such a life

writing after a long time.. 3years to be precise..

have been following current events : sania mania, IPL-3, mayawati garlanding..
what makes news and what makes it click..probably we r more interested in intruding in other ppl lives(camera men zoomed in & out to shoot shoaib inside sania's home in hyd)... or we are intrested in talking about sins, renounciation & blah blah and then renounce all that to get urself rewarded by some earthly pleasures(Garland anyone)..

i think of becoming a celebrity these days..even if i call my dog by some superstar name(pun intended) i would be in news.. even if i didnt have any affair, i would be in news for having a fling.. even if i go to mussorie to chill out with my family members i would be in news(sachin)... gosh i need such a life.. i mean sach(in) a life..

Rush of Adrenaline

A turbojet engine is a type of internal combustion engine often used to propel aircraft. The processes in a turbojet engine can be briefly described as follows:
• Air is drawn into the rotating compressor via the intake and is compressed, through successive stages, to a higher pressure before entering the combustion chamber.

• Hot combustion products leaving the combustor expand through a gas turbine, where power is extracted to drive the compressor.

• The gas stream is then expanded to ambient pressure via a propelling nozzle, producing a high velocity jet as the exhaust.
Indeed, the entire process is similar to a four-stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously, but in different sections of the engine. The efficiency of a jet engine is strongly dependent upon the overall pressure ratio (combustor entry pressure/intake delivery pressure) and the turbine inlet temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a relatively small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The high-speed exhaust of a turbojet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine , commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves.






Types of Jet Engines
There are a large number of different types of jet engines, all of which achieve propulsion from a high speed exhaust jet.
Turbojet
Generic term for simple turbine engine
Simplicity of design, efficient at supersonic speeds (~M2)
Basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy
Turbofan
First stage compressor greatly enlarged to provide bypass airflow around engine core.
More efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperature
Greater complexity. Top speed is limited due to the potential for shockwaves to damage engine.
Rocket
Carries all propellants and oxidants onboard, emits jet for propulsion
Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed , thrust/weight ratio over 100, exhaust, no turbine in hot exhaust stream.
Needs lots of propellant- very low specific impulse. Extreme thermal stresses of combustion chamber can make reuse harder. Extraordinarily noisy
Turboprop (Turboshaft similar)
Strictly not a jet at all — a gas turbine engine is used as powerplant to drive propeller shaft.
High efficiency at lower subsonic airspeeds , high shaft power to weight
Limited top speed, somewhat noisy, complex transmission
Propfan/Unducted Fan
Turboprop engine drives one or more propellers. Similar to a turbofan without the fan cowling.
Higher fuel efficiency, potentially less noisy than turbofans, could lead to higher-speed commercial aircraft.
Development of propfan engines has been very limited, typically more noisy than turbofans, complexity

Advantages

• Very high power-to-weight ratio, compared to reciprocating engines
• Smaller than most reciprocating engines of the same power rating
• Moves in one direction only, and doesn't vibrate, so very reliable
• Simpler design.

Disadvantages
• Cost is much greater than for a similar-sized reciprocating engine
• Very high-performance, strong, heat-resistant materials needed
• Requires precision in manufacturing and other processes.









Components

Sections
o Intake
o Compression
o Combustion
o Exhaust

Cold Section:
Air intake (Inlet) —At supersonic flight speeds, shockwaves form in the intake system and reduce the recovered pressure at inlet to the compressor. So some supersonic intakes use devices, such as a cone or ramp, to increase pressure recovery, by making more efficient use of the shock wave system.
Compressor or Fan — The compressor is made up of stages. Each stage consists of vanes which rotate, and stators which remain stationary. As air is drawn deeper through the compressor, its heat and pressure increases. Energy is derived from the turbine , passed along the shaft.

