In spark ignition engines, a device is required to ignite the compressed air-fuel mixture at the end of compression stroke. Ignition system fulfills this requirement. It is a part of electrical system which carries the electric current at required voltage to the spark plug which generates spark at correct time. It consists of a battery, switch, distributor ignition coil, spark plugs and necessary wiring.
A compression ignition engine, i.e. a diesel engine does not require any ignition system. Because, self ignition of fuel air mixture takes place when diesel is injected in the compressed air at high temperature at the end of compression stroke.
There are three types of ignition systems which are used in petrol engines.
In battery ignition system, the current in the primary winding is supplied by a battery whereas it is supplied by a magneto in magneto ignition system.
Battery ignition system is used in cars and light truck. Magneto ignition system is used in some scooters. Both the systems work on the principle of mutual electromagnetic induction. Electronic ignition systems use solid state devices such as transistors and capacitors.
Battery ignition system consists of a battery of 6 or 12 volts, ignition switch, induction coil, contact breaker, condenser, distributor and spark plugs. A typical battery ignition system for four cylinder SI engine has been shown in Figure
The primary circuit consists of battery, switch, primary winding and contact breaker point which is grounded. A condenser is also connected in parallel to the contact breaker points. One end of the condenser is grounded and other connected to the contact breaker arm. It is provided to avoid sparking at contact breaker points so as to increase their life.
The secondary ignition circuit consists of secondary winding distributors and spark plugs.
All spark plugs are grounded.
The ignition coil steps up 12 volts (or 6 volt) supply to a very high voltage which may range from 20,000 to 30,000 volts. A high voltage is required for the spark to jump across the spark plug gas. This spark ignites the air-fuel mixture as the end of compression stroke. The rotor of the distributor revolves and distributors the current to the four segments which send the current to different spark plugs. For a 4-cylinder engine the cam of the contact breaker has four lobes. Therefore, it makes and breaks the contact of the primary circuit four times in every revolution of cam. Because of which current is distributed to all the spark plugs in some definite sequence.
The primary winding of ignition coil has less number of turns (e.g. 200 turns) of thick wire. The secondary winding has relatively large number of turns (e.g. 20,000 turns) of thin wire.When ignition switch in turned on, the current flows from battery to the primary winding. This produces magnetic field in the coil. When the contact point is open, the magnetic field collapses and the movement of the magnetic field induces current in the secondary winding of ignition coil. As the number of turns in secondary winding are more, a very high voltage is produced across the terminals of secondary.
The distributor sends this high voltage to the proper spark plug which generates spark for ignition of fuel-air mixture. In this way, high voltage current is passed to all spark in a definite order so that combustion of fuel-air mixture takes place in all cylinders of the engine.
A ballast register is connected in series in primary circuit to regulate the current. At the time of starting this register is bypassed so that more current can flow in this circuit. The breaker points are held by a spring except when they are forced apart by lobes of the cam
This system consists of a magneto in place of a battery. So, the magneto produces and supplies current in primary winding. Rest of the system is same as that in battery ignition system. A magneto ignition system for a four cylinder SI engine has been shown in Figure .
The magneto consists of a fixed armature having primary and secondary windings and a rotating magnetic assembly. This rotating assembly is driven by the engine.
Rotation of magneto generates current in primary winding having small number of turns. Secondary winding having large number of turns generates high voltage current which is supplied to distributor. The distributor sends this current to respective spark plugs. The magneto may be of rotating armature type or rotating magnet type. In rotating armature type magneto, the armature having primary and secondary windings and the condenser rotates between the poles of a stationary horse shoe magnet. In magneto, the magnetic field is produced by permanent magnets.
Functions of various components used in battery (coil) ignition and magneto-ignition systems are discussed here in brief.
It is an important component of electrical system. The battery supplies the necessary current to the primary winding of ignition coil which is converted into high voltage current to produce spark. It also supplied current to run the starting motor when engine is cranked for starting. A battery stores energy in the form of chemical energy and supplies it for running lights and other accessories of an automobile. Lead-acid battery is commonly used in most of the automobiles.
The ignition coil is step up transformer to increase the voltage form 12 volt or 6 volt to 20000- 30000 volts. It consists of a primary winding and a secondary winding wound on a laminated soft iron core. Primary winding contains about 300 turns made of thick wire. Secondary consists of about 20000 turns of thin wire. In a can type coil, secondary is wound on the soft core over which primary is wound. This assembly is housed in a steel casing fitted with a cap. The cap is made of insulating material. The terminals for electrical connections are provided in cap. To save the windings from moisture and to improve insulation, windings are dipped in oil.
Contact breaker is required to make contact and break contact of the primary circuit of ignition system. It consists of two contact breaker points as shown in Figures. One point remains fixed while the other can move. A cam is sued to move the movable point. As cam moves, the contact is made and broken alternately. Primary circuit breaks when the breaker points open. Magnetic field collapses due to this. This produces high voltage current in the secondary winding which is supplied to the distributor. This current is distributor to proper spark plug where it produces spark for ignition of fuel-air mixture.
The function of the condenser in the ignition system is to absorb and store the inductive current generated in the coil. If condenser is not provided, the induced current will cause arcing at the breaker points. This will cause burning of the breaker points.
The distributor sends the high voltage current, generated in the secondary winding, to the proper spark plug at proper time. If the automobile is having a four cylinder engine, it will have four spark plugs.
