Before describing the various types of ignition systems, it is necessary to explain the fundamentals and their role in these systems. The following is a general overview of the basic principles and theory of ignition system design.
It is only an explanation about how the various systems function, and does not specifically refer to any particular brand. As such, the design of an ignition system depends on the following factors:
Triggering is arranged so that the ignition coil is charged in sufficient time before the actual ignition point. This requires the formation of a dwell period (coil saturation time) in the ignition system. The energy to be released as a spark is usually stored in a coil as magnetic energy (with conventional systems). In other cases, this can be replaced with a capacitor as electrostatic energy, such as in a capacitive discharge ignition system (CDI), in which case the role of the coil changes to simply that of an energy transfer device. The high tension results from disconnecting the primary inductor from the power supply followed by transformation.
The high tension is then applied via the distributor to the cylinder currently performing the working stroke. All this combines to produce the required firing voltage, which is determined by the cylinder pressure, a by product of the inlet charge and compression, combined with the gap, temperature and shape of the spark plug electrode. The ignition system will then only deliver the voltage necessary to fire the spark plug. If all is well, the mixture will be successfully ignited. If insufficient energy is available, ignition does not occur, thus allowing a misfire. This is why adequate ignition must be provided.
The ignition coil is the heart of the ignition system. It consists of three main components; I) The primary winding, II) A soft iron core, and III) The secondary winding. The primary current, as switched on and off by the ignition distributor, flows through the primary winding of the ignition coil. The magnitude of the current is determined by the battery voltage (during initial starting), and alternator voltage (when running), as well as ohmic resistance in the primary winding. This may be between 0.2 and 3 ohms depending on the type of coil.
When a spark is required, power is disconnected from the primary winding by the points or electronic module and it induces the stored energy to flow onto the secondary winding, which because of its much greater amount of turns compared with the primary, multiplies the voltage available to create the high tension spark. At the ignition point, the voltage at the high tension tower of the ignition coil rises sinusoidally (in waves). The rate of rise is determined by the energy stored in the ignition coil. The coil then fires the spark plug performing the working stroke via the distributor cap, using only the voltage necessary to do so.
To complete the ignition system and carry the high tension secondary current effectively to the spark plug, a good quality rotor button, distributor cap and plug wire is necessary.
There are three main types of plug wires as follows:
- Standard carbon core plug wires. These are cheap and inexpensive, but with extremely high resistance as the spark has to pass from particle to particle of the carbon core to reach the spark plug. This high resistance tends to shorten the spark duration available to ignite the mixture. The reason this type of lead is used in standard applications is because they offer high levels of electrical noise suppression, thus allowing the use of cheaper radios, engine management systems etc.. Standard engines also do not have the higher cylinder pressures associated with high performance applications, and as such, are easier to ignite. These leads are not suitable for high output ignitions as the carbon core tends to burn out easily and are often of inferior quality as they are a cheap, mass produced item.
- Pure wire core plug wires. This type of lead is the opposite extreme to the carbon core plug wire. They offer no resistance through a straight stainless or copper wire core. They were originally used with low output ignitions, such as points type systems, to try and maximise the spark available to the plug. However, problems such as cross firing, interference with radio, TV and other electronic equipment becomes a problem, especially if attempts are made to use them with high energy electronic ignition systems. They are specifically prohibited for this reason, and also because they damage electronic systems and void warranty.
- Induction wound / spiral core plug wires. These are the most effective type of plug wire in every respect, offering very low resistance allowing the longest duration spark possible, while still having high levels of suppression, thus avoiding problems such as cross firing and electrical interference. They are ideal for all applications, but are especially useful in high performance engines and particularly those using alcohol or LPG.
We can now relate to the different types of ignition systems used and understand how each type works. We will cover single point systems, twin point systems, OE variable dwell electronic ignition systems and capacitive discharge ignition systems.
Single Point Systems
Points triggered coil ignition systems are the simplest version of an ignition system. This means that the current flowing through the ignition coil is switched on and off mechanically via a contact in the ignition distributor, (contact breaker – points). These are controlled by the distributor cam, which has as many lobes as the engine has cylinders. The cam is shaped such that there results a dwell angle corresponding to the ignition and the sparking rate. This dwell angle is invariable throughout the entire speed range, thus resulting in less time for coil charging as engine rpm increases, which translates to less spark output. This is one reason why points type systems are not very well suited to high performance work.
