ATA-74 AIRCRAFT IGNITION SYSTEM It provides high intensity electric spark in to either cylinder or combustion chamber to fire (burn) the furl air mixture. Ignition system must deliver a high tension (20,000V) spark to each cylinder of reciprocating engine in a firing order at a predetermined number of degrees (5° to 17°,correct 0
e-gap angle is 11 ) ahead of TDC (Top dead center) position of the piston. Spark must jump the spark plug gap under al] operating conditions. Various Ignition Systems:
Battery ignition and magneto ignition. Magneto ignition further classified into high tension (above 600 Volts) and low tension (up to 600V). The other classification is single ignition system. Duel ignition system. Single type ignition magneto has one housing, one rotation magnet, one cam single coil (primary and secondary). Single breaker point one distributor, Single ignition leads and one spark plug in each cylinder. Dual type has one housing, one rotating magnet and single cam the other remaining parts are two (concession made FAA regulations part 33).
Used in automobile engine starting s tarting Cam and distributor are driven by engine (starter ). When ignition switch is closed current flows through primary coil through closed contact of breaker points to earth. Engine
operates cam, which opens the breaker contact current stops flowing, flux starts collapsing passes to spark plug through distributor contacts so high tension spark is provided. Ignition coil primary winding comprises of few turns of thick copper wire, secondary winding of very thin copper wire of many turns, The purpose of capacitor connected a cross breaker points are 1. It prevents arcing between the breaker points during opening 2. It speeds up the collapse of magnetic filed in primary winding. S ystem: High Tension Magneto Ignition System: Principle:
It works on the principle of permanent magnet generator and step up transformer in order to generate high tension voltage. types of magnetos Rotating Armature. When the windings rotates within the permanent magnets magnetic field
an AC EMF is induced Rotating Field: The permanent magnets (field) rotates r otates between fixed transformer windings. Polar inductor: Soft-iron inductor bars rotate between fixed permanent magnet and
transformer windings. HT magneto:
Construction: Magnetic circuit, primary coil assembly, secondary coil assembly.
Magnetic Circuit:
It consists of permanent multi pole rotating magnet soft iron core and pole shoe. Permanent magnet mode of material having high permeability like alnico or alcomax. It is in cylindrical shape Core and pole shoe made of soft iron core where both primary and secondary coils are wound. Permanent magnet is geared to a/c engine rotates in the gap between the pole shoe. When magnet rotates in pole shoe the magnetic flux of periodical reversal in nature is produced, passes through core thus EMF is induced. Primary Coil Assembly:
It consists of primary winding, capacitor and breaker assembly. Primary winding winding is made of few turns (180-200 turns) of heavy / copper wire enamel insulated one end ground to core other end to breaker points. Capacitor is connected across the breaker points. The purpose is (i)
To prevent arcing between the points when opened,
(ii)
Hastens the collapse of magnetic field about primary coil.
Breaker assembly consists of pivot less type breaker points. One of the points is stationary and other is moved by cam follower made of Micarta. The breaker actuating cam is directly driven by magneto rotor shaft or through a g ear train from rotor shaft. Secondary Coil Assembly:
It consists of secondar se condary y winding, distributor, ignition igniti on leads and spark plug. Secondary winding contains thousands thousands of turns of fine copper wire wound on the top of primary coil. Inside end of secondary connected to the outside of primary coil. Outside end of secondary brought out of coil assembly of high voltage contact spring loaded carbon brush in the center of distributor rotor presses against. This high voltage contact Secondary winding produces high voltage (18000-20000V) which is required for spark. Distributor:
It receives high tension current from secondary coil and distribute it in the correct sequence to appropriate cylinders. -
It consists of two parts. Revolving part is called called distributor rotor, rotor may be in in the shape
of disc, drum or finger made of insulating material with an embedded conductor. -
Static part of distributor is called called distributor distributor of insulating insulating material contained terminals and
terminal receptacles s in to which wiring that connects the distributor and spark plug is attached. -
The distributor numbers represents magneto sparking sparking order.
