Chapter 2

Fuel Systems

The most complicated part of any small gas turbine apart from the gearbox and mechanical drive system, is likely to be the fuel system. Small gas turbines in a similar way to much larger units make high demands on their fuel systems. The system is required to start and accelerate the engine, govern or control the engine at one or more speeds, maintain a given speed under load conditions and in many designs, the engine must be protected against excessive exhaust gas temperatures and compressor surging.

All small gas turbines are designed to operate on Jet fuel such as JetA, Jet A1 and JP4, these fuels are similar in many ways to Kerosene. Paraffin is a type of kerosene, as is 28 second heating oil and gas oil. Small engines will run happily on any of these fuels, tolerance of different fuels is one of the advantages of the gas turbine. As the combustion process is continuous and at constant pressure, there are no problems with deternation or pinking. Deternation or pinking which is associated with piston engines can significantly limit their performance. Certain designs intended for specialist stationary applications may be operated on natural gas.

Certain small engines can be operated on Diesel fuel or Petrol, the basic performance of the engine will be unaffected but prolonged use is not recommended for a number of reasons.

Diesel fuel contains sulphur which can deposit itself on components in the fuel system. Certain fuel pump components are often silver plated which are damaged by the sulphur content in the fuel.

Petrol even when cold gives off vapours which can make the operation of the engine hazardous, accumulated fuel could become explosive and ignited if the ignition system is inadvertently operated with the engine stationary, this could lead to a fire. Kerosene when cold is surprisingly difficult to ignite, testimony to this is the ferocity of high energy ignitors required to reliably light it. The fuel in a gas turbine also serves as a lubricant for the fuel pump, petrol is not as an effective lubricant as kerosene.

A typical gas turbine engine originating from an aircraft will require the fuel to be supplied to it under slight pressure. Aircraft installations normally feed engines from what is called a low pressure (LP) system. Centrifugal pumps submerged in the aircraft tanks pressurise the LP system to about 10 PSI. The pumps draw fuel through mesh screens so that no large foreign matter enters the LP system. Fuel to a small engine should be filtered to prevent any further small particles from entering the system. Automotive fuel injection filters and diesel injection filters are suitable for this purpose, the flow rate is higher than for automotive applications so depending on the cleanliness of the fuel, they will need replacing more often.

A stationary ground based engine will often benefit from using some sort of booster pump to draw fuel from a tank. It is possible to use an electric automotive pump such as an SU solenoid activated unit. Slight pressure will assist in bleeding the fuel system and removing any trapped air. Certain small gas turbine engines are sensitive to air bubbles which may be trapped the fuel, the bubbles result in a brief loss in fuel pressure which may cause an engine with a single burner to "flame out".

When constructing a low pressure fuel system to feed a gas turbine, a low pressure shut off cock should be included to stop the engine in an emergency. The stop cock may consist of a manual "Gate" type valve, or an electrically operated solenoid valve which closes when de-energised. This arrangement will ensure that if electrical power is lost to the engine, it will shut down safely.

Warning
Caution should be exercised when choosing a suitable booster pump, fuel injection pumps for cars should be avoided as they are capable of high pressure. The pump itself will operate satisfactorily but if no pressure relief valve is fitted, the high fuel pressure will effect the governing system of the engine, this may cause it to run fast.
Any positive displacement pump should be checked and fitted with a pressure relief valve to ensure that the output pressure does not exceed 20 PSI. Certain models of car fuel pump can exceed 100 PSI. An SU pump has a built in pressure limiting device as it was originally designed to feed a needle valve inside a carburettor.

Many engines will work satisfactorily from a gravity fuel feed, small gas turbines are very thirsty compared to piston engines, if prolonged operation is required a large fuel tank will be needed. To ensure satisfactory starting the tank will have to be placed above the engine which could be inconvenient. Fuel consumption can be up to 10 gallons per hour so the tank should be vented to prevent it becoming partially evacuated as the fuel is consumed.

