Bottom 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 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 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.

In a reciprocating engine, top dead centre of piston #1 is the point from which ignition system measurements are made and the firing order is determined. For example, ignition timing is normally specified as degrees of crankshaft rotation before top dead centre (BTDC). 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.

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Wikipedia