EDITORIAL

A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of  motion. This broad definition of jet engine generally refers to a gas turbine engine used to produce a jet of high speed exhaust gases for special propulsive purposes. The basic concepts date back to the first known gas turbine built by Hero of  Alexandria in 130 B.C.

Earlier aircrafts were powered by reciprocating type engines in its many different forms (rotary and static radial, air cooled and liquid-cooled inline). The aircraft was propelled by a propeller, driven by the engine, which was mounted on the front of an aircraft.

However, engineers were beginning to realize conceptually that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of power plant would have to be developed. This was the motivation behind the development of the jet engine.

Working principle:

A jet engine is basically a gas turbine working on a thermodynamic cycle called the Joule or the Brayton cycle shown in the fig.

The jet engine works in the open cycle configuration. It consists of a compressor, combustion chamber, and a turbine.

The compressor takes in ambient air and raises its pressure. In the combustion chamber, heat is added to the air, raising its temperature and hence its heat energy. The heat added to the air is accomplished by injecting fuel into the air, and burning it in a combustion chamber. The heat addition takes place at constant pressure as shown in the p-v diagram above. This working fluid is then available, at a high temperature and pressure, to be expanded through a turbine to develop mechanical energy, in a manner similar to the familiar steam turbine. Since ambient air enters the compressor and the gases of combustion are rejected to the atmosphere, the working medium must be continuously replaced. This cycle is termed an open cycle, and is a continuous flow process. However, part of the power developed by the turbine must be utilized to drive the engine accessories, as well as the compressor with only the remainder available as useful work. The engine's thrust comes from taking a large mass of air in at the front and expelling it at a much higher speed than it had when it entered the compressor. Thrust then, is equal to mass flow rate times change in velocity.

Therefore, the more air that an engine can compress and use, the greater is the power or thrust that it can produce.

Major Components of a Jet engine:

The inlet, compressor, combustion chamber, turbine and nozzle (exhaust) are the main components of a jet engine.

Inlet:

An inlet reduces the entering air velocity to a level suitable for the compressor. The air velocity is reduced by a compression process that increases the air pressure. The operation and design of the inlet are described in terms of the efficiency of the compression process, the external drag of the inlet, and the mass flow into the inlet. The design and operation of the inlet depend on whether the air entering the duct is subsonic or supersonic. As the aircraft approaches the speed of sound, the air tends to be compressed more, and at Mach 1, shock waves occur. Shock waves are compression waves, and at higher Mach numbers, these compression waves are stronger. Compression by shock waves is inefficient. In subsonic flow, there are no shock waves, and the air compression takes place quite efficiently. In supersonic flow, there are shock waves present. Shock waves and the compressibility of air then influence the design of inlets.

Compressor:

           The combustion of fuel and air at normal atmospheric pressure will not produce sufficient energy enough to produce useful work. The energy released by combustion is proportional to the mass of air consumed and its pressure. Therefore, higher pressures are needed to increase the efficiency of the combustion cycle. So the jet engines must rely upon some other means of compression.  Although centrifugal compressors are used in many jet engines, the efficiency level of a single stage is relatively low. The multistage of centrifugal compressor is better, but still do not compare with those axial flow compressors. Some small modern turbo shaft and turboprop engines achieve good results by using a combination of axial flow and centrifugal compressor.

Combustion chamber:

There are three basic types of burner systems in use today. They are Can type, Annular type and Can-annular type. Fuel is introduced at the front end of the burner. Air flows in around the fuel nozzle and through the first row of combustion air holes in the liner. The air entering the forward section of the liner tends to recirculate and move up stream against the fuel spray. During combustion, this action permits rapid mixing and prevents flame blowout which acts as a continuous pilot for the rest of the burner. There are usually has only two igniter plugs in an engine. The igniter plug is usually located in the up stream region of the burner. About 25 percent of the air actually takes part in the combustion process. The gases that result from the combustion have temperatures of 3500 degree F. Before entering the turbine, the gases must be cooled to approximately half this value, up to the designed values of temperature for the turbine materials involved. Cooling is done by diluting the hot gases with secondary air that enters through a set of relative large holes located toward the rear of the liner.

Turbine:

The turbine extracts kinetic energy from the expanding gases that flow from the combustion chamber. The kinetic energy is converted to shaft horsepower to drive the compressor and the accessories. Nearly three-fourths of all the energy available from the products of combustion is required to drive the compressor. The axial-flow turbine consists of a turbine wheel rotor and a set of stationary vanes stator. The set of stationary vanes of the turbine is a plane of vanes (concentric with the axis of the turbine) that are set at an angle to form a series of small nozzles that discharge the gases onto the blades of the turbine wheel. The discharge of the gases onto the rotor allows the kinetic energy of the gases to be transformed to mechanical shaft energy.

Like the axial compressor, the axial turbine is usually multistage& there are generally fewer turbine stages than compressor stages because in the turbine the pressure is decreasing (expansion process), whereas in the compressor the pressure is increasing (compression process). In each process (expansion or compression), the blades of the axial turbine or axial compressor act as airfoils, and the airflow over the airfoil is more favorable in the expansion process. The result is that one stage of turbine can power many compressor stages.

Exhaust nozzle:

The purpose of the exhaust nozzle is to increase the velocity of the exhaust as before discharge from the nozzle and to collect and straighten gas flow from the turbine. In operating, the gas turbine engine converts the internal energy of the fuel to kinetic energy in the exhaust gas stream. The net thrust (or force) of the engine is the result of this operation, and it can be calculated by applying Newton's second law of motion (with the thrust equation given above). For large values of specific thrust, the kinetic energy of the exhaust gas must be high, which implies a high exhaust velocity. The nozzle supplies a high exit velocity by expanding the exhaust gas in an expansion process that requires a decrease in pressure. The pressure ratio across the nozzle controls the expansion process, and the maximum thrust for a given engine is obtained when the exit pressure equals the ambient pressure.The two basic types of nozzles used in jet engines are the convergent and convergent-divergent nozzles.

In the next article different types of jet engines will be explained. So please keep checking this column for the next article.