Vikram Final

7/30/2019 Vikram Final

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Coil Ignition System

An ignition coil (also called a sparkcoil) is aninduction coilinanautomobile'signition systemwhichtransformsthebattery's12volts(6 volts in some older vehicles) to thethousands of volts (20 to 30 thousand volts or more) needed tospark the spark plugs. Some coils have an internal resistor toreduce the voltage and some rely on a resistor wire or anexternal resistor to reduce the voltage from the car's 12 voltwiring flowing into the coil. The wire which goes from theignition coil to thedistributorand the wires which go from the

distributor to each of the spark plugs are called spark plugwires orhigh tension leads.

This specific form of theautotransformer, together with thecontact breakerand acapacitor(still referred to in automobileparlance by its old name of "condensor"), converts low voltagefrom a battery into thehigh voltagerequired by spark plugs inaninternal combustion engine.
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Basic principles

When the contact breaker closes, it allows a current from thebattery to build up in the primary winding of the ignition coil.(The current does not flow instantly because oftheinductanceof the coil.) Once the current has built up to its

full level, the contact breaker opens. Since it has a capacitorconnected across it, the primary winding and the capacitor forma tuned circuit, and as the stored energyoscillatesbetween theinductor formed by the coil and the capacitor, the changingmagnetic field in the core of the coil induces a much largervoltage in the secondary of the coil. More modern electronic
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ignition systems operate on exactly the same principle, butsome rely on charging the capacitor to around 400 volts ratherthan charging the inductance of the coil.

Modern ignition systems

In modern systems, the distributor is omitted and ignition isinstead electronically controlled. Much smaller coils are usedwith one coil for each spark plug or one coil serving two spark

plugs (for example two coils in a four-cylinder engine, or threecoils in a six-cylinder engine). A large ignition coil puts outabout 20 kV, and a small one such as from a lawn mower putsout about 15 kV. These coils may be remotely mounted or theymay be placed on top of the spark plug (coil-on-plug orDirectIgnition). Where one coil serves two spark plugs (in twocylinders), it is through the "wasted spark" system. In thisarrangement the coil generates two sparks per cycle to bothcylinders. The fuel in the cylinder that is nearing the end of its

compression stroke is ignited, whereas the spark in itscompanion that is nearing the end of its exhaust stroke has noeffect. The wasted spark system is more reliable than a singlecoil system with a distributor and less expensive than coil-on-plug.

Where coils are individually applied per cylinder, they may all

be contained in a single molded block with multiple high-tension terminals. This is commonly called a coil-pack.

A bad coil pack may cause a slight or not so slight misfire, badfuel economy, loss in power, and in some rare cases loss ofbraking performance and power steering problems.
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A Short Course on Ignition Systems:The purpose of the ignition system is to create a spark that willignite the fuel-air mixture in the cylinder of an engine. It mustdo this at exactly the right instant and do it at the rate of up toseveral thousand times per minute for each cylinder in theengine. If the timing of that spark is off by a small fraction of asecond, the engine will run poorly or not run at all.

The ignition system sends an extremely high voltage to the

spark plug in each cylinder when the piston is at the top of itscompression stroke. The tip of each spark plug contains a gapthat the voltage must jump across in order to reach ground.That is where the spark occurs.

The voltage that is available to the spark plug is somewherebetween 20,000 volts and 50,000 volts or better. The job of theignition system is to produce that high voltage from a 12 volt

source and get it to each cylinder in a specific order, at exactlythe right time.

The ignition system has two tasks to perform. First, it mustcreate a voltage high enough (20,000+) to arc across the gapof a spark plug, thus creating a spark strong enough to ignitethe air/fuel mixture for combustion. Second, it must control thetiming of that the spark so it occurs at the exact right time andsend it to the correct cylinder.

The ignition system is divided into two sections, the primarycircuit and the secondary circuit. The low voltageprimarycircuit operates at battery voltage (12 to 14.5 volts) and isresponsible for generating the signal to fire the spark plug atthe exact right time and sending that signal to the ignition coil.
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The ignition coil is the component that converts the 12 voltsignal into the high 20,000+ volt charge. Once the voltage isstepped up, it goes to the secondary circuit which then directs

the charge to the correct spark plug at the right time.