Fig XX shows a basic gas turbine fuel system, it consists of a number of devices to control the fuel flow. Most of these devices are housed in one block which can be referred to as the fuel control unit

Fuel Pump
The pump is sometimes referred to a high pressure (HP) pump and is usually one of two types.-

Gear pumps are common in small gas turbines, two meshed gears turn inside an metal housing, the fuel enters the housing and travels around the outer parts of the gears between the teeth. The rotation of the gears produces a positive displacement as the fuel cannot return to the inlet due to the meshed portion of the gears between them. The pump gears have to run in close fitting tolerances to ensure sufficient fuel pressure is built up and to prevent the fuel from leaking back to the inlet. The end faces of the gears are sometimes mated to carbon or bronze seals to further reduce leakage. A carbon seal consists of a spring loaded carbon bush which mates up with a rotating component, the mating surfaces are highly polished to prevent friction. Carbon seals are quite common in small gas turbines, it they are disturbed during disassembly, care should be taken with them not to scratch the mating surfaces, they are also quite brittle. Naturally only one gear in a gear pump needs to be driven around by the engine. Many engines use gear pumps, Garrett engines, Lucas Aerospace GTSs and Man-Turbo units all make use of this type.

A second type of fuel pump uses a series of small pistons to pump the fuel, this arrangement is common in larger aircraft engines. A rotating disc or swash plate is shaped in such away that it varies in distance from a fixed point along its axis. As it rotates the disc impinges on one or more pistons and pushes them against a spring to provide a pumping action. Many larger engines are fitted with a lever mechanism to vary the pump stroke which controls the flow rate. This arrangement is more complicated than a gear pump but is capable of higher pressure.

The fuel pump is lubricated and also partly cooled by the incoming fuel, any foreign matter in the fuel can cause the pump components to wear and pick up, hence the need for good filtering. Fuel pumps should not be turned when dry due to the lack of lubrication, if an engine needs to be rotated, the pump should at least be kept "wet" by squirting oil such as WD40 into the pump inlet at regular intervals.

Fuel pumps on small engines usually form part of an integrated fuel control unit. The control unit will also incorporate various other devices all placed inside one casing. Fuel pumps normally incorporate some sort of pressure relief valve, when an engine is shut down the fuel down stream of the pump is cut off with a valve, the relief valve will open to limit the pressure build up. The pump flow capacity will normally be in excess of engine requirements so that the governor can function properly by spilling excess fuel back to the pump inlet under all load conditions.

Governor
Most gas turbine engines employ a governor to maintain the engine at constant speed. The high pressure fuel from the pump feeds into a rotating valve assembly (Often a ball valve) which under the action of centrifugal force opens at a predetermined speed, the valve then spills the high pressure fuel back to the pump inlet and hence reduces the flow to the burner(s). An excessive inlet fuel pressure will act against the valve and hold it closed until greater speed and hence centrifugal force is reached, causing the engine to over-speed. Some governors also use a second ball valve which opens at a slightly higher speed to act as a back up governor in case the first one fails.

Certain models of engine also employ various over-speed safety devices, such as centrifugal switches and trip mechanisms.

The Garrett GTP30 engine uses two bob weights to govern the speed. The two weights rotate and move outwards, at the same time elbows attached to them act upon a sleeve placed around a hollow shaft. HP fuel from a gear pump travels through the shaft and is prevented from escaping through openings placed in the shaft by the sleeve. The sleeve is spring loaded axially along the shaft against the bob weight force, eventually as the rotational speed rises the bob weight force overcomes the spring force and the sleeve moves, as the sleeve moves it uncovers the openings. The uncovered openings in the shaft allow fuel to escape reducing the delivery to the burner and the engine is governed. The spring force along the shaft can be varied by axially moving the end of the spring via a ball race, this allows for adjustment of the engine speed whilst the engine is operating.