Common:
Shaft — The shaft connects the turbine to the compressor, and runs most of the length of the engine There may be as many as three concentric shafts, rotating at independent speeds, with as many sets of turbines and compressors. Other services, like a bleed of cool air, may also run down the shaft.
Hot Section:
Combustor or Can or Flameholders or Combustion Chamber — This is a chamber where fuel is continuously burned in the compressed air.
Turbine — The turbine acts like a windmill, gaining energy from the hot gases leaving the combustor. This energy is used to drive the compressor (or props, or bypass fans) via the shaft, or even (for a gas turbine-powered helicopter) converted entirely to rotational energy for use elsewhere. Relatively cool air, bled from the compressor, may be used to cool the turbine blades and vanes, to prevent them from melting.
Afterburner or Reheat- Produces extra thrust by burning extra fuel, usually inefficiently, to significantly raise Nozzle Entry Temperature at the exhaust. Owing to a larger volume flow (i.e. lower density) at exit from the afterburner, an increased nozzle flow area is required, to maintain satisfactory engine matching, when the afterburner is alight.
Exhaust or Nozzle — Hot gases leaving the engine exhaust to atmospheric pressure via a nozzle, the objective being to produce a high velocity jet. In most cases, the nozzle is convergent and of fixed flow area.
Supersonic Nozzle — If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient Pressure) is very high, to maximize thrust it may be worthwhile, despite the additional weight, to fit a convergent-divergent (de Laval) nozzle. As the name suggests, initially this type of nozzle is convergent, but beyond the throat (smallest flow area), the flow area starts to increase to form the divergent portion.

The various components named above have constraints on how they are put together to generate the most efficiency or performance. For the engine optimisation for its intended use, important here is air intake design, overall size, number of compressor stages (sets of blades), fuel type, number of exhaust stages, metallurgy of components, amount of bypass air used, where the bypass air is introduced, and many other factors. For instance, let us consider design of the air intake.

Air Intakes
Types:
• Subsonic Inlets



Pitot intakes are the dominant type for subsonic applications. A subsonic pitot inlet is little more than a tube with an aerodynamic fairing around it.At zero airspeed (i.e., rest), air approaches the intake from a multitude of directions: from directly ahead, radially, or even from behind the plane of the intake lip.
At low airspeeds, the streamtube approaching the lip is larger in cross-section than the lip flow area, whereas at the intake design flight Mach number the two flow areas are equal. At high flight speeds the streamtube is smaller, with excess air spilling over the lip.
Beginning around 0.85 Mach, shock waves can occur as the air accelerates through the intake throat.Careful radiusing of the lip region is required to optimize intake pressure recovery (and distortion) throughout the flight envelope.
· Supersonic Inlets
Supersonic intakes exploit shock waves to decelerate the airflow to a subsonic condition at compressor entry.
There are basically two forms of shock waves:
1) Normal shock waves lie perpendicular to the direction of the flow. These form sharp fronts and shock the flow to subsonic speeds. Microscopically the air molecules smash into the subsonic crowd of molecules like alpha rays. Normal shock waves tend to cause a large drop in stagnation pressure.
2) Conical (3-dimensional) and oblique shock waves (2D) are angled rearwards, like the bow wave on a ship or boat, and radiate from a flow disturbance such as a cone or a ramp. For a given inlet Mach number, they are weaker than the equivalent normal shock wave and, although the flow slows down, it remains supersonic throughout. Conical and oblique shock waves turn the flow, which continues in the new direction, until another flow disturbance is encountered downstream.

Compressors



Axial compressors rely on spinning blades that have aerofoil sections, similar to aeroplane wings. As with aeroplane wings in some conditions the blades can stall. If this happens, the airflow around the stalled compressor can reverse direction violently. Each design of a compressor has an associated operating map of airflow versus rotational speed for characteristics peculiar to that type.
At a given throttle condition, the compressor operates somewhere along the steady state running line. Unfortunately, this operating line is displaced during transients. Many compressors are fitted with anti-stall systems in the form of bleed bands or variable geometry stators to decrease the likelihood of surge. Another method is to split the compressor into two or more units, operating on separate concentric shafts.
Another design consideration is the average stage loading. This can be kept at a sensible level either by increasing the number of compression stages (more weight/cost) or the mean blade speed (more blade/disc stress).