The Basics

All automobiles work on DC, or Direct Current. This meansthat current moves in one direction, from the positive batteryterminal to the negative battery terminal. In the case of the

automobile, the negative battery terminal is connected by aheavy cable directly to the body and the engine block of thevehicle. The body and any metal component in contact with itis called the Ground. This means that a circuit that needs tosend current back to the negative side of the battery can beconnected to any part of the vehicle's metal body or the metalengine block.

A good example to see how this works is the headlight circuit.The headlight circuit consists of a wire that goes from thepositive battery terminal to the headlight switch. Another wiregoes from the headlight switch to one of two terminals on theheadlamp bulb. Finally, a third wire goes from a secondterminal on the bulb to the metal body of the car. When youswitch the headlights on, you are connecting the wire from thebattery with the wire to the headlamps allowing battery current

to go directly to the headlamp bulbs. Electricity passes throughthe filaments inside the bulb, then out the other wire to themetal body. From there, the current goes back to the negativeterminal of the battery completing the circuit. Once the currentis flowing through this circuit, the filament inside the headlampgets hot and glows brightly. Let there be light.


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The Mechanical Ignition System

The distributor is the nerve center of the mechanical ignitionsystem and has two tasks to perform. First, it is responsible fortriggering the ignition coil to generate a spark at the preciseinstant that it is required (which varies depending how fast theengine is turning and how much load it is under). Second, thedistributor is responsible for directing that spark to the propercylinder (which is why it is called a distributor)

The circuit that powers the ignition system is simple andstraight forward. When you insert the key in the ignition switchand turn the key to the Run position, you are sending currentfrom the battery through a wire directly to the positive (+) sideof the ignition coil. Inside the coil is a series of copper windingsthat loop around the coil over a hundred times before exitingout the negative (-) side of the coil. From there, a wire takes

this current over to the distributor and is connected to a specialon/off switch, called the points. When the points are closed,this current goes directly to ground. When current flows fromthe ignition switch, through the windings in the coil, then toground, it builds a strong magnetic field inside the coil.

This distributor cam rotates in time with the engine, making onecomplete revolution for every two revolutions of the engine. Asit rotates, the cam pushes the points open and closed. Everytime the points open, the flow of current is interrupted throughthe coil, thereby collapsing the magnetic field and releasing ahigh voltage surge through the secondary coil windings. Thisvoltage surge goes out the top of the coil and through the high-tension coil wire.


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Now, we have the voltage necessary to fire the spark plug, butwe still have to get it to the correct cylinder. The coil wire goesfrom the coil directly to the center of the distributor cap. Under

the cap is a rotor that is mounted on top of the rotating shaft.The rotor has a metal strip on the top that is in constant contactwith the center terminal of the distributor cap. It receives thehigh voltage surge from the coil wire and sends it to the otherend of the rotor which rotates past each spark plug terminalinside the cap. As the rotor turns on the shaft, it sends thevoltage to the correct spark plug wire, which in turn sends it tothe spark plug. The voltage enters the spark plug at the

terminal at the top and travels down the core until it reachesthe tip. It then jumps across the gap at the tip of the sparkplug, creating a spark suitable to ignite the fuel-air mixtureinside that cylinder.

The ignition switch

There are two separate circuits that go from the ignition switchto the coil. One circuit runs through a resistor in order to stepdown the voltage about 15% in order to protect the points frompremature wear. The other circuit sends full battery voltage tothe coil. The only time this circuit is used is during cranking.Since the starter draws a considerable amount of current tocrank the engine, additional voltage is needed to power thecoil. So when the key is turned to the spring-loaded start

position, full battery voltage is used. As soon as the engine isrunning, the driver releases the key to the run position whichdirects current through the primary resistor to the coil.On some vehicles, the primary resistor is mounted on thefirewall and is easy to replace if it fails. On other vehicles, mostnotably vehicles manufactured by GM, the primary resistor is a


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special resistor wire and is bundled in the wiring harness withother wires, making it more difficult to replace, but also moredurable.

The Distributor

When you remove the distributor cap from the top of thedistributor, you will see the points and condenser. Thecondenser is a simple capacitor that can store a small amountof current. When the points begin to open, the current flowingthrough the points looks for an alternative path to ground. If

the condenser were not there, it would try to jump across thegap of the points as they begin to open. If this were allowed tohappen, the points would quickly burn up and you would hearheavy static on the car radio. To prevent this, the condenseracts like a path to ground. It really is not, but by the time thecondenser is saturated, the points are too far apart for thesmall amount of voltage to jump across the wide point gap.Since the arcing across the opening points is eliminated, the

points last longer and there is no static on the radio from pointarcing.

