Red marquis at 19:04, 13 May 2010

2010-05-13T19:04:39Z

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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''').		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 unicycles, bicycles, tricycles, various types of machine presses, [[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.		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 unicycles, bicycles, tricycles, various types of machine presses, steam locomotives, 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.

	==Internal combustion engine==		==Internal combustion engine==

Red marquis at 19:04, 13 May 2010

2010-05-13T19:04:10Z

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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 unicycles, bicycles, tricycles, various types of machine presses, [[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.

==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 marks 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.

Finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders will give the [[engine displacement]].

==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 locomotives normally have two cylinders with the [[Crank|cranks]] set at right angles 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.

[[Category:Engine technology]]

Dead centre - Revision history

Red marquis at 19:04, 13 May 2010

Red marquis at 19:04, 13 May 2010