A Palouste engine uses (P2) air from the compressor which balances the forces from various spring loaded fuel valves. The air feeding the valves is bled to atmosphere by a centrifugal valve which prevents the pressure from exceeding a certain limit and the engine exceeding a predetermined speed.

Acceleration limiter Valve
The purpose of this valve is to control the acceleration of the engine during starting and also a transition form idle to full speed. If a gas turbine accelerates too quickly two possible problems can exist. Firstly the operation of the compressor can suffer, if excessive fuel is supplied to the combustion chamber, the resulting combustion and expansion of the gases in it will present an excessive load on the compressor. The aerodynamics in the compressor are quite finally balanced, if the engine tries to accelerate too quickly these aerodynamics break down and the compressor suffers what is referred to as a "Surge" condition. Surging in small engines should not normally occur, unless the fuel control system has been tampered with and the settings disturbed. If it does occur, the engine may cough and splutter or make a low frequency humming noise. Compressor surge usually results in higher than normal exhaust temperatures, also smoke and flames may briefly appear at the air intakes.

A second problem associated with acceleration is exhaust temperature. As an engine accelerates there is always more fuel than air compared to a steady state running condition. The faster the engine accelerates the more fuel is burnt and the hotter the exhaust temperature will be. By limiting the maximum possible acceleration of the engine, the temperature can be kept to within a safe limit.

Small gas turbines, particularly gas turbine starters can start and accelerate very quickly. The fuel system is set up so that they achieve maximum acceleration before surging or over heating occurs. The Plessey "Solent" gas turbine starter can reach full speed in under 10 seconds.

The acceleration limiter valve works by balancing compressor air (P2) and fuel pressure against a spring. As the engine gathers speed, the P2 pressure gradually rises, this pressure compresses the spring which progressively opens a needle valve to allow more fuel to the burner system . A portion of fuel is always needed to reach the burner so that it is available for start up, this is because the P2 pressure at the relatively low starting rpm will be almost zero. The spring pressure on the acceleration control valve can normally be adjusted and is set up during manufacture and testing. The acceleration limiter valve should not normally need adjusting, care must be exercised if the settings are to adjusted, for stationary applications certain engines may benefit from a reduction in acceleration to reduce temperatures.

Shut Off Valve
To start and stop an engine a valve is normally placed in the fuel feed to the burners, this valve is sometimes referred to as an HP (High Pressure) cock. The valve may be opened as soon as the engine begins rotating or may be delayed until the engine has gathered speed and air is flowing through it. Solenoid valves are often used as HP cocks, normally 24V DC is required to hold them open while the engine operates, to shut down the engine the current in the valve solenoid is cut off. Solenoid valves are quite useful for stationary applications, complete failure of a 24V supply to the engine will result in it shutting down, a fail safe condition.

Certain Rover 1S models use a gate valve which is operated by a motor driven actuator, this is used to control the fuel supply. The actuator consists of an electric motor and gearbox, the motor runs briefly to open or shut the valve and is then cut off by limit switches mounted inside the actuator. Electrical power is required to open and shut the valve, this arrangement does not "Fail Safe" in the same way that solenoid valves do. Failure of the DC supply during operation of the engine will leave it running, instead a LP cock can be used to stop the engine by starving it of fuel. Due to the thirst of gas turbines, stopping the fuel supply to them will almost instantly stop them, unlike petrol engines which will continue to run off fuel stored in a carburettor.

When a Lucas GTS shuts down, a valve shuts off the fuel supply to the burners and opens the burner fuel line to the outside atmosphere. The instantaneous pressure in the combustion chamber back purges all the fuel from the burners and associated pipe work, the fuel is then dumped to atmosphere. This process prevents deterioration of static fuel in the hot burners which can lead to carbon build up. Another Lucas model uses a separate electric air pump to perform the same function.

A number of stationary engines including the Rover firepump use a simple hand operated shut off cock to supply fuel to the engine.