The points require periodic adjustments in order to keep theengine running at peek efficiency. This is because there is arubbing block on the points that is in contact with the cam andthis rubbing block wears out over time changing the point gap.

There are two ways that the points can be measured to see ifthey need an adjustment. One way is by measuring the gapbetween the open points when the rubbing block is on the highpoint of the cam. The other way is by measuring the dwellelectrically. The dwell is the amount, in degrees of camrotation, that the points stay closed.


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On some vehicles, points are adjusted with the engine off andthe distributor cap removed. A mechanic will loosen the fixedpoint and move it slightly, then retighten it in the correct

position using a feeler gauge to measure the gap. On othervehicles, most notably GM cars, there is a window in thedistributor where a mechanic can insert a tool and adjust thepoints using a dwell meter while the engine is running.Measuring dwell is much more accurate than setting the pointswith a feeler gauge.

Points have a life expectancy of about 10,000 miles at which

time they have to be replaced. This is done during a routinemajor tune up. During the tune up, points, condenser, and thespark plugs are replaced, the timing is set and the carburetor isadjusted. In some cases, to keep the engine runningefficiently, a minor tune up would be performed at 5,000 mileincrements to adjust the points and reset the timing.

Ignition Coil

The ignition coil is nothing more that an electrical transformer.It contains both primary and secondary winding circuits. Thecoil primary winding contains 100 to 150 turns of heavy copperwire. This wire must be insulated so that the voltage does not

jump from loop to loop, shorting it out. If this happened, itcould not create the primary magnetic field that is required. The

primary circuit wire goes into the coil through the positiveterminal, loops around the primary windings, then exits throughthe negative terminal.

The coil secondary winding circuit contains 15,000 to 30,000turns of fine copper wire, which also must be insulated from


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each other. The secondary windings sit inside the loops of theprimary windings. To further increase the coils magnetic fieldthe windings are wrapped around a soft iron core. To withstandthe heat of the current flow, the coil is filled with oil which helps

keep it cool.

The ignition coil is the heart of the ignition system. As currentflows through the coil a strong magnetic field is built up. Whenthe current is shut off, the collapse of this magnetic field to thesecondary windings induces a high voltage which is releasedthrough the large center terminal. This voltage is then directedto the spark plugs through the distributor.

Ignition Timing

The timing is set by loosening a hold-down screw and rotatingthe body of the distributor. Since the spark is triggered at theexact instant that the points begin to open, rotating thedistributor body (which the points are mounted on) will change

the relationship between the position of the points and theposition of the distributor cam, which is on the shaft that isgeared to the engine rotation.

While setting the initial, or base timing is important, for anengine to run properly, the timing needs to change dependingon the speed of the engine and the load that it is under. If wecan move the plate that the points are mounted on, or we could

change the position of the distributor cam in relation to the gearthat drives it, we can alter the timing dynamically to suit theneeds of the engine..


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Ignition Wires

These cables aredesigned to handle 20,000to more than 50,000 volts,enough voltage to toss youacross the room if youwere to be exposed to it.The job of the spark plugwires is to get thatenormous power to the spark plug without leaking out. Sparkplug wires have to endure the heat of a running engine as well

as the extreme changes in the weather. In order to do theirjob, spark plug wires are fairly thick, with most of that thicknessdevoted to insulation with a very thin conductor running downthe center. Eventually, the insulation will succumb to theelements and the heat of the engine and begins to harden,crack, dry out, or otherwise break down. When that happens,they will not be able to deliver the necessary voltage to thespark plug and a misfire will occur. That is what is meant by

"Not running on all cylinders". To correct this problem, thespark plug wires would have to be replaced.

Spark plug wires are routed around the engine very carefully.Plastic clips are often used to keep the wires separated so thatthey do not touch together. This is not always necessary,especially when the wires are new, but as they age, they canbegin to leak and crossfire on damp days causing hard starting

or a rough running engine.


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Mechanically timed ignition

Distributor cap

Mostfour-stroke engineshave used a mechanically timedelectrical ignition system. The heart of the system is thedistributor. The distributor contains a rotatingcamdriven by the

engine's drive, a set ofbreakerpoints, a condenser, a rotor anda distributor cap. External to the distributor is theignition coil,the spark plugs and wires linking the distributor to thesparkplugsand ignition coil.

