General, ignition

Fig 1: Identifying Normal Ignition Timing Graph

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 2: Identifying Ignition Function Timing Graph

Courtesy of SAAB-SCANIA OF AMERICA, INC.

This ignition function is the one normally used when driving.

The ignition timing is regulated depending on the engine load and speed (air mass per combustion mg/c). The objective of this ignition regulation is to obtain the correct combustion pressure buildup.

The combustion pressure should normally attain its maximum value at around 16--17 degrees ATDC. In order to achieve this, the fuel/air mixture must ignite slightly earlier. Normal ignition timing at 2000 RPM, light load, can be around 25--28 degrees BTDC so that the igniting flame has time to expand and allow the pressure in the cylinder to reach its maximum at the correct point in time.

The expansion rate of the flame varies with the conditions in the combustion chamber, heat, pressure and turbulence. The ignition timing must therefore vary over the engine's load/speed range.