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Post by poofables on Apr 20, 2012 9:40:49 GMT -6
Calender, a few things that ntrenn missed that I wanted to touch on. I think that when you are talking about compression ratio, you are actually talking about the volume of the combustible mixture in the cylinder. No matter how much is in there, it will still be compressed the same amount. However, if there is more volume in the cylinder the pressure of the compressed mixture will be higher. Regardless, the EGR will not affect this, the EGR is basically a passive system. It is not a forced induction system like a turbo charger or supercharger. there will still be a vacuum (negative pressure) in the intake manifold with or without the EGR. Watch the output of a MAP sensor as you disconnect and reconnect an EGR. Again, the post cat o2 sensor does not affect engine management. It's basically just the babysitter for the emissions system. You are right in saying that it is in place to make the cat last longer and to make sure it doesn't over heat. However it does this only by letting the user know that there is a problem that could damage the cat via the check engine light. I am sure there are plenty of articles out there on this, or you could talk to someone who is into cars or take that class you mentioned if you don't want to take my word for it. The GM training and the SAE papers that I read over are not correct? The post O2 sensors can offset the way the PCM fuels the engine. Do you have proof that they don't? email me and I will send you some information stating that it does caleditor@pcmcalibrators.com I am at work so my resources are limited. Here is a discussion and an article. sites.google.com/site/mikefocke2/oxygensensorswww.bimmerboard.com/forums/posts/832060The post cat sensor is not used to adjust fuel trim because in an ideal situation it essentially sees nothing because the cat has used all of the remaining oxygen in the exhaust to burn off waste gasses. |
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Post by caleditor on Apr 20, 2012 10:05:46 GMT -6
I see the issue you are looking at a Beemer forum
This is just one of the many documents that I have. This is from GM's Service Information
I found this in SI
DTC Descriptors
DTC P2096: Post Catalyst Fuel Trim System Low Limit Bank 1
DTC P2098: Post Catalyst Fuel Trim System Low Limit Bank 2
Circuit Description
Fuel trim bias is used to keep the post catalyst air/fuel ratio within a predetermined range. This allows optimal catalyst efficiency under light load conditions at a steady cruise. The engine control module (ECM) constantly monitors how lean or rich the fuel trim bias is commanded, to determine if the fuel trim bias is greater than a calibrated amount.
Conditions for Setting the DTC
The lean correction limit for a condition causing a rich air/fuel ratio has been exceeded for greater than 4 seconds or for a cumulative of 30 seconds.
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Post by caleditor on Apr 20, 2012 10:08:06 GMT -6
I have GM document that back this up
One problem with cyberspace myths become fact the more it is told
The rear oxygen sensor, located after the catalyst, is used for fuel trim corrections on OBD-II vehicles.
By virtue of its location, the rear sensor is generally protected from high temperatures and much of the contamination that affects the front oxygen sensors.
In addition, the rear sensor sees exhaust gases that are equilibrated – they have already been converted by the catalyst so that there is very little residual oxygen.
This allows the rear sensor to respond to much smaller changes in exhaust gas oxygen content.
In turn, it then possible for the rear sensor voltage to remain near the 0.45 volt switchpoint.
This characteristic allows the rear sensor to be used for fuel control.
Under steady rpm and load conditions, the short term fuel trim bias can be adjusted so that the rear sensor voltage is maintained near the 0.45 volt switchpoint.
This ensures that the catalyst is getting a stoichiometric exhaust gas mixture, despite any shift in the front sensor switchpoint.
The rear fuel trim corrections are learned in KAM (Keep Alive Memory).
Internally, this system is known as Fore Aft Oxygen Sensor Control (FAOSC). Note that FAOSC learns and reacts very slowly because the catalyst, with its large/slow oxygen storage and release characteristic, is part of the control loop.
Also, this system cannot be used with a "y-pipe" exhaust where a single rear sensor would try to adjust dual front sensors.
Rear O2s if you will are a fine tune of the commanded fuel flow but are very much part of the model used in the PCM's math to correct AFR for closed loop
This means running no rear O2s or Simms will effect how the PCM computes what the fuel trims are.
Another issue is being the 02s hardware logic is expecting to see under 1 volt return from 02 and if O2s are removed the unused pins of 02 wiring connector are now prone to pick up EMI or RFI noise and confuse the PCM.
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Post by lanceslambos on Apr 20, 2012 10:38:07 GMT -6
you all are talking about how more efficient & powerful cars have got & i'm fairly certain that post cat 02 sensor reading affects performance by sending a rich or lean signal to the PCM just as pre cat 02. if after adjusting for this with no change in X#of miles, it goes into closed loop. (thus one example of how the technology has advanced in this area.)
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Post by sall on Apr 20, 2012 10:47:36 GMT -6
I was under the impression the post-cat O2 sensor is to monitor the cat performance/efficiency.
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Post by wireless on Apr 20, 2012 11:04:18 GMT -6
The two sensors are used for comparisons.
The pre-cat measures emissions before the cat (obviously), while the post-cat is used for the after.
Look at it in a sort of before/after scenario; without one working, both are null and void. However, the second one is what measures cat efficiency and sends the necessary signals.
