Modern ignition systems
Mechanically timed ignition
Until recently, most four-stroke engines used a mechanically timed electrical ignition system. The heart of this mechanically timed ignition system is the distributor, containing a rotating cam synchronised to the engine, a set of breaker points, a condenser, a rotor and a distributor cap.
The distributor is connected to the ignition coil and the spark plugs via a highly insulated ignition wiring system.
The electrical power source is the ubiquitous lead-acid battery that is charged while the engine is running by a dynamo or alternator.
The rotating engine operates contact breaker points interrupting or modulating current flow to an induction or ignition coil.
The ignition coil consists of the primary and secondary windings of a transformer on a common magnetic core. They form 2 separate wiring circuits and are joined together at common point that connects to the battery via a current limiting resistor. The primary coil is also connected to the points within the distributor. The secondary is connected to the spark plugs through the distributor cap and rotor.
The rotating cam controls the opening and closing of the breaker points and the Primary is fed with current when the breaker points close. This induces a magnetic field to form.
As the engine turns it reaches the top of its compression cycle. This is synchronised to be the point when the breaker opens, switching off the Primary circuit. The magnetic field collapses and generates a high voltage in the coil's secondary windings. A condenser capacitor holds the excess energy until it can be released causing a spark across the gap in a spark plug. A rotor, within the distributor cap, sequentially connects the coil's secondary windings to one of the engine's spark plugs.
Electronic ignition
The breaker points are subject to mechanical wear through continuous rubbing against the cam during each open-close cycle. Oxidation and burning of the contact surfaces occurs with sparking across the gap. Compensating for wear, the system needs regular adjustment to maintain optimal firing performance.
Electronic ignition (EI) solves these problems where the breaker points are replaced by a rotor angle sensor such as a Hall effect magnetic field sensor that triggers a switching device such as a thyristor, for shunting a large flow of current through the coil. The distributor and spark plugs remain the same as the mechanical system.
The lack of moving and rubbing parts compared with the mechanical system leads to greater reliability and longer service intervals. Older cars could be retrofitted with an EI system in place of the mechanical one. Sometimes, a modern distributor will fit the older engine with no other modifications.
Engine management Systems - EMS
In a modern Engine Management System, electronics control virtually everything - fuel delivery, ignition timing and the firing order. The Primary sensors on the system are engine angle checking Crank Position or Top Dead Center TDC, airflow into the engine and accelerator position for fuel demand.
The modern electronic circuitry creates a mini-computer or Electronic Control Unit (ECU) that determines which cylinder needs fuel, how much, opens the appropriate injector valve, delivers the fuel, causes a fuel ignition spark at just the right moment to burn it.
Some EMS designs still use the coil, distributor and spark plug system. Now its is possible to use special spark plugs containing their own coil for Direct Ignition. Modern EMS systems even monitor other important engine efficiency parameters such as temperature, the level of free oxygen in the exhaust. This allows greater control of the engine to minimise un-burnt or partially combusted fuel and other noxious gases - the so-called SOx NOx Gases, leading to much cleaner and more fuel-efficient engines.