UNDERSTANDING IGNITION WAVEFORMS

 

by Bernie C. Thompson

 

 

AUTOMOTIVE TEST SOLUTIONS, INC. 2004 © All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission of the publisher.

 

 

 

Energy. What a loaded word! Mankind has been in search for this since the dawn of our creation. First mankind found fire, perhaps a bolt of lightening fell from the sky and gave mankind his first taste of energy. From that point forward, man has been in search of new and better forms of energy.

 

The internal combustion engine is perhaps one of the more useful forms of energy that man has created. From its humble beginning, this energy source has been transformed many times to create more power and to be more efficient. This energy source has been accepted world wide and it is used in many ways, but none more so than in the automobile. The internal combustion engine comes in two forms; compression ignition and spark ignition.

 

We will be analyzing the spark ignition system. It is important to understand how the energy is released in the engine. In the internal combustion engine the air or vaporized air/fuel mixture is drawn into the cylinder where it is then compressed. When air is compressed, the molecules are forced into a smaller space where they hit each other causing friction which then produces heat. The compression is a way to put energy in the form of heat in the fuel molecule. It takes energy to hold together the different atoms that form the molecular chain. For example, if you were holding onto another person and were being spun around very fast, you would expend energy just to hang on to the other person. In order for the fuel to release this energy, the fuel molecule must separate or break apart and then reform into a different molecular structure. Once the fuel molecule is broken, the energy that it used to stay together is no longer needed. This energy is now released. This freed energy is what powers the internal combustion engine. The compression from the cylinder is not enough energy to separate the fuel molecule by itself. The heat that is transferred into the fuel molecule makes it unstable, but more force will have to be applied to separate the atoms. For example, if two huge muscular wresters were locked together in combat it would not be easy to separate them. In order to separate these men you would have to apply more force to them than they are using to hold onto each other. If the referee wanted to separate the wresters and they were unwilling to part, he would have to use force. A stun gun that applied a spark of 100,000 volts would work. This potential energy is greater than the energy the wrestlers are using to hold on to each other with, so they would let go and separate. In the cylinder, even though the compression creates heat energy, more energy is needed to separate the molecular structure of the fuel.

 

 

Just like the referee with the wrestlers, we will need to use force in the way of electrical energy or high voltage spark to separate the fuel molecules and release their energy. But rather than a stun gun, we will use an ignition system.

 

There have been many different ignition systems used to supply a high energy spark to ignite the air/fuel mixture. The most popular system in use today is the step up transformer. The step up transformer uses a low voltage, high current pole, to create a high voltage, low current pole. This is done by using two different coils or windings of wire. The first coil is the primary; the second coil is the secondary (above). The primary is wound around a core for magnetic amplification. In newer transformers this core will be made of many plates of a ferrous metal, usually a soft iron, layered or laminated together. This gives better amplification than a solid core. The primary winding uses larger diameter wire with fewer windings. This allows the primary to have a very low resistance value. The secondary uses small diameter wire with many more windings. This makes the secondary have a high resistance value. The automotive coil is usually wound approximately 1 to 100, in other words, for every 1 winding of the primary the secondary has 100 windings. The primary winding usually has 1 to 4 ohms of resistance; where as, the secondary winding usually has 8,000 ohms to 16,000 ohms of resistance. The primary and the secondary are electromagnetically coupled so anything that affects either winding is mirrored in the other winding. The principal that the step up transformer uses is electromagnetic induction. To understand how this transformer works, let’s look at the waveform produced by this device. This is the open circuit voltage or source voltage. The reason this is referred to as an open circuit voltage is the circuit has not been completed. There is no current flowing through the primary circuit at this point (Figure 3 Part A). The voltage drops abruptly when the module driver is turned on, thus completing the primary circuit to ground (Figure 3 Part B). This voltage drop will come very close to ground.

 

 

The initial voltage drop will depend on whether the electrical device used is a transistor or a MOSFET. If a transistor is used the voltage drop will be .7 volts to 1 volt. This is due to the resistance across the transistor’s gate. A MOSFET will have less resistance across its gate causing less voltage drop, usually about .1 volt to .3 volts. The initial voltage drop is the voltage that is remaining in the circuit to push the current across the resistance of the module driver or gate (Figure 3 Part C). Once the module closes the driver, current starts to flow through the primary winding circuit. When current flows through a coil winding, all of the current is used to create a magnetic field around the winding (Figure 4). This magnetic field build up is inductance. The magnetic field is proportional to the inductance and the current or, in other words, the larger the current the larger the magnetic inductance. This current flowing through the primary coil winding allows the energy to build a magnetic field. As the magnetic field is building, it moves across the primary and secondary windings. This induces voltage in both the primary winding and the secondary winding. However, the effect of this induction is different within the two windings. As the magnetic field builds and moves across the secondary winding this induces electromotive force and frees electrons. This can be seen in the secondary waveform when the module driver closes; there are voltage oscillations at the beginning of the circuit being completed (Figure 5 with Figure 3). This is caused by the magnetic field moving across and inducing voltage in different windings contained within the secondary coil winding. Between the coil winding, or turns, there is capacitance. Capacitance occurs when two conductors are separated by space and current is flowing through the conductor. Electrical potential builds between the two conductors, the size of the conductors and the distance between these two conductors determines the capacitance. The turns in the coil winding are made of a conductor and space exists between these turns or windings. This allows the capacitance between the coil turns to build electrical energy.