The cap of the distributor is connected to the secondary winding of coil. It has a rotor which rotates and comes in contact with the terminals (4 in number for 4 spark plugs) placed around the rotor. As the rotor comes in contact with the terminals (numbered 1, 2, 3 and 4 in Figures), the current is passed to the respective spark plug at proper time when spark is needed.
The function of the ignition switch is to connect the battery and starting motor in the automobiles having self starting system.
Example : In car, jeep, etc.
Its function is to connect battery to induction coil in the battery ignition system.
The function of the spark plug is to produce spark between its electrodes. This spark is used to ignite the fuel-air mixture in the spark ignition (SI) engines.
Magneto is used in magneto ignition system. Magneto is a kind of generator to provide electrical energy to run the ignition system. It is replacement of battery for ignition. When it is rotated by the engine, it produces high voltage current to be supplied to spark plugs through the distributor.
The purpose of spark advance mechanism is to assure that under every condition of engine operation, ignition takes place at the most favorable instant in time i.e. most favorable from a standpoint of engine power, fuel economy and minimum exhaust dilution. By means of these mechanisms the advance angle is accurately set so that ignition occurs before TDC point of the piston. The engine speed and the engine load are the control quantities required for the automatic adjustment of the ignition timing. Most of the engines are fitted with mechanisms which are integral with the distributor and automatically regulate the optimum spark advance to account for change of speed and load. The two mechanisms used are:
The centrifugal advance mechanism controls the ignition timing for full- load operation. The adjustment mechanism is designed so that its operation results in the desired advance of the spark. The cam is mounted, movably, on the distributor shaft so that as the speed increases, the flyweights which are swung farther and farther outward, shaft the cam in the direction of shaft rotation. As a result, the cam lobes make contact with the breaker lever rubbing block somewhat earlier, thus shifting the ignition point in the early or advance direction.
Depending on the speed of the engine, and therefore of the shaft, the weights are swung outward a greater or a lesser distance from the center. They are then held in the extended position, in a state of equilibrium corresponding to the shifted timing angle, by a retaining
spring which exactly balances the centrifugal force. The weights shift the cam either or a rolling contact or sliding contact basis; for this reasons we distinguish between the rolling contact type and the sliding contact type of centrifugal advance mechanism.
The beginning of the timing adjustment in the range of low engine speeds and the continues adjustment based on the full load curve are determined by the size of the weights by the shape of the contact mechanisms (rolling or sliding contact type), and by the retaining springs, all of which can be widely differing designs. The centrifugal force controlled cam is fitted with a lower limit stop for purposes of setting the beginning of the adjustment, and also with an upper limit stop to restrict the greatest possible full load adjustment.
Vacuum advance mechanism shifts the ignition point under partial load operation.
The adjustment system is designed so that its operation results in the prescribed partial load advance curve. In this mechanism the adjustment control quantity is the static vacuum prevailing in the carburetor, a pressure which depends on the position of the throttle valve at any given time and which is at a maximum when this valve is about half open. This explains the vacuum maximum.
The diaphragm of a vacuum unit is moved by changes in gas pressure. The position of this diaphragm is determined by the pressure differential at any given moment between the prevailing vacuum and atmospheric pressure. The beginning of adjustment is set by the pre- established tension on a compression spring. The diaphragm area, the spring force, and the spring rigidity are all selected in accordance with the partial –load advance curve which is to be followed and are all balanced with respect to each other. The diaphragm movement is transmitted through a vacuum advance arm connected to the movable breaker plate, and this movement shifts the breaker plate an additional amount under partial load Ignition Systems condition in a direction opposite to the direction of rotation of the distributor shaft. Limit stops on the vacuum advance arm in the base of the vacuum unit restrict the range of adjustment.
The vacuum advance mechanism operates independent of the centrifugal advance mechanism. The mechanical interplay between the two advance mechanisms, however, permits the total adjustment angle at any given time to be the result of the addition of the shifts provided by the two individual mechanisms operates in conjunction with the engine is operating under partial load.
The simple requirement of a spark plug is that it must allow a spark to form within the combustion chamber, to initiate burning. In order to do this the plug has to withstand a number of severe conditions. Consider, as an example, a four-cylinder four-stroke engine with a compression ratio of 9:1, running at speeds up to 5000 rev/min. The following conditions are typical. At this speed the four-stroke cycle will repeat every 24 ms.
_ End of induction stroke –0.9 bar at 65 ° C.
_ Ignition firing point –9 bar at 350 ° C.
_ Highest value during power stroke –45 bar at 3000 ° C.
_ Power stroke completed –4 bar at 1100 ° C.
Besides the above conditions, the spark plug must withstand severe vibration and a harsh chemical environment. Finally, but perhaps most important, the insulation properties must withstand voltage pressures up to 40kV.
The centre electrode is connected to the top terminal by a stud. The electrode is constructed of a nickel-based alloy. Silver and platinum are also used for some applications. If a copper core is used in the electrode this improves the thermal conduction properties.
The insulating material is ceramic-based and of a very high grade. Aluminium oxide, Al2O3 (95% pure), is a popular choice, it is bonded into the metal parts and glazed on the outside surface. The properties of this material, which make it most suitable, are as follows:
_ Young’s modulus: 340kN/mm2.
_ Coefficient of thermal expansion: 7.8 x10K-1.
_ Thermal conductivity: 15–5W/mK (Range 200–900 ° C).
_ Electrical resistance: 1013Ω/m.
