What gas octane should I use in a 2007 FJR

[conwest]

I think we still have some confusion (at least I do) on pre-ignition. The word before is not well-defined in this discussion in my opinion.Before definition 1: Ignition will occur at time t0. Somewhere in the combustion chamber the air/fuel mixture auto ignites at a time before t0 (in the same cobustion cycle).

Before definition 2 Ignition occurs (from the plug as it should). At time t_P the flame from this ignition should reach point P in the combustion chamber, but before that can happen auto ignition occurs at P. In other words auto-ignition could occur in the combustion chamber at one point at the same time as ignition from the spark plug.

I have been using definition 2, but with the wording some of you are using it sounds like you are using definition 1.

...are you kidding me? For the pupose of brevity, preignition is when the fuel/air mixture "lights up" sooner than it should. Nothing more, nothing less. QUOTE from charisma.,:"Pre-ignition, however, is self-ignition before the spark event and can be disastrous to an engine by trying to compress an already expanding/exploding mixture. I doubt if octane can help much -- this severe condition? FWIW -- my $.02 (end of quote) . Higher octane fuel resists this from happening (once again, assuming all the other operating parameters are in proper spec) hence, knocking is quelled/eliminated. On another note: it`s the lack of succinctness on the part of some writers around here that confuse the **** out of people.The subjects of "octane" and "knocking" are simple. h34r: BTW...does anybody know what Jestal`s profession is? Just curious that`s all.... :assassin:

 

[v65]

I think we still have some confusion (at least I do) on pre-ignition. The word before is not well-defined in this discussion in my opinion.Before definition 1: Ignition will occur at time t0. Somewhere in the combustion chamber the air/fuel mixture auto ignites at a time before t0 (in the same cobustion cycle).

Before definition 2 Ignition occurs (from the plug as it should). At time t_P the flame from this ignition should reach point P in the combustion chamber, but before that can happen auto ignition occurs at P. In other words auto-ignition could occur in the combustion chamber at one point at the same time as ignition from the spark plug.

I have been using definition 2, but with the wording some of you are using it sounds like you are using definition 1.

...are you kidding me? For the pupose of brevity, preignition is when the fuel/air mixture "lights up" sooner than it should. Nothing more, nothing less. QUOTE from charisma.,:"Pre-ignition, however, is self-ignition before the spark event and can be disastrous to an engine by trying to compress an already expanding/exploding mixture. I doubt if octane can help much -- this severe condition? FWIW -- my $.02 (end of quote) . Higher octane fuel resists this from happening (once again, assuming all the other operating parameters are in proper spec) hence, knocking is quelled/eliminated. On another note: it`s the lack of succinctness on the part of some writers around here that confuse the **** out of people.The subjects of "octane" and "knocking" are simple. h34r: BTW...does anybody know what Jestal`s profession is? Just curious that`s all.... :assassin:

No I am not kidding you, as I see it the difference in the two definitions could make a huge difference. For example if pre-ignition happened to occur at a point near the electrode at a time near when ignition was supposed to occur, then what would the effect of the pre-ignition be? I would have to imagine that it would be no where near as devastating as pre-ignition at the edge of the piston (say at a point on a line perpendicular to the wrist pin so that force will try to rotate the piston in the cylinder) at the same time as ignition from the plug. Now you have a load on the piston that it was not designed for, and two ignitions that are going to meet somewhere to no good end.

So if pre-ignition is self-ignition before the spark event, what is self-ignition after or at the same time as the ignition event in another part of the cylinder? I would have called this detonation, but now we have the definition of detonation as an explosion that burns too fast, its not pre-ignition according to your definition, so what is it then?

BTW...does anybody know what Jestal`s profession is?

Jestal is an engineer with GM, he tests engines for a living. He is generally acknowledged around here as knowing his stuff from an academic and real world experience point of view. When I get a chance to learn something from him I do, that is why the questions. They were not meant for you, sorry if you found it upsetting. BTW, who are you?

