Difference between revisions of "Top dead centre"

Latest revision as of 20:58, 25 May 2010

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Top dead center

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-In a [[reciprocating engine]], the '''dead centre''' is the position of a piston in which it is farthest from, or nearest to, the [[crankshaft]]. The former is known as  '''top dead centre''' ('''TDC''') while the latter is known as '''bottom dead centre''' ('''BDC''').
-More generally, the dead centre is any position of a [[crank]] where the applied force is straight along its axis, meaning no turning force can be applied.  Many sorts of machines are crank driven, including [[unicycle]]s, [[bicycle]]s, [[tricycle]]s, various types of [[machine press]]es, [[steam locomotive]]s, and other [[steam engines]]. Crank-driven machines rely on the energy stored in a [[flywheel]] to overcome the dead centre, or are designed, in the case of [[Engine configuration|multi-cylinder engines]], so that dead centres can never exist on all cranks at the same time.  A [[steam locomotive]] is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.
-==Bicycles==
-[[Bicycle]] cranks have dead centres at approximately 12 o'clock and 6 o'clock where simple pushing down of the pedal will not turn the [[chainwheel]], but the rider's leg is able to apply tangential force at the pedal to overcome it. [[Fixed gear bicycles]] (without a [[freehub]]) use the [[momentum]] of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.
-==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 vehicle|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:
-* In the [[V-twin]] configuration, the two pistons reach TDC at different times, equal to the [[angular displacement]] between the cylinders.
-* In the [[flat twin]] configuration, two opposing pistons reach TDC simultaneously, which is also called 0° displacement.
-* In the [[straight-4]] configuration, the two end pistons (pistons 1 and 4) reach TDC simultaneously, as do the two centre pistons (pistons 2 and 3), but these two pairs reach TDC with an angular displacement of 180°. Similar patterns are found in almost all [[straight engine]]s with even numbers of cylinders, with the two end pistons and two middle pistons moving together (not necessarily 180° out of phase however) and the intermediate pistons moving in pairs in mirror-image around the centre of the engine.
-* In the flatplane [[V8 engine|V8]] and many larger [[V engine]]s, the piston motion within each bank is similar to that of a straight engine, however in the [[crossplane]] V8 and all [[V10 engine|V10]] engines the motion is far more complex.
-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.
-By finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders you have, you will get your engine displacement.
-==Steam engine==
-As [[steam engine]]s are commonly [[Horizontal plane|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 [[Cylinder (steam locomotive)|cylinders]] with the [[Crank (mechanism)|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 centre.
-{{Automobile configurations}}

Difference between revisions of "Top dead centre"

Latest revision as of 20:58, 25 May 2010

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