|
|
Types of Air-Breathing Engines | |
page 1 | |
|
|
There are four basic types of air-breathing engines (turbines): the turbojet, the turboprop, the turbofan, and the ramjet. Each has its advantages and disadvantages for specific cruise speeds. Engineers look for two things when designing a jet engine: thrust to weight ratio and fuel consumption. Most aircraft are designed for low fuel consumption, even though it means lower thrust capabilitiy. Some aircraft, such as fighter jets need a lot of thrust and are not as concerned about the amount of fuel used, if the mission requires it. Engineers recommend which engine would work best. Turbojet: The original jet engine, discussed in the History section, became known as the turbojet. This engine completely changed air transportation. It greatly reduced the expense of air travel and improved aircraft safety. The turbojet also allowed faster speeds, even supersonic speeds. It had a much higher thrust per unit weight ratio than the piston-driven engines, which led directly to longer ranges (flight distances) and higher payloads (more passengers and baggage). As it happened, it also has 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. Most aircraft do not use an afterburner, because they use so much fuel. Fighter aircraft with afterburners only use them when absolutely necessary. If a pilot runs too long with the afterburner on, he or she risks running low on fuel before the mission is completed. Remember, from the components section, that temperature is a very important factor when designing the turbine. The exhaust cannot be too hot or it will melt parts (such as the blades) in the turbine. However, the hotter the exhaust the more thrust there will be. The engineers use a technique called "turbine blade cooling". This allows hotter than normal exhaust from the combustor to enter the turbine engine. Cool air from the compressor is fed into hollow turbine blades, so they won't become overheated and warp or break. The cooling must be controlled very carefully to get maximum thrust. The turbojet engine is the most popular engine for most high-speed aircraft, in spite of the higher fuel consumption. When high speed and performance are important, the cost of fuel is less important. Military fighters and fast business jets use turbojet engines. Turboprop: Soon after the first turbojets were in the air the turboprop engine was developed. This engine design produces two thrusts, one with the propeller and the other through exhaust. A large gear box makes it possible for the turbine to turn a large propeller at high speed, producing the first thrust. The large gear box has many moving parts (that could break) and can get in the way of the air stream going into the engine. As the propeller speed increases, the tips of the blades may approach supersonic speeds. If this happens, the flow may separate and shocks may form, decreasing the air flow into the engine. For these reasons this type of engine is still restricted to slower speeds because of the large propeller and the gear box. 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. (This is the second thrust, after the propeller) 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 compared to 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. 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. A turbofan engine can fly at transonic speeds up to Mach 0.9. 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. Like the turboprop engine, the turbofan has low fuel consumption compared to a turbojet. The turbofan engine is the engine of choice for high-speed, subsonic commercial airplanes.
Ramjets: Below Mach 1.0 a compressor is very much needed as a component of an air-breathing engine. As an airplane increases its speed past Mach 1.0 the air pressure created from the speed of the air flow decreases the need for a compressor. As speeds approach Mach 3.5 - 4.0, a compressor isn't even needed. The ramjet is the most efficient engine because it has less components. 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.
A ramjet does have limitations. The first is that it will not work at less than supersonic speeds; another engine must first power the aircraft to supersonic speeds. Another limitation is the burning of the fuel and air mixture in the combustor. The ramjet inlet must slow the air flow from the supersonic speeds to subsonic speed for ignition in the burner. As the ramjet approaches Mach 6.0 the air coming into the burner is too hot to burn! This is due to the friction created as the supersonic air is slowed at the inlet to subsonic speed. At this speed not enough thrust is being generated to continue performance. There is a proposed solution to the ramjet's speed limitation (Mach 6.0). It is called supersonic combustion ramjet (SCRAMJET). Instead of slowing the air flow down to subsonic speeds for combustion, the SCRAMJET will ignite the air while still supersonic (thus avoiding the friction at the inlet). Fuel must still be injected into the airstream to be ignited. Unfortunately, today's fuels do not ignite quickly enough. The development of a workable fuel injection system for the SCRAMJET is still in its early stages.
Web Hosting Provided By The National Business Aviation Association. Explore Space ... Not Drugs! |