EDIT: Forgot this:

can be disastrous to an engine by trying to compress an already expanding/exploding mixture

Better go change the timing on my vehicles, the advance is going to screw them up.

 

[Rogue]

BTW...does anybody know what Jestal`s profession is?

Jestal is an engineer with GM, he tests engines for a living. He is generally acknowledged around here as knowing his stuff from an academic and real world experience point of view. When I get a chance to learn something from him I do, that is why the questions. They were not meant for you, sorry if you found it upsetting. BTW, who are you?

IMO (But I think most on the forum will agree) Jestal really knows his sh*t, I know he is human and could be wrong at times but I will take his advise until he is proven wrong.

 

[conwest]

Quote: "BTW, who are you?

...check the profile.

 

[jestal]

now that my double secret probation suspension is over.....

conwest, a couple of things to keep in mind before proceding with the discussion....

We are talking about spark ignited gasoline engines here. I'm not sure what syntax around the term pre-ignition is being confused with something chemists or refinery engineers or others might use in their jargon, but, in the world of engine engineers pre-ignition means the mixture is ignited (somehow) before the spark ignited it normally. Read any text on the subject and the answer is the same...."Taylor and Taylor" has a good discussion of it in their engineering manuals. Pre-ignition, to engine engineers, does not mean that the mixture just spontaneously combusted nor does it refer to the end gas in the chamber auto-igniting due to heat and pressure. Pre-ignition refers to an abnormal combustion event that happens before the spark plug arcs and ignites the mixture normally.

Your earlier comments are confusing to me as to exactly what you are trying to prove....i.e...the guy with the "knock" that was fixed by changing to premium fuel". Understand that no one can hear pre-ignition. Pre-ignition is silent. If someone heard a "knock" then they heard detonation. Detonation happens AFTER the spark ignites the mixture normally and detonation makes noise. If a knock is heard and premium fuel eliminated it then that is EXACTLY what I was expressing in the earlier posts. Higher octane fuel resists detonation more and will prevent detonation in most ordinary cases thus eliminating the knocking noise. Higher octane fuel has no effect on pre-ignition, however. What you described in your example is detonation, not pre-ignition.

Detonation makes noise often described as knocking or pinging. It is the extremely high pressure spike "ringing" or exciting the walls of the combustion chamber. Much like a hammer striking the head. Pre-ignition has no such extreme pressure rise, thus it makes no noise. No one can ever hear pre-ignition. That is why pre-ignition is so insidious. The piston mysteriously has a hole in it suddenly with no noise and no warning. That is pre-ignition, not detonation, and increasing the fuel octane rating has no effect on pre-ignition itself. Pre-ignition does end up with very very high (abnormally high) pressures in the combustion chamber but the pressure occurs thru normal burn of the mixture, albeit ignited earlier than normal, while the piston is well before top dead center. So, the piston is trying to compress expanding, very hot gases apres combustion has started which causes the damage. But no sudden, extremely sharp spike in pressure, like with detonation, to cause the ringing noise.

For making best power/efficiency the fuel/air mixture is almost always ignited before top dead center while the piston is moving upward, still, toward top dead center. As an example, an engine at full throttle and 6000 RPM might need 25 to 35 degrees of "spark advance" to make best power.....that means the spark is ignited 25 to 35 degrees before the piston reaches top dead center. This is necessary because an operating spark igntion engine is always a compromise between the ideal gas laws and mechanical necessity. The piston is going to move up and down at a rate prescribed by the geometry of the engine and cylinder. This rate will change with RPM. The mixture is going to burn and expand at a rate prescribed by the ideal gas laws. This could change with fuels, turbulence in the chamber, etc. Those two phenomenon do not necessarily overlap perfectly so compromise is necessary. If the mixture could "explode" or burn all at once then it might be fine to ignite it at TDC or thereabouts. It doesn't. It burns at a rate and thus takes time to finish the burn. The ideal thing is to find the conditions where the pressure in the chamber increases to the max at the prime moment to take maximum mechanical advantage acting on the piston/rod/crank. Thus spark advance is necessary.

