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For the lathe tool, see lathe center. For the author of the "Top Dead Center" column, see Kevin Cameron (journalist).

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, gasoline engines, diesel engines, 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 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.


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.

Reciprocating engine[edit]

In a reciprocating engine, top dead centre of piston #1 is the datum point from which ignition system measurements are made and the firing order is determined. For example, ignition 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 combustions, 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 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 - but one piston will be at TDC of the compression stroke, the other on TDC of the exhaust stroke.
  • 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 engines 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 and many larger V engines, the piston motion within each bank is similar to that of a straight engine, however in the crossplane V8 and all V10 engines the motion is far more complex.

The concept of top dead centre is also extended to pistonless rotary engines, 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.[citation needed]

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[edit]

As steam engines are commonly horizontal, the relevant terms are front dead centre and back dead centre rather than "top" and "bottom".[citation needed]

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 restart. 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. Even larger engines might require the use of a barring engine.

Steam locomotives normally have two cylinders with the cranks set at right angles so one piston will always be off the dead centre and no starting assistance is required.[citation needed]

Other machines[edit]

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.[citation needed]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Dead_centre — Please support Wikipedia.
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The Guardian

The Guardian
Thu, 07 Aug 2014 03:05:26 -0700

"Nothing will come of nothing," said King Lear. Too true. And yet something does come of nothing in this piece of edgy contemporary theatre created by Dublin's Dead Centre and directed by Ben Kidd, best known for his Headlong production of Spring ...
Irish Times
Thu, 21 Aug 2014 17:11:15 -0700

But Horan, a regular visitor and participant, points to a diversity of aesthetics and scope among other Irish participants, including the virtuoso performance of Olwen Fouéré in her Joyce-inspired Riverrun or Dead Centre's challenging Lippy, both at ...

The Guardian (blog)

The Guardian (blog)
Fri, 22 Aug 2014 02:56:15 -0700

Then, with the scoreboard showing 90 minutes, Ian Stewart picked up possession 25 yards from goal and dead centre. Seeking Stainrod, he darted in a pass that was cleverly stepped over, and there was Micklewhite behind him, haring into the box and in on ...
Times of India
Sun, 17 Aug 2014 15:50:01 -0700

LUCKNOW/NEW DELHI: Flood fury continued unabated in Uttar Pradesh claiming 28 lives and displacing over 60,000 people in nine districts bordering Nepal till Sunday evening. As Rapti, Saryu and Ghaghra rivers continue to flow above the danger mark, ...

Net Newsledger

Net Newsledger
Thu, 21 Aug 2014 17:22:30 -0700

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Bluebird Banter
Wed, 20 Aug 2014 15:02:42 -0700

Marcus Stroman entered the game on his fifth pinch-running appearance right before Colby Rasmus homered to almost dead centre to make it a 9-5 game. Stroman scored the first run of his career, joining a select club of Blue Jays pitchers who have scored ...
The Australian
Thu, 21 Aug 2014 07:03:45 -0700

Every now and again during the course of the past 125 years, residents of Queenstown in Tasmania, and workers at the mine that sits dead centre in the town, have thought the jig was up. Disasters, including the big one that killed 42 workers in 1912 ...

Irish Examiner

Irish Examiner
Tue, 19 Aug 2014 18:30:00 -0700

Shannon Airport sits almost dead centre on WWA's coastal profile. It has huge headroom to raise its passenger throughput and could evolve as a key WWA hub that feeds tourists in to the system. Could the terminal facilities not evolve to provide more ...

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