Anatomy of a car engine

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ELECTROMAGNETIC FIELDS:

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Electricity and magnetism are two separate but closely related forces. This is demonstrated by the fact that magnetic lines of force are produced around magnets, and also around conductors carrying electrical current. When electrical current is passed through a conductor, there will always be a magnetic field surrounding the conductor. The strength of this magnetic field depends upon the amount of current flow. The higher the amperage, the greater the magnetic strength.

If two conductors are arranged side by side and current passes through both conductors in the same direction, the magnetic field around each conductor will be in the same direction. As a result, the magnetic fields will combine to form one stronger field surrounding both conductors. This causes the two conductors to be drawn together or attracted to each other. If the current is in opposite direction, the magnetic fields surrounding the two conductors will oppose each other and result in a repelling action. This is the principle involved in the operation of an electric motor such as a starter motor on a vehicle.

If a conductor is wound into a coil, the current passing through it will flow in the same direction in all turns. The magnetic field produced by each turn combines with the field produced by adjacent turns, resulting in a strong continuous field lengthwise around and through the coil. The polarity of the field produced by the coil depends upon the direction of current flow, and the direction in which the coil is wound. The strength of the magnetic field depends upon the number of wire loops and the amount of current passed through the coil. This is known as AMPERE TURNS.

The strength of the magnetic field around the coil can be materially strengthened by placing a core of soft iron inside the coil. Because the iron is a much better conductor for the magnetic lines of force than is the air, the field becomes more concentrated and much stronger. Electromagnetic relays using this basic design are used in many applications in the electrical system of the automobile.

ELECTROMAGNETIC FIELDS

POWER LOSS:

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Before a successful tune-up can be performed, it must be determined if the engine is in a satisfactory mechanical condition. An engine that has burned or leaking valves, worn piston rings, sticking valve lifters, leaking cylinder head gaskets or other mechanical malfunctions, will not perform efficiently even after being tuned-up. These conditions must be corrected before an engine can be tuned to perform satisfactorily and efficiently.

Since the power developed by an engine on its power stroke is largely dependent on the efficiency of the compression stroke, and because of the testing convenience afforded, the compression stroke is used for testing engine condition.

During the compression stroke the air-fuel mixture is compressed in the tightly sealed combustion chamber. Should any openings be created by burned valves, leaking gaskets, or worn piston rings, the reduced amount of the air-fuel mixture would proportionately reduce the power output of the engine.

Leakage at any point in the combustion chamber will effect efficient engine operation. Leaking intake valves will allow a portion of the air-fuel mixture to be pushed back into the manifold during the compression stroke and less fuel will be available for the power stroke. During the power stroke, the expanding gases will leak past the burned valves and less pressure will be available on the head of the piston. Also burned gases will be forced into the intake manifold to mix with the air-fuel mixture. A diluted air-fuel mixture will then be available for the next intake stroke and consequently less power will be developed by the engine. If the exhaust valve is burned, the expanding gases will leak through it and less power will be available from the cylinder.

Any leakage past the piston rings will also affect the power of the engine. During the compression stroke, part of the air-fuel mixture will be forced into the crankcase and cause oil contamination. The power stroke will also force burned gases into the crankcase. These gases will overheat some of the oil, turning it to carbon and the oil will become contaminated.

A leaking head gasket will permit water to be drawn into the cylinder during the intake stroke. During the compression and power strokes, gases will be forced from the combustion chamber into the cooling system and cause the engine to overheat. Also, a less dense air-fuel mixture will be available for the power stroke.

It is obvious that conditions of compression stroke leakage are proportionately reflected in engine power loss and must be corrected before an engine can be properly tuned.

POWER LOSS

CONDENSER ACTION:

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The functions of the ignition condenser are to reduce the amount of arcing across the points thereby preventing excessive metal transfer from one point to the other and to quickly stop the flow of current in the primary coil winding so that the maximum voltage can be generated in the secondary coil winding.

The condenser can store a certain amount of energy. Then the breaker points open and the induced voltage in the coil tries to keep current flowing across the breaker points, the condenser will absorb the electrical energy until the breaker points have opened sufficiently so that an arc cannot occur. By the time the condenser becomes fully charged, the points have opened too far for arcing to take place.

By preventing the arc from occurring across the breaker points, the condenser brings the primary current flow to a sudden stop. This causes a very sudden collapse of the magnetic field, so that the lines of force cut through the windings in the coil with great speed. The voltage induced in the secondary winding forces current to jump the spark plug gap. The charge in the condenser surges back in a reverse direction through the primary circuit across the battery and builds up on the opposite plate of the condenser. The condenser then discharges in the opposite direction and charges the condenser once more in the original direction. Each time the condenser discharges, part of the energy is lost in overcoming the resistance of the circuit so that the oscillating current will die out, or nearly so, before the contact points close for the next build up.

