Ignition System Basics

The purpose of the ignition system is to provide the high voltage (push) necessary to make electricity jump the gap in the spark plug to ignite the air/fuel mixture in the cylinder, at the correct time. The ignition coil or coils are what turns the 12-15 volts of the vehicles battery and/or charging system into the 20,000-70,000+ volts necessary to jump the spark plug gap. Such high voltage is necessary because air has a high resistance. When the cylinder is on compression stroke, air and fuel are compressed into the combustion chamber. This causes more air to sit between the spark plug electrodes, increasing the resistance even further. Depending on the system, spark plug wires and a distributor may be needed to deliver the high voltage to the correct cylinder at the correct time. Modern ignition systems tend to have one ignition coil per cylinder. This eliminates the need for a distributor and (most of the time) ignition wires as well as (supposedly) prolonging the life of the ignition coils. (Different ignition system designs will be explained later on in the article) All ignition systems will need to know where the engine is in its cycle to determine which cylinder to fire. Distributors are timed and mechanically driven by the engine. They may have “points” that control the ignition coil, or they may have a sensor that can directly control the ignition coil or indirectly control the the ignition coil through a module. Modern systems use the camshaft position sensor (CMP) and/or crankshaft position sensor (CKP) to determine where the engine is in its cycle and controls the spark timing electronically.

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Pieces of an Ignition System

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Ignition Coils

ignition coilsIgnition coils can look very different depending on the application, but they all use the same basic principles to generate the voltage necessary for ignition. All ignition coils have 2 coils of wire that are used to generate the high voltage, a primary winding and a secondary winding, that are wrapped around an iron core. The primary winding has several coils of heavier wire that conducts battery current. The primary winding with the iron core create a magnetic field when current is flowing through the winding. The secondary winding can have 20,000+ windings of very thin wire. This winding has voltage induced into it from the electromagnetic field created by the primary winding and the iron core. The secondary winding will be connected to either a spark plug wire or the spark plug directly at one end and a ground source or another spark plug at the other end depending on the system. It is important to note that it is when the primary winding STOPS flowing electrical current and the electromagnetic field collapses through the secondary winding that voltage is generated. The time that current is flowing through the primary winding, to build the electromagnetic field, is called “dwell time.” Voltage is induced into the secondary winding because the magnetic field passes through it when it collapses. The extremely high voltage is created because of how fast the magnetic field collapses and passes through the secondary winding as well as the high number of windings in the secondary winding. Because of the high overall resistance of the secondary circuit (including the spark plug gap or gaps) amperage in the secondary circuit is kept very low. A system may have one ignition coil for all cylinders, one coil for every 2 cylinders or an individual coil for every cylinder depending on the application.

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Spark Plugs

Engine spark plugThe spark plugs use the high voltage generated by the ignition coil to deliver the spark necessary for combustion to the combustion chamber. A spark plug has a terminal on the top which is connected to a spark plug wire or an ignition coil directly. This terminal is connected to the centre electrode through a resistor. The resistor is in place to reduce EMI (electromagnetic interference) which can have a negative impact on many electronic devices. The resistor is surrounded by the insulator. The insulator is a ceramic piece that insulates the terminal, resistor and centre electrode from the shell. The shell is the backbone of the plug. It also provides the hex and the thread to install and remove the spark plug. The ground electrode or side electrode is placed a specific distance away from the centre electrode. It is connected to the spark plug threads in the shell, which thread into the engine’s head. Contact with the head provides a ground source for the ground electrode. Some spark plugs gave several ground electrodes. Electricity will follow the path of least resistance and tend to arc to the closest or cleanest ground electrode. The distance between the centre electrode and ground electrode is called spark plug gap and it determines the length of the electric arc generated by the spark plug. Recommended spark plug gap can be different from engine to engine. Too short of a gap may not provide enough of an arc to ignite the air/fuel mixture, which would cause a misfire. Too large of a gap will require a higher voltage from the ignition coil to jump the gap. This could overwork and overheat the ignition coil resulting in premature ignition coil failure. Also, the spark generated may not last long enough or be too weak to ignite the air/fuel mixture. The ceramic insulator makes sure that the electric arc jumps the gap instead of shorting through the plug itself directly to the shell. The connection between the insulator and the shell must be strong enough to hold in compression and combustion pressures.

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Spark Plug Wires/Ignition Cables

ignition cableSpark plug wires transfer the high voltage electric charge from the ignition coil to the distributor and from the distributor to the spark plugs or directly from the coil to the spark plug, depending on the system. Ignition cables are designed with very high resistance (usually 5,000-20,000 ohms depending on length and design). This is done to reduce television and radio interference as well as reduce the amperage flowing through the secondary circuit to prolong the life of the spark plugs. Ignition cables have boots on either end to keep water and dirt away from connections. The cables themselves are also protected by a thick coating to protect the internal wires as well as prevent the secondary circuit from shorting directly to the engine block.