-
When the high voltage in the distributor distributor flashes across across the the insulator insulator to ground (in stead of
arcing across the air gaps of distributor ) or this high voltage misdirected to spark plug Other than one that should be fired called flash over. -
Distributor is a part of magneto, distributor rotor is driven by gear from main
magneto shaft. Some cases distributor rotors are driven by engine gears train separately. Magneto and Distributor Speed:
Magneto Speed = Number of Cylinders 2 X Magneto Spark per revolution Distributor rotor is driven at half engine speed irre spective of magneto speed. Ignition Harness:
(Ignition leads) made of stranded copper or stainless steel wire enclosed in a rubber or silicone insulation. This insulation is covered with braided metal shield which is protected by tough plastic covering. Slick electro inc. produced ignition harness stranded wires are replaced by continuous spiral of wire impregnated with high voltage silicone rubber insulation. Two sizes of ignition harness is used 5mm a nd 7mm. 5mm is used more widely. Purpose of Ignition Harness:
1. It supports the wires and protects them from damage by engine heat, vibration or weather. 2. Minimizes the radio radio interference, which
is caused due to stray magnetic field. These These
fields are grounded by shielding. 3. Spark plug conducts the short impulse of high voltage current through the wall of combustion chamber. Two sizes of spark plug 5-8 - 24 shielded park plug or 3-4-20 all weather spark plug. All weather is more popular. Harness
testing:
Harness tester is used to test the leakage current the input is 110v 60Hz output is 015000V DC. Two ammeters are used for leakage current 0-100 µamp and 0-1000 µamp. Check is carried out for 10 seconds excessive leakage reading more than 50 µamps are break down is indicated by flashing break down indicator. Ignition leak to be removed put in contact with cylinder for good ground. Operation of Magneto (HT):
When the magnetically opposite poles perfectly aligned with the pole shoes magnetic lines of force (flux) linking with core is maximum. This position is called "Full register".
When no flux (magnetic lines) flows through the coil core such position is called Neutral position". When the pole is in between pole shoes (45°). Permanent magnet generator moved 90° again maximum flux links with core by direction changes as pole under pole shoe changed.
Primary breaker points closes at approximately at full register position. Completes the primary circuit induces primary current. The field produced by primary current is such that it opposes the cause produce it (flux change reversal in core due to rotation of permanent magnet (lenz law)). The primary current holds the resultant flux in core at a high value in one direction until rotating magnet in neutral position 7°-11° beyond neutral. As this point the breaker point opens high rate flux change occurs opening of contacts stops the primary current makes coil core to change the direction of field (reversal of field). This maximum flux change induces high voltage in secondary, which will be carried to distributor and to spark plug. So the spark of high voltage intensity jumps through spark plug thus igniting fuel air in cylinder head. The breaker points will open beyond neutral position, where greatest change in flux take place. the angle where it occurs (spark occurs) called "E-gap angle. E gap means "Efficiency gap". Where actual spark takes place. Ignition Switches:
Ignition systems are controlled by an ON-OFF switch in cockpit. When the ignition switch in OFF position, it completes the circuit by closing its contacts between magneto primary winding and earth (ground) thus short circuiting the contact breaker. In this condition on sudden collapse of primary flux even the magneto rotates (Actually in OFF position switch contacts are made).
When ignition switch in ON position (switch contacts are open actually) breaks the circuit between primary winding and ground thus normal operation of magneto takes place. In dual ignition systems, each magneto is controlled by a separate toggle switches or four position rotary switch controlling both magnetos. The four positions are OFF, LEFT, RIGHT, BOTH. The left and right position allow one system to be turned off at a time for carrying out magneto drop check' (when (when s/w is OFF RPM drop of engine should not be more than prescribed valve ) during ground g round run. Disadvantages of High Tension Ignition (HT Ign): 1. Flash Over: Jumping
of high voltage inside the distributor at hi gh altitudes due to less density
of air. 2. Moisture: Affects the conductivity of the high voltage 3.
High
Voltage Corona Effect: A condition of stress across any insulator exposed to high
voltage. This high voltage imposed between conductor and insulator jumps as an arc of light blue color due setting of electrical e lectrical stress on insulator. 4. Where number of cylinders are more the size s ize will become bigger. Low tension magneto ignition system LT
Magneto
Magneto assembly consists of rotating magnet and winding of fewer turns of copper wire wound on coil core. The output of inductor coil is low voltage AC. Distributor consists of cam, contact breaker assembly, capacitor and distributor, cam and distributor driven at half engine speed. Distributor rotor makes a rubbing contact with distributor segment. Cam operates contact breaker thus contact breaker interrupts. The magneto coil (induction coil) predetermined intervals of time. Transformer consists of primary and secondary windings. Distributor output is connected to primary winding. Distributor output is connected to primary winding. Transformer mounted spark plug by short high tension (HT) cables. Operation:
When magnet rotates flux is induced in core. This flux induces alternating low voltage by the rotation of cam operated contact c ontact breaker.
