Idle torque reserve
Most of the time an engine is running we want it to be as efficient as possible. A key step to obtaining optimum efficiency is operating at Maximum Brake Torque (also known as optimal torque, or MBT) timing in order to make the most of the cylinder pressure being generated. Retarding the timing from MBT at any point means that there is some untapped potential torque left in reserve. Its possible to intentionally build up a torque reserve simply by retarding the timing from the mechanically ideal MBT point.
When an engine is idling the ECU is constantly trying to find a way to balance all of the torques in the system. Friction, pumping losses, accessories, and the transmission all provide negative torque to the crankshaft that are constantly trying to slow the speed to a stop. To offset this, the combustion of of air and fuel in the cylinder is used to generate just enough torque to maintain a constant idle speed.
If the losses outweigh the the positive torque being generated by the combustion, engine speed drops below the target setpoint. Any time the losses are reduced while the combustion torque remains constant, the engine speed flares upward until a new equilibrium point is reached. The driver often changes the torque losses by pulling the vehicle into gear, turning on the air conditioning compressor, or applying a load to the power steering pump.
During all of these operations, there is an expectation that the engine will not stall or otherwise surprise the driver. Since the ECU cannot always predict when these losses will come and go, it must have some mechanism of dealing with their sudden appearance and disappearance. What the ECU needs is a bank of additional torque to draw from if there is any sudden increase in system losses at idle.
This is accomplished by normally running the engine at less-than-MBT timing at idle. Running at a lower ignition ignition advance angle means that at any moment the ECU has the option to increase torque output to the crankshaft to overcome any sudden increase in losses. This torque reserve is critical to maintain good idle control.
When setting up the ignition timing map on an ecu, its best to check for MBT timing at speeds just above idle. Chances are that the engine will have a similar value for MBT at idle, and this will give the calibrator a good idea of just how far back they are from MBT when idling. Its not unusual to operate an engine with 10 degrees or more of spark retard from MBT at idle to provide the necessary cushion of available torque.
Keeping this torque reserve available at idle goes a long way toward being able to to maintain a stable idle speed under a variety of conditions.