CONDENSER TESTING AND SELECTION:

Condensers are tested for resistance, capacity and leakage. The resistance test reveals the presence of any loose or high resistance connection in the pigtail or the case. The capacity test checks the microfarad capacity of the condenser. The insulation test stresses the insulating material with about 500 volts while the tester meter indicates the presence of current leakage through the insulation. A condenser that tests defective in any test must be replace.

The testing of a used condenser will reveal the presence of any defects but it cannot measure the amount of useful life left in the condenser. For this reason it is advisable to always install a new condenser when replacing the breaker points.

Some transfer of metal from one breaker point to the other is a normal action that occur in all ignition systems. The function of a condenser of correct capacity is to prevent any excessive metal transfer. The capacity of the condenser is an important specification. It has been selected to match the particular requirements of a given ignition system. When replacing the condenser be sure that new condenser has the microfarad capacity recommended in the specifications.

Typical specifications are: 0.18 – 0.23 mfd ; 0.21 – 0.25 mfd ; 0.25 – 0.285 mfd.

When a good condenser of the correct capacity is employed and a condition of excessive point metal transfer nevertheless exists, the condition may be due to:

· Excessive primary or secondary circuit resistance in the ignition system which will upset the system balance.

· A high voltage regulator setting which causes increased primary current flow resulting in everlasting and burning of the breaker points.

· Incorrect breaker point dwell angle.

· Continual high speed operation.

· Frequent periods of prolonged idling.

Locating and servicing the cause of the trouble will prevent a reoccurrence of the condition. In the case of the last two conditions, more frequent tune-ups should be recommended.

CONDENSER CONSTRUCTION:

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The condenser is constructed of layers of aluminum foil, insulated from each other by layers of high dielectric insulating material. One layer of aluminum foil extends beyond the insulating material on one side, while the other layer of foil extends beyond the insulating material on the other side. The layers of aluminum foil and insulating material are then rolled into a tight cylinder and inserted into the condenser case. The layers of aluminum foil extending at one side will contact the bottom of the case and represents the ground terminal of the condenser. The other layer of foil will contact a disc which is connected to the insulated lead of the condenser.

Current does not flow through a good condenser. If it does, the layers of foil are touching or there is a hole in the insulating material. If either condition exists the condenser is defective and must be replaced.

IGNITION COIL CONSTRUCTION:

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An ignition coil is composed of a core, two winding, a metal case and mounting bracket.

The core of the coil usually consists of thin soft iron strings or lamination. Its purpose is to increase the efficiency and output of the coil by promoting faster and more complete coil magnetic saturation. The soft iron core readily conducts magnetic lines of force, so less energy is used then if the magnetic lines of force had to travel through an air core.

The two winding are identified as a primary winding and a secondary winding. The primary winding consists of approximately 250 turns of relatively heavy wire, which is laminated with a special varnish. The secondary winding is wound inside the primary winding and consists of approximately 20,000 turns of very fine varnished wire. The many layers of secondary winding are insulated from each other by high dielectric paper. One end of the secondary winding is connected to the high tension tower, while the other end is connected to one of the primary terminals inside the coil.

Ignition coil are often filled with oil or special compound to provide additional insulation and to help dissipate the heat that is created by the transformation of battery voltage.

The dissipation of heat is very important in an ignition coil as heat tends to weaken insulation. An insulation results in partial or total coil failure.

IGNITION COIL CONSTRUCTION

IGNITION COIL ACTION:

When the ignition switch is turned on and the breaker points are closed, current flow in the primary circuit. As current flows through the primary winding of the ignition coil, a strong magnetic field is produced, with the aid of the core. When breaker points open, current ceases to flow through the primary winding of the coil and causes the magnetic field to collapse across the man thousands of turns of wire in the secondary winding. This action induces a very high voltage in the secondary circuit which forces current to jump the rotor and spark plug gap.

When a piece of wire is connected across a source of voltage, current will immediately reach a maximum value determined by the resistance of the wire itself. But this is not true when the same wire is wound into a coil as in the primary winding of the ignition coil. This characteristic of a coil conductor is called reactance or counter-electromotive force, and is due to the self-induced voltage in the coil.

When the breaker points close, current starts to flow in the primary winding. As the magnetic field begins to build up, the lines of force not through the primary winding and induces a voltage that opposes battery voltage. Therefore, it takes a definite period of time for the primary current to reach a maximum rate of flow after the breaker points close. This period of time is called a “build-up” time. When maximum current is flowing in the coil winding, the maximum magnetic field is present and the coil is said to be fully “saturated”.

