Aircraft Electrical System 3

Md. Nur Alam Course/Batch: FENDA 09Page 10
Student ID: 33
Task: 01
Operations of air start system of turbojet engine:
An air-starter on a turbine engine would typically consist of a radial inward flow turbine, or axial flow turbine, which is connected to the High Pressure compressor spool through the accessory gearbox, plus the associated piping and valves. Compressed air is provided to the system by bleed air from the aircraft's auxiliary power unit or from an air compressor mounted on ground support equipment.
Detail description of operation using diagrams
The air is admitted into the cylinder when the piston is just past TDC and continued until just before the exhaust valve opens. There is always more than one air start valve open: - a situation known as overlap. This ensures that the engine will start in any position. The opening of the main air start valves is controlled by a set of pilot valves located in the air start distributor, which in turn are timed to operate by a drive linked to the main camshaft. In the example shown, a small camshaft is used to control the opening and closing of the air start pilot valves.
 
The drawing shows the principle of operation of an air start system. Large air receivers are used to store the compressed air. The diagram shows the isolating valve open so air is being allowed as far as the automatic valve and the air start control valve.
When the engine is required to start, a low pressure air signal is sent to the air start control valve (which can also be hand operated in an emergency). The air pushes a piston down which opens the valve and allows high pressure air to flow to the pilot valve and the automatic valve operating pistons. The pilot valve is forced down onto the cam profile and the automatic valve opens and high pressure air is led to the main air start valves and the pilot valve. When the pilot valve cam follower is on the lowest point of the cam, air flows to the operating piston of the main air start valve for that particular cylinder, opening the valve and allowing high pressure air to flow into the cylinder.
When the pilot valve is lifted by the cam, the pilot valve vents and the main air start valve closes. When the start air signal is taken off the air start control valve, the system vents and the automatic valve shuts.
The image (animated) below shows the sequence of operations.

 An interlock blocking valve will operate, for instance if the turning gear is left in, and this will stop high pressure air from reaching the air start control valve and thus either the automatic valve or the pilot valve.
A slow turning valve is fitted. This will open instead of the main automatic valve if the engine has been stopped for more than 30 minutes during manoeuvering. It will only supply enough air to turn the engine over very slowly; this is a precaution in case a cylinder has had oil or water leak into it which would cause damage to the engine when starting. If the engine completes a full revolution on the slow turn, then the main automatic valve opens and the engine will start. (note: The operating system for the slow turning has been omitted for simplicity). [1]
http://www.marinediesels.info/2_stroke_engine_parts/Other_info/air_start.htm



Task: 02
Ignition systems of piston engine
Basically Convectional Ignition systems are of 2 types:
(a) Battery or Coil Ignition System, and
(b) Magneto Ignition System.
Both these conventional, ignition systems work on mutual electromagnetic induction principle.
Battery ignition system was generally used in 4-wheelers, but now-a-days it is more commonly used in wheelers also (i.e. Button start, 2-wheelers like Pulsar, Kinetic Honda; Honda-Activa, Scooty, Fiero, etc.). In this case 6 V or 12 V batteries will supply necessary current in the primary winding. Magneto ignition system is mainly used in 2-wheelers, kick start engines.(Example, Bajaj Scooters, Boxer, Victor, Splendor, Passion, etc.).In this case magneto will produce and supply current to the primary winding. So in magneto ignition system magneto replaces the battery. [2]
http://www.pilotfriend.com/training/flight_training/fxd_wing/ignition_system.htm
Magneto ignition system
A magneto uses a permanent magnet to generate an electrical current completely independent of the aircraft´s electrical system. The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder. The system begins to fire when you engage the starter and the crankshaft begins to turn. It continues to operate whenever the crankshaft is rotating.
Most standard certificated airplanes incorporate a dual ignition system with two individual magnetos, separate sets of wires, and spark plugs to increase reliability of the ignition system. Each magneto operates independently to fire one of the two spark plugs in each cylinder. The firing of two spark plugs improves combustion of the fuel/air mixture and results in a slightly higher power output. If one of the magnetos fails, the other is unaffected. The engine will continue to operate normally, although you can expect a slight decrease in engine power. The same is true if one of the two spark plugs in a cylinder fails.
The operation of the magneto is controlled in the cockpit by the ignition switch. The switch has five positions:
OFF
R—Right
L—Left
BOTH
START
With RIGHT or LEFT selected, only the associated magneto is activated. The system operates on both magnetos with BOTH selected.
You can identify a malfunctioning ignition system during the pretakeoff check by observing the decrease in r.p.m. that occurs when you first move the ignition switch from BOTH to RIGHT, and then from BOTH to LEFT. A small decrease in engine r.p.m. is normal during this check. The permissible decrease is listed in the AFM or POH. If the engine stops running when you switch to one magneto or if the r.p.m. drop exceeds the allowable limit, do not fly the airplane until the problem is corrected. The cause could be fouled plugs, broken or shorted wires between the magneto and the plugs, or improperly timed firing of the plugs. It should be noted that "no drop" in r.p.m. is not normal, and in that instance, the airplane should not be flown.
Following engine shutdown, turn the ignition switch to the OFF position. Even with the battery and master switches OFF, the engine can fire and turn over if you leave the ignition switch ON and the propeller is moved because the magneto requires no outside source of electrical power. The potential for serious injury in this situation is obvious.
Loose or broken wires in the ignition system also can cause problems. For example, if the ignition switch is OFF, the magneto may continue to fire if the ignition switch ground wire is disconnected. If this occurs, the only way to stop the engine is to move the mixture lever to the idle cutoff position, then have the system checked by a qualified aviation maintenance technician. [3]