Burners
As mentioned in the previous chapter, fuel is admitted to the combustion chamber through one or more nozzles called burners.

Burners are required to spray fuel into the combustion chamber in a very fine atomised form. Fuel under pressure is forced through a very small orifice which causes it to break up into tiny droplets. Compressed air from the compressor or a separate pump is sometimes mixed with the fuel inside the burner, this will assist the process of atomisation. Certain designs of burner incorporate a valve which shuts off at low pressures and helps to maintain atomisation. The Rover gas turbine uses compressed air from a motor driven pump to assist atomisation which improves ignition and starting performance. The burner air supply pump is known as an emulsion pump as it creates a fuel/air emulsion.

A cone shape spray pattern from a burner is normally required, as the engine ages this may deteriorate due to carbon deposits and abrasion from the fuel. The fuel has to be adequately filtered to ensure that no foreign matter enters the burner and clogs it up. Burner units are often fitted with small filters to further prevent damage due to foreign matter contaminating the fuel supply

Temperature limiter
It is important during the starting and heavy load operation of a gas turbine that it does not exceed a safe exhaust temperature limit (And hence turbine inlet temperature). Many fuel systems employ a temperature regulating device to keep the exhaust temperature below a predetermined limit, this usually consists of a thermal sensing device which is placed in the exhaust jet pipe. As the exhaust temperature increases, the sensor operates a valve in the fuel system and reduces the fuel supply to the burner.

The Rover and Man-Turbo 6012 engines use a small mercury filled capsule which is mounted in the exhaust, the capsule is connected to a bellows inside the fuel control unit via a thin pipe. The bellows are spring loaded and mechanically connected to a valve, when the exhaust temperature rises above a certain value, the mercury expands and expands the bellows. As the bellows expand the valve spills fuel from the HP pump outlet to reduce the fuel delivery and the temperature.

Garrett engines use a pneumatic thermostat which is mounted in the exhaust. P2 air is supplied to the fuel control unit for acceleration limiting, this air also passes to a thermostatically operated valve. As the temperature rises the valve opens and starves the fuel control unit of P2 air causing it to reduce the fuel supply and thus bring the temperature down.

Idle Controls
Many small gas turbines do not have an idle speed, they simply start and accelerate to a governed maximum speed. Single shaft engines driving generators or hydraulic pumps are only loaded when full speed is reached, their is no need for an intermediate idle speed.

Certain engines are fitted with idle controls for various reasons. An idle speed is normally achieved by restricting the fuel flow to the burner. A restricting orifice is placed in the fuel supply to the burner, and a solenoid valve bypasses the orifice for normal full speed operation. Fuel flow can also be reduced by bleeding fuel from the burner back to the fuel pump inlet, this then lowers the engine speed.

To aid testing an engine the speed can often be varied and the engine "throttled" manually by using a needle valve to spill fuel from the burners. It should be noted that the valve will operate in the opposite sense to that what might be expected, as the valve is opened more fuel is bypassed from the engine and it consequently slows down.

A more sophisticated way of varying the engine speed and to establish an idle speed is to adjust the governor. The datum around which the governor works is dependant on centrifugal force balancing a spring force. On certain types of governor the spring force can be varied which will cause the engine speed to change. Once set the engine speed will remain constant and governed at that value.

Caution should be exercised if adjustments or modifications are to be made to a governor system. The malfunction of a governor could lead to over-speeding and consequently catastrophic failure.

Gas turbines as compared to piston engines generally have a small range over which their speed can be varied. A typical APU may run at 45,000 rpm and idle at 25,000 rpm, this is a speed range of less than 2:1. The fuel efficiency of a gas turbine is related to the amount of compression which takes place in the compressor. As the speed drops the pressure ratio falls off quickly and so does the fuel efficiency, there are few advantages in operating an engine at an idle speed.

