Ignition System Basic Operation

With a four stroke gasoline engine, we use the motion of a piston to suck in air and fuel.   Once the air/fuel mixture is trapped inside the cylinder we compress the mixture to raise its temperature close to what is required to start the air and fuel burning.   As the piston nears the top of the compression stroke the spark plug fires.   The heat of the spark provides enough extra energy to start the fuel burning.   The burning air and fuel drastically raises the temperature and pressure inside the cylinder and the piston is forced back down for the Power stroke.   This article will explain the basic operation of the gasoline engine ignition system.

To understand what causes the spark plug to fire we need a basic understanding of electricity.

Basic Elecricity

Electricity is moving electrons through an electrical circuit.   To get electrons to move through a circuit requires a power source to steal away electrons from the positive end of the circuit and add them to the negative end of the circuit.   The power source does not create the electrons; it just supplies the power to push electrons into one end of the circuit and pull them back out the other end.

Electrical power can come from moving a magnetic field past a coil of wire.   A generator uses spinning magnets to create electrical power.   Electrical power can also come from energy released during a chemical reaction.   A battery is a power source that uses a chemical reaction.

Moving electrons through a circuit is known as electrical current.   Electrical current is measured in Amps.

To get electrons to move through a circuit requires a force or electrical pressure.  Electrical pressure is measured in Volts.

How difficult it is to move electrons through a circuit is called resistance.  Electrical resistance is measured in Ohms.

It is easy to move electrons through a wire and other types of conductors.  Automobiles use 12 volt battery and 14 volt generator to move the electrons through their electrical circuits.  The gap of a spark plug has too much resistance for 14 volts to be able to move any electrons.  Modern engines may require 40,000 volts or more to reliably fire the spark plug.  To change the electrical pressure or voltage high enough to push electrons across the spark plug gap, gasoline engines use ignition coils.

Ignition Coil Theory

An ignition coil is really two separate coils of wire. They are known as the Primary (low voltage) coil and the Secondary (high voltage) coil.   The primary coil is made up of several hundred windings of relatively thick wire (with thin insulation).   The copper wire is thick because it carries many amps of electrical current.   The insulation is very thin because the voltage is quite low.  The primary ignition coil carries ten amps or more of current.   Any time electrical current flows through a wire it surrounds that wire with a magnetic field.   As current flows through the primary coil windings, the magnetism around each loop of the coil is focused or concentrated.   By tightly coiling the primary wire, and running many amps through those windings, we create a very strong magnetic field.  This magnetic field surrounds the secondary coil windings.

The secondary coil consists of several thousand windings of a very thin wire.  Very thin wire cannot flow very many amps of current and by creating several thousand loops we are forcing this coil to generate a high voltage.   Remember, the purpose of the ignition coil is to take the relatively low electrical pressure or voltage of the primary windings and create very high electrical pressure or voltage into the secondary windings.  It takes time for the primary magnetic field to build to its full strength.   This is called Dwell and is often measured in degrees of crankshaft rotation.  Once the primary coil windings have reached their full magnetic strength, the current flowing through those windings is turned off.  This will cause the magnetic field surrounding both the low voltage primary windings and high voltage secondary windings to rapidly collapse.   As the magnetic field in the primary windings collapses it moves rapidly past the secondary coil windings.  This will induce a very high voltage into the secondary coil windings.

This is a link to an introduction to the ignition coil with a good animation www.gill.co.uk/products/digital_ignition/introduction/5_ign_coils.asp

Any time you move a magnetic field moves past a coil of wire it induces electrical power into that coil.   How much power depends upon the strength of the magnetic field, and how fast it is moving.   Electrical power is a balance between Volts (electrical pressure) and Amps (electrical current).   The secondary windings of the ignition coil will develop just enough voltage to push the electrons through the secondary circuit.   For a modern ignition system that will be many thousands of volts.   How many volts generated will depend upon the entire circuit resistance.  Most of the resistance in the secondary circuit is found at the spark plug gap.  Air is an excellent insulator and electrons do not easily flow through an insulator.   In fact, as the electrons are forced through the air they literally burn their way through the air molecules.   This creates a tremendous amount of heat and this starts the air and fuel mixture inside the cylinder to burning.

