FIRE IN THE HOLE: Understanding Ignition Waveforms

Motor, May 2005 by Thompson, Bernie C

The ignition waveform is a window that allows you to see what's occurring in the combustion chamber. Join us for a closer look.

From its humble beginnings, the internal combustion engine has been transformed many times over to produce more power and to be more efficient. Todays internal combustion engine comes in two forms: compression ignition (diesel) and spark ignition. We will analyze the spark ignition (SI) system here. At this point, its still the dominant system in use in this countiy.

It's important to understand how energy is released in the SI engine. In an internal combustion engine, the air/iuel mixture is drawn into the cylinder, where it's compressed. As the air/fuel mixture is compressed, the molecules are forced into a smaller space. This causes them to run into each other, which creates friction and heat.

It takes energy to hold together the different atoms that form the molecular chain of the fuel molecules. In order for the fuel to release this energy, the fuel molecules must separate, or break apart, then reform into a different molecular structure with a lower energy state. Once the fuel molecules are broken apart, the energy used to hold everything together is no longer needed. This freed energy is what powers the internal combustion engine.

In an SI engine, cylinder compression alone does not provide enough energy to separate the fuel molecules. The heat that's transferred into the fuel molecules makes it unstable, but more force must be applied to separate the atoms contained in the fuel molecules. It would not be easy to separate two wrestlers locked together in combat. To separate them you'd have to apply more force than they're using to hold on to each other.

A stun gun that applied a spark of 100,000 volts would do the job. The potential energy supplied by the stun gun is greater than the energy the wrestlers are using to hold on to each other, so they would let go and separate. Even though the cylinder compression creates heat energy, more force is needed to separate the fuels molecular structure and release its energy. That force is supplied by a high-energy spark from an ignition system.

Many different types of ignition systems have been used to supply the high-energy spark necessary to ignite the air/fuel mixture. The most popular system in use today is the step-up transformer, which uses a low-voltage, high-current pole to create a high-voltage, low-current pole. This is accomplished with two different coils, or windings, of wire. The first coil is the primary and the second coil is the secondary (Fig. 1). The primary is wound around a core for magnetic amplification. In newer transformers, this core is composed of many plates of a ferrous metal (usually 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 smaller diameter wire with many more windings to produce a higher resistance value. The automotive coil is usually wound at a ratio of approximately 1:100. In other words, for each turn of the primary winding, the secondary has 100 winding turns. The primary winding resistance is normally in the range of 1 to 4 ohms, while the secondary winding usually has a resistance of 8000 to 16,000 ohms.

The primary and secondary windings are insulated from each other via transformer oil or epoxy. Transformer oil can hold off a breakdown voltage of only 20kV to 25kV, so in newer high-energy transformers, vacuum-sealed epoxy that can hold off a breakdown voltage of 50kV is used instead. The primary and secondary are electromagnetically coupled, so anything that affects one winding is mirrored in the other.

The step-up transformer uses electromagnetic induction to produce the necessary spark energy. To understand how the transformer works, lets look at the waveform produced by this device, beginning with waveform segment A in Fig. 2 below. (We'll keep referring to this waveform.) This is the open-circuit voltage, or source voltage, because the circuit has not been completed. There's no current flowing through the primary circuit at this point. The voltage then drops abruptly when the module driver is turned on, thus completing the primary circuit to ground (waveform segment B). This voltage drop will come very close to ground.

The initial voltage drop depends on whether the driver used to control the current is a transistor or a MOSFET. If a transistor is used, the voltage drop will be .7 to 1 volt. This is due to the resistance across the transistor's gate. A MOSFET has less resistance across its gate, causing a lower voltage drop of about .1 volt to .3 volt. The initial voltage drop is the voltage that remains in the circuit to push the current across the resistance of the module driver or gate (waveform segment 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 (Fig. 3). This magnetic field buildup is called inductance. The magnetic field is proportional to the inductance and the current. In other words, the larger the current, the larger the magnetic inductance.