If the breaker points remains closed for too short a period of time, maximum current flow will not be reached in the primary circuit and the maximum magnetic strength will not be attained. As a result, when the breaker points open, there will be less lines of force to cut through the secondary winding and coil output voltage will be reduced. This can cause the engine to misfire under certain operating conditions.

Reactance or counter-electromotive force not only opposes the build-up of current through the primary circuit, but also opposes any attempt to stop the flow of current. As the breaker points open, the magnetic field starts to collapse. The lines of force cut through the primary winding, but in the opposite direction from the build-up. This causes an induced voltage in the primary winding which is in the same direction as battery current and tends to keep current flowing. If current continues to flow when the breaker points open, there will be an arc between the breaker points. This arc has two very detrimental effects. First, it causes a transfer of metal from one point to the other, resulting in point pitting. Second, unless the flow of primary current is stopped quickly, the magnetic field will collapse gradually and the secondary winding output voltage will be considerably reduced.

To control the arc that takes place between the points as they separate and to quickly stop the flow of primary current to develop maximum coil voltage, condenser is connected across the points.

IGNITION COIL REPLACEMENT:

As previously stated, the ratio of coil secondary turns to primary turns is approximately 100 to 1. Therefore, a typical coil for a standard ignition system would have 200 turns of primary winding and 20,000 to 26,000 turns of secondary winding.

The coils used in transistorized ignition systems have a turns ratio of either 275 to 1 or 400 to 1. Because of the high current- carrying capacity of the heavier gauge transistor coil primary winding, approximately 95 turns of primary winding will be used in a coil having 26,000 turns of secondary winding.

It is very important when coil replacement is required that the proper replacement coil be installed on the engine. Mixing standard and transistor ignition coils or coils of the wrong polarity will be quickly reflected in poor engine performance or total ignition failure.

SPARK ADVANCE TIMING:

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To obtain full power from combustion, the maximum pressure must be reached just as the piston passes top dead center, and combustion must be completed by approximately 23 degrees after top dead center.

The fuel mixture ignited in the combustion chamber does not explode. It burns rapidly until the fuel is consumed. The time required for complete combustion is a small fraction of a second. For this reason ignition must take place before the piston passes top dead center.

As the engine speed increases, the piston moves through the compression stroke more rapidly, but the burning rate of the fuel mixture remains virtually the same. To compensate for the high piston speed, ignition must occur earlier in the compression stroke.

During the combustion process at 120 rpm, the crankshaft travel through 41 degrees of rotation, from the point of ignition, to 23 degrees after top dead center. The spark must occur at 18 degrees before top dead center. The same engine running at 3600 rpm will require 63 degrees of crankshaft rotation to complete combustion by 23 degrees after top dead center. This would require that the spark occur 40 degrees before top dead center. This is why spark advance is such an important factor in efficient engine operation.

AS ENGINE SPEED INCREASES SPARK MUST BE TIMED EARLIER

DWELL ANGLE:

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Dwell angle is the number of degrees the distributor cam turns during which time the breaker point are closed. During the dwell period, a magnetic field is built up in the primary winding of the coil. However, time is required to build up a full strength magnetic field. When a full strength magnetic field is produced, the coil is said to be “saturated”. To be assured of coil saturation at low engine speeds presents no problem because of the relatively slow rotation of the distributor cam. At high speeds, however, unless the distributor points are adjusted to provide a sufficient dwell period, coil saturation will not be attained.

In a six-cylinder engine running at idle speed of 400 rpm, the ignition system must produce 20 sparks per second to fire all of the cylinders. With an engine operating at this speed, dwell angle is not very critical because more than sufficient coil “saturation” time is available. However, with the engine running at 4000 rpm, or the equivalent of 90 miles per hour or more, 200 sparks per second would be required to fire all the cylinders. This is the speed at which dwell becomes extremely critical. If the dwell was reduced only slightly from the required amount, the engine would begin to misfire at high speed because the coil does not have time to becomes sufficiently “saturated”.

IGNITION DISTRIBUTOR

Although dwell is not critical at low speed, point gap becomes very important. With the engine cranking, there must be sufficient point gap or the points will arc excessively and the engine will not start readily. Also, if an engine is operated with too little point gap at low speed, the points will deteriorate rapidly. If the points open rapidly, and do not open wide enough, an arc will continue across the contact points using energy that would normally create a spark at one of the spark plug. When an arc occurs the engine usually misfires because the energy of the primary circuit is dissipated preventing sufficient secondary circuit voltage build-up.