The above list is intended as a guide only, as actual values can vary widely with slight manufacturing changes. The electrically conductive glass seal between the electrode and terminal stud is also used as a resistor. This resistor has two functions. First, to prevent burn-off of the centre electrode, and secondly to reduce radio interference. In both cases the desired effect is achieved because the resistor damps the current at the instant of ignition.
Flash-over, or tracking down the outside of the plug insulation, is prevented by ribs that effectively increase the surface distance from the terminal to the metal fixing bolt, which is of course earthed to the engine.
The material chosen for the spark plug electrode must exhibit the following properties:
_ High thermal conductivity.
_ High corrosion resistance.
_ High resistance to burn-off.
For normal applications, alloys of nickel are used for the electrode material. Chromium, manganese, silicon and magnesium are examples of the alloying constituents. These alloys exhibit excellent properties with respect to corrosion and burn-off resistance.
To improve on the thermal conductivity, compound electrodes are used. These allow a greater nose projection for the same temperature range, as discussed in the last section. A common example of this type of plug is the copper-core spark plug.
Silver electrodes are used for specialist applications as silver has very good thermal and electrical properties. Again, with these plugs nose length can be increased within the same temperature range.
The thermal conductivity of some electrode materials is listed for comparison.
_ Silver 407 W/m K
_ Copper 384 W/m K
_ Platinum 70 W/m K
_ Nickel 59 W/m K
Compound electrodes have an average thermal conductivity of about 200 W/m K. Platinum tips are used for some spark plug applications due to the very high burn-off resistance of this material. It is also possible because of this to use much smaller diameter electrodes, thus increasing mixture accessibility. Platinum also has a catalytic effect, further accelerating the combustion process.
Spark plug electrode gaps have, in general, increased as the power of the ignition systems driving the spark has increased. The simple relationship between plug gap and voltage required is that, as the gap increases so must the voltage (leaving aside engine operating conditions). Furthermore, the energy available to form a spark at a fixed engine speed is constant, which means that a larger gap using higher voltage will result in a shorter duration spark. A smaller gap
will allow a longer duration spark. For cold starting an engine and for igniting weak mixtures, the duration of the spark is critical. Likewise the plug gap must be as large as possible to allow easy access for the mixture in order to prevent quenching of the flame.
The final choice is therefore a compromise reached through testing and development of a particular application. Plug gaps in the region of 0.6–1.2 mm seem to be the norm at present.
Unit-4
Electrical and Electronic Ignition Systems
Electronic Ignition System
Electronic ignition is now fitted to almost all spark ignition vehicles. This is because the conventional mechanical system has some major disadvantages.
These problems can be overcome by using a power transistor to carry out the switching function and a pulse generator to provide the timing signal. Very early forms of electronic ignition used the existing contact breakers as the signal provider. This was a step in the right direction but did not overcome all the mechanical limitations, such as contact bounce and timing slip. Most (all?) systems nowadays are constant energy, ensuring high performance ignition even at high engine speed. Figure the circuit of a standard electronic ignition system.
Distributorless ignition systems (DIS) have been around for almost a decade now, and have eliminated much of the maintenance that used to be associated with the ignition system. No distributor means there's no distributor cap or rotor to replace, and no troublesome vacuum or mechanical advance mechanisms to cause timing problems. Consequently, DIS ignition systems are pretty reliable.
Even so, that doesn't mean they are trouble-free. Failures can and do occur for a variety of reasons. So knowing how to identify and diagnose common DIS problems can save you a lot of guesswork the next time you encounter an engine that cranks but refuses to start, or one that runs but is missing or misfiring on one or more cylinders.
If an engine cranks but won't start, is it fuel, ignition or compression? Ignition is usually the easiest of the three to check because on most engines, all you have to do is pull off a plug wire and check for spark when the engine is cranked. On coil-over-plug DIS systems, there are no plug wires so you have to remove a coil and use a plug wire or adapter to check for a spark.
If there's no spark in one cylinder, try another. No spark in any cylinder would most likely indicate a failed DIS module or crankshaft position sensor. Many engines that are equipped with electronic fuel injection also use the crankshaft position sensor signal to trigger the fuel injectors. So, if there's no spark and no injector activity, the problem is likely in the crank position sensor. No spark in only one cylinder or two cylinders that share a coil would tell you a coil has probably failed.
Distributorless ignition system used extensively by Ford incorporates all the features of electronic spark advance systems, except a special type of ignition coil is used in place of HT distributor. The system is generally used only on four- or six-cylinder engines, because the control system becomes highly complex for higher number of cylinders. It works on the principle of the lost spark. The spark distribution is achieved by the help of two double ended coils, fired alternately by the ECU. The ignition timing is obtained from a crankshaft speed and position sensor as well as through load and other corrections. When one of the coils is fired, a spark is delivered to two engine cylinders, either 1 and 4, or 2 and 3. The spark delivered to the cylinder on the compression stroke ignites the mixture as normal. Whereas the spark in other cylinder causes no effect, as this cylinder is just completing its exhaust stroke. Because of the low compression and the exhaust gases in the lost spark cylinder, the voltage only of about 3 kV is needed for the spark to jump the gap. This is similar to cap voltage of the more conventional
rotor arm. The spark produced in the compression cylinder is therefore not affected. It may be noted that the spark on one of the cylinders jumps from the earth electrode to the spark plug centre, whereas in others it jumps from the centre electrode. This is because the energy available from modern constant energy systems produces a spark of suitable quality in either direction. However, the disadvantage is that the spark plugs may wear more quickly with this system.
