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Top dead centre ('''TDC''') in a Piston Engine , is the position of a Piston in which it is farthest from the Crankshaft . The position closest to the crankshaft is known as '''bottom dead centre''' ('''BDC'''). INTERNAL COMBUSTION ENGINE Top dead centre is the Datum Point from which engine Timing measurements are made. For example, Ignition System timing is normally specified as degrees before top dead centre ('''BTDC''') although a very few small and fast-burning engines, require a spark just '''after top dead centre''' ('''ATDC'''), such as the Nissan MA Engine with hemispherical Combustion Chamber s, or Hydrogen Engines . Top dead centre for cylinder one is often marked on the crankshaft pulley, the Flywheel or Dynamic Balancer or both, with adjacent Timing Mark s showing the recommended ignition timing settings as decided during engine development. These timing marks can be used to set the ignition timing either statically by hand or dynamically using a Timing Light , by rotating the Distributor in its seat. In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on the Engine Configuration . For example:
The concept of top dead centre is also extended to Pistonless Rotary Engine s, and means the point in the cycle in which the volume of a combustion chamber is smallest. This typically occurs several times per rotor revolution; In the Wankel Engine for example it occurs three times for every one revolution of the rotor. STEAM ENGINE As Steam Engine s are commonly Horizontal , the relevant terms are front dead centre and '''back dead centre''' rather than "top" and "bottom". If a single-cylinder steam engine stops in either of the dead centre positions it must be moved off the dead centre before it will re-start. In small engines this is done by turning the Flywheel by hand. In large engines the flywheel is moved with a lever or "turning bar". Both operations must be done with care to avoid the operator becoming entangled in the machinery. Steam Locomotive s normally have two Cylinders with the Cranks set at Right Angle s so one Piston will always be off the dead centre and no starting assistance is required. OTHER MACHINES This term is also used in the realm of production equipment. A mechanical punch press employs a crankshaft similar to that found in an engine. In the punch press the crankshaft drives a ram which when it is farthest away from the platen of the press is considered to be in the position of top dead center. |
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