Settings are made for various operating conditions, according to fuel consumption, engine torque, exhaust emissions, knocking tendency, and driveability.
That means for example, that starting ability can be improved by making timing during cranking depend on cranking speed. Timing at low idle speeds can be set for low emissions, smoothness, and fuel economy. For part-load operation, the emphasis is on economy and driveability. At full load, it is set for maximum torque without detonation.
Using the digitally stored map, the ignition point is set for each operating condition, without affecting the settings for any other condition.
All of this raises an engine’s overall efficiency, for all operating conditions. Corrections to ignition timing, combined with those to the injector opening time, provide the optimum values to achieve the best combustion.
With higher compression ratios, there is more risk of detonation, and damage to the combustion chamber.
A knock sensor allows ignition timing to be calibrated to achieve the best setting for normal operating conditions. Knock or detonation can then be eliminated separately, using closed-loop knock control. When a detonation signal is received, the control unit retards the ignition setting, to bring detonation under control.
A turbocharged engine is different. Simply retarding the ignition could increase the already high temperatures at the exhaust-driven turbine. But reducing the boost pressure at the same time brings detonation under control quickly. As soon as knocking is detected, ignition is retarded, which works immediately. Reducing boost takes longer, but as soon as it takes effect, the control unit returns ignition timing to its optimum value.