CR melted piston discussion, why, why not, and for the love of god how not to..

[Begle1]

Disagree and why...

Just having more air doesn't mean that the fuel is going to burn any more or less efficiently. There is too many variables in this statement. If there is proper air/fuel ratio, there will be maximum efficiency.

Diesel Air/ Fuel Ratio for max power: 18-22:1 Is this always possible? no...

Did you ever put an O2 sensor in a common rail's exhaust to see what their actual air/fuel ratio was compared to a 12 or 24 valve?

"Air/ fuel ratio", as in a ratio of the total volume of fuel to the total volume of air in the combustion chamber, doesn't mean very much on a Diesel because the fuel is delivered over a long duration and must diffuse with the air on the spot. Your injection system producing a fine atomization over a good spray pattern has much more to do with having a lean or rich burning cloud of Diesel than the amount of air present. Yes, you must have sufficient air present for the lean burn to exist, but for a given amount of air the efficiency of the injection system is going to be what causes a lean or rich burn.

With a given amount of air flow, a P-pump is going to tend to burn "rich" and cool, because the fuel is not as well atomized and it has a hard time finding enough oxygen to react with. I think that with the right injection system and turbo set-up, a P-pump truck can run jut as efficiently as a common rail.

I really wouldn't think so, it'd be interesting to see though. If you set the same emissions standards for a mechanical and a common rail engine, the constant high pressure and precision of a common rail should be a clear advantage.

a common rail is going to run "leaner" and hotter because the fuel diffuses through the oxygen exceedingly well. This happens, because of the increased atomization effect with a common rail injection system. The better the atomization, the better the burn, the better the burn, the hotter the fire, the hotter the fire the better it runs.. Shouldn't it? that's why it was invented.

Yes, that's entirely correct. We're on the same page there. Note that the burn is going to be faster, hotter and better than a P-pump's burn regardless of the amount of air in the cylinder.

An Audi R10 race car has a 5.5 liter v12 Common rail engine with a diesel particulate filter.. It has twin turbos with a 1.57" inducer and manages to produce peak 700hp and continuous 600-650hp. All this with no smoke. Why because of the advanced common rail injection system designed for the engine. It was designed to race for an extended period of time at sustained maximum RPM at max fuel......

If what you say is true than all the common rail guys have to do is put big nozzles in, turn down their rail pressures, decrease duration, and have one injection event... As RPM increases Cylinder Pressure decreases.

Common rails burning hotter is ultimately a desireable trait, because it indicates a faster and more thorough burn, which is what lets them make more power with less fuel. The inherent downside is that you need a way to better deal with the heat.

Tuning down the efficiency of a common rail system, by reducing injection events, going to bigger injectors and reducing rail pressure, would solve the problem of excess heat, but it would also be removing the advantages of the common rail system. In the short term it could net bigger power gains, but intentionally introducing inefficiency to keep your engine together is probably not the most effective long-term path. It's a pointless road to try to out P-pump a P-pump with a common rail.

The Audi team has obviously found out how to cope with the hotter burns of the common rail system, and I'm willing to bet they didn't do so by making their fuel burn less well. They probably put a lot of work into head flow, piston bowl geometry, injector design and spray timing; the same stuff that needs to be done for the 5.9 common rail.

The Cummins Common rail Injection System was designed to make the power is had to make, while obeying emission standards, and staying quiet so the average 'Joe Shmo' can have a quiet, powerful, dependable truck... Chips and boxes increase timing and duration, but do not get rid of multiple events. (that I know of, enlighten me if there is) These multiple injection events coupled with increased timing and increased amounts of fuel is what cause melted pistons in common rails.. These extra events were added for noise emissions and exhaust emmisions. If the first injection event doesn't burn and stays raw in the chamber and is ignited by the second event there might as well be a OXY-Acetylene torch aimed on the lip of your pistons..

Cylinder temperature and pressure gets so high in common rails, because of the multiple injection events coupled with high timing and duration... With that being said, diesel fuel burns very slow compared to other fuels. With this scenario, the actual combustion is fighting against the upward travel of the piston, resulting in extremely high temperatures and pressures. (this is much like high timing in P-pump trucks, however, with a mechanically controlled injection pump, there is one event and has a relatively short duration) So you have not only one event with increased timing, you have multiple..