However, the dwell angle does change as the follower on the breaker points wears out. It causes the points to open later than normal, thus retarding the ignition timing and causing loss of power and economy. This is one of the reasons contact points need to be renewed regularly and the dwell angle checked. Remember, a V8 engine operating at 5000 rpm is switching the coil primary current on and off through the points 20,000 times per minute. This is also about the maximum rpm from this type of system. Another reason why maintenance is required is contact erosion (pitting) due to the primary voltage continually passing through the points, causing inadequate charging of the coil due to defective contact.
These single point systems employ conventional mechanical type centrifugal ignition advance shaft assemblies, comprising the main shaft, an ignition cam which pivots on the main shaft, advance weights which act on the ignition cam and primary and secondary springs to control the rate of advance when the weights are acting on the cam. The role of this mechanical advance system is to vary the timing according to the engine rpm, to suit the engine’s changing requirements throughout the entire rpm range. Further to this, they employ a vacuum advance canister, which acts on the points plate, to alter the timing dependant on the engine’s manifold vacuum and therefore load, to complement the mechanical advance.
Ignition coils used by single point systems are particular to their fixed dwell operation. They usually employ a resistor type coil (the resistor being built into the wiring loom), whereby the input voltage is limited by this resistor to 7-10 volts. This is necessary with points type systems as a higher voltage prematurely erodes the contacts on the contact points. Most of the factory coils provide adequate performance on standard engines, and replacing them with aftermarket type coils yields little or no benefit unless the O.E coil is faulty.
Twin Point Systems
Twin point systems are basically an extension of the single point system. They use two sets of points, still with a single coil, and by virtue of their staggered nature create extra dwell time. The dwell however is still fixed. The ignition coil is induced to fire when the second set of points begin to open. Twin point systems still suffer the same problems as single point systems in that points wear and regularly require attention. They provide a slightly higher output and spark duration although they are still no match for current technology electronic systems. Twin point systems predominantly utilise mechanical advance only. They employ the same type of coils as single point systems.
OE Variable Dwell Electronic Systems
OE variable dwell electronic systems represented a significant leap forward in ignition technology at the time. The distributor for these systems discards the conventional points and replaces it with a reluctor assembly and control module. The reluctor comprises of a permanent magnet with as many pole pieces as cylinders in the engine, a pick up coil with inductive wiring and a trigger wheel. The trigger wheel is comparable to the breaker cam of the contact breaker system. The number of teeth on the trigger wheel also corresponds to the number of cylinders in the engine. The signal created by this induction type pulse generator is the basis from which the on time of the dwell angle is determined by the control module. Consequently, this type of system is usually known as variable dwell. It allows a higher primary current to pass to their particular ignition coil, creating good intensity, combined with a long duration spark, to achieve greater total spark energy throughout the entire RPM range. These distributors continued to employ conventional mechanical and vacuum advance, the same as points systems.
The ignition coil used for these systems is specifically designed to cope with their variable dwell operation. They employ greater primary and secondary windings as compared to points type coils, and would burn out points if used in such a system. Original versions used either an oil filled design (and looked similar to points coils, but sometimes employed a male high tension tower), or were of a transformer design. These original coils usually offer good output and long term reliability.
Capacitive Discharge Ignition (CDI) Systems
The essential feature of CDI systems and what differentiates them over conventional electronic systems, is that the ignition energy is stored in the electrical field of a capacitor at approximately 400 volts. The storage capacitor is charged either with a constant current or with pulses. Regardless of the method, the charging stage contains a small transformer which boosts the voltage level to approximately 400 volts in order to achieve the required stored energy results. At the ignition point the thyristor is triggered. The capacitor then discharges via the thyristor to the ignition coil. The main advantage of CDI systems is their virtual insensitivity to electrical shunts (eg: fouled plugs) in the ignition circuit.
Originally designed in the late fifties, they were intended for higher revving applications, where their high intensity / short duration spark increased the total spark energy available over points systems, and was able to ignite mixtures more effectively. Another feature of some of these CDI systems is their multiple sparks at low RPM (usually under 3000 RPM, and particularly below 1000 rpm), increasing the total spark energy available. However, these multiple sparks always diminish with increasing RPM until a single spark is left above 3000 RPM. CDI systems can be triggered either by breaker points, (ie: an original single point distributor – as the role of the points is not critical with the CDI system) or inductive type pulse generator, (as found in many O.E and aftermarket electronic distributors). CDI systems also employ coils specific to their method of operation and should not be used with coils from other systems and vice-versa. They must also never be used with straight wire plug leads as this will cause massive interference with the units themselves and other electrical equipment as well as arcing to earth through the leads, thereby voiding any warranty.