Reciprocating Engines AVIA 1065 Ignition Systems PDF

RECIPROCATING ENGINES AVIA 1065 Ignition Systems 1  basic requirements are similar regardless of the type of engine must deliver a high-tension spark across the electro...

RECIPROCATING ENGINES AVIA 1065 Ignition Systems 1  basic requirements are similar regardless of the type of engine must deliver a high-tension spark across the electrodes of each spark plug in each cylinder of the engine in the correct firing order a certain number of degrees ahead of the top dead center position of the piston – as measured by crankshaft travel in degrees of rotation, the spark occurs in the cylinder output voltage of the system must be adequate to arc the gap in the spark plug electrodes under all operating conditions The spark plug is threaded into the cylinder head with the electrodes Engine Ignition and exposed to the combustion area of the engine’s cylinder Electrical Systems 2  Ignition systems two classifications magneto-ignition systems single magneto-ignition system, usually consisting of one magneto and the necessary wiring, was used with another single magneto on the same engine. Dual magnetos generally use one rotating magnet that feeds two complete magnetos in one magneto housing. An example of each type is shown in Figure 4-1 https://www.youtube.com/watch?v=_dyDU8 unkqw https://www.youtube.com/watch?v=gIdXLMV P6VU Engine Ignition and Electrical Systems 3 Engine Ignition and Ignition systems two classifications Electrical Systems FADEC electronic Full Authority Digital Engine Control (FADEC) systems for reciprocating engines Engine is controlled electronically More common with new Aircraft, example: Diamond Diesels etc Throttle input not connected to a linkage on a fuel control unit 4 Engine Ignition and magneto-ignition systems Electrical Systems Figure 4-1 5 Engine Ignition and Electrical Systems Aircraft magneto-ignition systems can be classified as either high-tension or low-tension low-tension magneto systems are not common anymore due to improvements in materials and shielding high-tension magneto systems are the most widely used aircraft ignition system 6  Engine Ignition and Electrical Systems Magneto-Ignition System Operating Principles The magneto is a special type of engine-driven alternate current (AC) generator It uses a permanent magnet as a source of energy By the use of a permanent magnet (basic magnetic field), a coil of wire (concentrated lengths of conductor) and relative movement between them, current is generated in the wire 7 Engine Ignition and Electrical Systems 8 Recip. Engine Ignition Systems Operation of a High-tension Magneto system 9 Engine Ignition and Electrical Systems 10 Engine Ignition and Electrical Systems Magneto-Ignition System Operating Principles the magneto generates electrical power when – engine rotating the permanent magnet – inducing a current to flow in the coil windings – current flowing through the coil windings generates its own magnetic field that surrounds the coil windings at the correct time, this current flow is stopped – the magnetic field collapses across a second set of windings in the coil – a high-voltage is generated and used to arc across the spark plug gap 11  Magneto-Ignition System Operating Principles Magneto operation is timed to the engine so that a spark occurs only when the piston is on the proper stroke at a specified number of crankshaft degrees before the top dead center piston position Engine Ignition and Electrical Systems 12 Engine Ignition and Electrical Systems magneto systems can be divided into three distinct circuits Magnetic primary electrical circuits secondary electrical circuits 13  Magnetic Circuit The magnetic circuit consists of a permanent multi-pole rotating Engine Ignition and magnet, a soft iron core, and pole shoes Electrical Systems The magnet is geared to the aircraft engine and rotates in the gap between two pole shoes to furnish the magnetic lines of force (flux) necessary to produce an electrical voltage 14 Engine Ignition and Electrical Systems Magnetic Circuit The poles of the magnet are arranged in alternate polarity so that the flux can pass out of the north pole through the coil core and back to the south pole of the magnet 15 Engine Ignition and Electrical Systems Magnetic Circuit When the magnet is in the position shown (full register) in Figure 4-3A, the number of magnetic lines of force through the coil core is maximum because two magnetically opposite poles are perfectly aligned with the pole shoes This position of the rotating magnet is called the full register position and produces a maximum number of magnetic lines of force, flux flow clockwise through the magnetic circuit and from left to right through the coil core Engine Ignition and Electrical Systems Magnetic Circuit When the magnet is moved away from the full register position, the amount of flux passing through the coil core begins to decrease This occurs because the magnet’s poles are moving away from the pole shoes, allowing some lines of flux to take a shorter path through the ends of the pole shoes Engine Ignition and Electrical Systems Magnetic Circuit As the magnet moves farther from the full register position, more lines of flux are short circuited through the pole shoe ends As the magnet is moved clockwise from this position, the flux lines flow through the coil core in the opposite direction[Figure 4-3C] At the 