![]() The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. Setting the correct ignition timing is crucial in the performance of an engine. 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. 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. In a vast majority of cases, the angle will be described as a certain angle advanced before top dead center (BTDC). The combustion gases 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. ![]() The need for advancing (or retarding) the timing of the spark is because fuel does not completely burn the instant the spark fires. In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke. Pressure in cylinder pattern in dependence on ignition timing: (a) - misfire, (b) too soon, (c) optimal, (d) too late.
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