Chapter 1

Basic Operation of a Small Gas Turbine

There have been many books written on the subject of gas turbine theory and it would be beyond the scope of this document to go into it in detail here. References will be given at the end of this document for those who need to pursue the theory further. A basic practical description relating to how a small gas turbine engine works follows.

Heat engines all work by utilising the expansion of a fluid when it is heated, this fluid is usually air or a gas. If the air is compressed before it is heated, then the amount of subsequent expansion can be increased. In a piston internal combustion engine, the expansion of gas is used to create a force upon a piston, the piston moves and turns a flywheel via a crank. A gas turbine is very similar, the expanding heated gas acts upon a turbine wheel which can be used to provide power. In a piston engine, the flywheel pushes the piston upwards to compress the ingested air ready for expansion. In a gas turbine, air is compressed using a rotating impeller, which is driven by the turbine.

A very important feature of a gas turbine was first suggested by Sir Frank Whittle. The exhaust from a gas turbine can be directed through a nozzle to propel the engine forward. A combination of the momentum transfer of the mass of air flowing through the engine and the pressure across the exhaust nozzle creates a reaction on the engine, this reaction produces thrust. This is how a turbo-jet engine works, the "load" on the gas turbine is created by the exhaust nozzle, the engine "pumps" its own exhaust gases through it. All turbo-jet engines are gas turbines, but not all gas turbines are used in this way and can be considered to be turbo-jet engines.

The main difference between a piston engine and a gas turbine is that the processes in a gas turbine are simultaneous and continuous, in a piston engine these processes occur individually as a number of cycles. If you study the temperatures and pressures within heat engines, more detail differences between turbines and pistons become apparent. Aerodynamics also play a key role in gas turbine operation, the flow of gases around turbine blades and compressors is crucial to there efficient operation.

All gas turbine engines consist of a number of fundamental component parts, broadly speaking, these component parts can be likened to the operating cycles of a piston engine. The main component parts in a gas turbine engine consist of a compressor, A combustion system or chamber, and one or more turbines, in addition, the areas where the air enters the engine and the exhaust leaves the engine are also important. The compressor and turbine are always connected with a common shaft which is supported by bearings. The components of the engine from the compressor onwards i.e. the combustion chamber, turbine nozzle, turbine and exhaust are sometimes referred to as the "Hot Section" components of an engine.

Air Intakes
The air intake(s) on a small engine can be found at the front or to the side of the unit (the front in most cases is at the opposite end from where the exhaust gases emerge). Air is guided into an engine by a simple bell-mouth or by one or two converging ducts. Certain designs incorporate a partial enclosure built around the air intake which forms a plenum chamber. A Gearbox is used to drive a mechanical load and various engine accessories, this is normally placed at the front of the engine and the air is drawn in around it. The rear or exhaust end of a gas turbine is generally hotter than the front, so all the temperature sensitive devices are grouped towards the front. Certain designs use the intake air to help cool the lubricating oil via an oil cooler radiator. Air intake filters are relatively rare but certain models of ground based stationary engines can be equipped with large industrial type filter screens.

A considerable amount of noise emerges from the air intake when a gas turbine operates, it is possible to fit devices to help silence this noise. Gas turbine silencers can be quite bulky and are generally only found on ground based stationary engines.

For their size all gas turbine engines draw large amounts of air into the compressor. Considerable suction is present in the air intake and may create a hazard. There is also a small pressure drop in the region of the compressor intake. The addition of intake screens, enclosures or silencing devices may increase this pressure drop which could adversely effect the operation of the engine if attention is not paid to it. In the case of propulsion engines there is arguably a very small additional thrust placed upon the engine due to the intake area static pressure being below that of atmospheric.

Compressor
All small gas turbine engines are fitted with a centrifugal compressor. A centrifugal compressor consists of a rotating impeller wheel which throws the air outwards as it spins at high speed. Curved blades at the centre of the wheel, called rotating inlet guide vanes induce the air into the wheel at the front and then pass the air between radial blades which guide the air outwards. In most small engines these blades are straight radiating out from the rotational centre of the wheel but in certain designs the blades are curved back at the circumference of the wheel, this has the effect of improving the efficiency of the compressor.