The system is powered by alead-acid battery, which is chargedby the car's electrical system using adynamooralternator. The

engine operates contact breaker points, which interrupt thecurrent to an induction coil(known as the ignition coil).

The ignition coil consists of two transformer windings sharing acommon magnetic corethe primary and secondary windings.

An alternating current in the primary induces alternating
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magnetic field in the coil's core. Because the ignition coil'ssecondary has far more windings than the primary, the coil is astep-up transformer which induces a much higher voltageacross the secondary windings. For an ignition coil, one end of

windings of both the primary and secondary are connectedtogether. This common point is connected to the battery(usually through a current-limitingballastresistor). The otherend of the primary is connected to the points within thedistributor. The other end of the secondary is connected, viathe distributor cap and rotor, to the spark plugs.

Ignition Circuit Diagram

The ignition firing sequence begins with the points (orcontactbreaker) closed. A steady charge flows from the battery,through the current-limiting resistor, through the coil primary,across the closed breaker points and finally back to the battery.
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This steady current produces a magnetic field within the coil'score. This magnetic field forms the energy reservoir that will beused to drive the ignition spark.

As the engine turns, so does the cam inside the distributor. Thepoints ride on the cam so that as the engine turns and reachesthe top of the engine's compression cycle, a high point in thecam causes the breaker points to open. This breaks theprimary winding's circuit and abruptly stops the current throughthe breaker points. Without the steady current through the

points, the magnetic field generated in the coil immediately andrapidly collapses. This change in the magnetic field induces ahigh voltage in the coil's secondary windings.

At the same time, current exits the coil's primary winding andbegins to charge up thecapacitor("condenser") that lies acrossthe now-open breaker points. Thiscapacitorand the coilsprimary windings form an oscillatingLC circuit. This LC circuit

produces a damped, oscillating current which bounces energybetween the capacitors electric field and the ignition coilsmagnetic field. The oscillating current in the coils primary,which produces an oscillating magnetic field in the coil, extendsthe high voltage pulse at the output of the secondary windings.This high voltage thus continues beyond the time of the initialfield collapse pulse. The oscillation continues until the circuitsenergy is consumed.

The ignition coil's secondary windings are connected tothedistributor cap. A turningrotor, located on top of thebreaker cam within the distributor cap, sequentially connectsthe coil's secondary windings to one of the several wires
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leading to each cylinder'sspark plug. The extremely highvoltage from the coil's secondary - often higher than 1000voltscauses a spark to form across the gap of thespark plug.This, in turn, ignites the compressed air-fuel mixture within the

engine. It is the creation of this spark which consumes theenergy that was stored in the ignition coils magnetic field.

High performance engines with eight or more cylinders thatoperate at high r.p.m. (such as those used in motor racing)demand both a higher rate of spark and a higher spark energythan the simple ignition circuit can provide. This problem isovercome by using either of these adaptations:

Two complete sets of coils, breakers and condensers can beprovided - one set for each half of the engine, which is typicallyarranged in V-8 or V-12 configuration. Although the two ignitionsystem halves are electrically independent, they typically sharea single distributor which in this case contains two breakers

driven by the rotating cam, and a rotor with two isolatedconducting planes for the two high voltage inputs.

A single breaker driven by a cam and a return spring is limitedin spark rate by the onset of contact bounce or float at highrpm. This limit can be overcome by substituting for the breakera pair of breakers that are connected electrically in series but

spaced on opposite sides of the cam so they are driven out ofphase. Each breaker then switches at half the rate of a singlebreaker and the "dwell" time for current buildup in the coil ismaximized since it is shared between the breakers.TheLamborghiniV-12 engine has both these adaptations and
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therefore uses two ignition coils and a single distributor thatcontains 4 contact breakers.

A distributor-based system is not greatly different from amagneto system except that more separate elements areinvolved. There are also advantages to this arrangement. Forexample, the position of the contact breaker points relative tothe engine angle can be changed a small amount dynamically,allowing theignition timingto be automatically advanced withincreasingrevolutions per minute(RPM) and/orincreasedmanifold vacuum, giving better efficiency andperformance.

