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ignition system[ig′nish·ən ‚sis·təm]
The system in an internal combustion engine that initiates the chemical reaction between fuel and air in the cylinder charge by producing a spark. An ignition system for a multicylinder internal combustion engine has three basic functions: (1) to provide a sufficiently energetic spark to initiate the burning of the fuel-air mixture within each cylinder; (2) to control spark timing for optimum efficiency so that cylinder pressure reaches its maximum value shortly after the piston reaches the top of its compression stroke; and (3) to select the correct cylinder fired.
In an inductive ignition system, there are three possible types of control-vacuum-mechanical, electronic spark, or full electronic engine. Prior to spark discharge, electrical energy is stored inductively in the coil primary. The current to the coil primary winding is turned on and off by the ignition module in response to the spark-timing trigger signal. The current-off time marks the beginning of the sparking event. An accurate spark-timing schedule is a complex function of many engine variables, such as fuel-air composition, engine revolutions per minute (rpm), temperature, cylinder pressure, exhaust gas recirculation rate, knock tendency, and engine design.
The ignition coil stores electrical energy during the dwell (current-on) period and acts as a transformer at the end of dwell by converting the low-voltage-high-current energy stored in the primary to high-voltage-low-current energy in the secondary. The distributor selects the fired spark plug by positioning the rotor opposite the terminal connected to one spark plug. The plug selected depends on the cylinder firing order, which in turn depends on the engine design. The distributor is driven at one-half engine speed from the camshaft. See Spark plug
When high voltage (10–30 kV) is created in the coil secondary, a spark jumps from the rotor to a distributor cap terminal, establishing a conducting path from the ignition coil high-voltage terminal along a high-voltage wire to the spark plug. Each cylinder usually has one spark plug. (High-efficiency engines may have two spark plugs per cylinder and two complete ignition systems.) The plug electrodes project as far into the cylinder as possible. After high voltage is applied to the plug, an electrical discharge is generated between its two electrodes. The energy and temperature of this discharge must be sufficient to reliably ignite the fuel-air mixture under all encountered conditions of composition, temperature, and pressure.
Among the several other types of ignition systems for internal combustion engines are capacitive discharge, multiple-firing capacitive discharge, continuous sustaining, magneto, and distributorless ignitions. The input energy for capacitive discharge systems is stored on a capacitor at several hundred volts (generated by a dc-dc converter). A semiconductor switch (thyristor) controls the discharge of the capacitor into the primary winding. In a multiple-firing capacitive discharge ignition, the ignition module repetitively fires a capacitive discharge ignition during one spark event, increasing both the energy and effective time duration of the spark. In a continuous sustaining ignition, supplemental electrical power is added to the spark after it is established, resulting in electronically controlled extended duration rather than uncontrolled duration as for conventional ignitions. In a magneto ignition, electric current and energy are generated in the primary by relative rotational motion between a magnet and a coil (electromagnetic induction). High voltage is generated in the secondary when a set of contacts in the primary circuit is mechanically opened. Magnetos require no external source of electrical power.
The distributorless ignition system eliminates the need for mechanical distribution of spark energy by using a single coil for one, two, or four cylinders. For the two-cylinder-single-coil system, a double-ended ignition coil simultaneously fires a cylinder in a compression stroke together with a second in an exhaust stroke. The exhaust stroke cylinder accepts the waste spark to complete the electrical circuit through the engine block. A design variation uses alternating polarity high voltage from a special type of double-ended coil and four high-voltage rectifiers to fire four plugs. The rectifiers steer the voltage to the correct pair of plugs.
In diesel or compression ignition engines, sparkless ignition occurs almost immediately after fuel injection into the cylinder due to high in-cylinder air temperatures. High temperatures result from the high compression ratio of diesel engines. Mechanical or electronic injection timing systems determine ignition timing. See Combustion chamber, Diesel engine, Internal combustion engine