Dead Center
From Quad Hub Wiki
In a reciprocating engine, the dead center is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead center (TDC) while the latter is known as bottom dead center (BDC).
More generally, the dead center is any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied. Crank-driven machines rely on the energy stored in a flywheel to overcome the dead center, or are designed, in the case of multi-cylinder engines, so that dead centers 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 center for each cylinder occurs out of phase with the other one (or two) cylinders.
Internal Combustion Engine
Top dead center is the datum point from which engine timing measurements are made. For example, ignition system timing is normally specified as degrees before top dead center (BTDC) although a very few small and fast-burning engines require a spark just after top dead center (ATDC), such as hydrogen engines.
Top dead center 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 center 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.
