Turbojets are jet engines that are turbocharged. Turbofans are turbojets onto the front end of which a large fan is added.
Propeller engines are suited for small airplanes that travel for short distances. Turbojets provide transport capabilities that propeller engines cannot. The McDonnell FH-1 Phantom was the first all-jet airplane ordered by the U.S. Navy and the Navy’s first airplane to fly 500 miles per hour. Its first flight took place on January 26, 1945. On July 21, 1946, an FH-1 Phantom became the first jet-propelled combat aircraft to operate from an American aircraft carrier. The Phantom weighed 10,035 pounds, could accommodate one crew member, had a range of 695 miles, and was powered by two turbojets, the Westinghouse J30-WE-20, each of which could deliver 600 pounds of thrust. A year later, the thrust delivered by turbojets had doubled. By the end of the 1950’s, the turbojets could deliver thrusts that were twenty times that delivered by the J30-WE-20.
An appreciation of how a jet engine works requires understanding Bernoulli’s principle, Sir Isaac Newton’s third law of motion, and how these two ideas from physics come into play in the operation of a jet engine.
For ordinary commercial and pleasure flights, air is treated as an incompressible substance that has no viscosity, and the level flight is treated as steady. Under these circumstances, a principle of physics called Bernoulli’s principle states that the sum of three forms of energy remains constant. These three forms are kinetic energy (energy associated with the motion of air), potential energy (that associated with weight and elevation above or below a reference level), and the energy associated with pressure. Air is a very light substance; it is conventional to neglect its potential energy because its contribution is typically very small compared to those of the other two forms of energy. Accordingly, when kinetic energy increases, pressure energy must decrease by the same amount, and vice versa.
Sir Isaac Newton formulated three laws of motion. Propulsion generated by a jet engine operates according to Newton’s third law of motion. It states that for every action there is a reaction. The reaction is equal to the action in magnitude but opposite to it in direction. This law can be seen in operation in many ordinary ways. For instance, walking involves planting a foot and pushing backward (action). The ground provides the reaction by pushing forward against the foot in order to create the thrust that makes walking possible. When a surface is slippery, this really means that its ability to provide a reaction is limited and that walking on that surface has become treacherous. As another example, if one inflates a small balloon but does not tie the open end shut before releasing the balloon, after release, the air inside the balloon will want to rush out of it. As it does so, a thrust will be created, helping the balloon sail forward for some time until most of the air has escaped. A jet airplane works a little bit like the inflated balloon. In a jet, a mechanism must be created to introduce air into the plane and release it in such a way that thrust is continuously created.
A jet engine utilizes jet propulsion, which is a kind of propulsion in which the force needed to move the body of the aircraft comes from discharging a jet of fluid from the body at high speed. As the fluid jet leaves the body, it produces a reaction force against the body and it is this force that propels the body forward, according to Newton’s third law of motion.
Jet engines include turbojets, turboprops, ramjets, scramjets, and rockets. Rockets carry all they need with them to generate combustion. Therefore, they can operate in space because they do not need air to function. The other jet engines require air to operate properly. It is for this reason that they belong to a class called air-breathing propulsion, or simply air-breathing machines. They utilize the mechanical behavior of air in their operation.
The propulsion system of a jet engine consists of an inlet diffuser, a compressor, a fuel injection system, a combustion chamber (also called a combuster), a turbine, and an exhaust nozzle. A jet engine slows down incoming air as it enters the engine; that is, it causes a decrease in kinetic energy. Thus, the air pressure increases according to Bernoulli’s principle.
The air goes through a series of compressor blades that look somewhat like fan blades. These blades help push the air forward, giving it new energy by doing work on them. This work increases the pressure energy of the air and thereby adds to the total energy. This high-pressure air enters the combustion chamber, where it is mixed with fuel, ignited, and burned. The hot gases resulting from this combustion want to expand and they leave the combustion chamber at very high speed and pressure. On their way out of the engine, the hot gases go through the blades of a turbine. They drive the turbine by pushing against its blades like a high wind blowing past a windmill. The turbine drives the compressor because the two units are connected to each other by a shaft. The movement of the compressor compresses the air that enters the engine. As the exhaust gases leave the engine, they exert a thrust that propels the airplane forward while the gases travel backward, again in accordance with Newton’s third law of motion (action equals reaction). These exhaust gases leave the airplane in a fast-moving stream commonly called a jet. That is why this is called a jet engine.
