Festival of Speed at Cal Speedway /Auto Club Speedway April 20 2018 - 928s running?
08-10-2018, 04:35 AM
#436
Nordschleife Master
It basically shows what i am talking about and what has been said. both knocking and pre-ignition can cause extreme heat in the cylinders. pre-ignition usually causes a knock as well, and this knock is before TDC when pressures are high..normally, as you say, cylinder pressures are designed to be highest just after TDC.. But, during the knock,this is not the rule. with a pre-ignition , it usually happens before TDC and this causes damage due to the high , random pressure waves due to the explosion. with a knock, it can happen when pressures become high enough to increase fuel/air temps to ignite, this happens when the flame front is moving and pressure increases. when the right temp for ignition occurs, detonation can happen. this can be before or after TDC. when before, it tends to create greater forces on the moving components.
Pre-ignition doesn’t necessarily cause knock in low power density engines and/or that use very high octane fuel. Instead, it acts just like a second spark plug that fires way too early. Combustion can be entirely normal, other than timing and second firing location. Even if pre-ignition doesn’t cause knock, it can and does melt pistons.
Knock causes high frequency pressure oscillations in the cylinder. Charge also burns much faster in knock cycles, but usually this faster burn only happens in the tail end of the combustion process. Consequently, knock usually hurts small parts like spark plugs and ring land edges. It doesn’t heat up the piston meaningfully on its own.
08-10-2018, 05:21 AM
#437
Rennlist Member
Thread Starter
Pre-ignition causes significant heating up of piston, while spark knock does not. This is comprehensively tested, so I’d classify it as a fact.
Pre-ignition doesn’t necessarily cause knock in low power density engines and/or that use very high octane fuel. Instead, it acts just like a second spark plug that fires way too early. Combustion can be entirely normal, other than timing and second firing location. Even if pre-ignition doesn’t cause knock, it can and does melt pistons.
Knock causes high frequency pressure oscillations in the cylinder. Charge also burns much faster in knock cycles, but usually this faster burn only happens in the tail end of the combustion process. Consequently, knock usually hurts small parts like spark plugs and ring land edges. It doesn’t heat up the piston meaningfully on its own.
08-10-2018, 07:29 AM
#438
Rennlist Member
The very point I have been trying to establish in this thread is that by inference the pistons [with coked oil on the underside] have been heating up to somewhere in the region of 350C [or more?] and extreme [i.e. destructive] knock has been taking place.
As Tuomo correctly says, pre-ignition does not mean knock will take place and knock does not mean pre-ignition has taken place. Every failure has a reason and this engine did not fail because Greg built an engine with an incorrect rod offset as clearly demonstrated by the rod failure point- hopefully that concept has been well and truly binned.
A pre-ignition event does not burn anything like a conventional ignition. First of all, by definition it ignites before the correctly timed ignition. Secondly the point of initiation is probably anywhere but where it should be so the combustion dynamics are probably screwed up relative to design. Thirdly in all probability, if and when the remaining mixture detonates there could be more of it and if so, potential for a more intense "bang". Add to that the very point I made about such detonations happening earlier in the cycle wherein the energy drives even more temperature increase on top of the pre-ignition increase and it is not too difficult to visualise how a destructive chain of events comes about.
Interesting to note how a conventional gasoline piston has a temperature in the region of 280C and a diesel has a peak piston temperature in the region of 360C- compare that to my estimate of the piston temperature based on that required to coke the lube oil- coincidence?
The other point to consider is that not all knocks are the same. Induce severe or heavy knocks, superimpose such on high metal temperatures that significantly reduce the UTS and kaboom!
Remember this motor was not a standard 928 rather it was a one off custom design rated for 110 octane racing fuel. Considering the above, is it a reasonable premise that a fuel of unknown [10 points lower?] octane and nothing else caused this chain of events?
As Tuomo correctly says, pre-ignition does not mean knock will take place and knock does not mean pre-ignition has taken place. Every failure has a reason and this engine did not fail because Greg built an engine with an incorrect rod offset as clearly demonstrated by the rod failure point- hopefully that concept has been well and truly binned.
A pre-ignition event does not burn anything like a conventional ignition. First of all, by definition it ignites before the correctly timed ignition. Secondly the point of initiation is probably anywhere but where it should be so the combustion dynamics are probably screwed up relative to design. Thirdly in all probability, if and when the remaining mixture detonates there could be more of it and if so, potential for a more intense "bang". Add to that the very point I made about such detonations happening earlier in the cycle wherein the energy drives even more temperature increase on top of the pre-ignition increase and it is not too difficult to visualise how a destructive chain of events comes about.
Interesting to note how a conventional gasoline piston has a temperature in the region of 280C and a diesel has a peak piston temperature in the region of 360C- compare that to my estimate of the piston temperature based on that required to coke the lube oil- coincidence?
