Every engine, whether a single-cylinder engine or a V8 engine, has a top dead centre. This position is a reference point for engine timing and indicates when a spark plug should fire, igniting the fuel in the cylinder's combustion chamber. The top dead centre is when the piston is at the very top of its compression stroke. At this position, the intake and exhaust valves in the cylinder head are closed and the air-fuel mixture is compressed in the cylinder. The TDC sensor monitors a reference point on the camshaft that indicates where the top dead centre is on a cylinder, usually cylinder number one. It sends a signal to the engine control module which, in turn, communicates to send a spark to the cylinder at the top dead centre. The spark ignites the fuel and the power stroke begins as the piston is forced downward.