There are many different kinds of engines. They are classified a number of different ways. Classification hinges upon how the essential components are designed and the role they play. For example, if combustion takes place inside an enclosure, the engine is called an internal combustion engine. However, if it occurs in the open, the engine is called an external-combustion engine. Internal combustion engines are very common. They are used in planes. They are designed to produce work at high efficiencies. The two main types of internal combustion engines are spark-ignition engines and compression-ignition engines. In spark-ignition engines, the fuel-air mixture is ignited using a spark. In compression engines, there is no spark at all and ignition of the fuel-air mixture is achieved spontaneously. This is achieved by increasing the temperature and pressure of the air inside the combustion chamber using compression. Spark-ignition engines are also called gasoline engines (petrol engines in Great Britain). Compression-ignition engines are also called diesel engines. In order to achieve satisfactory performance of compression-ignition engines, one must control the air motion and the fuel injection in a proper manner. Propeller engines are well suited for low-speed flights, but they do not work as well for high-speed flights for two important reasons: as the forward speed of an aircraft increases, the thrust that propellers provide to move the craft forward decreases and the drag resistance associated with their operation increases. Jet engines do not have these limitations. They were introduced to provide power to aircraft that move at high speeds because they work better at these speeds. Direct-injection engines have less air motion than indirect-injection engines, so, to compensate for this, direct-injection systems use multiholed nozzles with high (three times as high) pressures during the injection process. Ideally, internal combustion engines should have high efficiency, high output and rapid combustion, generate no pollution, and be very quiet. It is very challenging to design engines that would achieve all of these goals. In compression-ignition engines, volume requirements are higher but the combustion process is slower than in spark-ignition engines. For these reasons, the maximum speeds of compression-ignition engines are much lower than those of spark-ignition engines.
Two effective techniques were found to increase the output of compression-ignition engines: supercharging and turbocharging. An engine is supercharged by supplying pressurized air to it. When the air at the inlet to an engine is pressurized, the mass flow rate of air into the engine increases. Typically, this is associated with an increase in the flow rate of fuel to the engine. These two factors lead to increases in power output and efficiency. Compressors need power to do their work. When a compressor is driven from the crankshaft of an engine, the arrangement is called a supercharger. However, when it is driven by the turbine, the arrangement is called a turbocharger, and the affected engine is said to be turbocharged. Thus, turbocharging is a particular form of supercharging in which a compressor is driven by an exhaust gas turbine. There are thermodynamic advantages to turbochargers over superchargers, principally because the former utilize exhaust gas energy during the so-called blow-down. Turbocharging also reduces the weight per unit output and increases fuel economy.
Work done by the turbine is just sufficient to drive the compressor. Hot gases enter the turbine where they are expanded to a pressure that allows the work done on the turbine to equal that done by the compressor. The pressure of the exhaust gases for the turbine is greater than that of the surrounding air, so the hot gases are expanded further in a nozzle so that their pressure will be lowered to that of the surroundings. The gases leave at high velocity, and thrust is generated by the change in momentum that the gases undergo.
Nine characteristics are used to describe a turbojet: its weight, its length, its diameter, the number of stages its compressor has, the number of stages its turbine has, the thrust it can deliver at takeoff, the best thrust it can deliver at cruising speeds, its speed range, and its specific fuel consumption. The specific fuel consumption of a turbojet is the amount (in weight) of fuel it consumes per unit weight of thrust delivered, per hour.
The characteristics of the latest models of turbojets are protected carefully by manufacturers because these machines are used for purposes of defense and surveillance. However, older models are available in museums of flight. For example, in the late 1950’s, Pratt & Whitney designed the J-58 turbojet engine to be used by the U.S. Navy. Its axial compressor had nine stages, its turbine had two stages, it had an afterburner, and it could provide a takeoff thrust of 32,500 pounds. It weighed 6,000 pounds and operated above 85,000 feet at speeds three times that of sound (Mach 3). The J-58 is currently in the museum of flight at Wright-Patterson Air Force Base.
The turbojet is very successful at increasing the speed of the air that enters the engine, which increases its kinetic energy, but another way to increase the total energy of the air is to increase the amount of air that enters the engine. A turbofan achieves this by mounting a large fan at the inlet to the engine. The design of the turbojet is modified accordingly. The addition of the fan creates two different paths that the incoming air can use to travel through the engine before leaving it. Some of the air follows the path that it would use in a conventional turbojet: from the inlet diffuser, it goes successively through the compressor, or compressors, the combustion chamber, the turbine, and the exit nozzle. The remaining air bypasses the compressor altogether: it goes from the inlet diffuser, around the engine, and directly to the back of the plane. In doing so, it converts the pressure that it stored in the inlet diffuser directly into kinetic energy. Here, again, air leaves the engine traveling faster than when it entered it, and as this air leaves the fan, it exerts thrust on it. This mechanism provides a second thrust that is added to that due to the operation of a turbofan as a conventional turbojet. If one looks at modern jumbojets at airports, their engines look like huge fans. Chances are very high that these engines are turbofans.
Barsoum, Michel, et al. “The MAX Phases: Unique New Carbide and Nitride Materials.” American Scientist 89, no. 4 (July/August, 2001). This article discusses the role of ceramics in the production of heat-tolerant materials that could be used in jet engines in the future. Bloomfield, Louis. How Things Work: The Physics of Everyday Life. New York: John Wiley & Sons, 1997. A good reference for the general reader, requiring some knowledge of basic physics to be fully appreciated. Stone, Richard. Introduction to Internal Combustion Engines. 2d ed. Warrendale, Pa.: Society of Automotive Engineers, 1994. A somewhat technical book with many formulas and a wealth of material about engines that is quite accessible to the general reader. Wright, Michael, and M. N. Patel, eds. How Things Work Today. New York: Crown, 2000. A short reference for the general reader, it is well written, well illustrated, and easy to read.