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Ignition timing, in a spark ignition internal combustion engine (ICE), is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke. The need for advancing the timing of the spark is because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand, and the angular or rotational speed of the engine can lengthen or shorten the time frame in which the burning and expansion should occur. In a vast majority of cases, the angle will be described as a certain angle advanced ''before top dead center'' (BTDC). Advancing the spark BTDC means that the spark is energized prior to the point where the combustion chamber reaches its minimum size, since the purpose of the power stroke in the engine is to force the combustion chamber to expand. Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke. Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled in real time by an engine control unit use a computer to control the timing throughout the engine's RPM and load range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range. Early cars required the driver to adjust timing via controls according to driving conditions, but this is now automated. There are many factors that influence proper ignition timing for a given engine. These include the timing of the intake valve(s) or fuel injector(s), the type of ignition system used, the type and condition of the spark plugs, the contents and impurities of the fuel, fuel temperature and pressure, engine speed and load, air and engine temperature, turbo boost pressure or intake air pressure, the components used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine. ==Background== The spark ignition system of mechanically controlled gasoline internal combustion engines consist of a mechanical device, known as a distributor, that triggers and ''distributes'' ignition spark to each cylinder relative to piston position—in crankshaft degrees relative to top dead centre (TDC). Spark timing, relative to piston position, is based on static (initial or base) timing without mechanical advance. The distributor's centrifugal timing advance mechanism makes the spark occur sooner as engine speed increases. Many of these engines will also use a vacuum advance that advances timing during light loads and deceleration, independent of the centrifugal advance. This typically applies to automotive use, marine gasoline engines generally use a similar system but without vacuum advance. In 1972, Chrysler introduced an electronic ignition system (as standard equipment on some production cars, standard across the board by 1973) that replaced the mechanical contact breaker ("points"). This provided for a stronger ignition spark, as well as virtually eliminating variations in ignition timing due to wear associated with breaker points. Electronic control of ignition timing was introduced a few years later in 1975-'76 with the introduction of Chrysler's computer-controlled "Lean-Burn" electronic spark advance system. By 1979 with the Bosch Motronic engine management system, technology had advanced to include simultaneous control of both the ignition timing and fuel delivery. These systems form the basis of modern engine management systems. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ignition timing」の詳細全文を読む スポンサード リンク
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