However it is necessary to check periodically the maximumopening gap of the breaker(s), using a feeler gauge, since thismechanical adjustment affects the "dwell" time during which thecoil charges, and breakers should be re-dressed or replacedwhen they have become pitted by electric arcing. This systemwas used almost universally until the late 1970s,

whenelectronicignition systems started to appear.

Ignition System Coil

The coil is a simple device -- essentially a high-voltage

transformer made up of two coils of wire. One coil of wire iscalled the primary coil. Wrapped around it is the secondary coil.The secondary coil normally has hundreds of times more turnsof wire than the primary coil.
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Current flows from the battery through the primary winding ofthe coil.

The primary coil's current can be suddenly disrupted bythe breaker points, or by a solid-state device in an electronicignition.

If you think the coil looks like anelectromagnet, you're right --but it is also an inductor. The key to the coil's operation is whathappens when the circuit is suddenly broken by the points. Themagnetic field of the primary coil collapses rapidly. Thesecondary coil is engulfed by a powerful and changingmagnetic field. This field induces a current in the coils -- a veryhigh-voltage current (up to 100,000 volts) because of thenumber of coils in the secondary winding. The secondary coilfeeds this voltage to the distributor via a very well insulated,high-voltage wire.Finally, an ignition system needs a distributor.
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Ignition System Distributor

The distributor handles several jobs. Its first job is to distribute

the high voltage from the coil to the correct cylinder. This isdone by the cap and rotor. The coil is connected to the rotor,which spins inside the cap. The rotor spins past a series ofcontacts, one contact per cylinder. As the tip of the rotorpasses each contact, a high-voltage pulse comes from the coil.The pulse arcs across the small gap between the rotor and thecontact (they don't actually touch) and then continues down thespark-plug wire to the spark plug on the appropriate cylinder.

When you do a tune-up, one of the things you replace on yourengine is the cap and rotor -- these eventually wear outbecause of the arcing. Also, the spark-plug wires eventuallywear out and lose some of their electrical insulation. This canbe the cause of some very mysterious engine problems.

Older distributors with breaker points have another section inthe bottom half of the distributor -- this section does the job ofbreaking the current to the coil. The ground side of the coil isconnected to the breaker points.


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Acamin the center of the distributor pushes a lever connectedto one of the points. Whenever the cam pushes the lever, itopens the points. This causes the coil to suddenly lose itsground, generating a high-voltage pulse.

The points also control the timing of the spark. They may havea vacuum advance or a centrifugal advance. Thesemechanisms advance the timing in proportion to engine load orengine speed.

Spark timing is so critical to an engine's performance that mostcars don't use points. Instead, they use a sensor that tells the

engine control unit (ECU) the exact position of the pistons. Theengine computer then controls a transistor that opens andcloses the current to the coil.

In the next section, we'll take a look at an advance in modernignition systems: the distributorless ignition.

Spark Plug

The spark plug is in the center of the four valves in eachcylinder.The spark plug is quite simple in theory: It forces electricity toarc across a gap, just like a bolt oflightning. The electricity
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must be at a very high voltage in order to travel across the gapand create a good spark. Voltage at the spark plug can beanywhere from 40,000 to 100,000 volts.

The spark plug must have an insulated passageway for thishigh voltage to travel down to the electrode, where it can jumpthe gap and, from there, be conducted into the engine blockand grounded. The plug also has to withstand the extreme heatand pressure inside the cylinder, and must be designed so thatdeposits from fuel additives do not build up on the plug.

Spark plugs use a ceramic insert to isolate the high voltage atthe electrode, ensuring that the spark happens at the tip of theelectrode and not anywhere else on the plug; this insert doesdouble-duty by helping to burn off deposits. Ceramic is a fairly


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poor heat conductor, so the material gets quite hot duringoperation. This heat helps to burn off deposits from theelectrode.

Some cars require a hot plug. This type of plug is designedwith a ceramic insert that has a smaller contact area with themetal part of the plug. This reduces the heat transfer from theceramic, making it run hotter and thus burn away moredeposits. Cold plugs are designed with more contact area, sothey run cooler.

The difference between a "hot" and a "cold" spark plug is in theshape of the ceramic tip.

The carmaker will select the right temperature plug for eachcar. Some cars with high-performance engines naturallygenerate more heat, so they need colder plugs. If the spark

plug gets too hot, it could ignite the fuel before the spark fires;so it is important to stick with the right type of plug for your car.

Next, we'll learn about the coil that generates the highvoltages required to create a spark.


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