The compressor impeller is surrounded by a ring of stationary vanes or blades which receive the vary fast moving air which is "slung out" from it, this device is called a diffuser. The diffuser vanes slow down the air and remove some of the rotation from it, the effect of this is to raise the static pressure of the air. Air leaving the compressor diffuser sometimes travels through addition vanes to further remove any rotational movement before it is passed to the combustion system. The combined effect of the rotating impeller and the stationary diffuser is to raise the air pressure by as much as four times atmospheric pressure (The pressure at the intake). To do this, the compressor has to rotate at high speed, a wheel measuring about 5 inches in diameter may spin as fast as 60,000 rpm and in doing so considerable energy is required to drive it. When the air is compressed mechanical work is done, as much as two thirds of the energy developed in a gas turbine is used to drive the compressor. When air or any gas is compressed its temperature increases, this is because mechanical work is carried out on it, air leaving a gas turbine compressor is raised in temperature for this reason. A typical compessor discharge pressure may be in the region 40 PSI and at a temperature of 180 degrees C.

The compressor rotor operates inside a casing which is normally fabricated from aluminium or magnesium. The compressor case may often incorporate a reduction or accessory gearbox and may form the air intake area. A convergent duct or bell-mouth is usually formed in front of the rotating central "eye" of the compressor.

In order to get reasonable efficiencies from small centrifugal compressors, the mechanical running clearances have to be kept very small. The impeller has to maintain clearances of only a few thousandths of an inch between the rotating blades or vanes and the surrounding casing. The circumference of the compressor wheel runs close to the diffuser mounting assembly so that all the air flows into the diffuser and does not leak out and reduce the efficiency. A gap exists between the actual diffuser blades and the tips of the rotating compressor blades, this is to prevent turbulence and allow for a smooth transition of air into the diffuser. In order to prevent unwanted resonance the numbers of blades or vanes on the on the compressor wheel and the diffuser assembly are different. A typical number of compressor blades may be 13 full blades and 12 half blades and 15 corresponding diffuser blades.

Compressor wheels are normally made of aluminium alloy and in many Garrett engines they are made from Titanium. The rotating inlet guide vanes are sometimes made from steel or stainless steel, in this case the compressor wheel it is of a two part construction. The steel guide vanes help guard against foreign object damage (F.O.D.). F.O.D. is caused by the engine ingesting foreign matter during operation, small particles may hit the guide vanes and then go through the engine without significant harm. Larger particles ingested into the engine can cause damage to the compressor and possibly cause the whole engine to catastrophically fail. Particles can become briefly trapped between the compressor and its housing as it rotates, this process can damage the internal surface causing scratches and scoring, this will also increase the operating clearances and hence reduce the efficiency of the compressor. Air intake grills or meshes normally protect against F.O.D. (And probing fingers!), they should only be removed with caution.

Aluminium compressor wheels are vulnerable to corrosion especially in older engines, engines which have been lying around in scrap yards or engines of unknown history. It is always worth checking the compressor and intake area for corrosion and evidence of damp storage conditions. When inspecting the air intake area of a small gas turbine attention should be paid to the rotating inlet guide vanes, F.O.D may be indicated by gouges, nicks or even missing portions of blades. A gas turbine engine should never be operated with a damaged compressor as it may have become mechanically weakened or is out of balance.

The compressor system in certain engine designs is deliberately made to operate at a larger capacity than that which is required to run the engine, this feature enables the engine to supply an external feed of compressed air. A portion of air flowing through the engine is bled from the compressor as it runs, the air bleed although at only moderate pressure (45 PSI) exhibits high mass flow (Large volume of air per second flowing) and so contains quite high kinetic energy, this is sufficient to drive a small external turbine. This supply of compressed air can be used for aircraft engine starting or cabin air-conditioning. Many gas turbine engines are designed to provide compressed air and shaft horsepower simultaneously, usually the shaft output is de-rated during air bleeding. The engine is de-rated because less air flow is available within the engine to keep it cool below safe limits.

