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[Essex_Andy]

Fab'd a blanking plate out of 3mm sheet and the throttle response is noticeably better and according to the trip computer MPG at a steady 3K revs (75-ish mph) has dropped from low 40's to low 30's...is this normal?

[Markjay]

See:

http://www.omegaowners.com/forum/YaBB.pl?num=1171216958/0#12

Apparently EGR is supposed to (among other things) improve fuel consupmtion, so I guess this is normal...

[Essex_Andy]

Thanks

I'll see how it goes and what effect on my wallett...may well end up removing the plate

[omegaV6CD]

See:

http://www.omegaowners.com/forum/YaBB.pl?num=1171216958/0#12

Apparently EGR is supposed to (among other things) improve fuel consupmtion, so I guess this is normal...

I beg to differ on that.
Thermodynamically speaking the EGR would never improve consumption, this is marketing tricks to sell products. Yes blanking an the EGR would slightly increase your pumping losses(negligible difference) but the cooler charge in addition with the advanced ignition should give a pretty good consumption.
The EGR is there only to reduce NOx and in case that the fuel has high content in sulphur then it will eat the engine nicely due to formation of sulphuric acid.

[Markjay]

See:

http://www.omegaowners.com/forum/YaBB.pl?num=1171216958/0#12

Apparently EGR is supposed to (among other things) improve fuel consupmtion, so I guess this is normal...

I beg to differ on that.
Thermodynamically speaking the EGR would never improve consumption, this is marketing tricks to sell products. Yes blanking an the EGR would slightly increase your pumping losses(negligible difference) but the cooler charge in addition with the advanced ignition should give a pretty good consumption.
The EGR is there only to reduce NOx and in case that the fuel has high content in sulphur then it will eat the engine nicely due to formation of sulphuric acid.

I was only quoting what I read, and since it was on the Internet then it must be true  

[Marks DTM Calib]

you might get 1-2mpg tops when cruising......little more.....

[familyman]

just a note. the egr reduces NOx by reducing the combustion chamber temperatures (this is the bug bear of deisel engines high combustion temps lots of NOx.) so as the combustion chamber temp increases the point at wich the egr valve would operate and cool the gasses is taken away. at that point the o2 sensor sees an incorrect mixture that is reqiured by the ecu ( to much oxygen, weak mixture ) and the ecu dumps more fuel into the cylinder to cool it and bring the oxygen level into spec. Hence poorer economy, although by how much is debatable, i guess it would depend on your driving style, a lot of cruising would make the egr operate a lot where as full bore driving would decrease the amount the egr is used but then your mpg would be bad anyway
catch 22

[Markjay]

just a note. the egr reduces NOx by reducing the combustion chamber temperatures (this is the bug bear of deisel engines high combustion temps lots of NOx.) so as the combustion chamber temp increases the point at wich the egr valve would operate and cool the gasses is taken away. at that point the o2 sensor sees an incorrect mixture that is reqiured by the ecu ( to much oxygen, weak mixture ) and the ecu dumps more fuel into the cylinder to cool it and bring the oxygen level into spec. Hence poorer economy, although by how much is debatable, i guess it would depend on your driving style, a lot of cruising would make the egr operate a lot where as full bore driving would decrease the amount the egr is used but then your mpg would be bad anyway
catch 22

So are you saying that with EGR you have worst fuel consumption than without it?  

[Marks DTM Calib]

just a note. the egr reduces NOx by reducing the combustion chamber temperatures (this is the bug bear of deisel engines high combustion temps lots of NOx.) so as the combustion chamber temp increases the point at wich the egr valve would operate and cool the gasses is taken away. at that point the o2 sensor sees an incorrect mixture that is reqiured by the ecu ( to much oxygen, weak mixture )

There is no reason why the raised combustion temps will cause the Lambda to see a week mixture....in reality the high compression ratio of the V6 and elevated (not by much though)temps will ensure that mix is more efficiently consumed (remember that EGR gases are inert....)