Dwell angle directly affects ignition timing. Under certain conditions the rubbing block on the movable breaker arm may wear. As a result, the dwell angle increases which in turn causes the ignition timing to be late.

One of the largest single causes of breaker point failure is the lack of cam lubricant. Point rubbing block wear can be appreciably reduced by applying a thin film of high-temperature cam lubricant to the distributor cam when servicing the distributor. It is important that the proper lubricant be used since it must be able to adhere to the cam surface at high cam speed; resist melting at high temperature; resist chemical reaction with the polished steel cam; effectively control moisture to prevent rust formation on the cam; resist drying out with age. Ignition point spring tension plays an important part in the performance of the ignition system, and must be within specified limits. Excessive pressure causes rapid rubbing block and cam wear, while insufficient pressure will permit high-speed point bounce which in turn, will cause arcing and burning of the points and misfiring of the engine.

CONTACT POINT DWELL TEST

DISTRIBUTOR ASSEMBLY:

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The distributor assembly is made up of several subassemblies: the cap, rotor, breaker plate assembly supporting the breaker points and condenser, cam, centrifugal mechanism, vacuum advance unit and the distributor base or body.

The distributor cap and rotor are used to distribute the high voltage current developed in the coil to each spark plug in firing order sequence. The distributor cap is keyed to the distributor body to maintain the correct relationship between the distributor cap towers and the rotor. The rotor is keyed to distributor cam to be in the correct position to transfer the high voltage current to one of the distributor cap high voltage towers when the points open.

The breaker plate supports the breaker points at the correct position for the cam to open them. In most distributors, the breaker plate is moveable and is mounted on a center bearing or side pivot. Since a moving part can develop high resistance at the bearing surface, a flexible insulate wire is used to connect the points to the primary circuit. Also, a grounding wire is used between the plate and distributor housing.

The distributor cam opens the breaker points to interrupt the primary circuit. The cam has the same number of lobes as the engine has cylinders as the points must be opened to produce a high voltage surge for each power stroke. The distributor cam is actuated by the weights of the centrifugal advance mechanism.

DISTRIBUTOR ASSEMBLY

The centrifugal advance mechanism varies the position of the cam in relation to engine speed. The mechanism has weights that are thrown out against calibrated springs to advance the breaker cam as speed increases. The advance mechanism is usually mounted in the distributor housing below the breaker plate.

The vacuum advance unit advances the ignition timing in addition to the advance provided by the centrifugal unit. The vacuum unit is controlled by manifold vacuum and consists of a metal chamber in which a flexible air-tight diaphragm is located. A link extends from one side of the diaphragm to the breaker plate assembly or distributor housing. If the distributor is equipped with a moveable breaker plate, the vacuum advance mechanism rotated the plate to advance the ignition timing. If the plate is rigidly mounted in the distributor, the vacuum advance unit rotates the complete distributor assembly for advance.

The lower drive end of the distributor shaft is equipped with either an offset grove or tang or with a gear, depending on the manner in which the distributor is driven. The reason the tang is offset is to assist in correct installation of the distributor when replacing it in the engine.

The base assembly or body serves as a housing for all the parts mentioned and also serves as the bearing for the distributor shaft. The bearing may be cast iron, bronze or ball bearings. The base is also the adapter for installing the distributor assembly in the engine block. The base can be rotated in either direction to make the initial timing adjustment.

How the ignition system works

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The ignition system is employed to generate a very high voltage (from 18 to 30 thousand volts) to jump across the spark plug gap to ignite the air-fuel mixture inside the combustion chamber of a gasoline engine.The ignition system consist of two main circuits.

1. PRIMARY CIRCUIT:

Primary circuit is also called the low voltage or the low tension circuit. This is the circuit through which current flows at battery or generator voltage. The ignition switch, resistor, ignition coil primary winding, breaker points and the condenser are all in the primary circuit.

2. SECONDARY CIRCUIT:

The secondary ignition circuit is also called the high voltage or high tension circuit. The coil secondary winding, distributor rotor, distributor cap, high tension leads, and the spark plugs are all part of the secondary circuit.

THE IGNITION SYSTEM

HOW IGNITION SYSTEM WORKS:

When the ignition switch is turned on and the breaker points are closed, current flows in the primary circuit. As current flows through the primary winding of the ignition coil, a strong magnetic field is produced in the coil. When the breaker points open, current through the primary winding of the coil is stopped, and the magnetic field around the coil winding collapses. These collapsing lines of force cut across both the primary and secondary winding inducing a very high voltage in the secondary winding. The high voltage so induced current to jump the spark plug gap.