Although large flow compressors are usually all-axial, the rear stages on smaller units are too small to be robust. Consequently, these stages are often replaced by a single centrifugal unit. Very small flow compressors often employ two centrifugal compressors, connected in series. Although in isolation centrifugal compressors are capable of running at quite high pressure ratios (e.g. 10:1), impeller stress considerations limit the pressure ratio that can be employed in high overall pressure ratio engine cycles.
Increasing overall pressure ratio implies raising the high pressure compressor exit temperature . This implies a higher high pressure shaft speed, to maintain the datum blade tip Mach number on the rear compressor stage. Stress considerations, however, may limit the shaft speed increase, causing the original compressor to throttle-back aerodynamically to a lower pressure ratio than datum.

Combusters

• Great care must be taken to keep the flame burning in a moderately fast moving airstream, at all throttle conditions, as efficiently as possible.
• Since the turbine cannot withstand stoichiometric temperatures, resulting from the optimum combustion process, some of the compressor air is used to quench the exit temperature of the combustor to an acceptable level.
• Air used for combustion is considered to be primary airflow, while excess air used for cooling is called secondary airflow. Combustor configurations include can, annular, and can-annular.


Turbines
• In a turbojet almost two thirds of all the power generated by burning fuel is used by the compressor to compress the air for the engine.
• Because of its significantly higher entry temperature, the turbine pressure ratio is much lower than that of the compressor .The turbine needs fewer stages than the compressor, mainly because the higher inlet temperature reduces the deltaT/T (and thereby the pressure ratio) of the expansion process.
• The blades have more curvature and the gas stream velocities are higher
Designers must, however, prevent the turbine blades and vanes from melting in a very high temperature and stress environment. Consequently bleed air extracted from the compression system is often used to cool the turbine blades/vanes internally. Other solutions are improved materials and/or special insulating coatings. The discs must be specially shaped to withstand the huge stresses imposed by the rotating blades. They take the form of impulse, reaction, or combination impulse-reaction shapes. Improved materials help to keep disc weight down.


Afterburners (reheat)


Due to temperature limitations with the gas turbines, jet engines do not consume all the oxygen in the air ('run stochiometric'). Afterburners burn the remaining oxygen after exiting the turbines, but usually do so inefficiently due to the low pressures existing at this part of the jet engine, however this gains thrust, which can be useful.
Nozzles
The primary object of a nozzle is to expand the exhaust stream to atmospheric pressure, thereby producing a high velocity , relative to the vehicle. If the fully expanded jet has a higher impulse than the moving aircraft, there will be a forward thrust on the airframe.
Simple convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e. smallest flow area), in the jet wake. Although much of the gross thrust produced will still be from the jet momentum, additional (pressure) thrust will come from the imbalance between the throat static pressure and atmospheric pressure.


At high nozzle pressure ratios, the exit pressure is often above ambient and much of the expansion will take place downstream of a convergent nozzle, which is inefficient. Consequently, some jet engines (notably rockets) incorporate a convergent-divergent nozzle, to allow most of the expansion to take place against the inside of a nozzle to maximise thrust. However, unlike the fixed con-di nozzle used on a conventional rocket motor, when such a device is used on a turbojet engine it has to be a complex variable geometry device, to cope with the wide variation in nozzle pressure ratio encountered in flight and engine throttling. This further increases the weight and cost of such an installation.
Thrust Reversers

