A timing mark is an indicator used for setting the timing of the
ignition system of an
engine, typically found on the
crankshaft
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
pulley (as pictured) or the
flywheel, being the largest radius rotating at crankshaft speed and therefore the place where marks at one degree intervals will be farthest apart.
On older engines it is common to set the
ignition timing using a
timing light, which flashes in time with the ignition system (and hence engine rotation), so when shone on the timing marks makes them appear stationary due to the
stroboscopic effect
The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples (as opposed to a continuous view) at a sampling rate clos ...
. The ignition timing can then be adjusted to fire at the correct point in the engine's rotation, typically a few degrees before
top dead centre and advancing with increasing engine speed. The timing can be adjusted by loosening and slightly rotating the
distributor in its seat.
Modern engines usually use a
crank sensor
A crank sensor (CKP) is an electronic device used in an internal combustion engine, both petrol and diesel, to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control the fue ...
directly connected to the
engine management system
An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by re ...
.
The term can also be used to describe the tick marks along the length of an
optical mark recognition
Optical mark recognition (also called optical mark reading and OMR) is the process of reading information that people mark on surveys, tests and other paper documents.
OMR is used to read questionnaires, multiple choice examination papers in the ...
sheet, used to confirm the location of the sheet as it passes through the reader. See, for example, U.S. Patent 3,218,439 (filed 1964, granted 1965), which refers to a timing track down the left side of the form, and U.S. Patent 3,267,258 (filed 1963, granted 1966), which refers to a column of timing marks on the right side of the form.
The term can also be used to describe the timing patterns used in some
barcodes, such as
PostBar,
Data Matrix,
Aztec Code, etc.
{{Automotive engine , collapsed
Ignition systems
Synchronization
Engine technology