The primary ballast resister is essentially a current compensating device consisting of a resistor unit or wire located in the primary ignition circuit. The compensating action is obtained because, at low engine speeds, the current flows for longer period of time. This heats up the resistor, thereby raising its resistance and reducing current flow. This action serves to keep the coil primary winding cooler and improves distributor breaker point life. At high speeds, the current flows for shorter periods of time which lets the resistor cool and increases the current flow in the primary winding of the coil. This action permits maximum secondary voltage to be obtained.

Because of the lowered battery voltage resulting from the starter load on some vehicles, the ballast resister is bypassed while the starting system is in operation. This is done to provide higher secondary voltage for starting.

STATOR WINDING TESTS:

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Stator winding are checked for open circuit and ground circuit.

To check for open circuit, touch the prods of 110 volt test lamp to the stator windings terminals as illustrated. If the lamp does not light up, the stator winding is open. To complete the test, move one test lamp prod to the other stator winding terminal.

To check for a ground circuit, touch one prod of the 110 volt test lamp to the stator frame and the other test prod to any of the stator winding terminals. The test lamp should not light. If it does light up, the stator winding is grounded.

If an ohmmeter is available, these tests may be conducted by measuring resistance values.

Visual inspection for or charred winding relation should also be conducted at this time.

STATOR WINDING TESTS

FIELD WINDING TESTS:

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The rotor field winding may be checked electrically for open circuit, ground circuit and short circuit.

To check for open circuit, touch a 110 volt test lamp prod to reach slip ring. If the lamp fails to light, the field winding is open.

To check for ground circuit, touch one 110 volt test lamp prod to either slip ring and the other test lamp prod to the rotor shaft. If the lamp lights, the field winding is grounded.

To check for short circuit, connect a 12 volt battery and an ammeter in series with the two slip rings. The ammeter should indicate approximately 2 amperes. An ammeter reading above this value indicates shorted field coil winding. High output alternators will have a higher field current draw.

If an ohmmeter is available, these three tests can be conducted by measuring resistance values.

FIELD WINDING TESTS

DIODE TESTS:

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Due to the Manner in which the three stator windings are connected, two windings are always being used at any one time. Only two diodes are being used at any one time. If any diode is defective, causing one phase of the 3 phase winding to be missing, alternator output is decreased by approximately two-thirds because any single missing phase undesirably influences both of the other phases.

The diodes on some makes of alternators can be tested with the alternator assembled and on the engine. Other makes have to be disassembled for diode testing.

Diodes can be readily checked using a diode tester. When using this test instrument follow the manufacturer's instructions.

Diodes may also be tested with the 12 volt test lamp. Touch the prods of the test lamp leads to the diode case and diode lead and then reverse the test lamp prods as previously explained. If the diode is good, the test lamp will light in only one test. If the lamp fails to light in both tests or lights in both tests, the diode is defective.

Another method of checking diodes is with an ohmmeter. After the stator leads have been disconnected, each diode can be tested for shorts and opens. Touch one ohmmeter prod to the diode case and the other prod to the diode lead and observe the ohmmeter reading. Then reverse the ohmmeter prods and again observe the meter readings. A diode in good condition will have one high reading and one low reading. If both readings are very low, or if both readings are very high, the diode is defective. Push and pull the diode leads gently while testing it to detect loose connections. Test all six diodes in the same manner. The readings of the diodes in the insulated heat sink will be opposite from those in the grounded heat Sink or end frame.

When diode is found defective, it is advisable to replace all 3 diodes in the end frame or heat sink using the correct procedure and the proper removal and installation tools. Diode must never be hammered into position as the impact can easily crack the Silicon wafer. Be sure to test the condenser as it may also be damaged.

TRANSISTORS:

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A transistor is a device which acts as an electrical switch but has no moving parts. Current flow through the transistor can be controlled mechanically by a set of contacts or electrically by reversing the circuit polarity.

A transistor is made up of three small sections of material fused together like a wafer or Sandwich and placed in a container. The sections are referred to as "N" or "P" type material. The arrangement of the sections determines the transistor's polarity. The transistor may be known as a PNP transistor or as a NPN transistor according to the positions of the wafers. The middle wafer serves as the base and therefore dictates the polarity.

The transistor symbol is illustrated on the chart. The circle represents the container. The letters represent the three elements of which the transistor is composed -- E is the Emitter, C is the Collector, and B is the base. The arrow on the Emitter symbol indicates the direction of current flow through the transistor.