 

and the ecu dumps more fuel into the cylinder to cool it and bring the oxygen level into spec. Hence poorer economy, although by how much is debatable, i guess it would depend on your driving style, a lot of cruising would make the egr operate a lot where as full bore driving would decrease the amount the egr is used but then your mpg would be bad anyway
catch 22

The ECU does not add fuel to aid cooling........it has little or no contol on engine cooling at all....reality is that the main cooling would come from the air coming in of which there is 16 times the mass when compared to fuel!

Reality is that the EGR setup on these units (much later 4 pots etc are VERY different) adds very little in the way of inert gas (exhaust fumes) so the effect is minimal....

[Markjay]

The explanation provided in the link I posted seem to make sense, i.e. the EGR introduces inert gasses so there is less oxygen and the ECU accommodates by reducing the fuelling, in effect there's less mixture or as the website says it is reducing the effective engine volume/capacity. So it would behave like a smaller engine and consume less fuel...

But the website is generic and does not relate specifically to GM or the V6, so as Mark DTM says in our case the EGR implementation may be such that the entire issue is highly theoretical with little actual effect on fuel consumption…

[Marks DTM Calib]

Consider this.....

The energy required to overcome transmission loses, rolling resistance, engine losses, windage, drag etc are pretty constant at a constant speed.....this energy comes from the fuel air mix.....adding an inert gas will dilute the mix giving an effective cc reduction and hence lower cylinder temps.....but, you still need the same amount of energy to overcome this constant loss....the only thing you might gain is through better volumetric efficiency as the throttle will be open wider and the pumping losses will be less....which would be minimal.

This is backed up by there being very little difference in fuel consumption between 2.0, 2.5 and 3.0 engines when cruising at a constant speed....

[familyman]

The ECU does not add fuel to aid cooling........it has little or no contol on engine cooling at all....reality is that the main cooling would come from the air coming in of which there is 16 times the mass when compared to fuel!

Reality is that the EGR setup on these units (much later 4 pots etc are VERY different) adds very little in the way of inert gas (exhaust fumes) so the effect is minimal....
[/quote]
I didn't mean to say that the ecu adds fuel for cooling but that by the chamber temp being higher than normal for the given amount of engine conditions and the fueling required you would have a lean burn situation, and lean burn increases combustion temp the o2 sensor would see the lean burn and tell the ecu to richen the mixture for the given conditions. But the by product would be the increase in fuel would lower the combustion temp. Your quiet right the ecu doesn't doesn't add fuel to cool the engine its just a by product of gettihg correct mixture ratio. and the changes are so small they wouldn't show on the gauge or to the coolant temp anyway but would affect the level of precat emission

[Marks DTM Calib]

I didn't mean to say that the ecu adds fuel for cooling but that by the chamber temp being higher than normal for the given amount of engine conditions and the fueling required you would have a lean burn situation, and lean burn increases combustion temp the o2 sensor would see the lean burn and tell the ecu to richen the mixture for the given conditions. But the by product would be the increase in fuel would lower the combustion temp. Your quiet right the ecu doesn't doesn't add fuel to cool the engine its just a by product of gettihg correct mixture ratio. and the changes are so small they wouldn't show on the gauge or to the coolant temp anyway but would affect the level of precat emission

You dont get a lean burn due to temperature increases, that happens due to a lean mix......(anything upto 60:1!) even under higher cylinder temps and full EGR you are still running at the stoicheometric ratio....its not as if this ratio changes due to the temp increase.

[TheBoy]

So, should blanking the EGR increase (marginally) the economy, or reduce it?

[Marks DTM Calib]

Pretty much zero effect.....maybe 1mpg tops....due to pumping losses increasing slightly...

The key thing to consider is that exhaust gases are inert to the combustion cycle....they have zero fuel and oxygen in them......