The ignition system provides a spark between the spark plug electrodes. The spark must occur at precisely the right time in the engine cycle, and it must have sufficient energy to bridge the gap and ignite the air-fuel mixture under all operating conditions.
The energy required can be obtained from the vehicle's electrical system, but the nominal battery voltage of 12 volts must be increased or " stepped up " to provide a firing voltage of many thousands of volts. This high firing voltage causes the spark gap to become electrically conductive, enabling an ignition spark to occur.
It must have sufficient heat energy to ignite the mixture so that it can continue to burn by itself. Exactly how much energy is required varies according to the condition of the mixture, and the pressure in the cylinder at the end of the compression stroke.
With an engine at normal operating temperature, and under a light load, a mixture with a ratio close to the ideal of 14.7 : 1 ignites readily. However, suddenly depressing the accelerator to increase speed, or to maintain speed when hill-climbing, causes cylinder pressures to rise. This increases the firing voltage needed.
The ignition system is designed to have reserve energy available in excess of it's normal requirements so that it is able to produce ignition, even when conditions are unfavorable.
Most light vehicle ignition systems are of the inductive type: they use an induction coil with primary and secondary windings to "step-up " the nominal 12 volts at the battery to the required firing voltage.
Summary
An automotive ignition system is divided into two electrical circuits -- the primary and secondary circuits. The primary circuit carries low voltage. This circuit operates only on battery current and is controlled by the breaker points and the ignition switch. The secondary circuit consists of the secondary windings in the coil, the high tension lead between the distributor and the coil (commonly called the coil wire) on external coil distributors, the distributor cap, the distributor rotor, the spark plug leads and the spark plugs.
Process
The ignition principles are utilised with a sequences of advents with the following components. The sequence here is a generic one and can and does from various manufacturers and component suppliers.
Ignition coil
An ignition coil (also called a spark coil) is an electrical device in a automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs. This specific form of the induction coil converts current from a battery into the high voltage required by spark plugs in a internal combustion engine.
Firing order
The firing order is the sequence of sparking of the spark plugs in a reciprocating engine, or the sequence of fuel injection in each cylinder in a Diesel engine. Choosing an appropriate firing order is critical to minimizing vibration and achieving smooth running, for long engine fatigue life and user comfort.
In a straight engine the spark plugs (and cylinders) are numbered, starting with #1, from the front of the engine to the rear. In most cars the front of the engine also points to the front of the car, but some manufacturers (Saab, Citroën) in some models place the engine 'backwards', with #1 towards the firewall.
In a V engine the right bank is numbered first, followed by the left bank.
In a radial engine the cylinders are numbered around the circle, with the #1 cylinder at the top. There are almost always an odd number of cylinders, as this allows for a constant every-other-piston firing order: for example, with a single bank of 7 cylinders, the order would be ...2-4-6-1-3-5-7-2....
The numbers are usually cast on the cylinder head or the intake manifold or the valve cover(s).
In a conventional engine, the correct firing order is obtained by the correct placement of the spark plug wires on the distributor. In a modern engine with an engine management system and direct ignition, the Electronic Control Unit (ECU) takes care of the correct firing sequence.
High Tension Leads
High tension (HT) leads consist of stranded metallic wire or graphite core conductors wrapped in an insulated cover or sheath. The purpose of the leads is to conduct the electrical impulse or surge from the coil to the spark plugs via the distributor. Because of the extremely high voltages required to create an arc-over for each spark plug, the HT leads must have sufficient conductivity to carry a charge without any leakage. The higher the voltage the higher the potential leakage, thence the utilization of the silicon based HT leads.
Resistance across each lead varies in proportion with length, but the generic standard for each wire is around 500 ohms per foot (300mm). This resistance allows a appropriate electrical transmission while restricting or limiting the amount of Radio Frequency (RF) interference.
Each end of the high tension leads feature a brass fitting, either designed for connectivity with the distributor cap or spark plug terminal. It goes without saying, it is imperative that any connection of this manner is clean and tight to prevent voltage loss.
Distributor cap
Distributor caps are used in automobile engine to cover the distributor and its internal rotor. The rotor switches a high sparking voltage to the spark plugs so that these fire in correct sequence.
