Chapter 5:  Determination of open-loop vs. closed-loop operation

So how does the ECU decide when 'normal' conditions exist and when 'acceleration' conditions prevail? The primary trigger that switches the ECU from one control method to another is a simple lookup table, which compares throttle (accelerator) position against engine speed. These signals are both provided to the ECU from various sensors in the engine. A second trigger is change in throttle position (delta-TPS) - a large enough change in throttle position over a short enough period of time will also cause the ECU to enter open-loop mode.

Closed loop operation occurs when the engine is idling, or running at near-constant throttle position at near-constant engine speed. During idle, the ECU actually has two goals: one, to deliver the correct fuel, and two, to maintain the idle speed at 750 RPM. The fuel is handled by controlling the injectors and using the oxygen sensor for feedback. The idle speed is controlled by the idle speed control motor (ISC), using the crankshaft angle sensor as feedback.

[Why the crankshaft angle sensor, and not the tachometer? The tachometer signal is generated by the ECU, not from the engine. The ECU determines RPM by watching the crankshaft angle sensor, and runs the tachometer in the instrument cluster.]

Relatively large changes in throttle position at a given RPM will cause the ECU to change to open-loop operation. Once the throttle position and RPM are back "in sync" with each other, the ECU will generally switch back to closed-loop operation. This can occur if the throttle position moves again, or if the engine speed begins to level out at the new throttle position. However, there are some special cases that may change this behavior.

Above a certain RPM level, the ECU will run open-loop regardless of throttle position, except when the throttle is completely closed (full deceleration). Those with tendancies towards high highway cruising speeds will have noted the large decrease in fuel economy above certain speeds. Above a certain RPM, the ECU is providing all that extra gas to the engine, some of which is blown straight out the exhaust pipe. As you might imagine, this radically decreases fuel economy.

The critical RPM depends on the model of car, air temperature, atmospheric pressure, and a few other factors. It is identical for front-wheel-drive turbo DSMs than all-wheel-drive turbos, and tends to hang around 4000-4500 RPM. Because FWD cars are geared slightly differently than AWD cars, FWD cars will achieve a slightly higher cruising speed before hitting open-loop operation than AWD cars.

In the event of a sensor failure, the ECU may have no choice but to use the open-loop method in order to keep the engine running. An excellent example of this is when the MAS air volume sensor fails. Although the temperature and barometric pressure sensors may continue to function in the MAS, the ECU now does not know the volume of air is entering the engine. This causes the ECU to run in open-loop mode, which provides the guaranteed minimum required amount of fuel and the maximum safety for the engine.

In this case, the process is a little more sophisticated than that. In order to have some idea of the air volume, the ECU uses a small pre-programmed table to guess at the correct airflow based on throttle position and RPM. Otherwise, the engine could not run at all. This table, of course, is programmed conservatively in order to safeguard the engine. This value is then used to further processed to pick the open-loop fuel quantity. Still, the process is open-loop, as the ECU does not use the oxygen sensor signal as feedback to regulate fuel flow.