The Blackburn Palouste engine discharges air as it runs and the air bleed is only shut off during starting by a special valve. During starting the compressor efficiency is very low so the airflow through he engine has to be maximised to prevent it from overheating.

Certain versions of the Rover 1S series gas turbine are equipped with an air valve which is closed during starting and normal running, when an air supply is required the valve then opened.

Another air bleed engine the Microturbo Saphir discharges air continuously at all speeds even during start up, this engine is sometimes referred to as an "Air Generator" or "Air Producer".

Combustion system

Air emerging from the compressor diffuser is then passed to the combustion system where fuel is burnt, this provides the power which drives the engine. In the combustion chamber the air is heated, it expands, gains kinetic energy, and the pressure remains almost constant. The combustion arrangements in small engines basically consist of three types, a can type, an annular type or a radial type.

1. Can type combustion chamber. This is the simplest of combustion arrangement and consists of a cylindrical metal can shaped housing containing a metal liner or flame tube. Air from the compressor is ducted to the combustion chamber by the casing surrounding the compressor diffuser outlet. This casing may surround all the hot parts of the engine and hence reduce the outside surface temperature to an acceptable level.

The combustion chamber liner consists of a heat resistant metal cylinder which is closed at one end by a dome shaped cap. Fuel is sprayed into the combustion chamber from a nozzle at the closed end where it is ignited by a special spark plug. Air from the compressor flows around the outside of the liner between it and the outer combustion chamber casing. A series of holes or orifices in the liner admit the air into the central region where it sustains combustion. The exhaust gases exit through the open end of the liner and are ducted to the turbine. The holes in the liner are arranged to provide air for the actual combustion of the fuel and also additional larger holes towards the open end admit air for cooling of the combustion products. The flame temperature inside the liner may reach 2000 degrees centigrade which is far too hot for the rest of the engine to cope with so the cooling air is required. Only about 25% of the air entering a gas turbine is burnt the rest is used for cooling. The detailed arrangements of the holes becomes quite complicated, they are required to produce swirl within the flame tube which assists in the complete and stable combustion over a range of operating conditions. The surface of the liner is also kept cool by rings of very small holes which provide a boundary layer of cool air within the flame tube. The hole sizes are arranged so that the pressure drop between the outside and inside of the flame tube is kept to a minimum, any drop in pressure at this point will reduce the efficiency of the engine.

The flame tube/liner arrangement holds a flame within the combustion chamber which unlike a piston engine burns continuously. Ignition of the fuel is only required once during starting of the gas turbine.

The flame tube is made of a corrosion and heat resistant metal such as stainless steel or nimonic. Over a period of time the tube may became distorted due to thermal stresses and heat cycles. Small amounts of distortion depending on the engine type can be permitted. Cracks due to thermal fatigue can also form in the flame tube and often between the holes, these may be repaired by welding if not too severe. The overhaul manual for a particular engine will normally detail acceptable limits of distortion and cracking for the combustion chamber components.

The combustion chamber liner will be suspended at a number of points inside the engine. The liner will normally be aloud to "Float" slightly and may appear loose, this is to allow for thermal expansion and so prevent the liner from distorting unnecessarily.

During the operation of the combustion chamber, carbon deposits may build up in the flame tube and around the burner nozzle. Certain engines run cleaner than others but if excessive carbon deposits build up, the combustion process can be detrimentally effected. Carbon around the burner may upset the fuel spray pattern and reduce the atomisation quality. Carbon can also block or modify the air holes which leads to less efficient air distribution. If the spray pattern is very poor the resulting flame may continue outside the combustion chamber area and create an uneven temperature distribution around the turbine and associated components, this will lead to eventual burning of components and failure. It is possible to "De-carbonise" combustion chamber components, great care is needed as devices such as fuel burner nozzles and ignitor plugs should be not brought into contact with abrasive materials.

