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Old 01-20-2009, 12:42 PM   #1 (permalink)
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Electronic Engine Control Algorithm. Is this Correct?

Is this pretty much how modern electronic engine control algorithms work?

The engine management control program's goal is to hold the air/fuel mixture at a constant target value as measured by the O2 sensors. The program is written as a closed loop feedback loop with feed forward. A regular feedback control would wait for the O2 sensors to move away from the target O2, then start changing fuel rate to get the O2 back to target. This control is relatively slow. Feed forward is provided by using a mass air flow sensor. The feed forward speeds up control. In fact, if the feed forward were perfect, there would be no reason for feedback.

The manufacturers want a certain target level of richness/leanness in the combustion chamber. They use O2 sensors to measure this, and decide on a target O2 level for the control program. You also have a starting air/fuel ratio target. So, you step on the throttle, the throttle plate opens, and the MAFS sees air flow, and uses the ratio target to set the fuel flow. The O2 sensor reading should be on target, but if it's not, it will change the air/fuel ratio setpoint.

It continues to work like this. As long as the O2 readings are at target, the algorithm maintains things at a constant air/fuel ratio. When the MAFS sees a change in air flow, fuel flow is changed per the current air/fuel target. This continues unless the O2 moves away from its target. At this point, the control program starts to change the air/fuel target.

A bunch of preset tuning factors are needed to coordinate all this stuff going on. These factors are a function of time delays and anticipated rates of change of the variables.

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