System Components
The distributorless ignition system contains three main components such as the electronic module, a crankshaft position sensor and the distributorless ignition coil. Many systems use a manifold absolute pressure sensor, integrated in the module. The module functions almost in the same way as the electronic spark advance system.
The crankshaft position sensor operates in the similar way to the one described in the previous section. It is also a reluctance sensor positioned against the front of the flywheel or against a reluctor wheel just behind the front crankshaft pulley. The tooth pattern uses 36-1 teeth, which are spaced at 10 degree intervals, with a gap for the 36th tooth. The missing tooth is located at 90 degrees before TDC for numbers 1 and 4 cylinders. This reference position is located a fixed number of degrees before TDC for calculating the timing or ignition point as a fixed angle after the reference mark.
The distributorless ignition coil (Fig. 16.56) has a low tension winding, which is supplied with battery voltage to a centre terminal. The appropriate half of the winding is then connected to earth in the module. The high tension windings are separate and are specific to cylinders 1 and 4, or 2 and 3. Figure 16.57 shows a typical Ford distributorless ignition coil. The Citroen 2 CV has been using a double ended ignition coil together with contact breakers for many years.
Fault Diagnosis
The distributorless ignition system is highly reliable, specifically because it does not have any moving parts. The normal manufacturers servicing schedule should be adhered to for the replacement of spark plugs (often after 19,200 km operation). Some problems may be faced when trying to examine HT oscilloscope patterns, due to the lack of a king lead. This can be overcome by using a special adapter and shifting the sensing clip to each lead in turn. An ohmmeter can be used to test the distributorless ignition coil. The resistance of each primary winding should be 0.5 Q and the secondary windings between 11 and 16 kQ. The coil produces open circuit voltage in excess of 37 kV. The plug leads have integral retaining clips to prevent water ingress and vibration problems. The maximum resistance for the HT leads is 30 kQ per lead. Except for the octane adjustment on some models no service adjustments are possible with this system. This adjustment involves connecting two pins together on the module for normal operation, or earthing one pin or the other to change to a different fuel. The actual procedure as specified by the manufacturer for each particular model should be followed.
Electronic Ignition System is as follow :
It mainly consists of 6-12 V battery, ignition switch, DC to DC convertor, charging resistance, tank capacitor, Silicon Controlled Rectifier (SCR), SCR-triggering device, step up transformer, spark plugs.
A 6-12 volt battery is connected to DC to DC converter i.e. power circuit through the ignition switch, which is designed to give or increase the voltage to 250-350 volts. This high voltage is used to charge the tank capacitor (or condenser) to this voltage through the charging resistance. The charging resistance is also so designed that it controls the required current in the
SCR.
Depending upon the engine firing order, whenever the SCR triggering device, sends a pulse, then the current flowing through the primary winding is stopped. And the magnetic field begins to collapse. This collapsing magnetic field will induce or step up high voltage current in the secondary, which while jumping the spark plug gap produces the spark, and the charge of air fuel mixture is ignited.
Advantages
Disadvantages
The development of synthetic piezo-electric materials producing about 22 kV by mechanical loading of a small crystal resulted in some ignition systems for single cylinder engines. But due to difficulties of high mechanical loading need of the order of 500 kg timely control and ability to produce sufficient voltage, these systems have not been able to come up.
Due to the increased emphasis on exhaust emission control, there has been a sudden interest in exhaust gas recirculation systems and lean fuel-air mixtures. To avoid the problems of burning of lean mixtures, the Texaco Ignition system has been developed. It provides a spark of controlled duration which means that the spark duration in crank angle degrees can be made constant at all engine speeds. It is a AC system. This system consists of three basic units, a power unit, a control unit and a distributor sensor. This system can give stable ignition up to A/F ratios as high as 24 : 1.
Triggering is arranged so that the ignition coil is charged in sufficient time before the actual ignition point. This requires the formation of a dwell period (coil saturation time) in the ignition system. The energy to be released as a spark is usually stored in a coil as magnetic energy (with conventional systems). In other cases, this can be replaced with a capacitor as electrostatic energy, such as in a capacitive discharge ignition system (CDI), in which case the role of the coil changes to simply that of an energy transfer device. The high tension results from disconnecting the primary inductor from the power supply followed by transformation.
The high tension is then applied via the distributor to the cylinder currently performing the working stroke. All this combines to produce the required firing voltage, which is determined by the cylinder pressure, a byproduct of the inlet charge and compression, combined with the gap, temperature and shape of the spark plug electrode. The ignition system will then only deliver the voltage necessary to fire the spark plug. If all is well, the mixture will be successfully ignited. If insufficient energy is available, ignition does not occur, thus allowing a misfire. This is why adequate ignition must be provided.
The need for higher mileage, reduced emissions and greater reliability has led to the development of the electronic ignition systems. These systems generate a much stronger spark which is needed to ignite leaner fuel mixtures. Breaker point systems needed a resistor to reduce the operating voltage of the primary circuit in order to prolong the life of the points. The primary circuit of the electronic ignition systems operates on full battery voltage which helps to develop a stronger spark. Spark plug gaps have widened due to the ability of the increased voltage to jump the larger gap. Cleaner combustion and less deposit have led to longer spark plug life.
On some systems, the ignition coil has been moved inside the distributor cap. This system is said to have an internal coil as opposed to the conventional external one.
