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In areciprocating engine, thedead centre is the position of a piston in which it is either furthest from, or nearest to, thecrankshaft. The former is known astop dead centre (TDC) while the latter is known asbottom dead centre (BDC).[1]
More generally, the dead centre is any position of acrank where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, includingunicycles,bicycles,tricycles, various types ofmachine presses,gasolineengines,diesel engines,steam locomotives, and othersteam engines. Crank-driven machines rely on the energy stored in aflywheel to overcome the dead centre, or are designed, in the case ofmulti-cylinder engines, so that dead centres can never exist on all cranks at the same time. Asteam 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 more) 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 thechainwheel, but the rider's leg can apply tangential force at the pedal to overcome it.Fixed-gear bicycles (without afreehub) use themomentum of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.
In areciprocating engine, top dead centre of piston #1 is the point from whichignition system measurements are made and thefiring order is determined. For example,ignition timing is normally specified as degrees of crankshaft rotationbefore top dead centre (BTDC).[2] A very few small and fast-burning engines require a spark justafter top dead centre (ATDC), such as theNissan MA engine with hemisphericalcombustions, orhydrogen engines.[citation needed]
Top dead centre for cylinder one is often marked on the crankshaft pulley, theflywheel orharmonic balancer or both, with adjacenttiming marks showing the recommended ignition timing settings as decided during engine development. The timing marks can be used to set the ignition timing either statically by hand or dynamically using atiming light, by rotating thedistributor in its seat.
In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on theengine configuration. For example:
The concept of top dead centre is also extended topistonless 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 theWankel engine for example, it occurs three times for every one revolution of the rotor (although only once per revolution of the engine output shaft, since the output rotates at three times the speed 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 theengine displacement.
Assteam engines are commonlyhorizontal, 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 theflywheel 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 abarring engine.
Steam locomotives normally have at least twodouble acting cylinders, which enables thecranks to be set so that at least onepiston will always be off the dead centre and no starting assistance is required. In the common case of a two piston locomotive, the cranks are set atright angles, so that whenever one piston is at dead centre the other is in mid-stroke, and giving four equally spaced power strokes per revolution.[citation needed]
This term is also used in the realm of production equipment. A mechanicalpunch press employs a crankshaft similar to that found in an engine. In the punch press, the crankshaft drives a ram that when it is furthest away from the platen of the press is deemed to be in the position of top dead centre.[3]
The dictionary definition oftop dead center at Wiktionary