ULSD and timing = cracked pistons?

[CStraface]

What are we considering "high" egt's here?

 

[steved]

What are we considering "high" egt's here?

I was sort of thinking the same thing... I remember someone saying that touching 1500*F on a 3rd gen wasn't detrimental like it could be on an older truck... that they are meant for higher EGTs.



steved

 

[rbattelle]

Cetane is measure of how fast the fuel burns not neccessarily how fast it ignites.

Cetane

I think you're both right. Higher cetane gives smaller delay angle and low rate of pressure rise (slower burning). Lower cetane gives larger delay angle and high rate of pressure rise (fast burning and detonation).



In order to minimize detonation, delay angle must be as small as possible, so that conditions favorable to detonation don't have time to form.



In my opinion, then, higher cetane will give more power but reduced engine life (perhaps by an immeasurably small amount).



Injection Timing

Note that maximum indicated mean effective pressure (and maximum efficiency) occurs with peak pressure around 15° ATC. This suggests an injection start around 25° BTC.



But you need to limit your maximum pressure in order to avoid blowing the engine apart, so it's best to set your injection a little later (say between 15 and 20° BTC), which penalizes power and efficiency as a tradeoff for lower maximum pressure.



So devices that move the injection earlier (closer to ~25° BTC) will give you more power and more efficiency, but cause tremendous increases in the maximum pressure in the engine. This translates directly into shorter engine life. This is the "rest of the story" that you don't hear from companies that want to sell you a box that changes your injection timing.



Cetane + Injection Timing

It appears to me that higher cetane gives higher peak pressure. We've seen how early injection also gives higher peak pressure. The combination of the two will obviously decrease engine life that much more than the individual effect of each.



I think the order of magnitude of the increase in peak pressure due to early injection timing is greater than that due to increasing cetane number.



These are just my opinions, based heavily on Taylor's book The Internal Combustion Engine in Theory and Practice (vol. 2).



Ryan

 

[Vaughn MacKenzie]

If the engine doesn't have too much advanced timing, high cetane is more beneficial than harmful, because the fuel is burned in a smooth and even fashion, not in a more sharp, violent explosion.



The reason low cetane fuel produces more knock and clatter (especially in a non-common rail motor) is because it is slow to start burning. When it finally lights off the entire dose burns almost instantaneously, a virtual explosion, creating the sharp pressure spike and resulting clatter. Not ideal. What you do want is a fuel that begins burning ASAP, not one that takes awhile to light. This is more important in engines without pilot injection.



Because diesel fuel burns rapidly and more violently than gasoline, it has to be sprayed continously during the entire power stoke instead of being delivered in one quick dose. The injection starts spraying into the cylinder somewhere around 15-20 degrees BTDC (depending on RPM, type and brand of engine), continues spraying through TDC and ends several degress past TDC, depending on engine load. The more the load and fuel, the longer the injection duration. This is done to maintain an even, continuous burn, instead of a violent short explosion.



Here is a diesel timing diagram showing this engine begins injection at 10* BTDC and ends approx. 35* ATDC.

Good reading http://www.dieselduck.ca/machinery_page/diesel_engine/diesel_engine.02.htm#The four stroke ...





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Vaughn

 

[Walker]

Wow. Feels like I was in the TDR and a science class broke out.



So, what about the fact that the newer trucks have supposed decreased timing propgrams (ecm) to comply with all the emissions? I've always heard that the way to get back some of the mileage with the new motors is to "advance the timing" a little more.



So is the new ULSD with higher cetane, but lower BTU advancing timing at all; or just building higher pressures? Sorry, I missed that class.

 

[TMoe]

Good info, always way more than I need to know :-laf .

ULSD has no effect on timing does it? That's when fuel is injected by brain in some look up table calculated from some sensor inputs. Delay angle, when fuel ignites after injection gets smaller. Higher Cetane means burn starts sooner and burns more controlled and slower, simply re-iterating what I've read here as I know nothing. Lower BTU means you have to step on gas more to increase duration to get same hp, less mpg.



I am sure rail pressure plays into all of this duration vs power stuff also or is it fixed? Where does the dreaded 3rd event come in and how does that play in all this? Does the engine control, sensors, etc. , somehow compensate for different fuel cetane, btu, etc. , to keep temps down, etc, and protect my factory engine so I don't have to worry about it!

 

[betterthanstock]

im not worried, im running enough timing and fuel pressure, my truck sounds like a powerstroke when between gears over 30lbs of boost, it makes alot of power, gets decent mileage and never goes over 1350 deg, i drive it like its stolen, change oil every 3,000 miles, samples are clean and say change at 5,000, i think the failures are from abuse also, i toasted mine at 57,000, dropped valve seat, all 6 pistons were damaged, cracks, melting, scoring etc, this was right before the ulsd, i cant tell a difference with the ulsd, i have tried additives and i did pick up some mileage, but power and sound is the same, i guess i will worry when it blows up again, i already want to pull it again so i can do some different things/ideas, but it has to run till i buy a new 6. 4 power stroke so i can make the dodge a full blown toy since i wrecked my work truck Oo.

So then cracked pistons are "normal" for hot rodded trucks. Why are people complaining of dropped valve seats, when they need a full rebuild anyway? The engine was on its last legs with or without a dropped valve seat.

How did you learn of the higher Cetane with ULSD and what numbers they actually are in various areas?

I found that the new fuel is about 2-3 cetane higher. (about the same as adding cetane booster to the old one).



Stock engines run better on higher cetane, and it makes up for some of the loss of BTU. I was just referring to those running on the edge, driving by the EGT gauge.



I have suggested to some people with melted pistons to try "ceramic coated" pistons. Has anybody done that? Maybe we need a poll for that, and how they're holding up.

 

[RVTRKN]

I can help you out on this. The trucks with this failure are highly modified, sometimes with 500+ HP, towing at high speeds when these occur.

No issues with the Chrysler tuning. We are not worried about stock engines here.

When you play you pay I think is what the theme is here.

 

[CStraface]

Ok but either way we look at it, egts are egts, and timing is timing. So isn't it possible that this is just being over thought and that regardless of the cetane amount, our engines will be fine as long as we treat them good.



I mean we can't get all worked up over some boso who let his egts peg 2000 while towing 15000 up a grade with his TST on 9/9 and his BDDL on crazy larry stacked with his smarty and mp-8 and edge ez, while runnning advanced timing with his PDR cam, and then got the ulsd blues and a sudden trend of discouragement w/ cummins engines because he blew his baby up. "It's the diesel's fault my engine blew up!"

 

[TCluff]

Ok but either way we look at it, egts are egts, and timing is timing. So isn't it possible that this is just being over thought and that regardless of the cetane amount, our engines will be fine as long as we treat them good.



I mean we can't get all worked up over some boso who let his egts peg 2000 while towing 15000 up a grade with his TST on 9/9 and his BDDL on crazy larry stacked with his smarty and mp-8 and edge ez, while runnning advanced timing with his PDR cam, and then got the ulsd blues and a sudden trend of discouragement w/ cummins engines because he blew his baby up. "It's the diesel's fault my engine blew up!"

THATS WHAT I MEANT BUT DIDNT EXPLAIN IT THAT CLEAR !!!

 

[Giachino]

The latest smarty versions (Dec 31st release, stock injector programs) have more timming... . i'm now getting timming rattle at idle / low rpm (the previous 2 software versions didnt rattle at all)... . so yeah, I'm now concerned!