There are two important factors relative to the manner in which the transistor works. First: the Emitter-Collector circuit is the main current-carrying circuit. Second: current flow through the Emitter-Collector circuit is possible ONLY when there is current flow through the Emitter-Base circuit. Although the current flow in the base circuit may be only a fraction of the current flow in the Collector circuit, the Collector circuit cannot exist without the base circuit. It follows then, that an interruption of the light current flow in the base circuit will cause a stoppage in the heavy current flow in the Collector circuit in this manner the best circuit "triggers" the transistor and turn it ON or OFF.

As previously stated, the transistor is electrically controlled through the base circuit. A set of mechanical contact points, in series in the Emitter-Base circuit, can readily carry the light current flow in this circuit and serve to "make" and "break" the Base circuit.

Another method of electrical control is performed by reversing the current flow in the base circuit. The base circuit will carry current only when the proper voltage range and polarity is applied. When incorrect voltage or reversed polarity is applied the Base circuit is turned OFF and current flow in the Collector circuit is consequently stopped.

TRANSISTORS

DIODE:

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As previously stated, the diode is a current rectifying device. It serves as a one way electrical check valve which permits current flow readily in one direction but stops its flow in a reverse direction. Silicon die or Wafer in the diode possesses this electrical characteristic by virtue of the molecular construction of the metal.

The diode symbol is an arrow indicating the direction of current flow allowed by the diode. The bar indicates a one-way "gate" or block to current flowing in the opposite direction.

The cross-sectional view illustrates the position of the Silicon wafer in the bottom of the diode case. The case is made of rather heavy metal to serve both as a protection for the rather brittle silicon wafer and to effectively dissipate the heat, induced by the current flow through the diode. The case is tightly sealed during manufacture to prevent the entrance of moisture into the diode which would result in short circuiting of the unit. Moisture is readily drawn into any unit that operates at a temperature since it "breathes" as it heats and cools.

In all negative ground alternator charging systems the negative (case) diodes are pressed into the alternator grounded end-frame and the positive (case) diodes are pressed into a holder called a heat sink. The heat sink is usually made of die-cast aluminium because it possesses high heat dissipating qualities. It is mounted in, but electrically insulated from, the end-frame. The end-frame also serves to absorb the heat developed by the passage of current through the diodes.

A negative diode is properly identified as a negative case diode meaning the diode case is negative polarity and the diode lead is positive polarity. A positive case diode will have a positive polarity case and a negative polarity lead. Diodes are color coded for popularity identification. The part number of the diode is printed with the red dye on the positive diode and with the black dye for the negative diode. When part numbers are not used, a dab of red or black dye on the diode case identifies its polarity. Diodes used in heavy duty application are identified with a + (plus) or a - (minus) sign.

The Zener diode is used in many transistorized ignition systems and voltage regulators. Its basic function is to protect the transistors in the circuit from the harmful effects of high voltage. When a predetermined voltage is reached, the Zener diode " breaks down" and permits the passage of current in the opposite direction by providing a shunt circuit for the high voltage current. This breakdown voltage does not harm the Zener diode since it is designed to perform in this manner. When the voltage drops below the predetermined voltage, the Zener diode again blocks current.

Since the breakdown voltage of the Zener diode is lower than the voltage value to that would damage the transistor, the transistor is there by protected. The Zener diode can be constructed to "breakdown" at varying voltages.

DIODE SYMBOL

DIODE CROSS SECTION

TEMPERATURE CORRECTED HYDROMETER:

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Hydrometer floats are calibrated to indicate correctly only at 80 F temperature. It used at any other temperature, a correction factor must be applied. The reason for this lies in the fact that a liquid expands when it is heated and shrinks when it is cooled. This will cause a change in the density of the electrolyte solution which will raise or lower the specific gravity reading.

A thermometer is built into the temperature compensating-type hydrometer. The scale of this thermometer indicates the temperature of the solution. This reading should be used so that the proper temperature correction factor can be applied.

The table is based on an electrolyte temperature of 80°F. For other temperatures, correct the indicated reading by adding 4 points(.004) for each 10° above 80°F, and subtracting 4 points for each 10° that the electrolyte temperature is below 80°F.

TEMPERATURE CORRECTED HYDROMETER

For example: A specific gravity reading of 1.230 is obtained at a solution temperature of 10°F. If the electrolyte temperature is disregarded, the reading of 1.230 may be considered as low but acceptable. When the reading is temperature corrected, the true reading of 1.202(7*4=28 from 1.230) reveals that the battery is actually very low and definitely in need of charging.

