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Old 11-21-2021, 05:01 PM   #21 (permalink)
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I don't recall problems with pistons unless a broken retainer clip let's a valve drop onto the piston.

Traditional failure mode is unequal cooling on #1 that crisps a valve or blows out the cylinder top.

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Old 11-22-2021, 07:36 AM   #22 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
What about lean mixtures running hotter? Ever fry a piston top in a VW bug? In the diesel F250, you watch EGT on hard pulls because the aluminium stuff tends to melt.
Nope, the AFR gets so lean that there is not enough fuel to get it very hot in a diesel at it's most lean AFR, wich is usualy idle.
The more fuel you inject, the hotter it gets untill you reach the stochiometric AFR.
If you run it richer than that, you just produce lots of smoke.
So you'll get the highest EGTs under high load when the engine injects a lot of fuel.

On a sidenote:
The higher the combustion temperature, the more power you make.
But your engine doesn't like it when it gets too hot, it starts melting if your combustion temperatures get too high, and gasoiline engines start to knock/detonate when you push it too far.

The higher the difference between combustion and exaust temperature determines how efficient the engine is, higher expansion ratios can increase that.
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Old 11-22-2021, 08:39 PM   #23 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
There is no throttle so adding EGR doesn't help efficiency like in a gasser, because in a gasser the EGR allows the throttle to be open more.
Odd enough, now some Diesels are getting an actual throttle, mostly due to the emissions control devices, while some gassers got rid of the throttle. Yet EGR is still more effective on gassers...
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Old 12-24-2021, 11:28 AM   #24 (permalink)
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Sorry I'm late to this thread, I just got the email.

Quote:
Originally Posted by Autobahnschleicher View Post
A diesel engine usualy has no throttle.
Modern diesels use EGR to make the engine run richer without adding more fuel.
This results in lower NOx raw emissions, wich need to get converted to nitrogen and water by the SCR catalythic converter.
Not exactly. The purpose of the EGR is to reduce cylinder temp, which reduces NOx emissions. Any resulting change in AFR is just a side-effect.
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Old 12-24-2021, 11:32 AM   #25 (permalink)
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Originally Posted by Piotrsko View Post
If you don't notice your oil turning black from unburned fuel, you're not looking at the engine oil
I own some IDI engines and a common-rail. All of them have black oil, despite a disabled EGR and oil catch can. AMSoil sells a bypass oil filter that claims to remove soot and it's on my wish list.

We have a large mechanically-injected Cummins generator at work and the oil always looks pristine. I'm not sure what the secret is. Maybe because it never runs above 16% load except during semi-annual maintenance.

I drove a 2015 school bus with a Cummins 6.7 for two years and I never once saw the regen light turn on. Probably because I drove a rural route. I don't recall if the oil was black in that one.
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Old 12-24-2021, 11:37 AM   #26 (permalink)
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Originally Posted by clarknova View Post
I own some IDI engines and a common-rail. All of them have black oil, despite a disabled EGR and oil catch can. AMSoil sells a bypass oil filter that claims to remove soot and it's on my wish list.

We have a large mechanically-injected Cummins generator at work and the oil always looks pristine. I'm not sure what the secret is. Maybe because it never runs above 16% load except during semi-annual maintenance.

I drove a 2015 school bus with a Cummins 6.7 for two years and I never once saw the regen light turn on. Probably because I drove a rural route. I don't recall if the oil was black in that one.
The oil in diesels turns black quickly regardless of EGR as the raw particle emissions are pretty high and contaminate the oil.
Interestingly the opposite can be observed in CNG/LPG vehicles where the oil takes a long time to darken.
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Old 12-24-2021, 11:41 AM   #27 (permalink)
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Originally Posted by clarknova View Post
Not exactly. The purpose of the EGR is to reduce cylinder temp, which reduces NOx emissions. Any resulting change in AFR is just a side-effect.
The EGR is *not* lowering the EGTs as long as the engine isn't running rich.
Even cooled EGR is hot compared to ambient air/charge air after the intercooler.
The point of EGR in diesels is to get closer to stochiometric ratio to reduce the amount of oxygen that can react with nitrogen, reducing the amount of NOx produced.
However that's a tradeoff with particle emissions as these increase when you run less lean.
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Old 12-24-2021, 11:45 AM   #28 (permalink)
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Originally Posted by Blue Angel View Post
One thing I'm curious about is cold weather operation. A gas car runs rich to warm up the catalyst, which can take quite a while with light-footed driving, or while idling, and can result in fuel contaminating the oil. I don't believe diesels have this issue and would be much more efficient in the winter because of that, no? I didn't notice much change in fuel economy in the cold last winter and I'm wondering if that's why?
With the possible exception of throttle-fitted diesels, mentioned by another poster but completely unknown to me, a diesel engine is controlling idle by metering the fuel. Air flow is unrestricted. When the engine is cold, more of the fuel passes through unburnt (you may see white smoke on a cold start). This results in a less efficient burn and thus more fuel is required to maintain the idle. This natural feedback loop results in a richer AFR entering the cylinder, but this is not the direct result of any direct AFR control on the part of the ECM.

Some newer diesels have an exhaust choke that activates when cold to increase backpressure and thus temperature, to help warm the engine. This creates a very distinct and noticeable sound on some Powerstroke engines especially that sounds like a jet turbine even at idle.

As for cold idling, diesels are very efficient at idle, and this can lead to problems such as slobbering and glazing due to insufficient heat in the cylinder. This is why many bigger diesels are equipped with high-idle controls. Check out an informative video on YT by Adept Ape called "Is Idling Your Engine Bad? Does Idling Hurt Your Engine?". I'm not allowed to post links.
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Old 12-24-2021, 11:54 AM   #29 (permalink)
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Quote:
Originally Posted by Autobahnschleicher View Post
The EGR is *not* lowering the EGTs as long as the engine isn't running rich.
Even cooled EGR is hot compared to ambient air/charge air after the intercooler.
The point of EGR in diesels is to get closer to stochiometric ratio to reduce the amount of oxygen that can react with nitrogen, reducing the amount of NOx produced.
However that's a tradeoff with particle emissions as these increase when you run less lean.
I'm not an EGR expert but Wikipedia ("Exhaust gas recirculation") appears to disagree with your first and third statements. Yes, the recirculated gas is hotter than intake air, but it is lowering cylinder temps by reducing available oxygen, and the reduced cylinder temp is the mechanism of reduced NOx emissions. If you can show me a good source to refute this I will happily update the wiki page.
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Old 12-24-2021, 12:19 PM   #30 (permalink)
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Quote:
Originally Posted by clarknova View Post
I'm not an EGR expert but Wikipedia ("Exhaust gas recirculation") appears to disagree with your first and third statements. Yes, the recirculated gas is hotter than intake air, but it is lowering cylinder temps by reducing available oxygen, and the reduced cylinder temp is the mechanism of reduced NOx emissions. If you can show me a good source to refute this I will happily update the wiki page.
I could dig out a german textbook on internal combustion engines, but you'll likely not understand it because it's in german.
Regarding the temperature:
As long as all the fuel combusts, EGR does not lower combustion temperatures in *diesel* engines as the same amount of fuel combustion produces the same amount of heat.
It is different in gasoiline engines as here it dilutes the air/fuel mixture while keeping it stochiometric and thereby reduces combustion temperatures, where as in diesels it replaces intake air and enriches the air/fuel ratio.

In gasoiline engines it also improves efficiency under low load as it reduces pumping losses as the engine doesn't have to work against as much of a vacuum.
Meanwhile in diesel engines the engine doesn't have to work gaianst a vacuum to begin with.

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