I'd really like to see the numbers on total common rail degrees of duration (and timing) versus total P-pump degrees of duration (and timing). The injection pressure a P-pump delivers is mound-shaped over its duration; fuel begins being delivered at the injector's pop-off pressure, then pressure ramps up, peaks, then tapers down; a fraction of the fuel is always injected at a relatively low pressure, and that is the fuel that burns crappy and keeps temperatures down. The injection pressure that a common rail delivers is plateau-shaped; it doesn't go through those relatively low injection pressures and that's why the fuel all burns good and hot.

I wouldn't wager any money that a common rail injector spend's any more time open than a P-pump injector. It wouldn't have to be open as long to burn the same amount of fuel. Is it better to have the opening time in one long injection or multiple short ones? I would think that multiple short durations are more efficient, due to the fact that it decreases emissions and is pegged as causing meltdowns. The fact that things are melting means that the injectors are working GREAT, it's just the engine's fault it can't handle it.

Short term: tune down the injection system to where the engine can handle it. Long term: build the engine so it can handle the full brunt of the injection system.

 

[KILLER 'B']

when there is one event, id have to disagree that a certain amount of fuel burns any better or any worse..

 

[Billyram]

I don't believe there's much more that can be done to the CR programing that hasn't allready been tryed. "We're about as good as we are going to get" The HPCR injection has limitations that there's not much that can be done about. To go with one big injection event insted of multible events we would need injectors with much larger holes that would likely drop the rail pressure quite a bit. At idle or lower rpms you would be very smokey. I don't believe our current injectors will handle high rpms. "They aren't disigned to operate quick enough" It's going to take something other than what we have now to work with to catch up with the P-pump.
Billy

 

[apwatson50]

I wanted to add this information in here, as it relates to some lean/rich discussion. These aren't my words, I found them on another site after I google searched, but I think this person did a good job of describing how exactly the fuel burns as it is very different then a SI throttled engine.

"Direct-injected, compression-ignited engines always combust at near stoichiometric, if the engine is designed properly. The fuel injected will only combust when sufficient oxygen is present at the flame front. The fuel will continue to burn as long conditions are suitable within the combustion region (ie. oxygen, fuel and heat). If the available oxygen within the chamber is used up before the available fuel, soot is created. If the heat in the combustion zone is reduced sufficiently, due to expansion of the end gas and slow combustion, combustion halts and the remaining fuel and oxygen are dumped out the exhaust. If more oxygen is pumped into the working chamber than is necessary for the amount of fuel injected, then engine efficiency is reduced."

Aaron

 

[Spectre32]

I don't believe there's much more that can be done to the CR programing that hasn't allready been tryed. "We're about as good as we are going to get" The HPCR injection has limitations that there's not much that can be done about. To go with one big injection event insted of multible events we would need injectors with much larger holes that would likely drop the rail pressure quite a bit. At idle or lower rpms you would be very smokey. I don't believe our current injectors will handle high rpms. "They aren't disigned to operate quick enough" It's going to take something other than what we have now to work with to catch up with the P-pump.
Billy

The reason for CR technology is so you can have multiple injection events for various reasons. Comparing an inline pump to CR systems is useless. Yes the P-pump can fuel to more RPM's and the CR"S cant. The injectors cant fire fast enough to reach 4K or even 5K useable ranges. Thats an inhertent design with cummins. Ford/IH have piezo injectors that can fuel the 6.4's into whatever RPM region they want to. Eventually the 6.7 or the next gen engine after that will get the newer technology injectors...however I digress. CR technology is the way of the future and what it all comes down to is this: Cubic Dollars. Mechanical engines are a dime a dozen. I can go right now and find a 12V for under 2K or less. They are cheap to make power and they all have been proven to hold it. If you blow a block apart who cares, you can find another one on the cheap. The CR's(5.9) have have a short life span in terms of parts flying around, not to mention the limitations to which have been enlighted in this thread. So lets see... if i want to be competive NOW and I dont have $100,000 to invest into engine developlment... what do I do? I buy a 24V with P-pump or a 12V and be done. A stand alone ECM will help but you cant fix physics. The solinoids cant fire fast enough. For most everyday people having good power under 3800 RPMS is fine. If your in it to win it, your life could be starting at that range.