Low voltage impulses from magneto induction coil are supplied to distributor via a set of brushes and segment tracks. The distributor low tension output is supplied to transformer corresponding in number to the number of spark plugs. The secondar se condary y of transformer tra nsformer converts low tension (LT) to high tension (HT). Thus high voltage impulses provided to spark plug. LT magnetos are switched ON and OFF similar to HT magneto. Auxiliary ignition equipment:
Initial state of starting/cranking magneto rotates at low speed (80-100 RPM) produces low voltage which in sufficient to jump across spark plug. Auxiliary ignition equipment is used to provide high intensity spark during initial stages of starting where magneto speed is very low. Booster coil, induction vibrator are operated by battery. Battery not required for Impulse coupling Auxiliary ignition equipment which is operated by battery connected to right magneto provides retard spark to avoid kicking back when engine is started (spark later than the main magneto). Booster Coil:
Consists of primary secondary coils wound on soft iron core, set of contactor points and capacitor. Stationary contact is fitted with +ve terminal of battery. Voltage is applied when magneto s/w is placed in "Start" position, or automatically applied when the starter is engaged. This voltage flows through the primary winding via closed contact points to ground. As the current flows through primary coil sets up flux, magnetizes coil core, coil core attracts
movable contact point against the spring. Thus contact is broken, primary current stops; flux collapses spring brings back movable contact closed again. Primary current flows flux sets up. The results of expanding and collapsing magnetic field flux links with secondary which induces high voltage which is connected c onnected to main magneto distributor. Distributor
has
got two electrodes. The main
electrode
(main
linger) carries magneto output.
The
auxiliary
electrode (finger) carries booster coil voltage. The auxiliary
electrode
is
located such a way that it always trails the main electrode thus provides retarded spark during starting. The capacitor prevents arcing between contacts points during collapsing of magnetic field. Presently it is not used. Induction Vibrator or Starting Vibrator : Purpose:
1. It changes DC of the battery in to pulsating direct current and delivers it to the primary coil of magneto. 2. It serves as a relay for disconnecting auxiliary circuit when not in use Operation:
Battery +ve supply is connected to
vibrator
meshing
through
solenoid,
starter
circuit
is
energized. Current flows to relay coil to ground. Same time vibrator coil circuit is completed through closed contacts of vibrator coil and closed contacts of relay coil supply is available to magneto.
The current flow through magneto depends on primary contactor position if contactor points open current flows through primary coil. If contactor point closes current flows through ground. Vibrator coil cct closes core operates contact. So vibrating coil contact is opened supply cuts off to vibrator coil, spring closes the contact again current flows to vibrating coil. This operation takes place very rapidly. When this rapid succession of battery voltage applied to primary coil of magneto (primary contact are open). Magnetic field building and collapsing takes place, so EMF is induced in secondary. Secondary produces shower of sparks" across selected s park plug. Impulse Coupling:
Engines having a small number of cylinders (Four or Six), horizontally opposed air craft engine uses this methods. Impulse coupling is a small
spring
coupling magneto
loaded
between shaft
and
the the
engine drive gear. When the engine is not running driven by starter at slow speed
(Low
weights
in
RPM) the
fly
impulse
coupling contact a stop pin in the magneto housing. The pin holds the magnet shaft still as the engine continues to rotate. By that time engine has rotated far enough for the piston to reach the TDC (Top dead center). A projection on the body of impulse coupling wedges the fly weight off of the stop pin and the spring spins the magneto shaft fast enough to produce hot and late spark. This action takes place for every spark until engine starts running when RPM reaches high the impulse coupling acts as straight coupling for magneto. High Tension
Retard Breaker Magneto:
This system uses an additional breaker to obtain retarded spark for starting. It eliminates need for impulse coupling in light aircraft. Magneto s/w - Both: Starter s/w - ON. Starter solenoid L3 and starting vibrator coil L1 are energized. Closing relay contacts R1, R2 & R4. R3 connects right magneto to ground keeping it in operative during starting.
