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Top dead center in a gasoline engine. In a reciprocating engine, the dead centre is the position of a piston in which it is either 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). [1] Position of pistons
Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock, etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke. Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the ...
This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum (negative pressure) in the cylinder through its downward motion.
Process 1–2 is an adiabatic (isentropic) compression of the charge as the piston moves from bottom dead center (BDC) to top dead center (TDC). Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at top dead center.
Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs. Key: TDC = Top dead centre; BDC = Bottom dead centre; IO = Inlet valve opens
Ray-traced image of a piston engine. There may be one or more pistons. Each piston is inside a cylinder, into which a gas is introduced, either already under pressure (e.g. steam engine), or heated inside the cylinder either by ignition of a fuel air mixture (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (Stirling engine).
A variation coil-on-plug ignition has each coil handle two plugs, on cylinders which are 360 degrees out of phase (and therefore reach top dead center (TDC) at the same time); in the four-cycle engine this means that one plug will be sparking during the end of the exhaust stroke while the other fires at the usual time, a so-called "wasted spark ...
Note: All USDM gasoline Z20, Z22 and Z24 engines were known as NAPS-Z (NAPZ or NAPS-Z Nissan Anti-Pollution System), NAPZ motors had dual spark-plugs (two per cylinder) except the pre-82 versions and latest versions of the Z24i as fitted to the Pathfinder. All NAPZ engines sold in California reportedly had dual plug heads regardless of the year.