Does an egr help that much?

[George Tyler]

Exhaust gas recirculation - Wikipedia, the free encyclopedia

Intresting article

[George Tyler]

Originally Posted by mwebb

no
....................
so
Combustion pressure is reduced by the presence of the INert gases which do take up space but do not support combustion
the INert gases behave like a cushion , to reduce compression ratio and therefore combustion pressure
while EGR is operational

combustion temperature IS reduced because Combustion pressure IS reduced,
not the other way around........

Why do you say "compression ratio is reduced"? the volume of the cylinder at the bottom of the stroke is the same with or without EGR? as it is at the top?

then, pressure at the bottom is actually increased, as the vacuum is reduced by letting in exhaust gas, also, less power is reduced as less air flows through the trottle, so the throotle then has to be opened more to compensate as the same power is needed in the same conditions. the pressure after combustion is reduced as some of heat is used to heat up the included exhaust gas.

[George Tyler]

the other thing, what causes the pressure to increase after combustion? The heat produced. Until combustion the pressure is higher, a similar amount of heat is produced, but as that heat is heating an increased mass of gas the temperature rise is reduced.

[old jupiter]

Originally Posted by roflwaffle

Kinda.... Provided there isn't excessive EGR and the mixture is homogeneous, just as much fuel should be combusted as w/o EGR.

Yes . . . because the mixture has been RICHENED somewhat to make up for what's displaced by the exhaust back-filling. One of the frequent causes of mysterious part-throttle surging is a too-lean mixture caused by excessive EGR.

There was always at least a little EGR effect in engines at part-throttle, even before the EGR system was invented, due to incomplete cylinder-filling and exhaust scavenging (especially in engines with cheap and inefficient factory exhaust and intake manifolds. This called for an earlier lighting-of-the-fire using vacuum advance.

[jtbo]

In my turbo diesel, there is no throttle butterfly, EGR does clutter intake manifold with goo from exhaust, causing eventually lot of problems, when car was new it perhaps did help, but when it is making such a mess nothing works and in this engine type it is known to be cause of engine failures because stuff from intake manifold can get loose and big chunks enter between valve etc.

Also in stage that it is in many of these cars emissions will be also higher with EGR connected.

It is recommended that system is disabled by unplugging connector to EGR valve or regular cleanup of manifold and pipes, which does require removing cylinder head as there is no other way to get intake manifold out.

So it is not always good system.

One thing is bit puzzling, hotter burn = cleaner burn, this is what we are being taught here, but I did read that EGR lowers burn temperature, is this case again one of those were one parameter is lowered in expense of others?

As with any system, when they are operated as designed, things are nice, but with older tech conditions can be far from designed, most logical solution would be to make everything to designed specifications, but it seems to be that few cares about that, as long as tests are passed it is cool, no matter how system really runs.

[roflwaffle]

Originally Posted by old jupiter

Yes . . . because the mixture has been RICHENED somewhat to make up for what's displaced by the exhaust back-filling. One of the frequent causes of mysterious part-throttle surging is a too-lean mixture caused by excessive EGR.

An intake charge isn't richer due to additional fuel being adding when exhaust gases are added, unless for whatever reason the manufacturer can't get it running smoothly and has to add fuel, but that sounds more like a engine config specific thing.

Originally Posted by jtbo

One thing is bit puzzling, hotter burn = cleaner burn, this is what we are being taught here, but I did read that EGR lowers burn temperature, is this case again one of those were one parameter is lowered in expense of others?

That depends on what you mean by hotter burn. A weak spark on an SI engine or poor atomization from an injector on a CI engine is bad for emissions, but at the same time if the combustion reaction is too hot that's also bad for emissions.

[user removed]

There is research going on with gas ICEs where they are experimenting with controlling power output through EGR volume alone. In other words, instead of choking off air to the engine they are adding EGR to control speed and power output.

EGR allows for higher effective compression with lower manifold vacuum which means you have better "leverage" during combustion which increases efficiency.

Yes, the combination of positive crankcase ventilation (PCV) and EGR means you have a tar like substance accumulating in the intake manifold. It would be nice if manufacturers would make it possible to clean those passageways without a whole lot of effort.

regards
Mech

[user removed]

Also, when mwebb posts to a thread, I find his information to be relevant, well thought out, and almost absolutely correct.

regards
Mech

[old jupiter]

The Wikipedia article (Mr. Taylor's link, above) cites two effects as gains, that are actually net losses. "Reduced Throttling Losses," comes because the throttle must be opened a little more to make up for lost power. Put your foot to the floor and you can REALLY reduce throttling losses, but are you saving fuel? And "Reduced Heat Rejection," again should have been noted as a measure of the fact that your heat engine has a cooler fire inside it with EGR. What we'd like to do is make the MOST heat, and get the most out of that heat, with the smallest amount of fuel, emissions aside.

Mechanic, I'm not picking on anyone, and anyone reading Mr. Webb's posts can see he's a smart guy.

[George Tyler]

Originally Posted by old jupiter

The Wikipedia article (Mr. Taylor's link, above) cites two effects as gains, that are actually net losses. "Reduced Throttling Losses," comes because the throttle must be opened a little more to make up for lost power. Put your foot to the floor and you can REALLY reduce throttling losses, but are you saving fuel? And "Reduced Heat Rejection," again should have been noted as a measure of the fact that your heat engine has a cooler fire inside it with EGR. What we'd like to do is make the MOST heat, and get the most out of that heat, with the smallest amount of fuel, emissions aside.

Mechanic, I'm not picking on anyone, and anyone reading Mr. Webb's posts can see he's a smart guy.

Well, am engine is actually more efficient with a wide open throttle. Putting your foot to the floor in a standard car is an entirely different scenario, also, that is what you do basically with "pulse and Glide" to reduce consumption. I also wonder about the "heat rejection" part.