Current flows from +ve of BAT - s/w S1 ON - closed contact of R1 -> Relay Contact V1-coil L2 of starting vibrator-closed contact of R2 -> Left. Magneto to C2 -> Left magneto main breaker - g round. Energies L2 open V1 interrupting the current flow L 2 magnetic field around collapses. Vibrator points V1 closes current flow through th rough L2 vibrator. This operation continues. The interrupted battery current flows through retard main breaker of left magneto
BAT +ve - closed contact R4 - starter motor gets supply starts rotating. When engine reaches normal advance firing main breaker opens still the interrupted supply flow to ground through retard breaker points. Which, do not open till the retard firing position of the engine is reached. Which is 20° more travel retard points open the magneto primary coil not shorted and current produces in T1. Each time V1 open current through
V1
interrupted collapsing the field about T1 cuts
through secondary secondary and induces a high voltage surge used to fire the spark plug. V1 Opens and closes very rapidly a shower of sparks furnished to cylinder when both main and retard points are open. When engine beings to accelerate the spring loaded starter s/w released. L1L3 de energized, vibrator and retard breaker points in operative. Opens contact R3 making right magneto connected. Both magnetos fires normal running piston position.
Spark plug:
The function of spark plug in ignition system is to conduct a short impulse of high voltage current through the wall of combustion chamber. Inside the cylinder. It provides an air gap across which this impulse can produce an electric spark to ignite the fuel / air mixture. The three main components of spark plug are electrode, Insulator and out shell.
Electrode is made up of Nickel-clad copper electrode.
Outer shell shell threaded threaded to fit in cylinder made of finely mechanized steel. Outer shell plated to prevent prevent corrosion corrosion form engine gasses.
Copper gasket prevents cylinder gas pressure from escaping around the plug.
Insulator provides electrical insulation which is made of ceramic. Protective core around the electrode. It also transfers heat from ceramic tip or nose to the cylinder.
Reach:
The length of the threads on the outer shell classifies the spark plug. Short reach 1/2 inch thread. Long reach 13/16 inch. Heat
Range:
The heat range of spark plug refers to the ability of the insulator and center electrode to conductt heat away from its tip. conduc Hot spark spark plug has long insulator insulator path/nose: path/nose: Long heat path and tip operates at high temperatures used in engines whose temperatures are relatively low.
Clod spark plug has short path for heat used hot running high compression engines. It transfers heat rapidly to cylinder head. Capacitance effect of shield of ignition harness prolong the duration of spark. A 1500 resistor is installed in spark plug insulator between spark plug lead and center electrode reduces the duration of spark without affecting th e initial spark production.
Magneto Timing:
Ignition timing must be adjusted such a way that the following four conditions occur at same instant. 1.
The position in No. 1 cylinder must be in a position, a prescribed degrees before the top dead center (TDC) on compression stroke.
2.
The rotating magnet of magneto must be in "E" gap position.
3.
The breaker points must be just opening on the No. 1 cab lobe.
4.
The distributor finger must be aligned with the ele ctrode serving No. 1 cylinder.
Internal Timing of Magneto:
Adjustment of breaker points are opened (stoppage of primary current) at E' gap position where coil produces the greatest flux change. Attach timing scale to magneto housing, clamp a pointer over the commend of rotating magnet shaft. Set the pointer to "zero" mark on the scale for neutral position. Magnetos produced by slick air craft products have timing hole.
'Step' is cut on the end of breaker cam for checking internal timing of magneto for E' gap position. Pointed chamfered tooth, align with breaker p oints when breaker points about to open. o pen. 'E' gap
is correct when a timing pin in place and red marks visible through a vent hole in
the side of magneto is aligned. Magneto timing light across the breaker points goes off until timing light shows that breaker points are just opened. This in an important for internal timing ( E - gap setting). The distributor gear must meshed with drive gear by aligning the timing mark on distributor gear. External Timing Magneto to Engine Timing T iming
It is based on having the magneto in firing position for cylinder No.1 and installing it on engine which has been rotated until the cylinder No.1 is in the proper position for the ignition event. Timing devices like time - rate indicator or timing protractor or timing marks on engine case, accurately position the piston the correct number of degrees before TDC as specified by manufacturer.