 

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Distributor

distributorThe distributor contains a rotor and a cap that directs the secondary circuit voltage to the cylinder that is about to fire. Inside is the rotor. The rotor is driven at half the speed of the crankshaft, (because each cylinder fires once for every 2 full rotations of the crankshaft) which is why the distributor is commonly driven by the camshaft. An ignition cable will connect the centre of the rotor to the secondary circuit of the ignition coil through the centre terminal of the cap. As the rotor is driven by the engine, the tip of the rotor will line up with the other terminals in the cap. These terminals are in contact with the ignition cables that will carry the high secondary voltage to one of the spark plugs. The distributor must be timed so that the rotor terminal lines up with the correct terminal on the cap to fire the correct cylinder at the correct time (another reason they are commonly driven by the camshaft). Distributors usually have a triggering device to directly or indirectly (through a module) control when the ignition coil should fire. Old distributors have “points” which are physical contacts that open and close as the rotor spins to control the ignition coil primary circuit. Some have weights that pivot outward from the centrifugal force of the spinning rotor as the engine picks up speed to adjust ignition timing. Other distributors have a vacuum diaphragm which adjusts ignition timing based on engine vacuum/engine load as well as mechanical weights to detect engine speed. (Ignition timing will be explained later in the article)

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Ignition Control Module

The ignition module receives a signal from a triggering device and controls the primary circuit of the ignition coil(s). Its job is to know where the engine is in its cycle and fire the ignition coil or coils (depending on the system) accordingly. Most modern systems have made the ignition control module part of the PCM. That way the PCM can use other inputs to make slight adjustments to ignition timing. Many coil-on-plug and coil-near-plug ignition systems have a small ignition control module in each ignition coil. If the individual coils have 3 or 4 pins on electrical plug, there is a transistor inside that controls the primary ignition circuit based on commands from an external module.

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Triggering Device

triggering deviceTriggering devices monitor where the engine is in its cycle and either directly control the ignition coil or supply a module with a signal so it can control the ignition coil. The previously mentioned “points” system has physical contacts connected in series with the ignition coil primary circuit that are opened by the distributor to activate the ignition coil. This old system worked fine but the contacts in the distributor had a tendency to arc and burn. Replacing or at least cleaning points contacts were a common fix back in the day. Modern systems use PM generators and hall effect sensors to inform a module of the engines position so it can activate the ignition coil(s). Since theses modern systems do not have any physical contacts, they tend to last much longer.

 

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Types of Ignition Systems

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Distributor Ignition

distributor systemA distributor ignition system has one ignition coil for all the cylinders of the engine. That means that one ignition coil, must generate the high voltage for anywhere from 3 to 12 cylinders. That also means that if that one ignition coil fails, the engine will not start. The distributor directs the voltage to the appropriate spark plug and will usually contain the triggering device that controls the ignition coil. Spark plug wires must carry the high voltage from the ignition coil to the distributor and from the distributor to the spark plugs.

 

 

 

 

Advantages

  • -fairly simple
  • -cheap/easy to diagnose and repair

 

Disadvantages

  • -one coil for all cylinders
  • -one coil fails, engine can not run
  • -limited ignition timing control
  • -spark plug wires needed
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Waste Spark Ignition

waste spark ignition coilsIn a waste spark ignition system, one ignition coil is responsible for generating high voltage for 2 cylinders. Both ends of the secondary windings are in contact with an ignition cable that will carry the high voltage to the spark plugs. An ignition coil will provide spark for companion cylinders. Companion cylinders have pistons that will reach TDC (top dead centre) and BDC (bottom dead centre) at the exact same time. Many people describe them as “running together.” When they both reach TDC, one cylinder will be about to enter power stroke and the other will be on overlap. Both cylinders will get a spark whenever they reach TDC but since a spark on overlap does not produce combustion, it is considered wasted. (although it can be argued that the extra spark may burn a small amount of the hydrocarbons left over from combustion, reducing emissions) It is also important to note than in this system, one spark jumps from the ground electrode to the centre electrode of the spark plug and the other spark jumps from the centre electrode to the ground electrode of the spark plug.

 

Advantages

  • -each coil does less work than a in distributor system
  • -electronically controlled ignition timing
  • -engine will usually start if one coil fails (V6+)

 

Disadvantages

  • -each coil does more work than an individual coil system
  • -spark plug wires needed
  • -“wasted” spark
  • -can be difficult to diagnose problems
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Coil-Near-Plug Ignition

coil near plug ignitionIn a coil-near-plug ignition system, there is an ignition coil for every cylinder. The coils will usually sit on top of the valve cover, so they tend to get hot (but not as hot as in a coil-on-plug ignition system). Having one coil per cylinder greatly reduces the amount of work each coil has to do. It also allows ignition timing to be adjusted on each cylinder individually. This system still requires short ignition cables to carry the high voltage from the ignition coils to the spark plugs.