Ignitor plugs and burner heads should only be cleaned with soft materials soaked methalayted spirit or electrical cleaning solvents.

A single can type combustion chamber exhibits a number of advantages over other types. The main advantage is simplicity, a single burner is installed and the liner can be easily removed for inspections and maintenance. In small engines, achieving efficient combustion becomes difficult as the internal dimensions and burning length required are relatively small. A can type chamber holds a single relatively large flame for a given size of engine.

The disadvantage of a can combustor is that the engine may not be as compact as with other types. The engine will not be symmetrical about its axis, for some small starter units which may be mounted on larger engines, compactness, weight and minimum external dimensions are important.

A single can type combustion chamber is fitted to many Garrett engines and also the Rover 1S series uses a single can combustion chamber. The Perkins Mars/Solar T41 is another gas turbine which employs a single can type combustor. Occasionally engines use two can type chambers mounted diagonally opposite each other, an engine built by the company Auto diesels is constructed in this way.

2 Annular Combustion Chamber
An annular combustion chamber is required to do exactly the same job as a can type unit but is physically arranged in a different way to reduce the overall size of the engine. The combustion chamber liner is rapped around the axis of the engine and instead of one, a number of fuel nozzles are used.. The combustion chamber liner is often arranged to double back on itself, in which case it is referred to as a "reverse flow combustion chamber". A reverse flow combustion chamber saves on the overall length of an engine and provides a longer burning area for the gases to travel through.

A reverse flow combustion chamber arrangement results in a ring of fuel burners being placed at the back of the engine around the exhaust circumference. Fuel is distributed to the burners by a pipe manifold or by drillings in the combustion chamber outer casing. The spray patterns from the individual burners are of a cone or fan shape or in the case of a Lucas aerospace engine, five burners produce flat fan shaped patterns which are arranged in the form of a pentagon. One or more ignitor plugs are placed next to the burners to initiate combustion, once one burner lights up the flame spreads to the others rapidly.

Air holes are placed in the combustion chamber liner in a similar manner to that in the can type system. Small holes at the nozzle/burner end provide air for combustion and then larger holes down stream provide cooling air before the gases are passed to the turbine system. Air which has been bled from the compressor may also be supplied to the burners directly to assist in the atomisation process.

The annular combustion chamber reduces the overall size of an engine but it is more integrated into the engine construction and is therefor more difficult to remove and inspect.

The Rotax/Lucas Aerospace gas turbine starter/auxiliary power unit uses a reverse flow combustion chamber with five burners. The Microturbo Saphir and the related Plessey Dynamics "Solent" gas turbine starter also uses a reverse flow arrangement with eight burners. The Solar T62 "Titan" unit is fitted with a reverse flow combustion chamber and six burners plus torch ignitor.

3 Radial Combustion Chamber A third type of combustion system consists of a radial layout. The radial combustion chamber was pioneered by the French company Turbo-Mecca and can be found in a number of their designs. The German company Man-turbo (MTU) formally BMW and the Swiss company Saurer also used this combustion chamber layout.

The combustion chamber liner is arranged to form a radial chamber around the shaft which connects the compressor to the turbine, fuel is admitted to the chamber through the turning shaft from a series of radial drillings placed in it. As the shaft rotates at high speed, the fuel is thrown outwards and atomised into a fine disc shaped spray. The fuel is ignited and burned with a similar air distribution to that of the other combustion chamber types. The hot gases from the combustion process are guided by the liner outwards and along the engine axis or back towards the centre depending on the type of turbine wheel used. The whole radial liner is enclosed in an outer casing which is pressurised with air from the compressor.

Fuel has to be supplied to the combustion chamber through the engine shaft, this requires a seal arrangement at the cold end near the compressor to ensure that it does not leak out into the other engine systems i.e. the oil supply to the bearings.