Maintenance and Servicing of Magnetos
Breathing is a situation which occurs during the read adjustment of the system from low to high atmospheric pressure can result drawing in moisture laden air. Malfunctioning of carburetor, poor fuel distribution, too much valve over lap, leaking primer system or poor idle speed and mixture settings will show symptoms of faulty ignition for this sparks plugs are blamed. A thorough understanding of the various engine systems along with meticulous inspection and good maintenance methods can reduce such errors. The engine analyzer is an adoption of the laboratory oscilloscope. It is portable or permanently installed instrument. Whose function is to detect, located and identify engine operating abnormalities such are caused by faulty ignition. High energy ignition system:
High energy ignition units. Operation turbine engine ignition system, ignitor plugs. Two separate systems are required to start gas turbine engines. y
One system must be able to rotate the compressor and turbine (Starting system) at a speed where adequate amount of air (compressed) passes combustion system to mix with fuel from fuel spray nozzles.
y
Second system must be able to ignite the fuel air mixture in combustion system (Ignition system).
High energy discharge at ignition plug is needed to ignite fuel air mixture because of high altitude where low pressure exists or flame out of engi ne. High energy discharge is accomplished by means of storage capacitor termed as high energy capacitor discharge system. Energy is measure in joules : 1 joule 1 watt 1 sec. 1 Joule =
If the time is reduced, the energy in watts in very high During starting and relight 12 joules are required. Some engines are provided by low power discharge 4 joules (3 to 6 joules) continuously by one or both igniter plugs (continuous low power ignition system). Some other system relighting will be ON (By passing staring circuit) by pressure sensitive device which will be operated when internal pressure drops below pre determined value.
The two systems are low tension DC input. High tension Ac input. Some ignition system are rated up to 20 joules with 2000 Amps output. The three main components of HEIU are Ignition exciter, Ignition plug, Ignition lead. DC operated HEIU: Input 24-28 V DC
Induction Coil:
It consists of primary winding and secondary winding, both wound on soft iron core. Primary is connected to ground through trembler mechanism. Primary induces the pulsating flux with trembler mechanism. So that in secondary alternating voltage will be induced. Trembler Mechanism:
It consists of breaker point, capacitor connected across breaker points. Breaking and making of breaker points are achieved by a cam or with permanent magnet, which can be pulled when current flows through primary so breaker is open or transistor type. Trembler mechanism functions in conjunction with primary winding of induction coil. Breaker points operate for 200 times per second. So that battery DC voltage converted to pulsating DC in primary. So the EMF is induced in secondary. The purpose of capacitor is to reduce arcing across contacts. Rectifier:
Selenium rectifiers to allow the current to flow in one direction. Rectifier does not allow current to flow from - ve of secondary up to capacitor top side of secondary. It allows current vice versa.
Reservoir Capacitor:
The successive pulses of voltage produced in secondary charges reservoir capacitor. Through rectifier. When the voltage of reservoir capacitor reaches the predetermined value 2000volts is sufficient to breakdown the discharge gap by ionization so that capacitor discharges through igniter plug. Sealed discharge gap :
This gap is formed by two tungsten electrodes with in a chamber exhausted of air filled with inert gas and sealed to prevent oxidation which would otherwise will occur with large current. It takes around 1 second to charge the capacitor completely. The gap is preset to break down at 2000V. Choke:
Discharge is conducted through choke. It extends the duration of discharge through a high tension ignition lead to igniter plug where energy is released. The discharge is in the order of 12 joules with in 50 micro seconds of very high energy capable of vaporizing globules of fuel and overcoming carbonaceous deposits. Discharge Resistors:
There are fitted to ensure that any residual stored energy in reservoir capacitor is dissipated within ONE minute after unit being switched off. It also protect the ignition unit from overheating when ignition unit turn ON with igniter plug isolated or no igniter plug is connected. Safety Resistors:
These are fitted to enable the unit to operate safely even when high tension lead is disconnected disconnected or isolated by dissipating the spark t hrough it. Operation:
Primary of induction coil operated by trembler mechanism induces high voltage in secondary. This high voltage charges the reservoir capacitor through high voltage rectifier. When the
voltage in reservoir capacitor is equal to
the breakdown value of sealed
discharge gap the energy is discharged across the face of the igniter plug. AC operated HEIU:
AC Supply 115V 400Hz single phase is applied to the step up transformer Tl through noise filter.