A specific gravity reading of 1.235 is obtained at a solution temperature of 120°F. The reading itself may be interpreted as being rather low but when temperature corrected the reading is actually 1.251 (4*4=16 added to 1.235). This is specific gravity may be high enough for the battery to be restored to full charge by the car generator.

These examples indicate the importance of temperature correcting specific gravity readings to accurately interpret the true state of battery charge. To accurately test the true condition of the battery, a light load test or capacity test should be conducted after the specific gravity has been tested.

DIODE RECTIFIED OUTPUT:

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The diagram illustrates how the diodes rectify the alternating current flowing in either direction through any two stator winding so that direct current is always available at the output for battery terminal of the alternator.
Voltage induced in the winding "A" and "C" cause current to flow from the "A" coil of the stator through its positive diode, out of the "Bat" terminal of the alternator and to the battery. The return circuit is from the battery, to the alternator ground terminal, through the negative diode Connected to the "C" coil of the stator and back to the "A" coil where it started, completing the circuit.

DIODE RECTIFIED OUTPUT

When the alternating voltage induced in these winding is reversed, current will flow out of the "C" the stator coil of the alternator, through its positive diode, out of the "Bat" terminal of the alternator and to the battery. This rectified direct current flows to the battery in the same direction as it did previously. The return circuit is from the battery, to the alternator ground terminal, through the negative diode connected to the "A" stator coil and back to the "C" coil where it started, completing the circuit.
Current flow, in either direction, through the stator winding in the other illustrations can be traced by following the arrows from the stator coil winding, through the positive diode, to the battery, from the battery through the negative diode, through the stator winding back to its source.
These illustrations clearly show that by the action of the diodes, direct current is always flowing into the battery regardless of which stator winding are inducing the voltage or the direction of the current flow through the windings.

CHARGING SYSTEM

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The charging system has been developed to answer the need for increased generator output necessitated by the addition of electrically operated accessories on the modern car combined with low speed driving in congested traffic.

The alternator possesses the ability to produce current output at engine idle speed and low car speed. This factor makes it a superior charging unit to the direct current generator which must be rotated at reasonable speed before current output is developed.

The basic alternator charging system components include the battery, the self rectifying alternating current generator, a voltage limiting relay and interconnecting wiring. The circuit will include either an ammeter or an indicator light.

AC CHARGING SYSTEM

Every AC charging system is controlled by a voltage regulator. The battery initially supplies the current for the alternator field coil. At this time, and during idle and low speed operation, there is no voltage control problem. However, as alternator speed increases with engine speed, the voltage increased developed by the alternator would be imposed on the field coil. This increases the field strength and further raises the voltage output. Unless this voltage rise condition is kept under control, the high voltage developed will result in damage or shot in life expectancy and light bulb, relay coils, radio tubes, turn signal, flasher, breaker points, and other voltage sensitive units.

All alternator voltage regulator are of the double contact type discussed under generator regulator. In other alternator systems the regulator may contain, in addition to the voltage limiter, field relay which is used to complete the field circuit when the ignition switch is turned on. It may also include lamp relay when indicator light is used instead of an ammeter.

ALTERNATOR COMPONENTS:

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The alternator is composed of a rotor assembly, a stator assembly and two end-frame assemblies, one at the drive end of the alternator and one at the slip ring end.

The rotor assembly is composed of a field coil made of many turns off wire wound over and iron core which is contained between two iron segments with interlacing fingers. These fingers serve as magnetic poles. This assembly is press mounted on a steel shaft which turns in pre-lubricated anti-friction bearings. Two slip rings are mounted on one end of the shaft.

Each and of the field coil winding is connected to one of the slip rings. A brush rides on each slip ring. These brushes conduct battery current to the rotor winding to create the magneticfield required for current generation. This is necessary because the rotor is made of a metal alloy that does not retain residual magnetism and is therefore not self exciting.

ALTERNATOR COMPONENTS

The stator assembly is composed of a laminated iron frame and three sets of winding wound into slots in the frame. The manner in which these windings are wound and connected makes the alternator 3 phase unit. Constant voltage can be maintained since each of the three winding reaches its maximum voltage output at a different time. Three voltage impulses are induced for every turn of the rotor with this arrangement it whereas 1 voltage impulse would be induced if a single winding was used. One end of all three windings are connected together while the other end of each winding is connected to a pair of diodes, one positive and one negative.

When assembled, a very small air gap is present between the rotor poles and the stator to keep the magnetic field lines of force as strong as possible. As the rotor spins, the alternate north and south poles of the rotor fingers pass each loop in the stator winding inducing an alternate amperage and voltage in the winding. This alternating current is then rectified by the diodes.