The fuel is going to burn at a rate best described or quantified by time. The amount of "time" the piston is in a specific spot, however, is governed by the RPM of the engine further confusing the issue.

This has been studied for over a century and one thing has remained constant: location of peak pressure. The "location of peak pressure" in the chamber is always desired to be 14-15 degrees AFTER TOP DEAD CENTER. Depending on the burn rate of the fuel and the chamber and the speed and load of the engine the spark advance to obtain this is all over the map....but.....when it is right the location of peak pressure will always be at 14-15 degrees after top dead center.

If one were to look at the pressure in the chamber during a compression/power stroke one would see the pressure start to rise dramatically just before top dead center, increase dramatically until about 14 degrees after top dead center and then drop significantly as the piston moves down the bore. If detonation occurs, one would see a significant, sudden spike in the pressure imposed on the normal pressure curve. If pre-ignition had occured, one would see the pressure rise dramatically in the chamber WELL before top dead center with the location of peak pressure occuring at or before top dead center.

BTW...I am not making any of this up. You can read any college text book on combustion and get the same info. It isn't my idea or opinion....just my summation of the facts.

More later????

 

[twowheelnut]

Hey, lookie! You're back! Atta boy J! Good to see you posting. :good:

 

[jestal]

A fuel injected engine won't diesel unless it's a diesel. Carbs can diesel because the vacuum is pulling fuel into the firing chamber.
Back to a simple answer. 87 or higher is ideal.

BTW Just because you didn't hear it knock, doesn't mean it didn't. It just didn't knock hard enough.

Generally true...but....to pick a small nit.....

I have seen cases of fuel injected engines dieseling when the fuel vapor from the evaporative emission canister purge was excessive. The canister purge system will introduce significant fuel vapor into the engine even with key off (depending on the purge system design) which can lead to auto-ignition of the mixture and dieseling. Rare, but it does and can happen, thus saying that fuel injected engines cannot diesel is not totally correct.

Well...at the risk of venturing onto hallowed ground here, I for one must take issue with the following "Jestalism": Increased octane ratings will NOT prevent preignition. Exuse me? This is a blatant contradiction of every fuel industry analysis I`ve ever read/written. Nevermind me...just ask the guy who had a "knock" condition (assuming all the other pertinent parameters are in spec), who was advised to burn higher octane fuel and then, engine knock had disappeared. Simply put, in most cases engine knock is caused by preignition. The cure: higher octane. But then, I thought that was common knowledge by now. What gives Jestal?!

See above....the knock was detonation, not pre-ignition. Fuel octane rating has nothing directly to do with pre-ignition, only detonation.

Engine knock is NOT caused by pre-ignition. Preignition is silent and cannot be heard. Detonation is heard.

 

[jestal]

I think we still have some confusion (at least I do) on pre-ignition. The word before is not well-defined in this discussion in my opinion.Before definition 1: Ignition will occur at time t0. Somewhere in the combustion chamber the air/fuel mixture auto ignites at a time before t0 (in the same cobustion cycle).

Before definition 2 Ignition occurs (from the plug as it should). At time t_P the flame from this ignition should reach point P in the combustion chamber, but before that can happen auto ignition occurs at P. In other words auto-ignition could occur in the combustion chamber at one point at the same time as ignition from the spark plug.

I have been using definition 2, but with the wording some of you are using it sounds like you are using definition 1.

...are you kidding me? For the pupose of brevity, preignition is when the fuel/air mixture "lights up" sooner than it should. Nothing more, nothing less. QUOTE from charisma.,:"Pre-ignition, however, is self-ignition before the spark event and can be disastrous to an engine by trying to compress an already expanding/exploding mixture. I doubt if octane can help much -- this severe condition? FWIW -- my $.02 (end of quote) . Higher octane fuel resists this from happening (once again, assuming all the other operating parameters are in proper spec) hence, knocking is quelled/eliminated. On another note: it`s the lack of succinctness on the part of some writers around here that confuse the **** out of people.The subjects of "octane" and "knocking" are simple. h34r: BTW...does anybody know what Jestal`s profession is? Just curious that`s all.... :assassin:

If the subjects of knocking and octane are so simple then why is there so much misunderstanding of the facts???? Because......It isn't simple. Whole books have been written on the subject.