If you've ever had a post-cat o2 sensor go wrong, you'll notice that the vehicle starts to run like garbage lol
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Post by Aurora40 on Apr 21, 2012 7:33:20 GMT -6
Compression Ratio = The volume of the cylinder at the bottom of the stroke VS the volume when the piston is at the top of the stroke --- Correct? If you displace the initial volume with an inert gas then recalculate the CR it will be lower Correct? Volume is volume. The volume of a glass jar is the same whether it under vacuum, or full of 200psi air. The same is true of the cylinder's volume. If someone said "What is the volume of a 1"x1"x1" cube?", would you have to know how much exhaust gas was in it first? |
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Post by caleditor on Apr 21, 2012 11:35:59 GMT -6
Compression Ratio = The volume of the cylinder at the bottom of the stroke VS the volume when the piston is at the top of the stroke --- Correct? If you displace the initial volume with an inert gas then recalculate the CR it will be lower Correct? Volume is volume. The volume of a glass jar is the same whether it under vacuum, or full of 200psi air. The same is true of the cylinder's volume. If someone said "What is the volume of a 1"x1"x1" cube?", would you have to know how much exhaust gas was in it first? Correct -- You are getting it. So the Volume of the cylinder on the 4.0L is .50 L/Cyl and we all agree on that Now we all agree that the EGR allows Exhaust to be drawn back into the engine Correct? We also all agree that when the exhaust gas is drawn into the engine it will displace the air and the volume of the air in the intake manifold and the cylinder CORRECT? -- You can find this information commonly I hope that we can all agree that this exhaust gas does not enter all the cylinders equally. Let's throw out a number that I feel might be on the high side, but we will also use #7 & #8 Cylinders Let's say the Exhaust Gas Displaces 20% of the volume in the cylinder. ---- DO NOT THINK OF THE EXHAUST GAS & THE AIR CHARGE BEING MIXED ---- Now are cylinder volume is 20% less than .50 L/Cyl and this = .40 L/Cyl. Can we all agree that the exhaust gas would make the cylinder volume that the air charge occupies less? If this volume is say the 20% less and the .40 L/Cyl then when it is squished to the same squish it has before then the Compression Ratio would have dropped CORRECT? |
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Post by caleditor on Apr 21, 2012 11:38:14 GMT -6
Compression Ratio = The volume of the cylinder at the bottom of the stroke VS the volume when the piston is at the top of the stroke --- Correct? If you displace the initial volume with an inert gas then recalculate the CR it will be lower Correct? Volume is volume. The volume of a glass jar is the same whether it under vacuum, or full of 200psi air. The same is true of the cylinder's volume. If someone said "What is the volume of a 1"x1"x1" cube?", would you have to know how much exhaust gas was in it first? I like the jar comparison. That is a great way to look at it Thanks |
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Post by Aurora40 on Apr 21, 2012 11:49:52 GMT -6
If this volume is say the 20% less and the .40 L/Cyl then when it is squished to the same squish it has before then the Compression Ratio would have dropped CORRECT? I think you are not getting it. Compression ratio is simply the max volume / the min volume. This doesn't depend on how full of a gas they cylinders are. Also, .5L is the displacement, not the total volume. The total volume would include the combustion chamber. .5L would represent the amount the volume changed from max to min cylinder volume. Your example is incorrect. But if you can only think of it that way, then try this. You say EGR gas takes up 20% of the max volume when the piston is at the bottom of its travel. I.e. 20% of .5L. When the piston is at full compression, does not the compressed EGR gas still take up 20% of the now-smaller volume? How can the oxygen/fuel be squished to the same squish it had before, did the exhaust gas disappear? So is not the 80% that the oxygen/fuel occupy still at the same ratio? |
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Post by caleditor on Apr 21, 2012 12:15:38 GMT -6
If this volume is say the 20% less and the .40 L/Cyl then when it is squished to the same squish it has before then the Compression Ratio would have dropped CORRECT? I think you are not getting it. Compression ratio is simply the max volume / the min volume. This doesn't depend on how full of a gas they cylinders are. Also, .5L is the displacement, not the total volume. The total volume would include the combustion chamber. .5L would represent the amount the volume changed from max to min cylinder volume. Your example is incorrect. But if you can only think of it that way, then try this. You say EGR gas takes up 20% of the max volume when the piston is at the bottom of its travel. I.e. 20% of .5L. When the piston is at full compression, does not the compressed EGR gas still take up 20% of the now-smaller volume? How can the oxygen/fuel be squished to the same squish it had before, did the exhaust gas disappear? So is not the 80% that the oxygen/fuel occupy still at the same ratio? I would have argued the fixed mechanical point endless 15 years ago, but when I was at the GM training center this topic was brought up. The instructor went over it for hours. I understand how this theory works. Since that time I have read several SAE documents on EGR and VVT concerning NOX. These SAE documents explain it much better than I am. I will look for a copy again. |
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Post by ntrenn on Apr 21, 2012 15:30:43 GMT -6
EGR rates on modern gasoline engines are extremely low - in the single digit percentages. All you have to do is look at how small the feed tube is in comparison to the intake plenum. It takes very little to change the combustion characteristics. On diesel engines, they are using so much EGR that they have to cool the EGR.
40 - dead on example.
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Post by shelbyracing on Apr 21, 2012 20:28:09 GMT -6
A pound is a pound, a liter is a liter, and a compression ratio is a compression ratio, lol.  |
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