Basic Ignition Systems
The ignition system of an internal-combustion engine is an important part of the overall engine system. It provides for the timely burning of the fuel mixture within the engine. Not all engine types need an ignition system - for example, a diesel engine relies on compression-ignition, that is, the rise in temperature that accompanies the rise in pressure within the cylinder is sufficient to ignite the fuel spontaneously. All conventional petrol (gasoline) engines, by contrast, require an ignition system. The ignition system is usually switched on/off through a lock switch, operated with a key or code patch.
Contact ignition
The earliest petrol engines used a very crude ignition system. This often took the form of a copper or brass rod which protruded into the cylinder, which was heated using an external source. The fuel would ignite when it came into contact with the rod. Naturally this was very inefficient as the fuel would not be ignited in a controlled manner. This type of arrangement was quickly superseded by spark ignition, a system which is generally used to this day, albeit with sparks generated by more sophisticated circuitry.
Mechanical ignition
Most four-stroke engines have used a mechanical ignition system. Here, the power source is a lead-acid battery, kept charged by the car's electrical system, which generates electricity using a dynamo or alternator. The engine operates contact breaker points, which interrupt the current flow to an induction coil (known as the ignition coil) - a type of autotransformer. This steps up the voltage, which is fed via a rotating switch called a distributor to the spark plugs. This system is not greatly different from a magneto system, except that more separate elements are involved. There are also advantages to this arrangement, for example, the position of the contact breaker points relative to the engine angle can be changed a small amount dynamically, allowing the ignition timing to be automatically advanced with increasing revolutions per minute (RPM), giving better efficiency. This system was used almost universally until the late 1970s, when electronic ignition systems started to appear.
Basic Ignition
A basic ignition system consists of:
The ignition system provides high-intensity sparks at the spark plugs, to ignite the fuel charges in the combustion chambers. The sparks must be supplied at the right time, and they must have sufficient energy, over a range of conditions, to ignite the charges. The energy comes from the battery and alternator, and the voltage is increased by the ignition coil.
The system has two circuits. The primary, or low-tension circuit initiates the spark. The secondary, or high-tension circuit produces the high voltage and distributes it to the spark plugs.
There are three general types of ignition systems:
Electronic ignition
The disadvantage of the mechanical system is that it requires regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. Electronic ignition (EI) solves these problems. In an EI system, the contact breaker points are replaced by an angular sensor of some kind - either optical, where a vaned rotor breaks a light beam, or more commonly using a Hall effect sensor, which responds to a rotating magnet mounted on a suitable shaft. The sensor output is shaped and processed by suitable circuitry, then used to trigger a switching device such as a thyristor, which switches a large flow of current through the coil. The rest of the system (distributor and spark plugs) remains as for the mechanical system. The lack of moving parts compared with the mechanical system leads to greater reliability and longer service intervals. For older cars, it is usually possible to retrofit an EI system in place of the mechanical one.
During the 1980s, EI systems were developed alongside other improvements such as fuel injection systems. After a while it became logical to combine the functions of fuel control and ignition into one electronic system known as an engine management system.
Source: CDX Global & Wikipedia - en.wikipedia.org