The slip ring end-frame contains 6 diodes, which are electrical rectifying devices. The diodes, three negative and three positive, act as one way valves formatting current to pass freely in one direction but not in the other. By their combined action, the alternating current generated is rectified to direct current.

Book Index of automobile anatomy of petrol engine

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BATTERY SPECIFIC GRAVITY TEST:

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By using a hydrometer, the specific gravity of the electrolyte solution in a battery can be determined. The battery specific gravity is an indication of the battery state of charge. If the state of charge is low, the hydrometer will read low. If the state of charge is high, the hydrometer will read high. As an example, a reading from 1.260 to 1.280 indicates a fully-charged battery. A reading from 1.200 indicates a battery is in a discharged condition and cannot give satisfactory service.

The definition of specific gravity is the weight of the liquid compared to the weight of an equal volume of water. The specific gravity of chemically pure water at 80°F. is “one”. Therefore, by knowing the specific gravity of sulphuric acid, we can accurately measure the ratio of sulphuric acid to water in the battery electrolyte solution.

As a battery becomes discharged, a portion of the acid in the solution, becomes soaked into the plates, so to speak. This leaves the solution with a weaker acid content and consequently in a weaker state of specific gravity. When the battery is recharged, the acid is driven from the plates back into the solution, restoring the solution and the specific gravity to full strength. The hydrometer reading indicates either of these conditions thereby revealing the battery state of charge.

After activating a dry-charge battery, check the specific gravity. The gravity reading should be 1.260 or slightly higher. If the electrolyte level drops shortly after the initial fill, due to the plates and separators absorbing some of the solution, add more electrolyte to bring the solution up to the proper level. When so instructed, charge the battery at 15 amperes for 10 minutes before installing the battery to assure a full charge.

THE STARTER INSULATED CIRCUIT TEST:

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Loose or dirty connections or defective cables represent a power loss between the battery and the starter. Circuit resistance tests are made to determine if the insulated cable, switches, and ground connections can carry the current demanded by the starter. This resistance is indicated by a voltage drop. VOLTAGE DROP is the voltage expended in overcoming resistance in a given circuit. Permissible voltage drop in the average starter insulated circuit is 0.3 volt. The voltage drop allowed in this circuit is 0.1 of a volt per cable or switch. If the resistance is excessive, the result will be an extreme power loss in the starting system. When a starter motor is in operation, the high-amperage draw magnifies this seemingly low resistance value. This greatly reduces the efficiency of the entire starting system. Therefore, the circuit resistance tests are performed while the system is under normal cranking load.

To test the insulated circuit, one voltmeter lead is connected to the insulated battery terminal and the other voltmeter lead is connected to the large armature terminal on the starting motor or solenoid. Crank the engine and observe the voltmeter. If the voltage drop for the entire circuit is not excessive, objectionable resistance does not exist and no further testing is necessary. If the test results indicate excessive resistance, separate detailed tests of each component in the circuit must be conducted.

Starter insulated circuit tests

STARTING SYSTEM:

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A starter motor, ignition switch, battery, and cables, which comprise the starting circuit, provide the power for cranking the engine. The motor receives electrical power from the battery and converts it into mechanical power which is transmitted to the engine through a drive pinion gear and the flywheel ring gear. The ignition switch controls the operation by making and breaking the circuit between the battery and the starter motor.

THE STARTING SYSTEM

The battery, starter motor, ignition switch, and the wiring are all designed for the high current flow needed to produce efficient cranking power. The condition of these components is extremely critical, as even a small amount of resistance can cause a marked reduction in cranking ability. This indicates the necessity for testing the various components in the starting system.

The starter motor is connected in series with the battery. This is shown as the high-amperage circuit. The solenoid is the switch between the battery and the starter, and is activated by a low current carrying circuit known as the control circuit. The another work done by solenoid is to engage the drive pinion gear of starter motor with the flywheel ring gear, when the ignition switch starts to crank the engine.

STARTER GROUND CIRCUIT TEST:

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To test the starter ground circuit, connect one voltmeter lead to the ground post of the battery, and the other lead to a good ground on the starter motor. Operate the starter motor and note the voltage drop. Permissible voltage drop is 0.2 volt.

To make the solenoid circuit resistance test, connect one lead to the insulated battery post and the other lead to the proper control circuit terminal on the solenoid. Again note the voltage drop with starting system under load. Permissible voltage drop is 0.2 volt.

A higher than permissible voltage drop reading indicates at least one point of high resistance in the circuit. Separate detailed tests of each component and each connection in the circuit must be conducted. when the point of high resistance is located, the poor connection must be cleaned or tightened or the defective unit or cable must be repaired, as the case may be.