The Blackburn Palouste, Artouste and Nimbus all use this arrangement, as do other Turbo-Mecca engines and some Continental US built units. The radial layout makes for a symmetrical engine around its axis similar to the annular combustion chamber layout. The fuel pump required needs to maintain only moderate pressure, however due to centrifugal force generated by the high rotational speeds, the actual injection pressure into the combustion chamber is very high. A high fuel pressure helps to aid the atomisation process.

Turbines
The turbine wheel or wheels in a gas turbine serve a number of purposes, in all cases one or more turbines are used to drive the compressor via a connecting shaft, this is what keeps the engine going. External power from the engine can also be obtained from this shaft or an additional turbine can be used which is mechanically free from the turbine which is driving the compressor.

There are two basic types of turbine which are used in small engines, the radial-inflow turbine and the axial turbine. Depending on the design of engine either type can be found, notably in the Lucas Aerospace GTS/APU and Rover 2S150 both types are used in the same engine.

Axial Flow Turbine
The axial flow turbine wheel consists of a wheel with a number of small angled blades mounted around its circumference. The hot gases from the combustion chamber are directed onto the blades by a ring of similar static blades or vanes. These blades form a nozzle which ensures that the gases impinge on the turbine blades in the direction of rotation of the wheel. The turbine blades are a complex shape, the operating mechanism of the turbine is partly that of an impulse turbine and partly as a reaction turbine. As the gas strikes the turbine blade it gives up a portion of momentum to the blade causing it to move. The gas flow over the blades is such that the pressures on each side are different causing a reaction on the blade and forcing it two move. The blades can be considered to be airfoils or even tiny wings.

The combined effect of the stationary turbine nozzle and the rotating turbine wheel is to reduce the pressure, velocity and temperature of the gases flowing through it. Work is extracted by the turbine and so the energy in the gas stream is reduced. The turbine assembly may be considered to be similar to the compressor but working in reverse.

The axial flow turbine wheel turns inside a very close fitting shroud, the gap between the shroud and the turbine blade tips must be kept as small as is practicable. A small clearance between the blades and the shroud prevents significant amounts of gas
bypassing the turbine and reducing its overall efficiency. In small engines gaps of as little as 5 thou are possible.

The turbine wheel is made from nimonic alloy which is very tuff . It has to withstand
very high temperatures, resist corrosion, high rotational forces and remain within very tight mechanical tolerances. Over the life of the engine the wheel will experience many heat cycles when the engine is started and stopped, these thermal cycles must not have a detrimental effect on the wheel. Eventually over time the wheel may suffer cracking and the dimensions may change due to creep, periodically as part of an engine overhaul the wheel must be checked and inspected. The blade tips can also become eroded and burnt on high life turbine wheels which will reduce the efficiency.

Radial Flow Turbine
A radial flow turbine wheel is rather like the compressor wheel working in reverse. A ring of static vanes guide the hot gasses from the combustion chamber onto an impeller shaped wheel. The vanes act as a nozzle and direct the gases so that they impinge almost tangentially on to the radial blades of the impeller in the direction of rotation. The radial blades of the impeller also have a curved section towards the middle which guide the gases out along the turbine axis.

A radial flow turbine wheel is almost universally used in turbo-chargers, it is more efficient than a single stage axial flow wheel. It has the advantage that the running tolerances can be higher, this is because the clearances around the blades are less critical on this type of wheel.

Many small engines use radial inflow turbine wheels, they are very common in Garrett APUs, the German BMW 6012 uses one, Saurer GT15 is equipped with one and they can also be found in some Lucas/Rover engines.

Due to the favourable clearances which can be adopted with radial flow turbine wheels, the configuration allows for a particular mechanical engine layout. The compressor and turbine are mounted back to back on a common shaft and separated only by a baffle and a seal around the shaft. The bearings for the shaft are both placed outboard of the wheels in an "overhung or cantilever configuration". The advantage of this arrangement is that the bearings are well away from the "Hot Section" of the engine. This arrangement reduces the temperature of the bearings and exhibits advantages in terms of life and heat build up. The turbine wheel is placed furthest from the bearings and so any radial movement will be at a maximum at this point. To allow for a small amount of radial movement the clearances around the turbine wheel are relatively large, for this reason a radial inflow wheel is favoured. The gas flow around the wheels is also straight forward as it exits the compressor and enters the turbine around the circumference, keeping the hole configuration as short as possible.

