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worstmechanic 01-20-2009 12:42 PM

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.

some_other_dave 01-20-2009 03:10 PM

Nope, that's not really it.

The engine management figures out how much air the engine is taking in by using the MAP/MAF/whatever, the throttle position, intake air temp, and other sensors. It crunches those numbers to get an overall "how much air is going in" figure, and calculates a basic injector opening time. Other sensors, primarily the O2 sensor, then provide fine-tuning for that time value.

I do not know how much "authority" the O2 sensor feedback loop has, but it is definitely not the primary metering device. For one thing, most O2 sensors respond relatively slowly compared to the engine revolutions.

There are plenty of "special cases" built in, however. When the throttle goes from closed to more open, the engine adds extra "acceleration enrichment" so the engine doesn't bog or stumble when the throttle position changes. When the throttle is wide open, it adds "full throttle enrichment" for more power (after all, that's why most people push the pedal all the way down!). When the engine is cold, the mixture is also enriched because cold engines don't burn fuel as well as warmed-up ones.

In all of those cases, the engine goes "open loop" and ignores the O2 sensor feedback entirely. It just relies on the basic metering and temperature/RPM/whatever adjustments.

-soD

mikeross 01-08-2010 06:07 AM

Quote:

Originally Posted by worstmechanic (Post 84085)
Is this pretty much how modern electronic engine control algorithms work?

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.

so what are some symptoms or indications that the mass air flow sensor is not working properly? Thanks! I don't know much about engine control and stuff. Thanks!

DonR 01-08-2010 12:45 PM

When mine went bad, when you would let off the throttle to come to a stop. When you went to go again the engine had stalled.

If you think yours may be going bad, they do make MAF sensor cleaner you can use.

Don

tjts1 01-08-2010 02:18 PM

Quote:

Originally Posted by mikeross (Post 152856)
so what are some symptoms or indications that the mass air flow sensor is not working properly? Thanks! I don't know much about engine control and stuff. Thanks!

Check engine light, stalling, lack of power, poor thottle response, very poor cold running.

cfg83 01-08-2010 02:25 PM

some_other_dave -

Quote:

Originally Posted by some_other_dave (Post 84119)
...

There are plenty of "special cases" built in, however. When the throttle goes from closed to more open, the engine adds extra "acceleration enrichment" so the engine doesn't bog or stumble when the throttle position changes. When the throttle is wide open, it adds "full throttle enrichment" for more power (after all, that's why most people push the pedal all the way down!). When the engine is cold, the mixture is also enriched because cold engines don't burn fuel as well as warmed-up ones.

In all of those cases, the engine goes "open loop" and ignores the O2 sensor feedback entirely. It just relies on the basic metering and temperature/RPM/whatever adjustments.

-soD

In my experience, except for cold-starts, my ECU/PCM almost never leaves closed-loop. Maybe this is another benefit of how we drive. I can datalog, so I should double-check this. Would you make the claim that even when the ECU/PCM reports it's in closed-loop, it may not actually *be* in closed-loop?

What do you think of these explanations? :

Tips on Reading Gauges; Air-Fuel Ratio Monitor
Quote:

The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.

Since the oxygen sensor output is non-linear and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the PCM is in closed loop and trying to maintain a stoichiometric A/F ratio. The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the PCM ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.

What do you think of this? :

Tech - Closed Loop
Quote:

Closed Loop
In closed loop operation the ECU uses one or more oxygen sensors as a feedback loop in order to adjust the fuel mixture. This gives the name ‘closed loop’ from the closed feedback loop. The ECU won’t run in a closed feedback loop all the time, so ‘open loop’ is used to describe the operation of the ECU when the mixture is not being adjusted in this way (usually when the engine is cold or when running under high load).

In closed loop operation the ECU uses the oxygen sensor to tell if the fuel mixture is rich or lean. However, due to the characteristics of the oxygen sensor it can’t tell exactly how rich or lean, it only knows that the mixture is richer or leaner than optimum. The ECU will enrich the mixture if the oxygen sensor shows that the mixture is lean, and lean the mixture if it looks rich. The result of this is that the mixture will swing back and forward around the stoichiometric point.
...
OBD II Engines
OBD II engines use one oxygen sensor before the catalytic converter, and one oxygen sensor after the catalytic converter. The function of the second oxygen sensor is to determine if the catalytic converter is functioning. It does this by looking at the difference between the two oxygen sensors. If the catalytic converter is functioning correctly there will be a reduction in the exhaust oxygen content as carbon monoxide and carbon dioxide is catalyzed in the converter.
CarloSW2

some_other_dave 01-08-2010 03:46 PM

Quote:

Originally Posted by cfg83 (Post 152927)
In my experience, except for cold-starts, my ECU/PCM almost never leaves closed-loop. Maybe this is another benefit of how we drive. I can datalog, so I should double-check this. Would you make the claim that even when the ECU/PCM reports it's in closed-loop, it may not actually *be* in closed-loop?

I suspect that is primarily due to the way we drive. If you mashed your throttle pedal all the way down, you would likely see the engine management go open-loop, at least for a second or two. (I know the duration of acceleration enrichment is a tunable parameter in Megasquirt, and is often in the "several seconds" range.)

I would assume that any reporting that the ECU does of open/closed loop state would be accurate, as the state is maintained inside the ECU itself. There may be other factors affecting the indication (e.g., the OBDII bus only updates information every second, and the state is only open-loop for a half-second at a time) but I'd expect it to be reasonably accurate.

--The two articles you quote from match my experience and reading on this subject pretty well. Note that these are all describing the "Narrow Band" O2 sensor; the "Wide Band" type can actually tell how far off of "stoichiometric" you are, which gives more tuning options. (Which is why that type of sensor is often used in vehicles that use lean-burn technology.)

-soD

mwebb 01-08-2010 07:18 PM

fuel trim
short term fuel trim
long term fuel trim

LTFT is adjusted to keep STFT bouncing or cycling on zero
equal amounts up and down

if LTFT gets past a certain value , the system codes for rich or lean condition depending

if MR MAF sensor reports incorrectly LTFT adjusts to keep system at stoich to the limit described above



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