 

 

 

 

Advantages

  • -each ignition coil is responsible for only one cylinder
  • -electronically controlled ignition timing
  • -individual ignition timing can be controlled
  • -engine will still run if one ignition coil fails
  • -easy to diagnose problems

 

Disadvantages

  • -spark plug wires needed
  • -ignition coils run hot (but not as hot as in a coil-on-plug ignition system)
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Coil-On-Plug Ignition

coil on plug ignitionJust like in a coil-near-plug ignition system, there is one ignition coil per cylinder. However, in this system the ignition coils sit right on top of the spark plugs. This system works just like a coil-near-plug system except it does not need ignition cables. Since these coils sit partially inside the engine, they tend to run extremely hot.

 

 

 

 

Advantages

  • -each ignition coil is responsible for only one cylinder
  • -electronically controlled ignition timing
  • -individual ignition timing can be controlled
  • -engine will still run if one ignition coil fails
  • -easy to diagnose problems
  • -no spark plug wires needed

 

Disadvantages

  • -ignition coils run hot
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Quick Ignition Coil Math

Let’s say a V8 engine is running at 2,000RPM for one minute (2,000 overall revolutions of the engine). Keep in mind that each cylinder fires once every 2 revolutions of the engine.

  • Distributor Ignition- 2,000 revolutions of the engine results in 1,000 power strokes per cylinder. Since there are 8 cylinders on a V8 engine, 8,000 sparks must be produced. That means that one ignition coil must produce 8,000 sparks in one minute for a V8 engine to run at 2,000RPM.

 

  • Waste Spark Ignition- Since a waste spark system fires every time a piston reaches TDC, each ignition coil must produce a spark every revolution for its 2 cylinders. 2,000 revolutions of the engine means that a waste spark ignition coil must fire 2,000 times and produce 4,000 individual sparks (half are “wasted”) in one minute for a V8 engine (or any other even number cylinder count, since a waste spark ignition coil only cares about 2 cylinders) to run at 2,000RPM. 8,000 sparks still need to be created for the engine to run properly, (16,000 individual sparks will be created by 8,000 ignition coil firings on a waste spark system) but the load is spread across 4 ignition coils on a V8 engine.

 

  • Individual Coil Ignition- Individual coils only produce a spark for one cylinder. 2,000 revolutions of the engine results in 1,000 power strokes per cylinder. That is how many times an individual coil would have to fire to keep run a V8 engine at 2,000RPM for one minute. 8,000 sparks will still need to be created, but the load is spread across 8 individual ignition coils on a V8 engine.

 

As you can see, individual coils greatly reduce the amount of work each coil has to do.

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Ignition Timing

When spark occurs in the cylinder is important for engine performance. Ignition timing is measured by the angle of crankshaft rotation when the spark occurs. When ignition occurs, it takes a few milliseconds for the air/fuel mixture to ignite and cause a pressure increase in the cylinder. Because of this, and how fast the pistons move up and down in the cylinder, spark must occur near the end of the compression stroke so maximum cylinder pressure can occur at 10-25 degrees ATDC (after top dead centre) in the power stroke. The time it takes for the air/fuel mixture to ignite remains about the same, but piston velocity is constantly changing based on engine speed and engine load. As the engine speeds up, the pistons move faster in their cylinders and spark timing must occur earlier in the compression stroke. This is called advancing the ignition timing. If the engine is under load (going up a hill, towing a trailer, trunk full of groceries) the pistons slow down more than usual between power strokes. Because of this, spark timing must occur later in the compression stoke. This is called retarding the ignition timing. Some old distributor ignition systems with points have weights that would swing outwards as the engine speeds up and a vacuum diaphragm connected to a manifold vacuum source to detect engine load. With high manifold vacuum, engine load is low (idle and cruising) so timing is more advanced than it would be with engine RPM alone. When manifold vacuum is low, engine load is high (wide open throttle or going up a hill) so no additional advance is needed and timing is advanced according to engine RPM alone. Modern electronically controlled ignition systems may use a MAP (manifold absolute pressure) sensor to detect manifold vacuum and a CKP (crankshaft position sensor) to detect engine speed and position to adjust ignition timing accordingly. They will also use other inputs such as ambient air temperature, air mass, barometric pressure, throttle position, engine temperature as well as other inputs to calculate and fine tune ignition timing. One of the best engine improvements in relation to ignition timing is the knock sensors (KS). With the use of knock sensors, an PCM can advance the ignition timing until a knock is detected, then retard a couple of degrees and hold there. This is important when a driver switches the octane of fuel he/she is using (higher octane fuels burn slower so ignition timing should advance). It also helps to adjust for minor differences in cylinder wear because individual ignition timing can be adjusted. If one cylinder has slightly lower compression than the others, it may take slightly longer for the air/fuel mixture to ignite. If ignition timing is over-retarded, the engine may have lack of power and poor fuel efficiency. If the ignition timing is over advanced, combustion could take place as the piston is still moving up on compression stroke. This can cause severe engine damage such as a bent connecting rod or worse.

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