A second advantage of an overhung bearing arrangement is that the hot section components do not carry a bearing system and so may be removed for inspection without the rotor assembly and associated bearings and seals being disturbed.

Depending on the design of the engine one or more axial flow turbines or one radial inflow turbine are used to drive the compressor. Most small gas turbines consist of a single shaft layout, i.e. the rotational horsepower developed by the engine is extracted directly from this single shaft. A gearbox is normally used to reduce the rotational speed so that it can be used to drive a suitable load. Mechanical loads on small gas turbines usually consist of electrical generators, pumps or in some cases compressors. The gearbox is also used to drive various engine accessories like fuel and oil pumps. When using a single shaft engine, care must be taken to ensure that no significant mechanical load is placed upon the engine until it has successfully started and accelerated to governed speed. Certain types of DC generators are relatively stiff to turn due to high current brush gear, in this case a centrifugal clutch should be used so that the engine does not drive the generator until it has gathered speed, this also relieves the load on the starting system. One example of a single shaft gas turbine directly drives a second compressor, during starting the second compressor is blocked off by a valve and effectively stalled to prevent it loading the engine. Note: Compressors driven by free turbines must never be stalled or an over-speed condition and failure may result.

A remarkable system adopted by the company Saurer uses a fluid coupling to engage a load on to the GT15 engine. A fluid coupling connects the gas turbine to a compressor unit, during starting and other operations the fluid coupling is drained of fluid and so does not transmit drive to the compressor. When the compressor is required fluid is passed into the coupling and the drive is transmitted. The fluid used is in fact the fuel, the Saurer GT15 gas turbine uses fuel for lubrication, the power transmission and of course for combustion.

Free Power Turbine (Twin-Shaft)
A few small gas turbines adopt a twin shaft layout. Here a second turbine wheel mechanically free from the compressor turbine is used to extract power form the engine. Usually the engine accessories are driven from the compressor turbine to enable this part of the engine to start up. A gas turbine engine which supplies its hot gases to turn a free turbine is sometimes referred to as a "Gas Generator". As the gas generator runs up to speed the hot exhaust acting upon the free turbine gradually accelerates it up to speed, this can take place with a mechanical load already applied.

The main use for this arrangement is in gas turbine starter units (GTSs). A gas turbine starter is basically a small turbo shaft engine which is used to provide direct mechanical effort to spin up a larger propulsion engine in a similar way to that of an electric starter motor. The gas generator of the GTS is started electrically which intern spins a free turbine, the free turbine is connected to the propulsion engine through a reduction gearbox. As the main engine runs up to speed the starter can be shut down by means of a centrifugal switch, this operates when the free turbine reaches a pre-determined speed. A one way clutch prevents the started propulsion engine from continuously back-driving the turbine wheel after the GTS has shut down.

The Plessey "Solent" gas turbine starter unit is an example of a small free turbine engine. This unit was used to rotate the High Pressure spool in a RR Spey engine as fitted to the MD Phantom F4 aircraft. The Solent is intermittently rated to provide about 70 horsepower for the maximum duration of one minute. The intermittent rating is mainly due to a one shot oil system. The one shot oil system consists of a simple oil metering system operated by air from the compressor. On the Solent, the gas generator spins at 60,000 rpm and the free turbine cut of speed is about 50,000 rpm. A twin epicyclical gearbox with a ratio of 11:1 is used to provide a low speed output which drives into the RR Spey accessory gearbox.