These either consist of cups that swing across the end of the nozzle and deflect the jet thrust forwards(as in the DC-9), or they are two panels behind the cowling that slide backward and reverse only the fan thrust (the fan produces the majority of the thrust.)This is the case on many large aircraft such as the 747,C-17,KC-135,etc
Cooling Systems
All jet engines require high temperature gas for good efficiency, typically achieved by combusting hydrocarbon or hydrogen fuel. Combustion temperatures can be as high as 3500K (5841F), above the melting point of most materials.
Cooling systems are employed to keep the temperature of the solid parts below the failure temperature.
Air Systems
A complex around combustor and is injected into the rim of the rotating turbine disc. The cooling air then passes through complex passages within the turbine blades. After removing heat from the blade material, the air (now fairly hot) is vented, via cooling holes, into the main gas stream. Cooling air for the turbine vanes undergoes a similar process.
Cooling the leading edge of the blade can be difficult, because the pressure of the cooling air just inside the cooling hole may not be much different from that of the oncoming gas stream. One solution is to incorporate a cover plate on the disc. This acts as a centrifugal compressor to pressurize the cooling air before it enters the blade. Another solution is to use an ultra-efficient turbine rim seal to pressurize the area where the cooling air passes across to the rotating disc.Seals are used to prevent oil leakage, control air for cooling and prevent stray air flows into turbine cavities.
A series of (e.g. labyrinth) seals allow a small flow of bleed air to wash the turbine disc to extract heat and, at the same time, pressurize the turbine rim seal, to prevent hot gases entering the inner part of the engine. Other types of seals are hydraulic, brush, carbon etc.
Small quantities of compressor bleed air are also used to cool the shaft, turbine shrouds, etc. Some air is also used to keep the temperature of the combustion chamber walls below critical. This is done using primary and secondary airholes which allow a thin layer of air to cover the inner walls of the chamber preventing excessive heating.

Exit temperature is dependent on the turbine upper temperature limit depending on the material. Reducing the temperature will also prevent thermal fatigue and hence failure. Accessories may also need their own cooling systems using air from the compressor or outside air.
Air from compressor stages is also used for heating of the fan, airframe anti-icing and for cabin heat. Which stage is bled from depends on the atmospheric conditions at that altitude.
Fuel System
Apart from providing fuel to the engine, the fuel system is also used to control propeller speeds, compressor airflow and cool lubrication oil. Fuel is usually introduced by an atomized spray, the amount of which is controlled automatically depending on the rate of airflow.
So the sequence of events for increasing thrust is, the throttle opens and fuel spray pressure is increased, increasing the amount of fuel being burned. This means that exhaust gases are hotter and so are ejected at higher acceleration, which means they exert higher forces and therefore increase the engine thrust directly. It also increases the energy extracted by the turbine which drives the compressor even faster and so there is an increase in air flowing into the engine as well.
Obviously, it is the rate of the mass of the airflow that matters since it is the change in momentum (mass x velocity) that produces the force. However, density varies with altitude and hence inflow of mass will also vary with altitude, temperature etc. which means that throttle values will vary according to all these parameters without changing them manually.
This is why fuel flow is controlled automatically. Usually there are 2 systems, one to control the pressure and the other to control the flow. The inputs are usually from pressure and temperature probes from the intake and at various points through the engine. Also throttle inputs, engine speed etc. are required. These affect the high pressure fuel pump.

