Ok, I'm going to drag this subject back up to see if we can get more discussion. I really am curious. Let's hear it
Joe
Edit: I tried to copy & paste this but it came out as though I wrote the above article, which I did not. I brought it over from the 300 Six page.
Gentlemen,
I understand your angst about pinging engines and whilst my post was about the availability of Elf Oils in Perth I will offer something that may help to solve your problem of ‘pinging’. I agree that any uncontrolled detonation is destructive to engine components and needs to be eradicated, hence my effort to assist you. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject. Whilst generally the higher octane ratings equate to the reduction in the tendency for a given engine to ‘ping’, which you have explored, you have omitted one important avenue in the search for a solution ( as it seems you do not have the diagnostics available to you to explore the ECU programming / sensor malfunctions). Therefore I will suggest you explore spark plugs and heat ranges thereof to reach a solution to your problem.
DETONATION & PRE-IGNITION
( two are completely different and abnormal phenomenon.)
All high output engines are prone to destructive tendencies as a result of over boost, mis-fueling, mis-tuning and inadequate cooling. As engine makers push ever nearer to the limits of power output, they often come close to the margins where cylinder chamber combustion processes can quickly gravitate to engine failure. In a globalised, market mass manufacturing of these engines must attempt to address many variables in the local conditions so their products can adapt. Poor fuel quality, a range of driving conditions and unfortunately mass production tolerances are some factors. I will attempt define two types of engine failures, detonation and pre-ignition, that are as insidious in nature to users as they are hard to recognize and detect. There is NO repeat NO such thing as pre-detonation, only the mistaken slang that misnames pre-ignition and confuses the general layman.
First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion. This flame front is similar to dropping a pebble in a glass smooth pond with the ripples spreading out, the front should progress in an orderly fashion. The burn moves all the way across the chamber and, quenches against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. I repeat the mixture does not "explode" but burns in an orderly controlled fashion.
I introduce location of peak pressure (LPP) now to illustrate that there is a characteristic pressure build up (compression and combustion) and decay (piston downward movement and exhaust valve opening) during the combustion process that can be considered "normal" if it is smooth, controlled and its peak occurs at 14 degrees After Top Dead Centre. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.
SUMMARY
Our enlarged definition of normal combustion now says that the charge/bum is initiated with the spark plug, a nice even burn moves across the chamber, combustion is completed and peak pressure occurs at 14 ATDC. Pre-detonation does not exist.
WHAT GOES WRONG
Pre-Ignition ( not repeat not pre-detonation)
The definition of pre-ignition is the ignition of the fuel/air charge prior to the spark plug firing. Pre-ignition caused by some other ignition source such as an overheated spark plug tip, carbon deposits in the combustion chamber and, rarely, a burned exhaust valve; all act as a glow plug to ignite the charge.
Keep in mind the following sequence when analyzing pre-ignition. The charge enters the combustion chamber as the piston reaches BDC for intake; the piston next reverses direction and starts to compress the charge. Since the spark voltage requirements to light the charge increase in proportion with the amount of charge compression; almost anything can ignite the proper fuel/air mixture at BDC!! BDC or before is the easiest time to light that mixture. It becomes progressively more difficult as the pressure starts to build.
A glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. It is very conceivable that if you have something glowing, like a spark plug tip or a carbon ember, it could ignite the charge while the piston is very early in the compression stoke. The result is understandable; for the entire compression stroke, or a great portion of it, the engine is trying to compress a hot mass of expanded gas. That obviously puts tremendous load on the engine and adds tremendous heat into its parts. Substantial damage occurs very quickly. You can't hear it because there is no rapid pressure rise. This all occurs well before the spark plug fires. (Hence why I believe that you are not experiencing pre-ignition due to the sound that you have described)
Remember, the spark plug ignites the mixture and a sharp pressure spike occurs after that, when the detonation occurs. That's what you hear. With pre-ignition, the ignition of the charge happens far ahead of the spark plug firing, in my example, very, very far ahead of it when the compression stroke just starts. There is no very rapid pressure spike like with detonation. Instead, it is a tremendous amount of pressure which is present for a very long dwell time, i.e., the entire compression stroke. That's what puts such large loads on the parts. There is no sharp pressure spike to resonate the block and the head to cause any noise. So you never hear it, the engine just blows up! That's why pre-ignition is so insidious. It is hardly detectable before it occurs. When it occurs you only know about it after the fact. It causes a catastrophic failure very quickly because the heat and pressures are so intense. An engine can live with detonation occurring for considerable periods of time, relatively speaking. There are no engines that will live for any period of time when pre-ignition occurs. When people see broken ring lands they mistakenly blame it on pre-ignition and overlook the hammering from detonation that caused the problem. A hole in the middle of the piston, particularly a melted hole in the middle of a piston, is due to the extreme heat and pressure of pre-ignition. Other signs of pre-ignition are melted spark plugs showing splattered, melted, fused looking porcelain. Many times a "pre-ignited plug" will melt away the ground electrode. What's left will look all spattered and fuzzy looking. The center electrode will be melted and gone and its porcelain will be spattered and melted. This is a typical sign of incipient pre-ignition. The plug may be getting hot, melting and "getting ready" to act as a pre-ignition source. The plug can actually melt without pre-ignition occurring. However, the melted plug can cause pre-ignition the next time around.
SUMMARY
Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. Substantial damage occurs very quickly, you can't hear it except for the terminal event
Detonation
Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.
Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, and then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminium. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike. One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of kilometres. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output of the engine, the greater the sensitivity to detonation. Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. It's a snowball effect. That's why an overheating engine wants to detonate and that's why engine detonation tends to cause overheating.
Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, air/fuel mixture, fuel mixture, temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel. I want to reinforce the fact that the detonation pressure spike is very brief and that it occurs after the spark plug normally fires. In most cases that will be well after ATDC, when the piston is moving down pushing the piston like it's supposed to, and superimposed on that you get a brief spike that rings the engine.
SUMMARY
Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug. Damage occurs very slowly if at all, you can hear it as a ping.
Joe
Edit: I tried to copy & paste this but it came out as though I wrote the above article, which I did not. I brought it over from the 300 Six page.