A unit made by Lucas Aerospace is used to start the Pegasus engine in a Harrier. This unit drives through a free turbine and twin epicyclic gearbox to provide an output of 80 horsepower. The free turbine may also be used to drive a generator without it turning the Pegasus engine over. The free turbine is disconnected from the main engine and a power turbine governor is used to maintain an output speed of 12,000 rpm via a gearbox. In this case the gas generator spins at about 55,000 rpm, during starting the gas generator speed is increased to 77,000 rpm to provide extra power needed to start the Pegasus engine. The GTS can convert from one mode to another by making use of an elaborate system which applies a brake to the free turbine before coupling it to the stationary Pegasus engine via a dog clutch.

Great care must be taken when operating free turbine engines, if they are started with no load applied to the free turbine, it will over-speed with potentially catastrophic results. The Solent unit can only be operated in a stand alone mode if the free turbine and output gearbox are first removed. The engine then consists of a gas generator which will govern itself at 60,000 rpm. GTSs are normally tested on a special rig, the GTS mechanical output is used to rotate a large flywheel or dynamometer.

The Lucas engine has a governor driven from the free turbine, this allows the unit to be operated complete with no external load applied. This mode of operation is first selected by opening a solenoid valve which enables a power turbine governor. During a Pegasus starting operation the GTS bypasses the governor, this must not be allowed to happen if no load is applied to the output i.e. when the GTS is operated stand-alone.

Exhausts
Once the hot gases in a gas turbine have passed through the turbine(s) they are discharged to atmosphere. The exhaust emerges through a divergent duct which is sometimes bent around to direct the exhaust in a particular direction. The exhaust which is hot and travelling quickly is directed away from airframes, equipment and people. In the case of gas turbine starters, power is normally taken off rearward via a gearbox so the exhaust is ducted sharply around a 90 degree bend and out to the side. Vanes are sometimes placed in exhaust ducts to help guide the gases and reduce turbulence and eddies which could reduce the efficiency of the engine.

The exhaust temperature of small gas turbines can vary considerably from type to type and it also depends upon the operating conditions at the time. The exhaust gas temperature of an engine is a very important measure of the health and efficiency of the engine. During start up there is an excess of fuel and the compressor exhibits a low airflow, the engine will run hot until it gathers speed when more air becomes available for cooling. As a load is applied to a running engine, the airflow remains almost constant but more fuel is burned to maintain the same speed, this results in a higher exhaust temperature. All gas turbine engines have specified limits for maximum starting and running exhaust temperatures, these limits should not be exceeded. A typical starting temperature for a GTS could be as much as 750 degrees C, this is because they are required to start and run up as quickly as possible. Off load Garrett engines can run as cool as 280 degrees, off load the Rover 1S60 runs about 400 degrees, a Lucas GTS can run as hot as 500. As a gas turbine starts the exhaust temperature normally rises rapidly after "Light Up", reaches a peak and then settles back to a lower value when idle speed is reached.

Cooling
Most types of internal combustion engine require cooling, in the case of a piston engine they are normally air or water cooled. In the case of a the gas turbine engine, it can be considered to be "Self cooled". The working medium through out a gas turbine engine is air and there is large quantities of it. The majority of the air will flow from the compressor into the combustion chamber, this air is partly used for combustion and partly used to cool the combustion gases before they enter the turbine. Air is also used to cool a number of other areas within the engine.

In certain designs of engine, the vanes or nozzles which divert the combustion gases onto the turbine are actually hollow. Compressor air passes through the nozzles to cool them and to ensure that they remain within an acceptable temperature range. The turbine nozzle guide vanes are the first to receive gases from the combustion chamber, this is one of the hottest parts of the engine. As the gases pass through the turbine nozzles and turbine they reduce in temperature and expand as kinetic energy is extracted from them.

Air is also directed onto the surface of a turbine wheel to reduce its temperature and prevent excessive heat reaching the shaft to which it is attached. Air may be ducted to the turbine end bearing to keep it cool.

