Quote:
Originally Posted by mwebb
in my experience , the system any system
will switch the EGR flow off
right away
before it will add any enrichment .
and
why do you think more mass is entering the combustion chamber when the EGR is on as opposed to when EGR is off ?
MOST current systems use MAF sensors NOT speed density systems
Load is DECREASED with EGR operation
repeat your testing with your focus to see .
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You are right that most newer systems do turn off EGR flow when going into PE mode, but PE doesn't constitute the only enrichment possible.
The systems that still use AE routines do not turn off EGR.
At any rate, my statement was for Daox who seems to be interested in adding an extra, user-controlled, EGR system. I thought the information might be helpful in his tuning of his system.
When EGR is on, combustion efficiency drops, which requires more Oxygen and fuel in order to produce the same level of power asked of the engine.
Very basically, the calculated load would be ((old VE + extra VE) - EGR).
Actual mass moving through engine would be something closer to (old VE + extra VE + EGR)
By "VE" I mean the calculated air mass based on the sensor input (MAP or MAF).
EGR flow doesn't change actual loading against the engine, but EGR compensation does change calculated load, which is the only load that the ECM/PCM works with. The engine or ECM has no way to measure actual volumetric efficiency, nor does it need one. It only needs a way to reference the correct stored calibration data in order to keep fueling within a few percent of stoich, while the adaptive fueling routine does the rest.
As you can see in the first table in my first screen-shot, at any given calculated VE, with EGR-on the VE is multiplied by a reduction factor (calibration) that knocks down the original VE value - in this case about 35%, which in turn reduces the fueling value to match the predicted mass of Oxygen going into the engine.
In operation, if you start at a VE value of 35% with EGR off, but then turn on the EGR, you will need slightly more Oxygen to maintain the same power output level. This is achieved by a slightly larger throttle opening, which results in more intake air-flow, which then puts you up to a higher VE of say 54%. This new VE number is then multiplied by the EGR correction factor (54 * 0.65) to get the modified load value in order to calculate fueling.
At that point you have more total mass (necessary intake charge + EGR gases) going into the engine.
This increases effective charge compression because more mass is being squeezed and heated during the compression stroke. This is one reason that so much EGR can be added before combustion temp and efficiency drops to an unacceptable level. It's also the reason that such a high % of intake charge contamination results in such a relatively small loss in overall engine efficiency.
To propose that you can make an equal power level with LESS Oxygen going into the chambers would assume an increase in combustion efficiency. I think you'll find that an increase is extremely rare during EGR operation, especially the non-DC controlled systems. With small amounts of EGR you can see small increases in combustion efficiency under certain low-load conditions.
It can happen when:
a. peak combustion temp is not lowered a significant amount, and the amount of Oxygen is still high enough to maintain efficient combustion speed. Very little NOX reduction would be going on in this case.
b. The instantly added EGR spark advance correction precedes and/or outpaces the actual EGR flow creating higher peak combustion temp. This can happen as the EGR passages fill with particulate build-up, reducing the mass of EGR delivered. Again, NOX reduction would be minimal or non-existent.
c. The intake air charge is extremely cold and the EGR flow adds heat energy to the point that there is a net gain in combustion efficiency.
d. Some other scenario that I can't think of.
I will concede that my description of EGR operation isn't exactly what happens during 100% of EGR-on time, but it is fundamentally true for the majority of EGR-on time, even with newer systems. As I said before, what matters most is if you can get away with less engine power produced. In many operating conditions you can, but EGR operation during those times is rare or minimal. Again, I'm talking about EGR deliver strategy in factory form, not what you can achieve by fine tuning the system.
Any time you are discussing state-of-tune issues, different combinations can and will respond differently, as well as identical combinations under different operating conditions. That being the case, there will be instances where what I described doesn't hold true for some given period of operation time. It is quite rare to determine a tuning factor "rule" that you can't find an exception to if you try hard enough, or by accident through testing. That's why I don't discourage any sort of testing. An exception to a rule could result in a useful discovery.
I have no plans to re-test the EGR system on the Focus. I admit that I did NOT do extensive testing of that EGR function, nor did I trace the algorithm in the code. The only test with the Focus was for fuel mileage, day-to-day, and in highway test loops. EGR-off showed a very small gain in mileage on the highway, and a larger gain for day-to-day, which didn't surprise me in the least. The Focus is quite under-powered and responded very positively to increased power and throttle response. The difference was very noticeable as it often is. The wife even noticed the car being more snappy feeling. While throttle response is subjective and hardly conclusive, the less required TPS% and relative additional spark advance are quite measurable and significant. The PCM still thinks the EGR is active and functioning normally, though the EGR tube is blocked.
I did thoroughly test the EGR system on my 2001 S10 p/u. It does use a DC controlled system for precise control. Turning off the EGR in the calibration prevented the extra spark advance, so that wasn't a variable there. The result of EGR-off was a gain of 7.5% MPG on highway test loops. I did not test day-to-day mileage, but here again the subjective engine response was very noticeable. Being a V6 with an automatic trans in a 3500 lb truck, it too was under-powered for a good percentage of normal driving conditions. I was a little surprised at how much of a positive difference it made to turn the EGR off. Like you, I expected the newer system to do a better job.
I have yet to test or tune a vehicle where the EGR function added fuel efficiency, but I will certainly post up the information if I do. If I run into one it will probably inspire me to work with the EGR tuning to look for larger gains. I have never worked with a Geo Metro, but would be interested if the opportunity came up. I'm not working with EGR systems right now, but if one comes up in the future, what exactly would you want me to test or collect data from?
I'd assume you're thinking of TPS%, MAP%, MAF output, and calculated VE. I normally have to look at all of those closely during the tuning process. When something changes relative to the other, it normally requires a calibration adjustment.
Daox, if you see this as a thread hijack, please feel free to separate it out of your thread.
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