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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 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 This type of arrangement was quickly superseded by spark ignition, a system which is generally used to this day, albeit with sparks generated by more sophisticated circuitry.
Glow plug ignition
Glow plug ignition is used on some kinds of simple engines, such as those commonly used for model aircraft. A glow plug is a coil of wire (made from e.g. nichrome) that will glow red hot when an electric current is passed through it. This ignites the fuel on contact, once the temperature of the fuel is already raised due to compression. The coil is electrically activated for engine starting, but once running, the coil will retain sufficient residual heat on each stroke due to the heat generated on the previous stroke. Glow plugs are also used to aid starting of diesel engines.
The simplest form of spark ignition is that using a magneto. The engine spins a magnet inside a coil, and also operates a contact breaker, interrupting the current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are connected directly from the magneto output. Magnetos are not used in modern cars, but they are often found on mopeds, with 2-stroke engines and also in aircraft piston engines, where their simplicity and self-contained nature confers a generally greater reliability as well as lighter weight. Aircraft engines usually have multiple magnetos to provide redundancy in the event of a failure.
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 - a form of autotransformer. This steps up the voltage, which is fed via a rotating switch called a distributor to the spark plugs. This sytem 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.
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.
In an Engine Management System (EMS), electronics control fuel delivery, ignition timing and firing order. Primary sensors on the system are engine angle (crank or Top Dead Center (TDC) position), airflow into the engine and throttle demand position. The circuitry determines which cylinder needs fuel and how much, opens the requisite injector to deliver it, then causes a spark at the right moment to burn it. Early EMS systems used analogue computer circuit designs to accomplish this, but as embedded systems became fast enough to keep up with the changing inputs at high revolutions, digital systems started to appear.
Some designs using EMS retain the original coil, distributor and spark plugs found on cars throughout history. Other systems dispense with the distributor and coil and use special spark plugs which each contain their own coil (Direct Ignition). This means high voltages are not routed all over the engine, they are created at the point at which they are needed. Such designs offer potentially much greater reliability than conventional arrangements.
Modern EMS systems usually monitor other engine parameters such as temperature and the amount of pollution in the exhaust. This allows them to control the engine to minimise unburnt fuel and other noxious gases, leading to much cleaner and more effcient engines.
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