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Types of Air-Breathing Engines | |
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Although the engines will vary individually as far as component capabilities, there are 4 basic types of air-breathing engines: the turbojet, the turboprop, the turbofan, and the ramjet. Each has its advantages and disadvantages for specific cruise speeds. Two of the performance characteristics that engineers look at when designing an engine are the thrust to weight ratio and the fuel consumption. Depending on the specifications for the aircraft in question, an engineer might recommend one engine design over another. For example, for a civilian airplane for basic transport, an engineer might suggest that an engine that generates somewhat less thrust but at a low fuel consumption would be a better choice than an engine that produces more thrust at a significantly higher fuel consumption. For a military fighter, however, the same engineer might recommend the second, high thrust engine in spite of the fuel consumption, because the need for quick acceleration and a fast getaway is more important for the aircraft mission. Turbojet: As discussed in the History section, the turbojet was the original jet engine design. The development of the turbojet revolutionized transportation. It greatly reduced the expense of air travel and contributed to a major improvement in aircraft safety. In addition, it allowed for faster cruise velocities up to supersonic speeds. It had a much higher thrust per unit weight ratio than the early piston-driven engines, which led directly to longer ranges (flight distances) and higher payloads (more passengers and baggage). As it happened, it also turned out to have lower maintenance costs. The typical turbojet engine has all 5 of the components described in the previous section: an inlet, a compressor, a combustor, a turbine, and a nozzle. The figure below shows a basic turbojet schematic with the 5 components clearly identified. To get an increased thrust, an afterburner can be added to the turbojet. The figure below is the turbojet with an afterburner. Unfortunately, the increased thrust comes at the expense of an increased fuel consumption. Typically, transport aircraft (both civilian and military) do not use afterburners. Even fighters with afterburners use the afterburners judiciously; they are turned on for only a brief burst of thrust. If a pilot runs too long with the afterburner on, he or she risks running low on fuel before the mission in completed. The turbine inlet temperature is a primary design limitation for the turbojet. In order to get a hotter burn in the burner, beyond the maximum temperature allowable for the turbine, the design engineer may use a technique known as turbine blade cooling. This technique bleeds a small amount of the cooler air in the compressor, which then bypasses the burner and is fed back into the first 1 or 2 rows (called stages) of turbine blades. Typically, the air is fed through hollow turbine blades and pushed out along the trailing edge of the blade. Turbine blade cooling has been used successfully to increase the turbine inlet temperature above the maximum allowable temperature. Unfortunately, the engine performance is very sensitive to the amount of cooling air, and increased thrust is often at the expense of major cooling penalties. The turbojet is the engine of choice for most high-speed aircraft, in spite of the cost of the higher fuel consumption. When speeds up to and including supersonic velocities and maximum aircraft performance are important, the cost of the fuel is just part of the overhead. Military fighters and fast business jets use turbojet engines.
Following quickly in the footsteps of the new turbine engine designs, the turboprop was developed: the combination of the propeller engine with a turbine to power the propeller. This combination shows a high thrust power (the thrust times the airplane velocity) with respect to fuel consumption, but because of its limitations, it is used primarily to power medium-speed, moderate-size aircraft. Overall flight speed for this type of engine is still restricted to slower speeds because of the aerodynamics along the propeller blade. The local velocities increase along the blade from the hub to the tip, and if the flight speed is too high, the velocities may go supersonic near the tip. If this happens, the flow may separate and shocks may form, decreasing the effectiveness of the air flow into the engine. Also, the turboprop engine requires a very large gear box to efficiently translate the turning motion from the smaller turbine to the speeds necessary for the large propeller. This large gear box is heavy, requires extensive development for reliability and durability (so many moving parts that could break), and can obstruct the streamlines of the air flow into the engine. The sketch below shows the basic components of a turboprop engine. The propeller precedes the inlet and the compressor, but it serves the same purpose. It provides a large volume of high pressure air to the engine exhaust streams. An inlet and a compressor are used to send a part of the air flow to the burner. A turbine is used to power the propeller and the compressor, and the hot exhaust gases are accelerated out through the nozzle. There are essentially 2 exhaust streams contributing to the thrust: a stream of high pressure air generating a high velocity around the engine and a stream of high temperature air generating another high velocity from inside the engine. The total thrust is the sum of the 2 velocities minus the original velocity of the airplane times the very large rate of air flowing through each stream. Because only a small part of the air flow is actually burned inside the engine, the turboprop engine can generate a lot of thrust with a low fuel consumption. For the same thrust and flight speed, a turboprop will generally have a much lower fuel consumption than a turbojet engine. When an airplane is designed to fly at lower speeds, the turboprop is usually the engine chosen.
Turbofan: As engineers struggled to overcome the limitations of the turboprop engine for airplanes at higher speeds, a new design emerged: the turbofan. It can be described as a compromise between the turboprop and the turbojet engines. It includes a large, internal propeller (sometimes called a ducted fan) and 2 streams of air flowing through the engine. The primary stream travels through all of the components like a turbojet engine, while the secondary stream is usually accelerated through a nozzle to mix with the primary exhaust stream. Like the turboprop, the total thrust is the sum of the 2 velocity differences (exit velocity minus the original) times the air flow rates through each section. The figure below illustrates the design of a turbofan engine.
There are several advantages to the turbofan over the other 2 engines. The fan is not as large as a propeller, so the increase of speeds along the blades is less. Also, by enclosing the fan inside a duct or cowling, the aerodynamics are better controlled. There is less flow separation at the higher speeds and less trouble with shocks developing. An airplane with turbofan engines can fly at transonic speeds up to Mach 0.9. Because the turbofan has the fan and a large compressor, the gear box to translate the energy from the smaller turbine to the compressor/fan combination is much smaller and less complicated. While the fan is smaller than the propeller, it does suck in much more air flow than the turbojet engine, so it gets more thrust. If a turbofan engine and a turbojet engine are compared at the same thrust and flight speed, the turbofan, like the turboprop, will have a substantially lower fuel consumption. The turbofan engine is the engine of choice for high-speed, subsonic commercial airplanes. While it is possible to put afterburners into one or both streams, the slight additional thrust gained is at the expense of a large increase in the fuel consumption. The cost is so high, in fact, that they are rarely ever built into turbofan engines. Ramjets: The compressor pressure ratio (how much it raises the pressure) is also a major design consideration for an aircraft engine. As flight speeds increase past Mach 1.0 and go into the supersonic speed regime, the compressor pressure ratio needed for high thrust decreases. As speeds approach Mach 3.5 - 4.0, a compressor isn't even needed. The ramjet is the most efficient engine, and it is a much simplified engine. The ramjet doesn't have a compressor or a turbine, and it has a much higher tolerance to high temperatures. A schematic of a ramjet engine is shown below. It has an inlet, a burner, and a nozzle.
The proposed solution to this speed limitation is the development of the supersonic combustion ramjet (SCRAMJET). In stead of slowing the air flow down to subsonic speeds for combustion, the SCRAMJET will have combustion take place while the air stream is still supersonic. This requires that fuel be injected into the stream without disruptive shock waves, and the mixture of air and fuel must ignite and burn very quickly. Unfortunately, conventional fuels in use today do not ignite quickly enough, and the design of the fuel injection system is still a challenge. The development of the SCRAMJET engine is still in its early stages.
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