Secondary Current

When the magnetic field of the primary coil collapses, electrical power is generated in the secondary coil windings.   When a magnetic field moves past a coil of wire, it pulls electrons from one end of the wire and pushes them to the other end.   One end of the coil becomes negatively charged and the other end gets a positive charge.  The extra electrons on the Negative end of the coiled wire will attempt to return to Positive end of the coiled wire.   By design the electrons must leave the negatively charged end of the coiled wire, flow to the park plug, across the spark plug gap, and return to the positively charged end of the secondary coil winding.

The exact path of current flow is determined by the ignition system design.  The coil-on-plug design is the simplest and the most efficient.  With the coil-on-plug ignition electrons flow out the negatively charged end of the secondary coil windings, through a short plug wire, through the center electrode of the spark plug, across the plug gap, into the side or ground electrode and through the cylinder head back to the positively charged end of the secondary coil.

Ignition systems using a distributor have a more complicated current path.   Electrons leave the coil tower, go through the coil wire, into the center of the distributor cap, through the rotor, out through a spark plug wire, into the center electrode of the spark plug, across the plug gap, through the side or grounding electrode, through the cylinder head and ground system of the vehicle and back to the positively charged end of the secondary coil through the capacitance of the battery and/or capacitor in the distributor.

ignition wiring

Failure to Start

Here are the basic reasons of how the air/fuel can fail to start burning and cause the gasoline engine to not start.  If this problem is found in only one cylinder the engine may run however that cylinder will misfire.

#1 The compression can be too low inside the cylinder.  Low compression will not heat the air fuel mixture enough.  In effect the air fuel mixture will be too cold for the heat from the spark plug firing to start the combustion (burning) process.   For a non-starting engine, a slipped or broken timing belt is the most common cause for the compression to become too low to start the engine.  Checking for vacuum while cranking the engine can identify this problem.

#2 There can be not enough fuel in the cylinder.  If there is no fuel injected, the engine cannot run.   If the fuel pressure is low, or the injectors are not left on long enough the lean fuel mixture may start to burn right at the plug gap, but the fuel molecules are spread too thin and extra air creates insulation that keeps the flame from spreading inside the cylinder.   For a non-starting engine, a fuel pump that does not turn on, or is worn out is the most common cause of not enough fuel to run the engine.   If this is the reason for an engine to not start, adding propane while cranking should allow the engine to start, or at least try to start.  Low battery voltage will not allow the injectors to turn on quickly enough and the fuel mixture will become too lean.   Also improper sensor data to the computer may cause the injectors to not stay on long enough.   This will cause a lean mixture that can cause a misfire, or no-start condition.

#3 There can be too much fuel in the cylinder.  This rich mixture will have insufficient oxygen to keep the flame burning.  It drowns out the flame.   Excessive fuel pressure, or incorrect sensor data to the PCM (computer) can cause to much fuel to be injected into the cylinder.  For a non-starting engine, removing and inspecting a spark plug to see if it is wet with fuel can identify too much fuel as the cause for the no-start.   A fuel system pressure test and examining scan tool data can help identify the cause of too rich of a fuel mixture.

#4 The primary coil does not turn ON.  With no electricity flowing through the primary coil windings, no magnetic field can build to transfer electrical power to the secondary coil.   This can be as simple as a blown fuse and can be caused by an open circuit at any point in the primary ignition circuit.   Checking the primary coil positive terminal with a Voltmeter or test light will identify a blown fuse or open circuit in the primary ignition circuit.   A typical ignition primary circuit includes the battery, a fuse or fuse link, the ignition switch, the ignition coil, a power transistor (inside the ignition module), and a ground path back to the battery.   Older systems will use a ballast resistor or resistor wire between the ignition switch and the ignition coil.