Electronic Ignition systems are not as complicated as they may first appear. In fact, they differ only slightly from conventional point ignition systems. Like conventional ignition systems, electronic systems have two circuits: a primary circuit and a secondary circuit. The entire secondary circuit is the same as in a conventional ignition system. In addition, the section of the primary circuit from the battery to the battery terminal at the coil is the same as in a conventional ignition system.
Electronic ignition systems differ from conventional ignition systems in the distributor component area. Instead of a distributor cam, breaker plate, points, and condenser, an electronic ignition system has an armature (called by various names such as a trigger wheel, reluctor, etc.), a pickup coil (stator, sensor, etc.), and an electronic control module.
Essentially, all electronic ignition systems operate in the following manner: With the ignition switch turned on, primary (battery) current flows from the battery through the ignition switch to the coil primary windings. Primary current is turned on and off by the action of the armature as it revolves past the pickup coil or sensor. As each tooth of the armature nears the pickup coil, it
creates a voltage that signals the electronic module to turn off the coil primary current. A timing circuit in the module will turn the current on again
after the coil field has collapsed. When the current is off, however, the magnetic field built up in the coil is allowed to collapse, which causes a high voltage in the secondary windings of the coil. It is now operating on the secondary ignition circuit, which is the same as in a conventional ignition system.
Troubleshooting electronic ignition systems ordinarily requires the use of a voltmeter and/or an ohmmeter. Sometimes the use of an ammeter is also required. Because of differences in design and construction, troubleshooting is specific to each system. A complete troubleshooting guide for you particular application can be found in the Chilton's Total Car Care manual.
Wiring, Lighting and Other Instruments and Sensors AUTOMOTIVE WIRING
Electrical power and control signals must be delivered to electrical devices reliably and safely so that the electrical system functions are not impaired or converted to hazards. To fulfill power distribution military vehicles, use one-and two-wire circuits, wiring harnesses, and terminal connections.
Among your many duties will be the job of maintaining and repairing automotive electrical systems. All vehicles are not wired in exactly the same manner; however, once you understand the circuit of one vehicle, you should be able to trace an electrical circuit of any vehicle using wiring diagrams and color codes.
Tracing wiring circuits, particularly those connecting lights or warning and signal devices, is no simple task. The branch circuits making up the individual systems have one wire to conduct electricity from the battery to the unit requiring it and ground connections at the battery and the unit to complete the circuit. These are called ONE-WIRE CIRCUITS or branches of a GROUND RETURN SYSTEM. In automotive electrical systems with branch circuits that lead to all parts of the equipment, the ground return system saves installation time and eliminates the need for an additional wiring to complete the circuit. The all-metal construction of the automotive equipment makes it possible to use this system.
The TWO-WIRE CIRCUIT requires two wires to complete the electrical circuit- one wire from the source of electrical energy to the unit it will operate, and another wire to complete the circuit from the unit back to the source of the electrical power. Two-wire circuits provide positive connection for light and electrical brakes on some trailers. The coupling between the trailer and the equipment, although made of metal and a conductor of electricity, has to be jointed to move freely. The rather loose joint or coupling does not provide the positive and continuous connection required to use a ground return system between two vehicles. The two- wire circuit is commonly used on equipment subject to frequent or heavy vibrations. Tracked equipment, off-road vehicles (tactical), and many types of construction equipment are wired in this manner.
Some vehicle application requires a separate insulated-cable system for both the feed and the return conductors. It is also safer because with separate feed and return cables, it is practically impossible for the cable conductors to short even if chafed and touching any of the metal bodywork, as the body is not live since it is not a part of the electrical circuits. From the safety reasons, an insulated return is essential for vehicles transporting highly flammable liquids and gases, where a spark could very easily set off an explosion or a fire. The vehicles, such as coaches and double-decker buses use large quantity of plastic panelling. For these vehicles an insulated return is more reliable and safer. The insulated return off course uses extra cable that makes the overall wiring harness heavier, less flexible, and bulky, consequently increases the cost to some extent.
Earth Return.
All electrical circuits incorporate both a feed and a return conductor between the battery and the component requiring supply of electrical energy. The vehicle with a metal structure can be used as one of the two conducting paths. This is called as the earth return (Fig. 13.51). A live feed wire cable forms the other conductor. To complete the earth-return path, one end of a short thick cable is bolted to the chassis structure while the other end is attached to one of the battery terminal posts. The electrical component is also required to be earthed in a similar way. Only one battery-to-chassis conductor is necessary for a complete vehicle’s wiring system and similarly any number of separate earth-return circuits can be wired. An earth-return system, therefore, reduces and simplifies the amount of wiring so that it is easy to trace electrical faults.
Positive and Negative earthing
In the beginning, it was the general practice of earthing the negative terminal of the battery, whereas the positive current was supplied to the electrical units. The negative earthing system is still used in the cars of American make.
In some countries, the negative earth system has been replaced by the positive earth system. This is because the positive earth system possesses certain advantages over the negative earth system. These advantages concern the temperature of the central spark plug electrode and the corrosion of some parts, it is well known fact that the positive terminal of the leadacid battery is attacked by the liberated gases. If this is the live terminal and the negative terminal earthed, the exposed part of the positive will become corroded.
Further it is also a well known fact that the positive point of the spark plug wears away more quickly than the negative point. In view of this fact, the central electrode of the plug will wear away quickly if made electrode of the plug will wear away quickly if made positive when compared with the metal electrode of the shell. Alternatively, the central electrode of the plug will have much longer life if made negative by earthing the positive terminal of the battery.