Typically, pre-ignition is initiated by some ignition source, most often the glowing plug tip or center electrode. The industry standard tests for determining the onset of pre-ignition measure the ionization across the spark plug center electrode to the ground electrode. It is possible to cause pre-ignition from other sources but, by far and away, the most common is the spark plug center electrode....i.e....too hot of a spark plug for the conditions.

Pre-ignition occurs well before top dead center in the compression stroke. At that point the fuel octane rating is hardly being stressed.....so why would the mixtute "spontaneously combust" due to too low of an octane rating then? Answer: it doesn't. Pre-ignition occurs because something lights the mixture and increasing octane rating will not prevent this.

Lack of succinctness???? :lol: Don't blame lack of reading comphrehension on someone else.

Hey, lookie! You're back! Atta boy J! Good to see you posting. :good:

I figured it would annoy the admins more if I kept posting than if I just left. This way they have to read everything I post to keep me in line. Otherwise they would not have anything to do. :lol:

 

[otrfjr]

now that my double secret probation suspension is over.....

Glad to see you back, Dr. J

 

[warsw]

now that my double secret probation suspension is over.....

conwest, a couple of things to keep in mind before proceding with the discussion....

We are talking about spark ignited gasoline engines here. I'm not sure what syntax around the term pre-ignition is being confused with something chemists or refinery engineers or others might use in their jargon, but, in the world of engine engineers pre-ignition means the mixture is ignited (somehow) before the spark ignited it normally. Read any text on the subject and the answer is the same...."Taylor and Taylor" has a good discussion of it in their engineering manuals. Pre-ignition, to engine engineers, does not mean that the mixture just spontaneously combusted nor does it refer to the end gas in the chamber auto-igniting due to heat and pressure. Pre-ignition refers to an abnormal combustion event that happens before the spark plug arcs and ignites the mixture normally.

Your earlier comments are confusing to me as to exactly what you are trying to prove....i.e...the guy with the "knock" that was fixed by changing to premium fuel". Understand that no one can hear pre-ignition. Pre-ignition is silent. If someone heard a "knock" then they heard detonation. Detonation happens AFTER the spark ignites the mixture normally and detonation makes noise. If a knock is heard and premium fuel eliminated it then that is EXACTLY what I was expressing in the earlier posts. Higher octane fuel resists detonation more and will prevent detonation in most ordinary cases thus eliminating the knocking noise. Higher octane fuel has no effect on pre-ignition, however. What you described in your example is detonation, not pre-ignition.

Detonation makes noise often described as knocking or pinging. It is the extremely high pressure spike "ringing" or exciting the walls of the combustion chamber. Much like a hammer striking the head. Pre-ignition has no such extreme pressure rise, thus it makes no noise. No one can ever hear pre-ignition. That is why pre-ignition is so insidious. The piston mysteriously has a hole in it suddenly with no noise and no warning. That is pre-ignition, not detonation, and increasing the fuel octane rating has no effect on pre-ignition itself. Pre-ignition does end up with very very high (abnormally high) pressures in the combustion chamber but the pressure occurs thru normal burn of the mixture, albeit ignited earlier than normal, while the piston is well before top dead center. So, the piston is trying to compress expanding, very hot gases apres combustion has started which causes the damage. But no sudden, extremely sharp spike in pressure, like with detonation, to cause the ringing noise.