Case and point: The folks at Audi have a V-12 engine that kick ass and takes names.. and spins some redonkolous RPM's. Yes its a V engine design and they allow for more RPM's opposed to inline 6's. As was stated, they use CR technology. If I had audi R&D dollars I too, could have some sort of CR engine that would wreck shop in whatever motorsport I choose. If mechanical injection was so much better you would have seen them use it. They did not use it , because its a dinosuar and we only use it cause we dont have the pocket books to do what they do.

 

[Begle1]

I wanted to add this information in here, as it relates to some lean/rich discussion. These aren't my words, I found them on another site after I google searched, but I think this person did a good job of describing how exactly the fuel burns as it is very different then a SI throttled engine.

"Direct-injected, compression-ignited engines always combust at near stoichiometric, if the engine is designed properly. The fuel injected will only combust when sufficient oxygen is present at the flame front. The fuel will continue to burn as long conditions are suitable within the combustion region (ie. oxygen, fuel and heat). If the available oxygen within the chamber is used up before the available fuel, soot is created. If the heat in the combustion zone is reduced sufficiently, due to expansion of the end gas and slow combustion, combustion halts and the remaining fuel and oxygen are dumped out the exhaust. If more oxygen is pumped into the working chamber than is necessary for the amount of fuel injected, then engine efficiency is reduced."

Aaron

Your right, I am wrong to use the terms "rich burn" and "lean burn". How about "incomplete burn" and "complete burn"?

I don't see a functional difference between the two; rich/ incomplete is inefficient and cool, lean/ complete is efficient and hot. But if it was "lean" or "rich" we would get varying amounts of carbon monoxide, which we don't. Perhaps there is a more radical temperature difference between "lean" and "rich" combustion versus "complete" and "incomplete" combustion.

The reason for CR technology is so you can have multiple injection events for various reasons. Comparing an inline pump to CR systems is useless. Yes the P-pump can fuel to more RPM's and the CR"S cant. The injectors cant fire fast enough to reach 4K or even 5K useable ranges. Thats an inhertent design with cummins. Ford/IH have piezo injectors that can fuel the 6.4's into whatever RPM region they want to. Eventually the 6.7 or the next gen engine after that will get the newer technology injectors...however I digress. CR technology is the way of the future and what it all comes down to is this: Cubic Dollars. Mechanical engines are a dime a dozen. I can go right now and find a 12V for under 2K or less. They are cheap to make power and they all have been proven to hold it. If you blow a block apart who cares, you can find another one on the cheap. The CR's(5.9) have have a short life span in terms of parts flying around, not to mention the limitations to which have been enlighted in this thread. So lets see... if i want to be competive NOW and I dont have $100,000 to invest into engine developlment... what do I do? I buy a 24V with P-pump or a 12V and be done. A stand alone ECM will help but you cant fix physics. The solinoids cant fire fast enough. For most everyday people having good power under 3800 RPMS is fine. If your in it to win it, your life could be starting at that range.

Case and point: The folks at Audi have a V-12 engine that kick ass and takes names.. and spins some redonkolous RPM's. Yes its a V engine design and they allow for more RPM's opposed to inline 6's. As was stated, they use CR technology. If I had audi R&D dollars I too, could have some sort of CR engine that would wreck shop in whatever motorsport I choose. If mechanical injection was so much better you would have seen them use it. They did not use it , because its a dinosuar and we only use it cause we dont have the pocket books to do what they do.

Just conjecturing, if one could figure out how to turn off the second main injection event per power stroke at a certain RPM, then that would help the heat problems and let you spin twice as fast before the injector solenoid reached its limit. No?

 

[KILLER 'B']

"Direct-injected, compression-ignited engines always combust at near stoichiometric, if the engine is designed properly. The fuel injected will only combust when sufficient oxygen is present at the flame front. The fuel will continue to burn as long conditions are suitable within the combustion region (ie. oxygen, fuel and heat). If the available oxygen within the chamber is used up before the available fuel, soot is created. If the heat in the combustion zone is reduced sufficiently, due to expansion of the end gas and slow combustion, combustion halts and the remaining fuel and oxygen are dumped out the exhaust. If more oxygen is pumped into the working chamber than is necessary for the amount of fuel injected, then engine efficiency is reduced."

