Chapter 4:  Open-loop operation on DSMs

As mentioned before, closed-loop operation is designed to produce maximum fuel economy under constant operating conditions, When conditions are not constant, fuel economy may not be the primary goal.

As owners of sports cars, DSM owners expect a certain amount (read: lots!) of power from their cars. And, as any racer will tell you, power and fuel economy are uneasy bedfellows.

In fact, in order to get maximum power from the engine, you have to make sure to keep it cold. In fact, even running the ideal 14.7:1 air/fuel ratio under high load, high RPM conditions can be a very bad thing for your engine. You want to make certain the engine runs cool enough to prevent it from self-destructing.

One way of achieving cooler temperatures is to inject more fuel into the cylinders. This extra fuel acts as a coolant, reducing combustion temperatures. The cooler temperatures also reduce the possibility of the air/fuel mixture igniting in the cylinder before the spark plug is fired, a condition called preignition or knock. This lets the engine controller advance the timing on the engine, resulting in more power.

To achieve this cooling effect, the programmers of the DSM ECUs decided to program in very conservative values for the required amount of fuel, which guarantees the (stock) engine will run safely no matter how hard the car is driven. As a result, the ECU is only concerned with delivering enough fuel to each cylinder - how much more than enough is not a concern.

To this end, the ECU still determines the amount of fuel to deliver from the signals delivered by the MAS. Because the engine is under acceleration, however, the fuel quantity selected by the ECU is larger than before. Also, since the fuel amount is guaranteed to be enough (by design), the ECU no longer checks the oxygen sensor to see how close it was to the 'correct' output. (The signal is still there - the ECU simply doesn't bother doing anything with it.)

This failure to check the oxygen sensor for sufficient fuel is the difference between open-loop and closed-loop operation. While under 'normal' conditions, the ECU checks the oxygen sensor for the results of its own actions - monitors the feedback, in other words. While under acceleration, the ECU does not use the feedback information, and is 'demoted' to open-loop operation for the duration of the acceleration period. When the acceleration ceases, the ECU returns to closed-loop operation once again.

While running in open-loop operation, the ECU is designed to provide an air/fuel ratio of about 11:1; less air, more fuel. (This will change somewhat with RPM and airflow, but that's close enough at the moment.) This is well above the switch point for the oxygen sensor. Consequently, the oxygen sensor remains fairly constant during open-loop operation, on the rich side. Viewing the signal with a voltmeter or air/fuel ratio meter will show a very high, relatively steady oxygen sensor output, in contrast to the 'bouncy' signal seen during closed-loop operation. This signal is normal and, in fact, eminently desireable - the higher the better (to a point).

Just as an aside, the ECU designers went one better than this in the cooling department. While under high loads, the ECU actually fires all four injectors in between the normal firing times. This extra fuel has no chance of being burned in the engine (at least until the valves open), but helps to keep things cool.

The behavior of the ECU during open-loop operation has serious implications for those who modify their engines. More on this in a bit.