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Isn't the RPM at idle set in the computer? Do any differences in fuel or oil will be compensated by the computer
adjusting to keep the idle at 750 or whatever is the setting in the software?
 

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Isn't the RPM at idle set in the computer?
I have no idea what the officially approved ideal idle speed would be other than the assumption it should be at a point just above where it is so slow that the engine stalls out. In terms of negative feedback systems, the higher the resistance of everything in the engine towards keeping the inertial movement of everything going forward, the higher the idle speed would need to be so that the increased inertia from faster movement (thus higher rpm) would be able to overcome the resistance until the next fuel combustion explosion drives the piston down to keep all the parts moving. Thus, to me, a higher RPM means there is more overall resistance. Whether that is wrong or not I don't know, but it is my assumption that the computer will try to use the least amount of fuel to keep the engine from stalling out. If it needs to spend more energy, then it will shove more fuel into the system and the rpm would go up?
 

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It seems to me that, after reading your comments on resistance and engine speeds, your logic seems to be inverted.

The computer manipulates engine speed by controlling the air and fuel being provided. When it commands an idle speed, one would think that it compares what is commanded with what is observed and adjusts accordingly. In that case, the idle would react to the load (aka resistance) upon it.

When my car is idling and the AC turns on, whether it's in gear or neutral, the computer commands more fuel and air to make up for the added load.

I'm not sure if it's observing the effect of the added load (aka resistance), or just adjusting the current fuel and air amounts by a predetermined amount, such as from a LUT (look-up table).

On a side point, the transmission uses LUTs to determine when to shift into and out of each gear. It has to look up different tables for the shift desired (e.g. 1-to-2, 2-to-3, 6-to-5, etc.), RPM, torque, road speed, and others (e.g. Hot, Cold, etc.) before any action is taken. Since it's a computer, it does this fairly quickly, but Torque Management also comes into play, so the whole mess takes more time than it really should.

All this really proves is that we can theorize all day long on how we think it should work. What need are facts rather than conjecture.

If the internal resistance slows the engine speed enough (down 25% from 600 to 450) and the computer does nothing about it, then it's not paying attention to the observed RPMs. It seems that it is just simply adjusting the fuel and air based on a LUT. But I could be mistaken . . .


To comment on what appears to be backwards logic:

If the internal resistance goes up, the RPMs will go down, not up. If it monitors the RPMs, then any change in resistance would be met with an appropriate amount of force, whether that means reducing it (as in the case of less resistance) or increasing it (as in the case of more resistance).

While driving a car or riding a bicycle, when you encounter a hill, you add power. The hill adds resistance to your effort to maintain forward movement, and your immediate response is to try harder by adding more power, either through the gas pedal or the bike's pedals.

The "least amount of fuel" is taken care of by the emissions systems providing feedback on how efficiently the fuel is being consumed. It does so by monitoring the exhaust stream before and after the catalytic convertor(s).
 
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