Another factor which plays an important role in the voltage requirements of a spark plug is the temperature of the negative electrode. The hotter this electrode is, the lower will be the voltage required for producing the spark, It has also been observed that more uniform voltages at the sparking points have been obtained with the central electrode being negative. Further, the metal rotor arm of the distributor, if made negative, will wear at a slower rate than if it were made positive.
There is an additional advantage of the positive earth method in the ignition coil elements the primary circuit voltage is added to the secondary circuit voltage, making it more economical.
Recently, with the adoption of alternators in place of generators, it has been observed that employing negative earth method is advantageous along with an ac current rectifier having
transistors and diodes. This has meant shifting back to the negative earth method. However it is worth mentioning that the important advantages of the positive earth for the ignition system still hold good.
The lighting circuit includes the battery, vehicle frame, all the lights, and various switches that control their use. The lighting circuit is known as a single-wire system since it uses the vehicle frame for the return.
The complete lighting circuit of a vehicle can be broken down into individual circuits, each having one or more lights and switches. In each separate circuit, the lights are connected in parallel, and the controlling switch is in series between the group of lights and the battery.
The marker lights, for example, are connected in parallel and are controlled by a single switch. In some installations, one switch controls the connections to the battery, while a selector switch determines which of two circuits is energized. The headlights, with their high and low beams, are an example of this type of circuit.
In some instances, such as the courtesy lights, several switches may be connected in parallel so that any switch may be used to turn on the light. When a wiring diagram is being studied, all light circuits can be traced from the battery through the ammeter to the switch (or switches) to the individual light.
Small gas-filled incandescent lamps with tungsten filaments are used on automotive and construction equipment. The filaments supply the light when sufficient current is flowing through them. They are designed to operate on a low voltage current of 12 or 24 volts, depending upon the voltage of the the vehicle will be of the single-or double-contact small one-half- candlepower bulbs to large 50- candlepower bulbs. The greater the candlepower of the lamp, the more current it requires when lighted. Lamps are identified by a number on the base. When you replace a lamp in a vehicle, be sure the new lamp is of the proper rating. The lamps within Lamps are rated as to size by the candlepower (luminous intensity) they produce. They range from types with nibs to fit bayonet sockets, as shown in lamp is also whiter than a conventional lamp, which increases lighting ability.
The headlights are sealed beam lamps that illuminate the road during nighttime operation. Headlights consist of a lens, one or two elements, and a integral reflector. When current flows through the element, the element gets white hot and glows. The reflector and lens direct the light forward.
Many modern passenger vehicles use halogen headlights. A halogen headlight contains a small, inner halogen lamp surrounded by a conventional sealed housing. A halogen headlamp increases light output by 25 percent with no increase in current. The halogen The headlight switch is an ON/ OFF switch and rheostat (variable resistor) in the dash panel ) or on the steering column. The headlight switch controls current flow to the lamps of the headlight system. The rheostat is for adjusting the brightness of the instrument panel lights.
Military vehicles that are used in tactical situations are equipped with a headlight switch that is integrated with the blackout lighting switch. An important feature of this switch is that it reduces the possibility of accidentally turning on the lights in a blackout.
With no lights on, the main switch can be turned to the left without operating the mechanical switch to get blackout marker lights (including blackout taillights and stoplights) and blackout driving lights. But for stoplights for daylight driving or headlights for ordinary night driving, you must first lift the mechanical switch lever and then turn the main switch to the right. The auxiliary switch gives panel lights when the main switch is in any of its ON positions. But it will give parking lights only when the main switch is in service drive (to the extreme right). When the main switch is off, the auxiliary switch should not be moved from the OFF position.
The dimmer switch controls the high and low headlamp beam function and is normally mounted on the floorboard or steering column. When the operator activates the dimmer switch, it changes the electrical connection to the headlights.
In one position, the high beams are turned on, and, in the other position, the dimmer changes them to low beam.
The headlights can be aimed using a mechanical aimer or a wall screen. Either method assures that the headlight beams point in the direction specified by the vehicle manufacturer. Headlights that are aimed too high can blind oncoming vehicles. Headlights that are aimed too low or to one side will reduce the operator's visibility.
Vehicles that operate on any public road must be equipped with turn signals. These signals indicate a left or right turn by providing a flashing light signal at the rear and front of the vehicle.
The turn-signal switch is located on the steering column. It is designed to shut off automatically after the turn is completed by the action of the canceling cam. A common design for a turn-signal system is to use the same rear light for both the stop and turn signals. This somewhat complicates the design of the switch in that the stoplight circuit must pass through the turn-signal switch. When the turn-signal switch is turned off, it must pass stoplight current to the rear lights. As a left or right turn signal is selected, the stoplight circuit is open and the turn- signal circuit is closed to the respective rear light.
The turn signal flasher unit creates the flashing of the turn signal lights. It consists basically of a bimetallic (two dissimilar metals bonded together) strip wrapped in a wire coil. The bimetallic strip serves as one of the contact points.
When the turn signals are actuated, current flows into the flasher- first through the heating coil to the bimetallic strip, then through the contact points, then out of the flasher, where the circuit is completed through the turn-signal light. This sequence of events will repeat a few times a second, causing a steady flashing of the turn signals.