90° position, another position of maximum flux is reached 18 Engine Ignition Magnetic Circuit and Electrical Figure 4-4 Systems 19 Recip. Engine Ignition Systems 20 Recip. Engine Ignition Systems 21 Reciprocating Engines BREAK! RETURN AT 3 22  Primary Electrical Circuit consists of – a set of breaker contact points – a condenser – an insulated coil – [Figure 4-5] Engine Ignition and Electrical Systems 23 Engine Ignition and Primary Electrical Circuit The coil is made up of Electrical Systems a few turns of heavy copper wire one end is grounded to the coil core the other end to the ungrounded side of the breaker points. [Figure 4- 5] The primary circuit is complete only when the ungrounded breaker point contacts the grounded 24 breaker point Engine Ignition and Primary Electrical Electrical Systems Circuit the condenser (capacitor) wired in parallel with the breaker points prevents arcing at the points when the circuit is opened hastens the collapse of the magnetic field about the primary coil 25 Engine Ignition and Primary Electrical Circuit Electrical Systems primary breaker closes at approximately full register position the primary electrical circuit is completed the rotating magnet induces 26 current flow in the primary circuit this current flow generates its own magnetic field in a direction that it opposes any change in the magnetic flux of the permanent magnet’s circuit Engine Ignition and Primary Electrical Circuit While the induced current is Electrical Systems flowing in the primary circuit, it opposes any decrease in the magnetic flux in the core. current flowing in the primary circuit holds the flux in the core at a high value in one 27 direction until the rotating magnet has time to rotate through the neutral position to a point a few degrees beyond neutral This position is called the E-gap position (E stands for efficiency) 28 Engine Ignition Primary Electrical Circuit With the magnetic rotor in E-gap position and the primary coil holding the magnetic field of the and Electrical magnetic circuit in the opposite polarity, a very high rate of flux change can be obtained by opening the primary breaker points Opening the breaker points stops the flow of current in the primary circuit and allows the magnetic Systems rotor to quickly reverse the field through the coil core 29  Primary Electrical Circuit Engine Ignition This sudden flux reversal produces a high rate of flux change in the core, that cuts across the secondary coil of the magneto (wound over and insulated from the primary and Electrical coil), inducing the pulse of high-voltage electricity in the secondary needed to fire a spark plug As the rotor continues to rotate to approximately full register position, the primary Systems breaker points close again, and the cycle is repeated to fire the next spark plug in firing order 30 Engine Ignition and Primary Electrical Circuit The sequence of events can now be Electrical Systems reviewed in greater detail to explain how the state of extreme magnetic stress occurs. With the breaker points, cam, and condenser connected in the circuit as shown in Figure 4-6, the action that 31 takes place as the magnetic rotor turns is depicted by the graph curve in Figure 4-7. https://www.youtube.com/watch?v= 9dVy5tf_V90&t=40s https://www.youtube.com/watch?v= 9JnI8oN4h8I Engine Ignition and Electrical Systems 32 33 Engine Ignition and Electrical Systems Primary Electrical Circuit Figure 4-6 34 Engine Ignition and Primary Electrical Circuit At the top (A) of Figure 4-7, Electrical Systems the original static flux curve of the magnets is shown. Shown below the static flux curve is the sequence of opening and closing the magneto breaker points. Note that opening and closing the breaker points is timed by the breaker cam. The points close when a maximum amount of flux is passing through the coil core and open at a position after neutral. 35  Primary Electrical Circuit Since there are four lobes on the cam, the breaker points close and open in the same relation to each of the four neutral positions of the rotor magnet Also, the point opening and point closing intervals are approximately equal Starting at the maximum flux position marked 0° at the top of Figure 4-7, the sequence of events in the Engine Ignition and following paragraphs occurs Electrical Systems 36  Primary Electrical Circuit As the magnet rotor is turned toward the neutral position, the amount of flux through the core starts to decrease. [Figure 4-7D] This change in flux linkages induces a current in the primary winding. [Figure 4- 7C] This induced current creates a magnetic field of its own that opposes the change of flux linkages Engine Ignition and inducing the current Electrical Systems 37 38 Primary Electrical Circuit Engine Ignition and Without current flowing in the primary coil, the flux in the coil core decreases to zero as the magnet rotor turns to neutral and starts to Electrical Systems increase in the opposite direction (dotted static flux curve in Figure 4-7D) But the electromagnetic action of the primary current prevents the flux from changing and temporarily holds the field instead of allowing it to change (resultant flux line in Figure 4-7D) 39 40 Primary Electrical Circuit As a result of the holding process, there is a very high stress in the magnetic circuit by the time the magnet rotor has reached the position where the breaker points are about to open. Engine Ignition and The breaker points, when opened, function with the condenser to interrupt the flow of current in the primary coil, causing an extremely rapid change in flux linkages. Electrical Systems The high-voltage in the secondary winding discharges across the gap in the spark plug to ignite the fuel/air mixture in the engine cylinder Engine Ignition and Primary Electrical Circuit Each spark actually consists of Electrical Systems one peak discharge, after which a series of small oscillations takes place. They continue to occur until the voltage becomes too low to maintain the discharge. 