Figure: magneto ignition system components.
http://www.free-online-private-pilot-ground-school.com/aircraft-powerplant.html

Battery ignition system:
It mainly consists of a 6 or 12 volt battery, ammeter, ignition switch, auto-transformer (step up transformer), contact breaker, capacitor, distributor rotor, distributor contact points, spark plugs, etc.Note that the Figure 4.1 shows the ignition system for 4-cylinder petrol engine, here there are 4-spark plugs and contact breaker cam has 4-corners. (If it is for 6-cylinder engine it will have 6-spark plugs and contact breaker cam will be a perfect hexagon).

The ignition system is divided into 2-circuits :
(i) Primary Circuit : It consists of 6 or 12 V battery, ammeter, ignition switch, primary winding it has 200-300 turns of 20 SWG (Sharps Wire Gauge) gauge wire, contact breaker, capacitor.
(ii) Secondary Circuit : It consists of secondary winding. Secondary Ignition Systems winding consists of about 21000 turns of 40 (S WG) gauge wire. Bottom end of which is connected to bottom end of primary and top end of secondary winding is connected to centre of distributor rotor.
Distributor rotors rotate and make contacts with contact points and are connected to spark plugs which are fitted in cylinder heads (engine earth).
(iii) Working : When the ignition switch is closed and engine in cranked, as soon as the contact breaker closes, a low voltage current will flow through the primary winding. It is also to be noted that the contact
beaker cam opens and closes the circuit 4-times (for 4 cylinders) in one revolution. When the contact breaker opens the contact, the magnetic field begins to collapse. Because of this collapsing magnetic
field, current will be induced in the secondary winding. And because of more turns (@ 21000 turns) of secondary, voltage goes unto 28000-30000 volts.
This high voltage current is brought to centre of the distributor rotor. Distributor rotor rotates and supplies this high voltage current to proper stark plug depending upon the engine firing order. When the high voltage current jumps the spark plug gap, it produces the spark and the charge is ignited-combustion starts-products of combustion expand and produce power. [4]
http://mecrockers.blogspot.com/2013/07/ignition-systems.html
Task: 02 (b)
Turbojet Engine starting control system:
The fuel system as explained above is one of the two 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 cartridge turbine starter or "cart starter". This is an impulse turbine impacted by burning gases from a cartridge, usually created by igniting a solid propellant similar to gunpowder. It is geared to rotate the engine and also connected to an automatic disconnect system, or overrunning clutch. The cartridge is set alight electrically and used to turn the starter's 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 (or sometimes Hydrazine) 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 anauxiliary power unit or APU. It is normally a small gas turbine. Thus, one could say that using such an APU is using a small gas turbine to start a larger one. Low pressure (40-70 psi), high volume 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. The starter is usually an "air turbine" type, similar to the cartridge starter, but uses APU's bleed air instead of the burning gases of the propellant cartridge. Most cart starters can also use APU air to turn them. 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 and ignition systems are shut off.
The APUs on aircraft such as the Boeing 737 and Airbus A320 can be seen at the extreme rear of the aircraft. This is the typical location for an APU on most commercial airliners although some may be within the wing root (Boeing 727) or the aft fuselage (DC-9/MD80) as examples and some military transports carry their APU's in one of the main landing gear pods (C-141).
Some APUs are mounted on wheeled carts, so they can be towed and used on different aircraft. They are connected by a hose to the aircraft ducting, which includes a check valve to allow the APU air to flow into the aircraft, while not allowing the main engine's bleed air to exit through the duct.
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.
Hydraulic pumps can also be used to start some engines through gears. The pumps are electrically controlled on the ground.
A variation of this is the APU installed in a Boeing F/A-18 Hornet; it is started by a hydraulic motor, which itself receives energy stored in an accumulator. This accumulator is recharged after the right engine is started and develops hydraulic pressure, or by a hand pump in the right hand main landing gear well. [5]

