Dead centre


Dead centre

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.

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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.

Reciprocating engine

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 combustion chambers, 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

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 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. 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

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]

References


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Look at other dictionaries:

  • dead centre — /dɛd ˈsɛntə/ (say ded sentuh) noun 1. (in a reciprocating engine or pump) either of two positions of the crank in which the connecting rod has no power to turn it, occurring when the crank and connecting rod are in the same plane, at each end of… …   Australian English dictionary

  • dead centre — noun the position of a crank when it is in line with the connecting rod and not exerting torque • Syn: ↑dead center • Hypernyms: ↑position, ↑spatial relation * * * dead centre noun 1. In a reciprocating engine or pump, either of the positions, at …   Useful english dictionary

  • dead centre — noun the position of a crank when it is in line with the connecting rod and not exerting torque …   English new terms dictionary

  • dead-centre — /dɛd ˈsɛntə / (say ded sentuh) adjective 1. completely on target. –adverb 2. right in the middle; accurately …   Australian English dictionary

  • Musgrave non-dead-centre engine — Park Street Mill engine in the Bolton Steam Museum …   Wikipedia

  • inner dead centre — noun The piston position, when the crank pin is nearest to the centre, at the beginning of the outstroke of a reciprocating engine or pump (also top dead centre) • • • Main Entry: ↑inner …   Useful english dictionary

  • bottom dead centre — see ↑outer dead centre under ↑outer • • • Main Entry: ↑bottom …   Useful english dictionary

  • outer dead centre — noun The position of the crank of a reciprocating engine or pump when the piston is at its nearest to the crankshaft (also bottom dead centre) • • • Main Entry: ↑outer …   Useful english dictionary

  • top dead centre — noun (of a reciprocating engine or pump) the piston position at the beginning of the outstroke, ie when the crank pin is nearest to the cylinder (also inner dead centre) • • • Main Entry: ↑top …   Useful english dictionary


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