Melting pistons.

[Fingers]

FWIW, as I have been pouring over some of these LBZ plots, I am impressed how nuts on the actual timing is when compared to those in the tune. Way better than my poor LLY's accuracy.

Makes me wonder how much "injection scatter" plays into burning/breaking parts. I know it is a factor in other engines.

 

[dmaxvaz]

pat, are you thinking that with rather large injectors and a lower rpm, the pistons might last longer? Do you think stock engines should benifit from this also? Do you think there will be alot of smoke with the 100% over injectors?

by the way guys, great tech thread. Probably one of the best ones ive read here

 

[LBZ]

FWIW, as I have been pouring over some of these LBZ plots, I am impressed how nuts on the actual timing is when compared to those in the tune. Way better than my poor LLY's accuracy.

Makes me wonder how much "injection scatter" plays into burning/breaking parts. I know it is a factor in other engines.

Could this be due in part to the external FICM efficiency at higher duty cycle when compared to the LBZ/LMM which have the injector control integrated in with the ECM?

I would think heat and the lower voltage would both work against you at the higher duty cycle making the timing control harder to control and keep accurate no?

A scope on the injector at higher rpm's could probably give an idea of your injection scatter right? Would be an interesting thing to see.

 

[SmokeShow]

I'm absolutely drunken with information. 'Bout get a chub reading all this stuff! Thanks guys! :hello:


How bout that CP... obviously it peaks. But how important is the curve before and after the peak. I mean how steep the curve is. How important is the ramp up to peak an ddown from peak in relation to the actual peak number? Is the peak number actually the most important number or what? Is it better for the ramp up and down to be flatter (within reason, I know it's not gonna be flat like a HP or TQ curve fro a dyno unless the sampling rate is astronomical) or is it ok to be steep? What tuning changes change that rate of increase and decrease up to peak?


Shoo. That's ^^^ a lot in one question. :angel:



C-ya

 

[McRat]

pat, are you thinking that with rather large injectors and a lower rpm, the pistons might last longer? Do you think stock engines should benifit from this also? Do you think there will be alot of smoke with the 100% over injectors?

To be honest, I'm not sure. I still have very limited experience running big injectors. Our last test run was stopped by other failures, but showed no erosion with big injectors and sub 4000 rpm, but I only got the truck up to 150mph before it quit.

I don't have enough data. We really need others who have melted pistons to step up and describe the conditions in which it happened. But I'm not sure others are going to discuss it much. Too much pressure to keep it silent. They will read threads like this to get info from them, but not add to them. I do know my experience with melting is not unique to my truck, nor unique in other brands.

If I understand it correctly, the 12v guys solve it by overfueling hard to use the heat of vaporization to cool the piston. But I have no experience with that, nor do I know whether they also melt aluminum pistons. We do know that one race organization reduced the track length from 1/4mi to 1/8th mile because of failures on high profile 12v ProStreet trucks, but so far they won't tell us what the failures were. The official reason is "safety", but they can't point to one safety problem that would cause that decision. The only serious crash was at an 1/8th mile track, and the existing safety equipment did it's job.

Much of this topic started back when folk were first cracking LBZ pistons. But cracking and melting are two separate issues. Forged pistons can stop cracking, but melting has to be addressed separately.

 

[Mike]

Just curious to know if diesel fuel can turn to steam and store energy. Seems too volatile to be used as a cooling agent. I know it's said to be done, just wondering how it works.

 

[dmaxvaz]

To be honest, I'm not sure. I still have very limited experience running big injectors. Our last test run was stopped by other failures, but showed no erosion with big injectors and sub 4000 rpm, but I only got the truck up to 150mph before it quit.

I don't have enough data. We really need others who have melted pistons to step up and describe the conditions in which it happened. But I'm not sure others are going to discuss it much. Too much pressure to keep it silent. They will read threads like this to get info from them, but not add to them. I do know my experience with melting is not unique to my truck, nor unique in other brands.

If I understand it correctly, the 12v guys solve it by overfueling hard to use the heat of vaporization to cool the piston. But I have no experience with that, nor do I know whether they also melt aluminum pistons. We do know that one race organization reduced the track length from 1/4mi to 1/8th mile because of failures on high profile 12v ProStreet trucks, but so far they won't tell us what the failures were. The official reason is "safety", but they can't point to one safety problem that would cause that decision. The only serious crash was at an 1/8th mile track, and the existing safety equipment did it's job.

Much of this topic started back when folk were first cracking LBZ pistons. But cracking and melting are two separate issues. Forged pistons can stop cracking, but melting has to be addressed separately.

Good stuff. I have a theory that if we ditched the cr stuff for competition purposes and add a v8 p-pump like the guy with the dmax pulling tractor, we would easily be able to over fuel it like the 12v guys, rev it higher/faster, and have less problems with the pistons and mechanical side of things. yes its a step backwards in technology, but it works more reliably in high hp aplications and probably be $$$ ahead.

 

[Accelerator]

Any agree that a stand alone ECM with separate injector drivers would make the injection events more reliable and repeatable without "scatter" and mis-timed fuel injection?

 

[McRat]

Just curious to know if diesel fuel can turn to steam and store energy. Seems too volatile to be used as a cooling agent. I know it's said to be done, just wondering how it works.

It's true for all liquids. You spray them into air, and they cool the air as they evaporate. Whether it works or not on a diesel engine, I don't know.

 

[Mike]

Any agree that a stand alone ECM with separate injector drivers would make the injection events more reliable and repeatable without "scatter" and mis-timed fuel injection?

We have that with the lb7 and the lly.