In many modern cars the fuel pump is usually electric and located inside the fuel tank. The pump creates positive pressure in the fuel lines, pushing the gasoline to the engine. The higher gasoline pressure raises the boiling point. Placing the pump in the tank puts the component least likely to handle gasoline vapor well (the pump itself) farthest from the engine, submersed in cool liquid. Another benefit to placing the pump inside the tank is that it is less likely to start a fire. Though electrical components (such as a fuel pump) can spark and ignite fuel vapors, liquid fuel will not explode (see flammability limit) and therefore submerging the pump in the tank is one of the safest places to put it. In most cars, the fuel pump delivers a constant flow of gasoline to the engine; fuel not used is returned to the tank. This further reduces the chance of the fuel boiling, since it is never kept close to the hot engine for too long.
The ignition switch does not carry the power to the fuel pump; instead, it activates a relay which will handle the higher current load. It is common for the fuel pump relay to become oxidized and cease functioning; this is much more common than the actual fuel pump failing. Modern engines utilize solid-state control which allows the fuel pressure to be controlled via
pulse-width modulation of the pump voltage. This increases the life of the pump, allows a smaller and lighter device to be used, and reduces electrical load.
Cars with electronic fuel injection have an electronic control unit (ECU) and this may be programmed with safety logic that will shut the electric fuel pump off, even if the engine is running. In the event of a collision this will prevent fuel leaking from any ruptured fuel line. Additionally, cars may have an inertia switch (usually located underneath the front passenger seat) that is "tripped" in the event of an impact, or a roll-over valve that will shut off the fuel pump in case the car rolls over.
Some ECUs may also be programmed to shut off the fuel pump if they detect low or zero oil pressure, for instance if the engine has suffered a terminal failure (with the subsequent risk of fire in the engine compartment).
The fuel sending unit assembly may be a combination of the electric fuel pump, the filter, the strainer, and the electronic device used to measure the amount of fuel in the tank via a float attached to a sensor which sends data to the dash-mounted fuel gauge. The fuel pump by itself is a relatively inexpensive part. But a mechanic at a garage might have a preference to install the entire unit assembly.
Most fuel gauges are operated electrically and are composed of two units- the gauge, mounted on the instrument panel; and the sending unit, mounted in the fuel tank. The ignition switch is included in the fuel gauge circuit, so the gauge operates only when the ignition switch is in the ON position. Operation of the electrical gauge depends on either coil action or thermostatic action. The four types of fuel gauges are as follows:
The THERMOSTATIC FUEL GAUGE, SELF-REGULATING contains an electrically heated bimetallic strip that is linked to a pointer. A bimetallic strip consists of two dissimilar metals that, when heated, expand at different rates, causing it to deflect or bend. In the case of this gauge, the deflection of the bimetallic strip results in the movement of the pointer, causing the gauge to give a reading. The sending unit consists of a hinged arm with a float on the end. The movement of the arm controls a grounded point that makes contact with another point which is attached to an electrically heated bimetallic strip. The heating coils in the tank and the gauge are connected to each other in series.
The THERMOSTATIC FUEL GAUGE, EXTERNALLY REGULATED differs from a
self-regulating system in the use of a variable resistance fuel tank sending unit and an external
voltage-limiting device. The sending unit controls the gauge through the use of a rheostat (wire wound resistance unit whose value varies with its effective length). The effective length of the rheostat is controlled in the sending unit by a sliding brush that is operated by the float arm. The power supply to the gauge is kept constant through the use of a voltage limiter. The voltage limiter consists of a set of contact points that are controlled by an electrically heated bimetallic arm.
The THERMOSTATIC FUEL GAUGE, DIFFERENTIAL TYPE is similar to the other type of thermostatic fuel gauges, except that it uses two electrically heated bimetallic strips that share equally in operating and supporting the gauge pointer. The pointer position is obtained by dividing the available voltage between the two strips (differential). The tank unit is a rheostat type similar to that already described; however, it contains a wire-wound resistor that is connected between external terminals of one of the gauges of the bimetallic strip. The float arm moves a grounded brush that raises resistance progressively to one terminal, while lowering resistance to the other. This action causes the voltage division and resulting heat differential to the gauge strips formulating the gauge reading.
The MAGNETIC FUEL GAUGE consists of a pointer mounted on an armature. Depending upon the design, the armature may contain one or two poles. The gauge is motivated by a magnetic field that is created by two separate magnetic coils that are contained in the gauge. One of these coils is connected directly to the battery, producing a constant magnetic field. The other coil produces a variable field, whose strength is determined by a rheostat-type tank unit. The coils are placed 90 degrees apart.
A pressure gauge is used widely in automotive and construction applications to keep track of such things as oil pressure, fuel line pressure, air brake system pressure, and the pressure in the hydraulic systems. Depending on the equipment, a mechanical gauge, an electrical gauge, or an indicator lamp may be used.
TEMPERATURE GAUGE
The temperature gauge is a very important indicator in construction and automotive equipment. The most common uses are to indicate engine coolant, transmission, differential oil, and hydraulic system temperature. Depending on the type of equipment, the gauge may be mechanical, electric, or a warning light.
The ELECTRIC GAUGE may be the thermostatic or magnetic type, as described previously. The sending unit (fig. 2-83) that is used varies, depending upon application.
Both the mechanical speedometer and the tachometer consist of a permanent magnet that is rotated by a flexible shaft. Surrounding the rotating magnet is a metal cup that is attached to the indicating needle. The revolving magnetic field exerts a pull on the cup that forces it to rotate. The rotation of the cup is countered by a calibrated hairspring.