http://en.wikipedia.org/wiki/Components_of_jet_engines#Engine_starting_system
http://www.freepatentsonline.com/6922625.html
Task: 03
Thermal fire detection system
A thermal switch fire detection system is a circuit in which one or more thermal switches are connected in an electrical circuit which also has a warning light and an aural alarm to warn the flight crew that an overheat condition exists in a particular area. If more than one thermal switch is in the circuit, the switches are connected in parallel.
This makes sure that if any one switch closes a warning is given. In some circuits the detectors are connected between two wiring loops, either of which may be supplied through a magnetic circuit breaker. A short circuit in the energized loop results in operation of the magnetic circuit breaker and the supply is then routed to the second loop. This prevents a false indication of fire. The system can withstand one fault, either an electrical open circuit or a short to ground without sounding a false alarm. A double fault must exist before a false fire warning can occur.

Switches operating at different temperature settings may look identical but, when doing maintenance or replacing the switches its vital to identify by part number in the Illustrated Parts Catalogue (IPC). The unit type is more suitable for localized overheat detection and the 'continuous loop' type better for fire detection. [6]


Thermal fire detectors use a set of temperature-sensitive resistors called thermostats that decrease in resistance as the temperature rises. One thermistor is sealed and protected from the surrounding temperature while the other is exposed. A sharp increase in temperature reduces the resistance in the exposed thermistor, which allows a large current to activate the detector's alarm. 
Optical fire detection system

In an optical smoke detector, a light source and light sensor are arranged so that the rays from the light source do not hit the light sensor. When smoke particles enter the light path, some of the light is scattered and redirected onto the sensor, causing the detector to activate an alarm. These detectors react quickly to visible smoke particles from smoldering fires, but are less sensitive to the smaller particles associated with flaming or very hot fires. [7]
Differences between thermal and optical fire detection systems
Thermal fire detection
Optical fire detection
These are used primarily by large commercial or industrial facilities.
These are used primarily by medium large commercial or industrial facilities,
Thermal fire alarms go off when an internal thermostat reaches a certain level. 
These use beam of light.
This is arguably the least safe type of fire alarm
This is very popular and reliable
They only detect a fire once it's gotten pretty strong.
They can detect fire earlier
Thermal fire alarms are most useful in areas where low levels of smoke,
It can work in relatively higher level of smoky area.
Dust or heat (factories, warehouses, etc) is normally present and can trigger a false alarm. 
It is not that vulnerable to dust of heat.
http://aviamech.blogspot.com/2012/02/fire-detector-typs.html
http://wiki.answers.com/Q/How_does_a_fire_alarm_work
Bibliography

http://www.marinediesels.info/2_stroke_engine_parts/Other_info/air_start.htm
http://www.pilotfriend.com/training/flight_training/fxd_wing/ignition_system.htm
http://www.free-online-private-pilot-ground-school.com/aircraft-powerplant.html
http://mecrockers.blogspot.com/2013/07/ignition-systems.html
http://en.wikipedia.org/wiki/Components_of_jet_engines#Engine_starting_system
http://www.freepatentsonline.com/6922625.html
http://aviamech.blogspot.com/2012/02/fire-detector-typs.html
http://wiki.answers.com/Q/How_does_a_fire_alarm_work






Unit title/no: Aircraft Electrical System [84]
Assignment title/no: Function and operation of aircraft propulsion electrical system [03]

[Type the document title]
[Pick the date]