 

[Mike]

It's true for all liquids. You spray them into air, and they cool the air as they evaporate. Whether it works or not on a diesel engine, I don't know.

Good point Pat. Seems we'd see a fire ball out the tail pipe once o2 is reached. But, may be too cool at that point.

 

[McRat]

Good point Pat. Seems we'd see a fire ball out the tail pipe once o2 is reached. But, may be too cool at that point.

Watch the videos, you will see flames coming out the pipes on many 12v max effort trucks.

 

[Mike]

Watch the videos, you will see flames coming out the pipes on many 12v max effort trucks.

I've watched a video of Kevin's truck pulling with flames out the stack. Didn't think it was over fueling only thought it was late timing. No smoke present.

 

[Mike]

It's true for all liquids. You spray them into air, and they cool the air as they evaporate. Whether it works or not on a diesel engine, I don't know.

Question to you Pat. At what ambient fluid temperatures does evaporation no longer work in specific air temps? Air temps in a cylinder are high but so are the fuel temps.

 

[McRat]

Question to you Pat. At what ambient fluid temperatures does evaporation no longer work in specific air temps? Air temps in a cylinder are high but so are the fuel temps.

Not sure.

Here's what MIGHT occur (no proof):

You consume nearly all the available oxygen in the cylinder, yet the fuel continues to spray in, or is just still trapped in the cylinder unburned. At that point, the fuel will drop the cylinder temp rapidly as the temps are much higher than the boiling point of diesel. Just like spraying water on a hot skillet. Yes, the heat of vaporization is lower with hydrocarbons than it is with water, but it's still very significant. Water is the only one I'm familiar with, and it's about 540 calories per gram, IIRC.

Here's how to put that in perspective:

To raise water from 99 C to 100 C takes 1 calorie per gram. To raise water from 100 C to 101C (going from liquid to vapor) takes 540 + 1 calories per gram, or it absorbs over 500 times as much heat as liquid water does. This is why dipping a red hot piece of steel in water cools it down so fast. If the steel is over 100 C, the water has 540 times the effect it would have if the steel was less than 100 C.

 

[Mike]

Now it's time to compare this to Diesel. Seems the diesel would help cool the air charge just before flash. Could also explain why diesel is so violent.

I really get a strong feeling it's not high fuel pressure causing the problem ( as you feel as well Pat ) but in fact a lack of pressure. " super hot " diesel splashing and impinging in the combustion chamber. I say it's time to move a rail pressure sensor closer to an injector with a separate monitor with faster refresh cycles to observe actual pressures.

 

[Mike]

In high compression gas engines we'd polish piston tops, round corners and polish all other combustion surfaces. This would decrease surface area as well as keep carbon from forming for longer in an attempt to limit detonation. Not saying detonation is the concern only that we want more piston surface area don't we. This would help dissipate more heat like a big heat sink. Oil being the cooling agent.

 

[gr8shot]

The fuel burn is not a simple on and off event. It ramps up exponentially, reaches a plateau and then fades off. Ideally, you look to get the leading edge of the burn's plateau just ATDC. All the usual factors will determine when that point is. Duration, Rail pressure, RPM, and Boost.

Yeah, it's ideal to have the peak of the combustion a good bit after TDC right? I mean the piston doesn't move a whole heck of a lot until around 40-45* ATDC. Most of the crankshaft motion before then is directed in the horizontal direction with very little vertical direction. So having the peak of the combustion before that wouldn't be as efficient I'd think, since most of the energy is spent trying to push the crankshaft down, when it can only go mostly sideways and a little bit down.

i'd think it's the same on the injection as well. The timing shouldn't be quite as critical given the small amount of piston movement in the last 30-40* of crank rotation before TDC, compared to the huge amount of movement from BDC up to the last 30-40* BTDC. It's more a matter of timing when the combustion starts rather than when the fuel is injected. Or is this implied and I'm just now catching on?

I'm just thinking back to my statics and dynamics classes, and trying to think a little more straightfoward about this. Maybe i'm way off base. But i'd think that too much timing will obviously start the combustion before the optimal time, which means the pistons will soak more of the heat, since the piston really isn't going anywhere substantial from 30* or so BTDC to 30* or so ATDC. I'd think that starting the combustion later would mean that the piston actually is moved by the combustion more, and has less time to absorb the heat, and more of the heat of combustion is transferred into work, rather than soaked up by the metal.

Like I said, I'm just thinking out loud. Im not an engine builder, nor have I burnt/melted any pistions. Just trying to remeber some things from school

 

[gr8shot]

Now it's time to compare this to Diesel. Seems the diesel would help cool the air charge just before flash. Could also explain why diesel is so violent.

I really get a strong feeling it's not high fuel pressure causing the problem ( as you feel as well Pat ) but in fact a lack of pressure. " super hot " diesel splashing and impinging in the combustion chamber. I say it's time to move a rail pressure sensor closer to an injector with a separate monitor with faster refresh cycles to observe actual pressures.

I'd think the opposite actually. More pressure creates better atomization, which means more surface area on the fuel droplets. this would mean the fuel absorbs more heat faster in its transition into vapor, and would cool the cylinder better. Just like crushed ice versus ice cubes. The crushed ice cools faster but is gone quicker too, while cubes cool slower but last longer.

 

[TrentNell]

exactly what i was saying 10 pages ago... i do think this is "a" primary cause of piston failure.

If that theroy was correct than it would also mean that any fuel pressure that could be used ( enough to make decent power anyway 20K min ) would be too much also , and that kills the theroy behind what you believe will fix it . It does re-inforce the decisions Pat has made to correct the problem though