Timing marks

In a piston engine, top dead centre (TDC) is the position of a piston in which it is furthest from the crankshaft.

Top dead centre is the datum point from which engine timing measurements are made. For example, ignition system timing is normally specified as degrees before top dead centre (BTDC) although a very few small and fast-burning engines, such as the Nissan MA10 with hemispherical combustion chambers, require a spark just after top dead centre (ATDC). Top dead centre for cylinder one is often marked on the flywheel or dynamic balancer or both, with adjacent 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.

In a multi-cylinder engine, pistons may reach top dead centre similtaneously 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 boxer twin configuration, the two pistons reach TDC similtaneously, which is also called 0° displacement.

• In the straight-4 configuration, the two end pistons (pistons 1 and 4) reach TDC similtaneously, as do the two centre pistons (pistons 2 and 3), but these two pairs reach TDC with an angular displacement of 180°. Similar patterns are found in almost all straight engines with even numbers of cylinders, with the two end pistons and two middle pistons moving together (not necessarily 180° out of phase however) and the intermediate pistons moving in pairs in mirror-image around the centre of the engine.

• In the flatplane V8 and many larger V engines, the piston motion within each bank is similar to that of a straight engine, however in the crossplane V8 and all V10 engines the motion is far more complex.

The concept of top dead centre is also extended to 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 the Wankel engine for example it occurs six times for every one revolution of the rotor.