41 Current flows in the secondary winding during the time that it takes for the spark to completely discharge. The energy or stress in the magnetic circuit is completely dissipated by the time the contacts close for the production of the next spark.  Primary Electrical Circuit Breaker assemblies, used in high-tension magneto- ignition systems, automatically open and close the primary circuit at the proper time in relation to piston position in the cylinder to which an ignition spark is being furnished. The interruption of the primary current flow is accomplished through a pair of breaker contact points made of an alloy that resists Engine Ignition and pitting and burning. Electrical Systems 42 Engine Ignition and Primary Electrical Circuit Electrical Systems Most breaker points used in aircraft ignition systems are of the pivotless type in which one of the breaker points is movable and the other stationary [Figure 4-8] The movable breaker point attached to the leaf spring is insulated from the magneto housing and is connected to the primary coil. [Figure 4-8] 43 Engine Ignition and Primary Electrical Circuit Electrical Systems The stationary breaker point is grounded to the magneto housing to complete the primary circuit when the points are closed and can be adjusted so that the points can open at the proper time Another part of the breaker assembly is the cam follower, which is spring- loaded against the cam by the metal leaf spring 44 Engine Primary Electrical Ignition and Circuit The cam follower is a Electrical Micarta block or similar material that rides the Systems cam and moves upward to force the movable breaker contact away from the stationary breaker contact each time a lobe of the cam passes beneath the follower. 45 Engine Ignition and Electrical Systems Primary Electrical Circuit A felt oiler pad is located on the underside of the metal spring leaf to lubricate and prevent corrosion of the cam. The breaker-actuating cam may be directly driven by the magneto rotor shaft or through a gear train from the rotor shaft. 46 Engine Ignition and Secondary Electrical Circuit Electrical Systems The secondary circuit contains the secondary windings of the coil, distributor rotor, distributor cap, ignition lead, and spark plug The secondary coil is made up of a winding containing approximately 13,000 turns of fine, insulated wire; one end of which is electrically grounded to the primary coil or to the coil core and the other end connected to the distributor rotor The primary and secondary coils are encased in a non-conducting material. The whole assembly is then fastened to the pole shoes with screws and clamps 47 Recip. Engine Ignition Systems 48 Engine Ignition and Secondary Electrical Circuit When the primary circuit is Electrical Systems closed, the current flow through the primary coil produces magnetic lines of force that cut across the secondary windings, inducing an electromotive force. When the primary circuit current 49 flow is stopped, the magnetic field surrounding the primary windings collapses, causing the secondary windings to be cut by the lines of force. The strength of the voltage induced in the secondary windings, when all other factors are constant, is determined by the number of turns of wire.  Secondary Electrical Circuit Since most high-tension magnetos have many thousands of turns of wire in the secondary coil windings, a very high- voltage, often as high as 20,000 volts, is generated in the secondary circuit The high-voltage induced in the secondary coil is directed to the distributor, which consists of two parts: revolving and stationary The revolving part is called a distributor rotor and the stationary part is called a distributor block The rotating part, which may take the shape of a disk, drum, or finger, is made of a non-conducting material with an Engine Ignition and embedded conductor Electrical Systems 50  Secondary Electrical Circuit Engine Ignition and The stationary part consists of a block also made of non- conducting material that contains terminals and terminal Electrical Systems receptacles into which the ignition lead wiring that connects the distributor to the spark plug is attached 51 Engine Ignition and Electrical Systems Secondary Electrical Circuit This high-voltage is used to jump the air gap of electrodes of the spark plug in the cylinder to ignite the fuel/air mixture As the magnet moves into the E- gap position for the No. 1 cylinder and the breaker points just separate or open, the distributor rotor aligns itself with the No. 1 electrode in the distributor block 52 Engine Ignition and Electrical Systems Secondary Electrical Circuit The secondary voltage induced as the breaker points open enters the rotor where it arcs a small air gap to the No. 1 electrode in the block Since the distributor rotates at one-half crankshaft speed on all