Electronic Control Units
It is possible to control a gas turbine engine by the use of electronics. Most of the engines referred to in this document were built in the 1960s, during this period transistorised electronics was still a relatively new thing. Several gas turbine manufacturers during this period fitted an electronic control system to their engine designs.

The basic electronic control system consists of a fuel pump, a control unit, a fuel metering valve and an electronic engine speed sensor. The electronic speed sensor provides the control unit with a signal which is proportional to the engine speed. The control unit is connected to the metering valve and the fuel pump supplies fuel through the metering valve to the burners. The control unit varies the electric current in the metering valve so that the fuel is regulated and the engine is governed at a constant speed. During start up, the control unit effects the current flow so that the engine receives the correct amount of fuel for starting and acceleration.

Metering valves vary in design but generally consist of a an electromagnet which moves a needle type valve. The electromagnet may also be "biased" with a permanent magnet, a spring or both. Electric actuators are also used and consist of a larger electromagnet or a solenoid type device. Metering valves and actuators are often polarity sensitive, care should be exercised so that they are not connected up the wrong way around. Metering devices may also be electrically fragile and should be operated with care and for example, simply connected to a 24V supply.

An electronic control system can be designed to control all the stages of engine operation and can protect the engine against overheating, over-speeding and compensate against changes in ambient temperature, pressure and supply voltage.

The electronic control system is normally housed in an enclosure and mounted close to the engine. The electronic control unit may be powered from a 24V supply. The electronics can become very sophisticated, a control unit can be considered to be an analogue computer, many signals such as temperatures, engine speed and pressures are all processed simultaneously.

Microturbo Saphir, Solar T62T32 and Saurer GT15 gas turbines employ electronic control and governing systems. Care should be exercised if attempting to use this type of engine especially if they are quite old. Electronic circuits can deteriorate over long periods of time and the effects of poor storage conditions can damage electronic components.

The Microturbo Saphir engine employs a metering valve which is fitted in the return fuel line from the burner system. The starter motor on this engine turns continuously and provides a speed signal from a small electrical generator which is mounted on the motor shaft. The speed signal feeds a control box which in turn is connected to the metering valve. The fuel pump fitted to this engine is driven by a separate electric motor and not by the engine as in most other designs.

The similar engine the Microturbo TRS18 employs a special device known as a "Moog" electronic valve which acts as a precision interface between the electronic and fuel systems.

The Saurer GT15 gas turbine engine employs a metering valve which is mounted axially at the end of the main compressor shaft. Fuel is supplied to the combustion chamber along the shaft which is hollow, the electronically controlled metering valve supplies fuel into the shaft by moving a small tube in and out. The position of the tube is controlled by the current flowing in an electro-magnet. Interestingly this design works backwards, a decrease in electrical current corresponds to an increase in engine rpm.

During the operation of an electronically controlled engine, the signal supplied to the metering valve can be measured. The results can be compared to that which is specified in the manufacturers operating manual. For example, a current of 70mA might be required to hold a Saurer GT15 engine at idle speed and 40mA at operating speed.

The Solar T62T32 engine is equipped with a hybrid fuel control system. An engine driven pump supplies fuel via an actuator driven throttle metering valve and a pneumatically operated valve. The acceleration of the engine is controlled by the P2 (compressor delivery pressure) air operated valve and an electronic control box supplies current to the actuator to govern the engine speed. A magnetic reluctance speed probe is placed in the reduction gearbox and picks up a signal created by holes drilled into the rotating epicyclic gear carrier. AT 60,000 rpm a signal of 2 KHz is generated by the probe.