#5 The primary coil does not turn OFF.  If the magnetic field in the primary ignition coil windings fails to collapse, there will be no power induced into the secondary windings.  Modern ignition systems use a power transistor to turn the primary coil windings ON and OFF.   Placing a test light between the ignition coil primary windings negative terminal, and the battery positive post should flash the test lamp On and Off.   If this does not happen the ignition coil is not getting the signal to turn off.   Some ignition coils do not have accessible primary winding negative terminals and you cannot use a test lamp to check these styles.  This power transistor is often part of an ignition module.   This ignition module will turn off the ignition coil primary windings when it gets the proper signal.   The signal originates from a crankshaft sensor and is modified by the PCM for proper ignition timing.   Using a scan tool can help determine if the cranking signal is present and the ignition module is getting the signal to turn On and Off the primary coil windings.

ignition wiring

#6 There is a short in the secondary ignition circuit.  When electric current from the secondary ignition coil finds a path of lower resistance it is called a short circuit.  In the secondary circuit the spark plug gap is the point of highest resistance.   If the electrons can find another way to return back to the coil, other than jumping the spark plug gap, there will be no spark.   Too much fuel or oil in or on the spark plug can foul the spark plug gap and the electrons will flow around the gap instead of jumping across it.   Spark plug wires can have damaged insulation and the electric current will bypass the spark plug completely.  Distributor ignition systems can have carbon tracking inside the distributor cap that allows the secondary ignition current to bypass the spark plug, or to travel to the wrong spark plug.   Sometimes the spark can short out under damp or wet conditions.  Using a spray water bottle to mist around the spark plug wires, ignition coil and distributor cap while the engine is running can help identify secondary ignition system insulation that is failing and allowing the spark to follow a short circuit instead of jumping the spark plug gap.

#7 There is an open in the secondary ignition circuit.  This will quickly cause the ignition coil is to self-destruct!  The most common cause for an ignition coil to become defective is an open circuit in the secondary current path.  Plug wires that are open circuit, or removed while the engine is cranking or running will create an open circuit.   Inside the ignition coil the voltage will go extremely high.   Voltage is electrical pressure and if a coil develops too much voltage it will break down the insulation between the windings of the secondary coil wire.   If this happens a coil may fail to function completely, or it may only fail to work when the engine is placed under a high load such as when rapidly accelerating or climbing up a hill.   Sometimes this will show up using an ohm meter to test the resistance of the coil windings however this is not a conclusive test.   Spark plug wires can be reliably checked with an ohm meter.   A typical spark plug wire will have several thousand ohms of resistance.  When spark plug wires get old they often go open circuit and this will show up when testing with an ohm meter.   If one wire is found to be defective, it is a good idea to replace all the plug wires.  It is also a good idea to replace the ignition coil if a spark plug wire is found to be open circuit.

Testing Ignition Coils
      Testing ignition coils can be done using an ohm meter.   An open circuit in either the primary or secondary coil windings will quickly identify a defective ignition coil.   More common is a coil that is partially shorted.   This may show up as lower than specified resistance when using on ohm meter.  Some shorts inside the coil only occur when the coil is under maximum demand or full engine load.   For this reason, using an ohm meter can identify a bad coil if it fails the resistance test, but does not guarantee a good ignition coil.
      Coil windings will increase in resistance as they heat, and read lower resistance as they cool.  For this reason ignition coil resistance specifications are given for a specific temperature.   This is usually room temperature (close to 70 degrees F). Warmer coils may read slightly higher resistance than specified.   Primary coil windings often have very low resistance and you should carefully calibrate (zero) your ohm meter to accurately read low resistance values.   Secondary coil resistance values are much higher and ohm meters are more accurate for this test.   Remember Open circuits will show a defective coil and resistance values that are lower than specified may point out a coil that operates, but is failing.   The best way to check an ignition coil is with a lab scope.   Spark testers with an adjustable air gap can also identify weak iginition coils.

Testing Spark Plug Wires
      Spark plug wires (often called high-tension wires) are rarely made of wire.  Instead they use a carbon ribbon with high resistance.   The coil and plug high voltage, secondary wires should always be given a careful visual inspection.   Look for hardened or cracked boots, and any signs of corrosion at each wire terminal end.   Any physical defect in one wire should call for replacement of ALL secondary ignition cables.   If one wire is worn out, the others will not be far behind.   Also a spark plug wire that becomes open circuit will often cause the ignition coil(s) to also fail due to the extra high voltages that will be produced by the ignition coil.   As spark plug wires wear, their resistance often increases.   Most spark plug wires have a specified resistance of several thousand ohms per inch however there are exceptions to this so always look up the specified resistance when checking spark plug wires.   If you find an open spark plug wire, or one with too much resistance be sure to recommend a complete set of wires to prevent furure ignition coil damage.