four-stroke cycle engines, the distributor block has as many electrodes as there are engine cylinders, or as many electrodes as cylinders served by the magneto 53 Engine Ignition and Secondary Electrical Circuit Electrical Systems The electrodes are located circumferentially around the distributor block so that, as the rotor turns, a circuit is completed to a different cylinder and spark plug each time there is alignment between the rotor finger and an electrode in the distributor block. The electrodes of the distributor block are numbered consecutively in the direction of distributor rotor travel. [Figure 4-10] 54 Engine Ignition and Secondary Electrical Circuit Electrical The electrodes of the Systems distributor block are numbered consecutively in the direction of distributor rotor travel. [Figure 4-10] 55 Engine Ignition and Secondary Electrical Circuit Electrical Systems The distributor numbers represent the magneto sparking order rather than the engine cylinder numbers. The distributor electrode marked “1” is connected to the spark plug in the No. 1 cylinder; distributor electrode marked “2” to the second cylinder to be fired; distributor electrode marked “3” to the third cylinder to be fired, and so forth. 56  Magneto and Distributor Venting Since magneto and distributor assemblies are subjected to sudden changes in Engine Ignition and temperature, the problems of condensation and moisture are considered in the design of these units. Electrical Systems Moisture in any form is a good conductor of electricity. If absorbed by the non-conducting material in the magneto, such as distributor blocks, distributor fingers, and coil cases, it can create a stray electrical conducting path. Engine Ignition and Electrical Systems Magneto and Distributor Venting If absorbed by the non- conducting material in the magneto, such as distributor blocks, distributor fingers, and coil cases, it can create a stray electrical conducting path. 58 Engine Ignition and Magneto and Distributor Venting Electrical Systems The high-voltage current that normally arcs across the air gaps of the distributor can flash across a wet insulating surface to ground, or the high- voltage current can be 59 misdirected to some spark plug other than the one that should be fired. This condition is called flashover and usually results in cylinder misfiring. This can cause a serious engine condition called pre-ignition, which can damage the engine Engine Ignition and Magneto and Distributor Venting Electrical Systems For this reason, coils, condensers, distributors, and distributor rotors are waxed so that moisture on such units stand in separate beads and do not form a complete circuit for flashover. Flashover can lead to carbon tracking, which appears as a fine pencil-like line on the unit across which flashover occurs. The carbon trail results from the electric spark burning dirt particles that contain hydrocarbon materials. 60 Engine Ignition and Magneto and Distributor Venting Electrical Systems The water in the hydrocarbon material is evaporated during flashover, leaving carbon to form a conducting path for current When moisture is no longer present, the spark continues to follow the carbon track to the ground This prevents the spark from getting to the spark plug, so the cylinder does not fire Magnetos cannot be hermetically sealed to prevent moisture from entering a unit, because the magneto is subject to pressure and temperature changes in altitude 61  Magneto and Distributor Venting Engine Thus, adequate drains and proper ventilation reduce the tendency of flashover and Ignition and carbon tracking Good magneto circulation Electrical also ensures that corrosive gases produced by normal Systems arcing across the distributor air gap, such as ozone, are carried away In some installations, pressurization of the internal components of the magnetos and other various parts of the ignition system is essential to maintain a higher absolute pressure inside the 62 magneto and to eliminate flashover due to high altitude flight  Magneto and Distributor Engine Ignition and Venting This type of magneto is used Electrical Systems with turbocharged engines that operate at higher altitudes. Flashover becomes more likely at high altitudes because of the lower air pressure, which 63 makes it easier for the electricity to jump air gaps. By pressurizing the interior of the magneto, the normal air pressure is maintained and the electricity or the spark is held within the proper areas of the magneto even though the ambient pressure is very low. Engine Ignition and Electrical Systems Magneto and Distributor Venting By pressurizing the interior of the magneto, the normal air pressure is maintained and the 64 electricity or the spark is held within the proper areas of the magneto even though the ambient pressure is very low.  Magneto and Distributor Venting Engine Ignition and Even in a pressurized magneto, the air is allowed to flow Electrical Systems through and out of the magneto housing. By providing more air and allowing small amounts of air to bleed out for ventilation, the magneto remains pressurized. 65 Regardless of the method of venting employed, the vent bleeds or valves must be kept free of obstructions. Further, the air circulating through the components of the ignition system must be free of oil since even minute amounts of oil on ignition parts result in flashover and carbon tracking The End 66