This is 100% true as well this is what i was trying to say... In not so many words, lol..

 

[apwatson50]

This is 100% true as well this is what i was trying to say... In not so many words, lol..

Yes and I kept reading people saying rich/lean etc... and knowing it isn't like that. But kept having a hard time finding the right words to describe it, so I stole someone else's words. lol

But yeah its completely different then SI throttled engines!

Air follows fuel in diesel, in SI throttled fuel follows air.

 

[bdp]

This thread is getting more and more entertaining by the day.


One injection event and full parameter control, thats all I have to say about this.

 

[Billyram]

Your right, I am wrong to use the terms "rich burn" and "lean burn". How about "incomplete burn" and "complete burn"?

I don't see a functional difference between the two; rich/ incomplete is inefficient and cool, lean/ complete is efficient and hot. But if it was "lean" or "rich" we would get varying amounts of carbon monoxide, which we don't. Perhaps there is a more radical temperature difference between "lean" and "rich" combustion versus "complete" and "incomplete" combustion.



Just conjecturing, if one could figure out how to turn off the second main injection event per power stroke at a certain RPM, then that would help the heat problems and let you spin twice as fast before the injector solenoid reached its limit. No?

Yes if you had the right kind of injectors. I believe the race engines use multible ejection events too so multible injection events may not be all of the problems we're having.
Billy

 

[triton]

I think I've already eliminated the pilot event cause my truck has combustion rattle like you wouldn't believe. I'm afraid if another event were to be eliminated..... the motor would fall a part from the rattle you'd get. That or blow up from trying to ignite to much at one time. You'd have to have some expert tuning for it to work IMO

 

[Spectre32]

Just conjecturing, if one could figure out how to turn off the second main injection event per power stroke at a certain RPM, then that would help the heat problems and let you spin twice as fast before the injector solenoid reached its limit. No?

I guess, however I do not know for sure. Do the Dmax guys get rid of any of there injection events? I dont know.. but if you reduce everything to one main injection event, then why have all the fancy gizmo's of a CR? You would gain some over a mechanical injected truck, by having timming adjustment on the fly and perhaps injection pressure but it seems like an unfavorable trade off(maybe). I doubt any of the highly advanced diesels have one injection event, infact they do not. The audi has all of the EPA crap on it so they must be running more than one to contend with everthing. However, if you look at the 5-6 injection events, most of them are for emmissions control purposes, so perhaps beaner is on the right path.

 

[MCummings]

The folks at Audi have a V-12 engine that kick ass and takes names.. and spins some redonkolous RPM's. Yes its a V engine design and they allow for more RPM's opposed to inline 6's. As was stated, they use CR technology.

RPM is RPM no matter inline, V, W, or Boxer. The injectors have to open and close at the same speed...

Merrick

 

[Spectre32]

Well I was looking at it fromt he staind point of 8 cylinders vs 6. The 8's as of right now int he non mechanical injection world can fuel to whatever RPM they desire, at least the the Fords can. I tought hr Dmaxes... maxed out at somewhere in the 4K region.

 

[bprumos]

This thread is getting more and more entertaining by the day.


One injection event and full parameter control, thats all I have to say about this.
__________________

agreed



theres all talks about temps but not how those temps got there?

what about cylinder pressures vs cylinder temps? aluminum melts at 1220*F, literally everyone on this thread has passed that temp many times. so the piston tops are obviously staying below that point most of the time. maybe the melting is occuring more in the cr's and not 12v and 24v because of higher cyl pressures due to a more efficient burn, injection events happening faster, and in mutiple events. that creates large pressure spikes. like stated before the 12v's injection pressures gradually raise, peak, and then dropoff during the event. this should realate to a more linier cyl pressure cuve and could relate to a better distribution of heat across the surface of the piston, and also should give it more time to absorb the heat and transfer it away from the top. whereas the cr's pressures peak much faster and drop off much faster due to injection pressures and injection event control, and this is happening 3-4 times throught the stroke, plus with an added pre-event that is effectively pre heating the survace of the piston for more punishment. that coupled with increased efficiency and bad bowl designs like stated in the 04.5-07's could explain something maybe? specialy if the 6.7's use 03-04.5' style bowls theres a reason for it. this also might explain why duramax pistons are cracking due to several large pressure spikes at high temps. but there not melting thier pistons as much due to better bowl design and or better injection controll.