 

 

The ringing is the energy changing between electrical energy and magnetic energy. Just like when a bell is struck, the ringing from the bell is loud when first struck and diminishes. These oscillations diminish into a steady curve that flattens out at the point the coil has become saturated. The saturation point will vary with the amount of current flowing through the primary. When the magnetic field builds and moves across the primary winding, the voltage that is induced into the primary winding frees electrons. However, since current is flowing through the primary winding these free electrons impede the current flow. For example; imagine a school hallway packed shoulder to shoulder with children running down the hallway as fast as they can run. Now imagine children entering the hallway from the classrooms located along this hallway. These children leaving the classrooms will impede the flow of children running down the hallway (above). Just like the children entering the hallway, the induced voltage in the primary winding creates resistance to the current flowing through the primary circuit. This induced resistance is called counter voltage.

 

Anytime there is resistance in a circuit there will be a voltage drop proportional to the resistance. This voltage drop can be seen at the bottom of the ignition coil waveform. This is what causes the slight rise at the bottom of the ignition coil waveform. If the oscilloscope voltage setting is lowered to magnify the bottom of the ignition primary coil waveform, the voltage drop of the waveform can be seen more clearly (Figure 7 Part D). Since the current flowing through the winding makes the resistance for the voltage drop, it mirrors the primary ignition coil waveform made with an inductive amperage clamp (Figure 7). Once the current rises to the point of the coil being fully saturated, the magnetic field is now fully built up completely surrounds the secondary winding. The saturation point of the ignition coil is based on the current flowing through it. The larger the current, the larger the magnetic lines of force. Likewise, the smaller the current, the smaller the magnetic lines of force.

 

The circuit then limits the current flowing through the primary winding, (Figure 3 Part E); however, the magnetic field still remains at full strength. Notice, that when current limiting is switched on, the voltage is still below the open circuit voltage (Figure 3 Part F). To accomplish this there is a resistor switched into the circuit to limit the current flowing through it. If the primary circuit has unwanted resistance, the time for the current limit to switch on will be longer. If the coil is shorted or has less resistance, the time for the current limit will be shorter. If the system is known, then this time to limit current will be an indicator of a problem. As the RPM of the engine is increased, the time between cylinder firing becomes shorter and the time to saturate the coil becomes less, thus, the current limiting will cease. [Note: Not all ignition systems have current limiting.] The module or PCM then commands the module driver off. This ends the current flowing through the primary winding. The magnetic field then begins to fall across the secondary winding.

 

 

When a magnetic field moves across a wire or winding, voltage is induced into that wire or winding. This induction makes electromotive force, EMF, which frees electrons and pushes these electrons through a circuit until they return to the secondary winding where they were produced. The amount of induction is proportional to the size of the magnetic field and the speed with which the magnetic field falls across the secondary winding. For a faster collapse of the magnetic field, a condenser is used. A condenser or capacitor will not allow direct current to pass through it to ground. However, alternating current is able to pass through these devices. A direct current that pulses very fast becomes alternating current and can pass through the condenser or capacitor. This allows the current in the primary coil circuit to pass through the condenser or capacitor to ground.

 

The condenser is connected to the primary winding (above). Once the current stops, the magnetic field falls back into the primary winding to stabilize the current within the winding. The faster the current in the primary winding dissipates through the condenser, the faster the magnetic field will collapse. The rapid movement of the magnetic field increases the induction within the secondary winding and the current, being pushed by a high voltage of up to 50,000KV, will look for a pathway or circuit.

 

 

The ignition coil’s secondary is connected to the spark plug. The electrons move to the spark plug gap, however, this is an open circuit. When high voltage is pushing electrons across an open it will form a corona, or a low energy field, across the gap. The electrons will then start to ionize across the open.

 

To start ionization a very high voltage is required. Once the ionization occurs, the free electrons and the ions form a pathway across a gap. They create plasma (above). The plasma creates an easier pathway across the open. The plasma is a hot ionized gas containing about equal numbers of positive ions and electrons. The plasma is affected by the gas and pressure that comprises it. This ionization is the breakdown voltage.

 

 

An ion is a positively or negatively charged atom and is the result of the atom having lost or gained one or more electrons. (Figure 3 Part G). This is the break down voltage or the amount of voltage that was required to push the electrons across the resistance which, in this case, is the spark plug gap. The wider the spark plug gap or the more resistance between the spark plug electrodes, the higher the breakdown voltage will be. Once the electrons have bridged the spark plug gap the resistance drops, decreasing the voltage required to maintain the electron flow across the spark plug gap. Notice the ringing that occurs as the electron flow starts after the break down voltage (Figure 3 Part H).

