Knowledge Building: Modern Ignition Systems

As the number of electrical components in today’s vehicles increases, there is a growing need not only to understand their function and operational characteristics, but also how they are assembled.

In many cases the technician can improve his understanding of a particular system not just by following the manufacturer’s service information, but by learning how the system operates from within and what comprises each individual component.

The ignition system’s primary purpose is to supply the spark to the engine for proper ignition of the air/fuel mixture in the combustion chamber. Today’s cars use an engine control module (ECM) to control ignition systems that use such designs as coil-on-plug to distribute the power to each individual cylinder. (On some vehicles, a distributor cap and rotor may still be used.) Both types of systems must do the same job: provide the power at the right cylinder at the precise time. The slightest error in timing will cause a rough-running engine.

In order for the spark to occur, the voltage to the spark plug must average between 20,000 and 50,000 volts. In some cases, the voltage can be even higher.

For the ignition system to operate, it must be capable of doing two jobs at the same time. The main job is to increase the voltage from the 12.4 volts provided by the battery to the more than 20,000 volts needed to ignite the air/fuel charge. The second job is to ensure that the voltage is delivered to the right cylinder at the right time.

The ignition coil is the source of the power, and acts as a power transformer. The coil housing holds the primary and secondary ignition circuit windings. The secondary circuit windings can range anywhere between 15,000 and 30,000 turns of copper wire. The steel plates that the wires are wound around are laminated, which insulates the circuit voltage from jumping or shorting out while reducing eddy currents. The wire is also enamel coated, which stops turn-to-turn shorts. There may be additional insulation such as moulded plastic used as well. The secondary ignition windings are seated inside the windings of the primary circuit. To aid in the buildup of magnetic fields within the circuit, a soft iron core is used.

The ignition coil is a component of the ignition system, where there are very little tolerances for manufacturer error. If any of the windings, the plastic protection, the soft iron core, or the cooling agent fails, extreme engine drivability problems may be the result. A customer can complain about many things such as misfiring, difficulty in starting, or rough running under all conditions.

Manufacturers have made many improvements to the ignition coils, both in performance and design. Noise abatement material has been added between laminations to reduce audible coil noise. The use of composite iron coils can provide programmable energy control. Lower primary inductance provides faster performance of the coil. Direct connection to the spark plug with a coil for each cylinder allows for improved dwell accuracy, which reduces module and coil temperatures–improving high-speed performance, enabling better heat dissipation, and allowing for greater reliability.

Coil designs and styles have changed as well. We are all familiar with waste spark coil packs; manufacturers have worked to reduce the size of these by up to 40% over older designs. The coil packs have been used in semi-direct mount ignition systems, which have been updated further to a variety of configurations such as coil-near plugs, plug-top coils, or coil-on-plug and pencil coils.

Further developments in coil design include ion sense ignition systems and multi-charge ignition systems. Ion sense ignitions feature direct in-cylinder measurement of combustion quality. This system uses conventional spark plugs as in-cylinder combustion quality sensors. This system offers more precise knock and misfire detection–determined by combustion outcome rather than crankshaft speed measurements.

Multi-charge coils are designed for engines with highly diluted fuel mixtures such as a stratified direct injection application. They allow for longer spark duration, more energy, and re-ignition in the event that combustion is extinguished when liquid is present, which can compensate for fuel spray variation.

Ignition control, whether ignition module or ECM-controlled, can come in two distinct designs where the operation is concerned: Inductive Discharge Ignition (IDI) or Capacitive Discharge Ignition (CDI).

In the CDI modules, energy for the spark is stored in a capacitor situated within the module. The energy is sent to the appropriate ignition tower for the spark plug at any time during the ignition cycle–as determined by the engine control module. In doing this, the control module can adjust the engine’s timing automatically as driving conditions change.

To prevent the stored energy from causing damage to the internal components of the ignition module, manufacturers now use many features to improve the reliability and longevity of their parts. The usage of copper heat sinks to aid in the dispersion of the heat created by the high electrical current helps improve the module’s ability to operate under adverse temperature conditions. The lead connections at all points within the module are now electronically welded; this dramatically improves the contact points at the connections and increases the module’s reliability where circuit integrity is a factor.

The CDI module’s internal capacitor is charged via the engine’s charging system. At the precise moment that the air/fuel charge is to be ignited, the engine control module sends a signal to the ignition module to stop charging the capacitor and release the current–sending it to the appropriate cylinder for an ignition event. For the current to be strong enough to ignite the air/fuel charge, the voltages in these systems can be as high as 40,000 volts (depending on the make of the vehicle). The ignition module must use a transformer located within its housing. The 12.4 volt signal is first raised to around 400-600 volts.

The increased voltage is then sent to the capacitor for charging and storage. The charging portion of the ignition module contains its own rectifier; this is used to eliminate any accidental release of energy to the ignition circuit.

When the engine control module sends the signal for an ignition event to occur, the capacitor will release the fully stored amount of voltage very quickly. The current first goes to a low-inductance ignition coil within the ignition module, where the current is ramped up to the 40,000 volts that is required to adequately fire the spark plugs.

As we see more and more “smart” ignition systems, knowing your parts and how they work is vital.

For more information on automotive technology, visit CARS OnDemand training at www.cars-council.ca.