just throwing an idea out there, maybe it could explain something

 

[triton]

another good theory. The melting point you mention is for pure aluminum though which these pistons are not. That's why they can handle 1400F+ for sustained periods

I truly believe duration was the cause of mine to melt. That and the lack of air at the time.

How bout looking at this from another angle. Stock trucks can typically handle sustained periods of heat.... why ? Computer controlled timing, duration and fueling/defueling. We do nothing but increase every one of those parameters with our trucks but, not in a controlled way. If the programming can be done to where events don't over lap either other and work with each other.... we'd be good. Otherwise, by pushing and pushing for more power without controlled programming.... we're going to continue to see melting pistons.

just some random rambling

 

[MCummings]

Stock trucks can typically handle sustained periods of heat....

This statement has me thinking. Stock trucks, even, the '01 and '02's were running 1,400 or even 1,450 Factory, and not melting down, now the 6.7's are running constant at 1,400+ while regening, yet, you add more rail pressure, more air, a cam, ported head, keep the EGT's at 1,300, and it melts down.... What else should we monitor? Oil Temps? Intake air temps? A more precise measurement of water temps?

Merrick

 

[triton]

duration. the events happen in 1/1000+/- of a second or faster and we've possibly changed it to 1/100th of a second+/- with larger injectors and programming. That extra amount of fuel being injected could play a part ????

 

[Devan Manis]

agreed



theres all talks about temps but not how those temps got there?

what about cylinder pressures vs cylinder temps? aluminum melts at 1220*F, literally everyone on this thread has passed that temp many times. so the piston tops are obviously staying below that point most of the time. maybe the melting is occuring more in the cr's and not 12v and 24v because of higher cyl pressures due to a more efficient burn, injection events happening faster, and in mutiple events. that creates large pressure spikes. like stated before the 12v's injection pressures gradually raise, peak, and then dropoff during the event. this should realate to a more linier cyl pressure cuve and could relate to a better distribution of heat across the surface of the piston, and also should give it more time to absorb the heat and transfer it away from the top. whereas the cr's pressures peak much faster and drop off much faster due to injection pressures and injection event control, and this is happening 3-4 times throught the stroke, plus with an added pre-event that is effectively pre heating the survace of the piston for more punishment. that coupled with increased efficiency and bad bowl designs like stated in the 04.5-07's could explain something maybe? specialy if the 6.7's use 03-04.5' style bowls theres a reason for it. this also might explain why duramax pistons are cracking due to several large pressure spikes at high temps. but there not melting thier pistons as much due to better bowl design and or better injection controll.


just throwing an idea out there, maybe it could explain something

I don't think that is the case. The common rails are quieter for the simple fact that the cylinder pressure curves are more linear and less spikey. The multiple injection events help to smooth out the cylinder pressure curve. The injection events of the 12v would be more linear, but one all the fuel in the cylinder combusts the pressure spike is out of controll which causes the rattle.

 

[bprumos]

these trucks stock are set to run as high cyl temps as possible ,while staying safe, to meet emmitions standerds as high heat helps to completely burn the fuel which = less emmitions. after tuning and such your going to far excede the stock parameters and run the risk of melting things like what has happened. my theory is that the different pistons and the post event in the later cr's are strictly there as a last minute mod to meet emmitions without radical change of engine desing and programming and in stock form like that there prolly on the edge of thermal efficiancy to get as hot and as clean a burn as possible. after serious modding you push it past these limits, but without being able to completely remove the two major components, the pre injection, and the piston bowl design. hence why the early cr's dont melt as often cause you have more room to play with without passing those limits cause it wasnt initially enginered as close to them as the later ones. if you guys understand what im trying to say