The influence of the hairspring and the rotating magnetic field on the cup produces accurate readings by the attached needle. The flexible shaft consists of a flexible outer casing that is made of either steel or plastic and an inner drive core that is made of wire-wound spring steel. Both ends of the core are molded square, so they can fit into the driving member at one end and the driven member at the other end and can transmit torque.
Gears on the transmission output shaft turn the flexible shaft that drives the speedometer. This shaft is referred to as the speedometer cable. A gear on the ignition distributor shaft turns the flexible shaft that drives the tachometer. This shaft is referred to as the tachometer cable.
The odometer of the mechanical speedometer is driven by a series of gears that originate at a spiral gear on the input shaft. The odometer consists of a series of drums with digits printed on the outer circumference that range from zero to nine. The drums are geared to each other so that each time the one furthest to the right makes one revolution, it will cause the one to its immediate left to advance one digit. The second to the right then will advance the drum to its immediate left one digit for every revolution it makes. This sequence continues to the left through the entire series of drums. The odometer usually contains six digits to record 99,999.9 miles or kilometers. However, models with trip odometers do not record tenths, thereby contain only five digits. When the odometer reaches its highest value, it will automatically reset to zero. Newer vehicles incorporate a small dye pad in the odometer to color the drum of its highest digit to indicate the total mileage is in excess of the capability of the odometer.
The electric speedometer and tachometer use a mechanically driven permanent magnet generator to supply power to a small electric motor. The electric motor then is used to rotate the input shaft of the speedometer or tachometer. The voltage from the generator will increase proportionally with speed, and speed will likewise increase proportionally with voltage enabling the gauges to indicate speed.
The signal generator for the speedometer is usually driven by the transmission output shaft through gears. The signal generator for the tachometer usually is driven by the distributor
through a power takeoff on gasoline engines. When the tachometer is used with a diesel engine, a special power takeoff provision is made, usually on the camshaft drive.
Electronic speedometers and Odometers are self-contained units that use an electric signal from the engine or transmission. They differ from the electric unit in that they use a generated signal as the driving force. The gauge is transistorized and will supply information through either a magnetic analog (dial) or light-emitting diode (LED) digital gauge display. The gauge unit derives its input signal in the following ways:
An electronic tachometer obtains a pulse signal from the ignition distributor, as it switches the coil on and off. The pulse speed at this point will change proportionally with engine speed. This is the most popular signal source for a tachometer that is used on a gasoline engine.
A tachometer that is used with a diesel engine uses the alternating current generated by the stator terminal of the alternator as a signal. The frequency of the ac current will change proportionally with engine speed.
An electronic speedometer derives its signal from a magnetic pickup coil that has its field interrupted by a rotating pole piece. The signal units operation is the same as the operation of the reluctor and pickup coil described earlier in this TRAMAN. The pickup coil is located strategically in the transmission case to interact with the reluctor teeth on the input shaft.
The horn currently used on automotive vehicles is the electric vibrating type. The electric vibrating horn system typically consists of a fuse, horn button switch, relay, horn assembly, and related wiring. When the operator presses the horn button, it closes the horn switch and activates the horn relay. This completes the circuit, and current is allowed through the relay circuit and to the horn.
Most horns have a diaphragm that vibrates by means of an electromagnetic. When the horn is energized, the electromagnet pulls on the horn diaphragm. This movement opens a set of contact points inside the horn. This action allows the diaphragm to flex back towards its normal position. This cycle is repeated rapidly. The vibrations of the diaphragm within the air column produce the note of the horn.
Tone and volume adjustments are made by loosening the adjusting locknut and turning the adjusting nut. This very sensitive adjustment controls the current consumed by the horn.
Increasing the current increases the volume. However, too much current will make the horn sputter and may lock the diaphragm.
When an electric horn will not produce sound, check the fuse, the connections, and test for voltage at the horn terminal. If the horn sounds continuously, a faulty horn switch is the most probable cause. A faulty horn relay is another cause of horn problems. The contacts inside the relay may be burned or stuck together.
The windshield wiper system is one of the most important safety factors on any piece of equipment. A typical electric windshield wiper system consists of a switch, motor assembly, wiper linkage and arms, and wiper blades. The description of the components is as follows:
The WINDSHIELD WIPER SWITCH is a multi position switch, which may contain a rheostat. Each switch position provides for different wiping speeds. The rheostat, if provided, operates the delay mode for a slow wiping action. This permits the operator to select a delayed wipe from every 3 to 20 seconds. A relay is frequently used to complete the circuit between the battery voltage and the wiper motor.
The WIPER MOTOR ASSEMBLY operates on one, two, or three speeds. The motor has a worm gear on the armature shaft that drives one or two gears, and, in turn, operates the linkage to the wiper arms. The motor is a small, shunt wound dc motor. Resistors are placed in the control circuit from the switch to reduce the current and provide different operating speeds.
The WIPER LINKAGE and ARMS transfer motion from the wiper motor transmission to the wiper blades. The rubber wiper blades fit on the wiper arms.
The WIPER BLADE is a flexible rubber squeegee-type device. It may be steel or plastic backed and is designed to maintain total contact with the windshield throughout the stroke. Wiper blades should be inspected periodically. If they are hardened, cut, or split, they are to be replaced.
When electrical problems occur in the windshield wiper system, use the service manual and its wiring diagram of the circuit. First check the fuses, electrical connections, and all grounds. Then proceed with checking the components.
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