It is possible to construct electronic control systems which may partially or fully control the engine fuel requirements. The simplest system is to govern the engine at constant speed. An electronic tachometer circuit creates a voltage (Or current) signal which is proportional to rpm. This signal is compared to a fixed or variable speed reference voltage, the difference between the two signals forms an error signal which is used to drive a throttle actuator or metering valve holding the engine speed constant. This arrangement only takes account of engine speed and will not allow the engine to be started. During start up the error signal will be very large and over-fuelling will occur. Electronic controls become very complex as additional circuitry is required to allow for engine starting, acceleration and protection. Control systems also exhibit time constants and droop characteristics which require compensation circuitry to enable stable operation. With knowledge of electronic control principals a control system may be built up, testing this is tricky as the engine itself may have to be operated as a test bed unless it can be simulated in some way.

Open Loop Operation

It may become necessary to operate a gas turbine engine without a complete fuel control system. This may be the case with electronic systems which may be missing, faulty or not fully understood. Mechanical systems may also be found to malfunction or suffer from missing components as may be the case with Surplus engines. Open loop operation is an aid to "Reverse Engineering" the fuel system.

The amount of fuel delivered to the combustion system may be controlled manually by hand provided sufficient instrumentation is available so that engine parameters may be closely monitored. In the case of electronically controlled engines a variable electrical signal may be provided to a fuel metering valve (electrical/fuel interface), the current is supplied from a variable power supply and the current controlled by hand. During engine operation the current is carefully controlled so that the engine speed and temperature remain inside acceptable limits.

The current supplied to a fuel metering valve should be carefully controlled so that the valve is not damaged. Certain types of valve are fragile and excessive current will burn them out. Ideally a variable constant current supply should be used with a maximum setting which prevents damage to the valve coil.

To start the engine, the engine is first spooled up by means of the starter during which no fuel at all may be supplied to it. The fuel supply is then enabled by either opening an HP Cock type valve or by the combination of opening a HP cock and supplying current to a fuel metering valve. The current to the valve is increased (May be decreased depending on the action of the valve) so that the engine may light up and then the current carefully increased further to accelerate the engine to the required speed. During this process recorded measurements may be made of the engine speed and the current supplied to the valve. A basic current/rpm characteristic is built up which is useful for further tests.

This type of "Open Loop" control system is very useful when testing engines and attempting to re-develop replacement fuel systems. Small gas turbine engines may rapidly accelerate and so it is recommended that an over-speed cut out system is employed. An electronic tachometer circuit is used to detect an over-speed condition and cut the current supplied to any fuel control valves and also may close off the engine HP Cock. Fail safe solenoid valves are useful here and should be used with a resetable relay circuit. A useful technique is to test and validate the over-speed cut out before operating the engine. A variable oscillator is used to supply the tachometer circuitry with a signal to simulate the running engine. The calibration of the tachometer and cut out system may be checked safely. The cut-out speed is initially set to an estimated self sustaining speed and as experience and confidence grows the cut-out speed may be increased.

Over-temperature circuits are useful when testing small gas turbines. An electronic circuit may be constructed which monitors the exhaust temperature and at a predetermined point operates a relay circuit to cut off the fuel in a similar way to the over-speed protection.

A mechanical open loop control may be used to supply fuel to a gas turbine. An electric high pressure fuel pump (As opposed to a low pressure booster pump) supplies fuel to the burner (S) and a hand operated needle valve is used to bleed fuel from the burner supply reducing the delivery to the engine. Solenoid valves and the use of an over-speed cut-out is recommended to protect the operator.

In all cases of open loop control, wet cycling (Operation with no ignition) and inspection of the burners may be necessary to obtain a "feel" for the fuel system characteristics. Various metering valves exhibit different and even reverse current/voltage/fuel characteristics. Over-fuelling or operation with insufficient atomisation in the combustion chamber may damage the engine and create a fire hazard.

During open loop control gas turbine engines generally exhibit a tendency for the rpm to wander, particularly during a warm up phase after starting. As the rpm varies the fuel pump delivery may also vary and so a mildly unstable condition exists. The oil temperature may also effect rpm and as the temperature rise the drag on bearings may reduce. The operator may find himself having to continually watch the engine rpm and adjust the fuel flow accordingly.