The other point to consider is that not all knocks are the same. Induce severe or heavy knocks, superimpose such on high metal temperatures that significantly reduce the UTS and kaboom!
Remember this motor was not a standard 928 rather it was a one off custom design rated for 110 octane racing fuel. Considering the above, is it a reasonable premise that a fuel of unknown [10 points lower?] octane and nothing else caused this chain of events?
08-18-2018, 03:14 PM
#440
Rennlist Member
Thread Starter
so greg, did i mis-read your post, but you are considering the rod as a potential issue or factor in the failure? or should it have survived the abuse that was put on the pistons, acting on the rod?
thanks for the update. Very interested in seeing as much as you can provide on this postmortem
thanks for the update. Very interested in seeing as much as you can provide on this postmortem
08-23-2018, 02:42 PM
#441
Rennlist Member
Thread Starter
At this point, all my efforts on this engine are purely a learning opprotunity.
In this engine failure, there's no doubt that severe knocking overheated, cracked, and distorted the pistons, causing complete engine failure. And since the only thing that was changed since the engine was successfully ran was a radical change in the octane of the fuel, we know the direct cause of the failure.
That could, certainly, be the end of the story.
However, I'm a student of these engines...always questioning, always learnimg, always making subtle changes. It is sometimes difficult to tell much about the limits of the parts inside an engine, from a running, perfectly functional engine. Failed engine parts provide me with way more information.That means I'm going to look at and inspect each part...especially anything with a visible flaw!
I run fairly light custom made piston pins in these engines, and it would not surprise me if one of these pins failed from the excessive loads that this engine was subjected to. It does surprise me to find a cracked rod I expect/want the rod to be stronger than the piston or the pin.
Should the connecting rod have the damage it does from this knocking, or was there a flaw in the metal that the rod was built from....or some other problem?
If there was a problem, can this be avoided in the future by some sort of a change?
While I din't have the ability to answer these questions, the scientists and engineers at Carrillo do. And these people are very realistic. They know that even very high quality metal can have flaws. They know that metal parts in an engine can and do fail. If it was a flaw in the forging, they will tell me that. If there is something in the design that was weak, they will tell me and that will be changed.
In this engine failure, there's no doubt that severe knocking overheated, cracked, and distorted the pistons, causing complete engine failure. And since the only thing that was changed since the engine was successfully ran was a radical change in the octane of the fuel, we know the direct cause of the failure.
That could, certainly, be the end of the story.
However, I'm a student of these engines...always questioning, always learnimg, always making subtle changes. It is sometimes difficult to tell much about the limits of the parts inside an engine, from a running, perfectly functional engine. Failed engine parts provide me with way more information.That means I'm going to look at and inspect each part...especially anything with a visible flaw!
I run fairly light custom made piston pins in these engines, and it would not surprise me if one of these pins failed from the excessive loads that this engine was subjected to. It does surprise me to find a cracked rod I expect/want the rod to be stronger than the piston or the pin.
Should the connecting rod have the damage it does from this knocking, or was there a flaw in the metal that the rod was built from....or some other problem?
If there was a problem, can this be avoided in the future by some sort of a change?
While I din't have the ability to answer these questions, the scientists and engineers at Carrillo do. And these people are very realistic. They know that even very high quality metal can have flaws. They know that metal parts in an engine can and do fail. If it was a flaw in the forging, they will tell me that. If there is something in the design that was weak, they will tell me and that will be changed.
so, was it pre-ignition or knocking that caused the damage.........since you talk about heat being the major contributing issue, it would have to be pre-ignition if we are to believe what has been posted on this thread. and, do you think the difference between 110 leaded and the mix of 110 and 91 octane could cause such a major knock and /or pre ignition? and since mark also ran this mixture for 10 years prior with no issues, why would it be an issue now? could the tune be more on the edge ? those pistons were all cracked. that's some huge force or intense heat (or both). rod's splitting top down the middle top down, is also pretty telling of some very unusual forces.
Interested to see what else you find so we all can learn from this bad situation.
08-23-2018, 09:23 PM
#442
Rennlist
Basic Site Sponsor
Basic Site Sponsor
Greg, does your engine management system allow you to retrieve the engine map/tune from the ECU? As you know, we use the Electromotive systems, and one point of contention that I have with them is that they do not. They have explained to me that the design was laid out for a spec racing series and they were told to design it specifically so that once the tune had been uploaded into the ECU it could not be read or reteived. Always sounded like BS to me, but I like their stuff and it works well for us - so I have put up with this one issue.
Just wondering whether you can actually "read" the tune that they were running from the ECU's.
Just wondering whether you can actually "read" the tune that they were running from the ECU's.
Both of these engines used stock ECU's with Sharktuning hardware and software. The programming can be easily read.
__________________
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
08-24-2018, 01:34 AM
#443
Rennlist Member
Thread Starter