 

This ringing or oscillation is created by the induction occurring across the windings and the capacitance between the turns. The transformer makes it very easy for the energy to change between electrical energy and magnetic energy. The breakdown voltage that starts the arc is very fast, about 2 nanoseconds, this fast energy spike starts the energy change between electrical and magnetic. The harder the spike to start the arc, the more oscillations will occur.

 

For example; just like a child in a swing, if the swing is stationary and you push it; the harder the push the higher the swing will move. The swing will then oscillate until the energy has dissipated (above). The swing can also be used as a model to show how the ignition coil changes electrical energy into magnetic energy and vise versa. The swing, being a mechanical device, needs a “push” or energy in order to move. Just like the coil’s discharge, or “push”, causes an energy spike. When the swing is in motion it is changing kinetic energy into mechanical energy and vise versa. Once the electrons establish flow, the voltage is stabilized and the oscillations will diminish into an even voltage (Figure 3 Part I). The electron flow is now established across the gap and it will continue until the secondary is depleted.

 

At the end of the spark duration note the rise in voltage as the spark burns out (Figure 3 Part J). This is caused by the plasma breaking down. The plasma creates an electrical pathway that has less resistance. As it diminishes, the resistance increases causing the voltage to rise. The induction that put electrical energy into the secondary coil winding is limited; much like filling a bucket of water, once the bucket is full that is all the water it can hold (left). The point of the ignition coil being fully saturated would be like the water bucket being totally filled.

 

 

If a water pump were used to pump the water out of the bucket under pressure then, the higher the pressure the quicker the water will be used up. Once the water is used up, there will not be any pressure left. This is like the secondary ignition coil. The more voltage or pressure the coil needs to push the electrons across the resistance in the circuit, the quicker the electrons are used up.

 

The time the electrons are bridging the spark plug gap is the burn time (Figure 3 Part G-J). This burn time will change according to the pressure it took to start the electrons flowing through the circuit. If the pressure is low the burn time will be longer. If the pressure is high the burn time will be shorter. Let’s use a piece of rope to demonstrate this. Assume the rope is a set length, like the bucket of water is a set amount, and it is positioned in the pattern that the breakdown voltage and burn time makes (Figure 12). Then if the length of rope that is used to make the vertical line is longer, the horizontal line will become shorter. Likewise, if the horizontal line becomes longer the vertical line will become shorter. If the entire length of rope is shorter, just like when the ignitions coil’s magnetic field is not fully saturated, the vertical and horizontal sections will be affected due to the limited amount of stored energy.

 

The breakdown voltage and burn time are influenced by the pressure or compression and the content of the gas that is in the cylinder. Under normal conditions this cylinder is filled with a gas comprised of ambient air; which is approximately 21% oxygen, 79% nitrogen, and hydrocarbons (gasoline) in a ratio of 14.7 parts air to one part hydrocarbons. The gas mixture in this cylinder is composed of atoms that will ionize or allow the spark to jump across the spark plug electrodes. Since these atoms ionize, if these atoms change the ionization will change. The amount of pressure or compression will change the density of the mixture which will also have an effect on the ionization. The turbulence within the cylinder will also change the characteristics of the ignition waveform. If any of these variables change; compression or pressure, turbulence, gas content, fuel, or water vapor, then the ionization that forms the plasma will change which in turn affects the spark waveform. This forms a window into the chamber so the technician can see what is occurring within the cylinder. Some examples are; lean air fuel ratio, rich air fuel ratio, pre-ignition, turbulence caused by cam timing or valves, EGR, water vapor caused by engine coolant leak, worn spark plugs, carbon tracking, resistance within the circuit, etc.. Once the technician learns how to view the waveform during the breakdown voltage and burn time, the waveform will reflect what is occurring within the cylinder.

 

 

At the point the electrical energy is not strong enough to keep the electrons flowing across the spark plug gap, the flow stops (Figure 3 Part J). Whatever energy is still left within the coil has to be absorbed by the ignition coil and the energy that is absorbed within the turns of the coil is dissipated by changes between electrical energy and magnetic energy. This is what causes the ringing at the end of the spark duration (Figure 3 Part K). The larger the voltage change and the more oscillations within the ringing, the more energy is left in the ignition coil. If there are no rings the energy of the ignition coil has totally dissipated. This ringing can be used to see how much energy was used or not used during the ignition coil discharge.

 

It is very important to understand each part of the ignition coil waveform and what went into making the amplitude or voltage change over time. Without an in-depth understanding of the waveform, you will not know what has failed. For a proper diagnosis of this waveform one must pay close attention to the smallest changes. Within the ignition coil’s waveform there is more information than any other waveform produced on the vehicle.

 

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