Fuel Control Unit (FCU)
This element is something like a mechanical computer. It determines the output of the fuel pump by a system of valves which can change the pressure used to cause the pump stroke, thereby varying the amount of flow.
Take the possibility of increased altitude where there will be reduced air intake pressure. In this case, the chamber within the FCU will expand which causes the spill valve to bleed more fuel. This causes the pump to deliver less fuel until the opposing chamber pressure is equivalent to the air pressure and the spill valve goes back to its position.
When the throttle is opened, it releases i.e. lessens the pressure which lets the throttle valve fall. The pressure is transmitted (because of a back-pressure valve i.e. no air gaps in fuel flow) which closes the FCU spill valves which then increases the pressure and causes a higher flow rate.
The engine speed governor is used to prevent the engine from over-speeding. It has the capability of disregarding the FCU control. It does this by use of a diaphragm which senses the engine speed in terms of the centrifugal pressure caused by the rotating rotor of the pump. At a critical value, this diaphragm causes another spill valve to open and bleed away the fuel flow.
There are other ways of controlling fuel flow for example with the dash-pot throttle lever. The throttle has a gear which meshes with the control valve (like a rack and pinion) causing it to slide along a cylinder which has ports at various positions. Moving the throttle and hence sliding the valve along the cylinder, opens and closes these ports as designed. There are actually 2 valves viz. the throttle and the control valve. The control valve is used to control pressure on one side of the throttle valve such that it gives the right opposition to the throttle control pressure. It does this by controlling the fuel outlet from within the cylinder.
So for example, if the throttle valve is moved up to let more fuel in, it will mean that the throttle valve has moved into a position which allows more fuel to flow through and on the other side, the required pressure ports are opened to keep the pressure balance so that the throttle lever stays where it is.
Fuel Pump
Fuel pumps are used to raise the fuel pressure above the pressure in the combustion chamber so that the fuel can be injected. Fuel pumps are usually driven by the main shaft, via gearing.
Turbopumps are very commonly used with liquid-fuelled rockets and rely on the expansion of an onboard gas through a turbine.
Ramjet turbopumps use ram air expanding through a turbine.
Engine Starting System
The fuel system as explained above, is one of the 2 systems required for starting the engine. The other is the actual ignition of the air/fuel mixture in the chamber. Usually, an auxiliary power unit is used to start the engines. It has a starter motor which has a high torque transmitted to the compressor unit. When the optimum speed is reached, i.e. the flow of gas through the turbine is sufficient, the turbines take over. There are a number of different starting methods such as electric, hydraulic, pneumatic etc.
The electric starter works with gears and clutch plate linking the motor and the engine. The clutch is used to disengage when optimum speed is achieved. This is usually done automatically. The electric supply is used to start the motor as well as for ignition. The voltage is usually built up slowly as starter gains speed.
Some military aircraft need to be started quicker than the electric method permits and hence they use other methods such as a turbine starter. This is an impulse turbine impacted by burning gases from a cartridge. It is geared to rotate the engine and also connected to an automatic disconnect system. The cartridge is set alight electrically and used to turn the turbine.
Another turbine starter system is almost exactly like a little engine. Again the turbine is connected to the engine via gears. However, the turbine is turned by burning gases - usually the fuel is isopropyl nitrate stored in a tank and sprayed into a combustion chamber. Again, it is ignited with a spark plug. Everything is electrically controlled, such as speed etc.


Most Commercial aircraft and large Military Transport airplanes usually use what is called an auxiliary power unit or APU. It is normally a small gas turbine. Thus, one could say that using such an APU is using a small jet engine to start a larger one. High pressure air from the compressor section of the APU is bled off through a system of pipes to the engines where it is directed into the starting system. This "bleed air" is directed into a mechanism to start the engine turning and begin pulling in air. When the rotating speed of the engine is sufficient to pull in enough air to support combustion, fuel is introduced and ignited. Once the engine ignites and reaches idle speed, the bleed air is shut off.
The APUs also provides enough power to keep the cabin lights, pressure and other systems on while the engines are off. The valves used to control the airflow are usually electrically controlled. They automatically close at a pre-determined speed. As part of the starting sequence on some engines fuel is combined with the supplied air and burned instead of using just air. This usually produces more power per unit weight.
Usually an APU is started by its own electric starter motor which is switched off at the proper speed automatically. When the main engine starts up and reaches the right conditions, this auxiliary unit is then switched off and disengages slowly.