The distributor cap is a prime example of a component that eventually succumbs to heat and vibration. But even if its bakelite housing has not broken or cracked, carbon deposits and eroded metal terminals can cause distributor-cap failure. However it is a fairly easy and inexpensive part to replace.
The distributor cap has 3 to 9 posts on it. One post is for the coil voltage coming into the distributor. The other posts go to each spark plug respectively.
On the inside of the cap there is a terminal that corresponds to each post. The plug terminals are arranged around the circumference of the cap. Some distributors have the outside terminals in a straight line.
Rotor
The rotor head (usually called "the rotor") is attached to the top of the distributor shaft which is driven by a gear on the engine's camshaft and thus synchronized to it.
The rotor is pressed against a carbon brush on the center terminal of the distributor cap. A spring is used to keep tension on the carbon point. On the inside of the cap, the coil terminal is in the center. The rotor is constructed such that the center tab is electrically connected to its edge, so the voltage coming in the coil post will travel through the carbon point to the center of the rotor, then to its edge. Some rotors have an integrated resistor in the center tab for suppression of radio interference.
As the rotor rotates, its edge passes each of the plug terminals that are arranged around the inside of the cap, according to the firing order, sending the secondary voltage to the proper spark plug.
Contact breaker
Breaker arm with contact points at the left. The pivot is on the right and the cam follower is in the middle of the breaker arm. A contact breaker (or "points") is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of non Diesel-powered internal combustion engines. The purpose of the contact breaker is to interrupt the current flowing in the primary circuit of the ignition coil. When this occurs, the collapsing current induces a current in the secondary winding of the coil, which has very many more turns. This causes a very large voltage to appear at the coil output for a short period - enough to arc across the electrodes of a spark plug.
The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the piston's compression stroke. The contact breaker is usually mounted on a plate that is able to rotate relative to the camshaft operating it. The plate is rotated by a centrifugal mechanism, thus advancing the timing (making the spark occur earlier) at higher revolutions. This gives the fuel time to burn so that the resulting gases reach their maximum pressure at the same time as the piston reaches the top of the cylinder. The plate's position can also be moved a small distance using a small vacuum-operated servomechanism, providing advanced timing when the engine is required to speed up on demand. This helps to prevent pre-ignition.
Contact breaker points suffer from wear - both mechanical (since they open and close several times every turn of the engine) and caused by arcing across the contacts. This latter effect is largely prevented by placing a capacitor across the contact breaker - this is usually referred to by the more old fashioned term condenser by mechanics. As well as suppressing arcing, it helps boost the coil output by creating a resonant LC circuit with the coil windings. A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise, needs regular adjustment, and at higher revolutions, its mass becomes significant, leading to poor operation at higher engine speeds. These effects can largely be overcome using electronic ignition systems, where the contact breakers are retrofitted by a massless sensor device.
Fixed Capacitor
Capacitors have thin conducting plates (usually made of metal), separated by a layer of dielectric, then stacked or rolled to form a compact device. Many types of Discrete capacitors are available commercially, with capacitances ranging from the picofarad range to more than a Farad, and voltage ratings up to many kilovolts. In general, the higher the capacitance and voltage rating, the larger the physical size of the capacitor and the higher the cost. Tolerances in capacitance value for discrete capacitors are usually specified as a percentage of the nominal value. Tolerances ranging from 50%(electrolytic types) to less than 1% are commonly available. Another figure of merit for capacitors is stability with respect to time and temperature, sometimes called drift. Variable capacitors are generally less stable than fixed types.
Capacitors are often classified according to the material used as the dielectric with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films (so-called electrolytic capacitors).
As electric charge accumulates on the plates of a capacitor, a voltage develops across the capacitor due to the electric field of the accumulated charge. Ever increasing work must be done against this ever increasing electric field as more charge accumulates. The energy (measured in joules, in SI) stored in a capacitor is equal to the amount of work required to establish the voltage across the capacitor, and therefore the electric field.
The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to the permittivity of the dielectric (that is, non-conducting) substance that separates the plates.
