in air for co mbustion to occur. The lack of a compressor means the engine needs to ram through air at speeds fast enough to cause the air to compress and combust on its own. This implies that it is impossible to be used on commercial aircrafts since commercial aircrafts only fly at subsonic speeds. The ramjet is used on missiles. From the table, it is seen that the ramjet has excellent thrust to weight ratio. However, the fuel consumption of the ramjet is very bad below supersonic speeds. The operating speed is between M2 to M5. When the speed is sufficient, the engine will switch to the ramjet for better efficiency.