Ignition
Usually there are 2 igniter plugs in different positions in the combustion system. A high voltage spark is used to ignite the gases. The voltage is stored up from a low voltage supply provided by the starter system. It builds up to the right value and is then released as a high energy spark. Depending on various conditions, the igniter continues to provide sparks to prevent combustion from failing if the flame inside goes out. Of course, in the event that the flame does go out, there must be provision to relight. There is a limit of altitude and air speed at which an engine can obtain a satisfactory relight.
For example, the General Electric F404-400 uses one ignitor for the combustor and one for the afterburner; the ignition system for the A/B incorporates an ultraviolet flame sensor to activate the ignitor.
It should be noted that most modern ignition systems provide enough energy to be a lethal hazard should a person be in contact with the electrical lead when the system is activated, so team communication is vital when working on these systems.
Lubrication System
A lubrication system serves to ensure lubrication of the bearings and to maintain sufficiently cool temperatures, mostly by eliminating friction.
The lubrication system as a whole should be able to prevent foreign material from entering the plane, and reaching the bearings, gears, and other moving parts. The lubricant must be able to flow easily at relatively low temperatures and not disintegrate or break down at very high temperatures.
Usually the lubrication system has subsystems that deal individually with the pressure of an engine, scavenging, and a breather.
The pressure system components are an oil tank and de-aerator, main oil pump, main oil filter/filter bypass valve, pressure regulating valve (PRV), oil cooler/bypass valve and tubing/jets. Usually the flow is from the tank to the pump inlet and PRV, pumped to main oil filter or its bypass valve and oil cooler, then through some more filters to jets in the bearings.
If the engine speed increases, the pressure within the bearing chamber also increases, which means the pressure difference between the lubricant feed and the chamber reduces which could reduce slow rate of oil when it is needed even more. As a result, some PRVs can adjust their spring force values using this pressure change in the bearing chamber proportionally to keep the lubricant flow constant.
Brayton Cycle
Ideal brayton cycle:
Isentropic process - Ambient air is drawn into the compressor, where it is pressurized.
isobaric process - The compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
Isentropic process - The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
Isobaric process - Heat Rejection (in the atmosphere).
.

Actual Brayton Cycle:
Adiabatic process - Compression.
Isobaric process - Heat Addition.
Adiabatic process - Expansion.
Isobaric process - Heat Rejection
Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies. In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.
The plot indicates how the cycle efficiency changes with an increase in pressure ratio, while the other indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values.
.

Figure : Brayton cycle efficiency


Methods to improve Efficiency
• Reheat: working fluid expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines. This has the advantage of increasing the power output possible for a given compression ratio.
• Regeneration: still-warm post-turbine fluid is passed through a heat exchanger to pre-heat the fluid just entering the combustion chamber. This allows for lower fuel consumption and less power lost as waste heat.
• Intercooling: working fluid passes through a first stage of compressors, then a cooler, then a second stage of compressors before entering the combustion chamber. While this requires an increase in the fuel consumption of the combustion chamber, this allows for a reduction in the specific heat of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall.
• Cogeneration systems: make use of the waste heat from Brayton engines, typically for hot water production or space heating.

Turbofan Engines
Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air not only to the engine core, but to a bypass duct. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.

Turbofans are used for airliners because they give an exhaust speed that is better matched to subsonic airliner's flight speed, conventional turbojet engines generate an exhaust that ends up travelling very fast backwards, and this wastes energy. By emitting the exhaust so that it ends up travelling more slowly, better fuel consumption is achieved. In addition, the lower exhaust speed gives much lower noise.
Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of less concern in military uses relative to civil uses. Multistage fans are normally needed to reach the relatively high fan pressure ratio needed for high specific thrust. Although high turbine inlet temperatures are often employed, the bypass ratio tends to be low, usually significantly less than 2.0.


Turboprop Engines

Most of a turboprop engine's power is used to drive a propeller, and the propellers used are very similar to the propellers used in piston or reciprocating engine-driven aircraft
A turboprop engine is similar to a turbojet, but has additional stages in the turbine to recover more power from the engine to turn the propeller. Turboprop engines are generally used on small or slow subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kts (926 km/h, 575 mph).
In its simplest form, a turboprop consists of an intake, compressor, combustor, turbine and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor. The hot combustion gases expand through the turbine. Part of the power generated by the turbine is used to drive the compressor. The rest goes through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop, the remainder comes from the conversion of shaft power to thrust in the propeller.
Turboprops are very efficient at modest flight speeds (below 450 mph), because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high performance Short-Takeoff and Landing (STOL) capability and efficiency at modest flight speeds is required.