The purpose of noise filter is to reduce radio interference. The high voltage induced in secondary of T1 (2000V) rectified by selenium diodes land 2. The current passing through diodes are being limited by limiting resistors RI and R2. The rectified output charges the storage capacitor C2 until the stored voltage reaches the ionization potential of the discharge gap. When this happens the discharge current flows through high tension transformer (Trigger transformer) primary coil T2. Trigger capacitor 4. This action induces high voltage around 2000V in the secondary of trigger transformer T2, which is sufficient to ionize. The igniter plug air gas and complete the path to storage capacitor.
The
trigger spark which occurs by the action of trigger transformer transformer T1 and and capacitor capacitor
creates the low resistance path and allows both trigger capacitor and storage capacitor fully discharges at the igniter plug. Bleeder resistors R3 & R4 provides the means of dissipating energy of the circuit in the event that the output of igniter unit is open circuited. They also serve to bleed off any residual charge on capacitor 4 between successive flash over and so provides constant level of triggering voltage from the secondary of winding of transformer T1. Turbine Engine Ignition System:
High by pass turbo fan JT 9D engine powers being 747. Two identical heat shielded shock mounted ignition exciter provide ignition system. The energy is supplied to recess gap igniter plug through HT lead.
Small amount of air is directed to cool exciters HT leads, igniter plugs. Input is 115V 1- 400Hz maximum current input is 2.5 Amps stored energy is 4 joules. AC power applied between A and B terminals.
Filter Circuit:
Consists of reactor (power choke) and feed through capacitors. It prevents high speed frequency feed back in to A/C electrical systems. Power choke limits spark rate variations over input voltage frequency range. The voltage is applied to primary winding of power transformer from filter. Doubler Circuit:
It consists of two solid state rectifiers, two capacitors (double capacitors) and resistors. High voltage induced in secondary fed to doubler circuit. Each change in polarity of pulse of DC voltage sent to storage capacitor, with successive pulses the storage capacitor assumes a greater and greater charge at increasing voltage. The purpose of resistors in doubler circuit is to limit the current passing through the rectifiers during those intervals of discharge of the storage capacitor when the voltage has reversed. When the charge of storage capacitor reaches the predetermined value, Set by spark gap in discharger tube. Storage capacitor's charge breaks down the gap. Current flows through primary winding of HT transformer and trigger capacitor. This flow of current induces high voltage in secondary enough to ionize the air gap in igniter plug. Thus plug becomes conductive, remaining charge in storage capacitor delivered to igniter plug as high current low voltage spark s park across the air gap. This high energy spark produce ignition under adverse conditions.
Bleeder resistor is provided to dissipate the energy in the circuit if the ignition plug is absent or fails to fire; it provides path to ground for any left out charge on the trigger capacitor between cycles. When stored capacitor discharges all its accumulated energy next cycle of energy commences. Ignition cable assembly consists of electrical lead contained in flexible metal braiding, coupling nuts at each end to facilitate the connections to ignition exciter and spark igniter plug. Igniter Plug:
Insulator made of aluminum oxide ceramic with a diamond like hardness. That
y
provides mechanical strength, high voltage insulation and rapid heat conductivity. y
Electrodes are of inconel chromel D, tungsten or any of Nickel all oys.
y
Shell body made of extremely high quality of stainless steel or inconel to resist bums in combustion chambers. Internal seals are made of ceramic materials.
y
Glow Plug Ignition:
System provides current to a hot coil element in each glow plug, which makes the plug glow red hot. The glow plug reaches an extremely high temperature very rapidly and ignites the fuel spray in combustion section during starting. s tarting. High
Energy Igniter Plug : Surface
Discharge Igniters
A pellet at the firing end of the plug has a semi conducting surface and during operation. This permits a minute electrical leakage from the center of electrode to body. There by heating the surface. Due
to
negative
temperature
coefficient of resistance the pellet provides a low resistance path for high
energy
operation,
which
discharges across the surface as a high intensity flash over as opposed to as spark jumping in air gap. High energy ignition unit (HEIU) gives 60 sparks per minute.