Balance
Compressors and turbines rotate at high speed, in the case of a Saurer GT15 APU the maximum speed is some 85,000 rpm. Before assembly the high speed rotating components of a gas turbine are accurately dynamically balanced. Dynamic balance refers to the balancing of a wheel radially and also along its axis. It is possible to statically balance a wheel so that the centre of mass lies at the rotational axis, however the mass distribution may not be even on each side of the wheel. If the wheel is considered as a whole it is balanced, but if each side of the wheel is considered separately an imbalance can be found. If the wheel is rotated an out of balance force is created which must be corrected.

Turbine wheels and compressors should be balanced on a dynamic balancing machine or rig. The machine rotates the wheels and by means of a number of transducers, it detects the vibration generated by any imbalance. The transducer signal is displayed on an oscilloscope or other indicating device. By placing small weights on to the wheel (Often plasticine), the balance can be improved and the vibration signal reduced. Once balanced with the temporary weights, the wheel is machined at a position radially opposite the location of the weights. The machining is carried out on both sides of the wheel and the balancing weights removed, the balance of the corrected wheel is then re-checked on the machine.

Turbine wheels and compressors are provided with regions of metal which are intended for removal to aid balancing. A circular ridge is provided on each side of a an axial turbine disc, portions of which are ground off when the wheel is balanced. A similar ridge is also provided on the rear face of a compressor wheel. Small holes may also be drilled into the central boss near the front of the wheel and small chunks can also be removed from the wheel circumference. When inspecting wheels missing portions should be checked carefully to ensure that they are identified as balancing points and not the results of damage.

Small gas turbine engines operate at such high speeds that the rotation of the main shaft gives rise to a characteristic whine. As the engine spools up to speed, a distinctive whistle is produced by the compressor, at governed speed this is relatively high at for instance 10 KHz, a lower whine can also be heard which is proportional to the rotation of the shaft. An engine turning at 60,000 rpm revolves a 1000 times per second, this gives rise to an audio tone of 1 KHz. On better balanced engines this note may be quieter than that of inferior units. Poor balance may manifest itself as harmonic sounds and resonances as the engine spools up and down. Bearing assemblies also play a part on the effects of balance, the bearings support the rotational shaft and so aid in dampening vibration and reasonances.

Efficiency
Compared to most other power plants small gas turbines are not very efficient sources of power. This is one of the reasons why there application is generally restricted to aircraft where they find advantages in terms of size, weight and the fact they burn the same fuel as the propulsion engines. Larger gas turbines rated at many thousands of horsepower offer much improved efficiency.

The main reasons for poor overall fuel efficiency in small gas turbines can be attributed to the following causes-

1. Gas turbines which operate with only a single stage compressor, raise the air pressure by a maximum of about four times. In heat engines the more the air is compressed the more it can expand when heated, also the more energy is released. Engines with higher pressure ratios burn less fuel as more energy is released during the increased expansion. The pressure drops as the gases flow through the turbines and eventually becomes atmospheric as it leaves the exhaust. More turbine stages are possible with higher pressures.

2. When the air is heated by combustion in a gas turbine the expansion is determined by the temperature rise. The higher the inlet temperature to the turbine, the more expansion can take place in the turbine and more power is released. If the gases are cooled before entering the turbine then energy is lost which is not available to develop mechanical power. Larger engines with more sophisticated cooled turbine nozzles can cope with a higher turbine inlet temperature.

When a small gas turbine engine runs, much of the energy resulting from burning of the fuel ends up as waste heat in the exhaust. It is possible to improve the efficiency of the engine by re-cycling some of this waste heat. A heat exchanger can be used to extract heat from the exhaust and use it to raise the temperature of the air entering the combustion chamber. Less fuel is required to be burnt in the combustion chamber for the same turbine inlet temperature and so the engine burns less fuel for a given power output.

Heat exchangers which are fitted to small surplus gas turbines are relatively rare, they are often complicated and bulky. The difficulty in successfully producing a heat exchanger was one of the reasons for the failure of several automotive gas turbine projects.

Certain Microturbo gas turbine engines are fitted with heat exchangers and also a variation Solar T41 unit incorporates one.