For making best power/efficiency the fuel/air mixture is almost always ignited before top dead center while the piston is moving upward, still, toward top dead center. As an example, an engine at full throttle and 6000 RPM might need 25 to 35 degrees of "spark advance" to make best power.....that means the spark is ignited 25 to 35 degrees before the piston reaches top dead center. This is necessary because an operating spark igntion engine is always a compromise between the ideal gas laws and mechanical necessity. The piston is going to move up and down at a rate prescribed by the geometry of the engine and cylinder. This rate will change with RPM. The mixture is going to burn and expand at a rate prescribed by the ideal gas laws. This could change with fuels, turbulence in the chamber, etc. Those two phenomenon do not necessarily overlap perfectly so compromise is necessary. If the mixture could "explode" or burn all at once then it might be fine to ignite it at TDC or thereabouts. It doesn't. It burns at a rate and thus takes time to finish the burn. The ideal thing is to find the conditions where the pressure in the chamber increases to the max at the prime moment to take maximum mechanical advantage acting on the piston/rod/crank. Thus spark advance is necessary.

The fuel is going to burn at a rate best described or quantified by time. The amount of "time" the piston is in a specific spot, however, is governed by the RPM of the engine further confusing the issue.

This has been studied for over a century and one thing has remained constant: location of peak pressure. The "location of peak pressure" in the chamber is always desired to be 14-15 degrees AFTER TOP DEAD CENTER. Depending on the burn rate of the fuel and the chamber and the speed and load of the engine the spark advance to obtain this is all over the map....but.....when it is right the location of peak pressure will always be at 14-15 degrees after top dead center.

If one were to look at the pressure in the chamber during a compression/power stroke one would see the pressure start to rise dramatically just before top dead center, increase dramatically until about 14 degrees after top dead center and then drop significantly as the piston moves down the bore. If detonation occurs, one would see a significant, sudden spike in the pressure imposed on the normal pressure curve. If pre-ignition had occured, one would see the pressure rise dramatically in the chamber WELL before top dead center with the location of peak pressure occuring at or before top dead center.

BTW...I am not making any of this up. You can read any college text book on combustion and get the same info. It isn't my idea or opinion....just my summation of the facts.

More later????

Jestal, Glad to see you back "really" :good: . I was beginning to think that I was going to lose my favorite person to argue with, I mean debate with :lol: .

I hate to admit it but this is a subject that I know very little about but find very interesting. I kind of have a feel for what Pre-ignition is and what can cause it to happen but I'm not sure I completely understand what detonation is and what causes it. I know that it seems to happen more often when the motor is hot, the timing is advanced too much or if you run too low of octane gas when the motor wants something with more. Why is this and what is really happening?

Thanks

 

[sprint_st]

...are you kidding me? For the pupose of brevity, preignition is when the fuel/air mixture "lights up" sooner than it should. Nothing more, nothing less. QUOTE from charisma.,:"Pre-ignition, however, is self-ignition before the spark event and can be disastrous to an engine by trying to compress an already expanding/exploding mixture. I doubt if octane can help much -- this severe condition? FWIW -- my $.02 (end of quote) . Higher octane fuel resists this from happening (once again, assuming all the other operating parameters are in proper spec) hence, knocking is quelled/eliminated. On another note: it`s the lack of succinctness on the part of some writers around here that confuse the **** out of people.The subjects of "octane" and "knocking" are simple. h34r: BTW...does anybody know what Jestal`s profession is? Just curious that`s all.... :assassin:

Just to clarify some things, PRE IGNITION can be taken quite literally, before ignition. Normal ignition is caused by a spark plug (the igniter if all is going right) that fires well in advance of Top Dead Center. My FJR is 5 degrees of rotation before TDC but I have seen older cars with much more. Something hotter than it should be in the combustion chamber causes the mixture to fire by itself, and it AIN'T compression like a diesel. Pre-ignition can be caused by the wrong heat range of plugs, usually by a long shot, or the completely wrong plug. It also can mean crap built up on the valves or combustion chamber. Maybe pissing on a forest fire will help but it won't put it out. Same for going up in octane, that may help but it isn't addressing the real problem.

Oh yea, my job, I was William T. Sherman's point man, retired.

 

[jestal]

Preignition is most likely to happen when the engine is under full load at high RPM. That condition will put the most heat into the spark plug, overheating the center electrode and causing it to initiate combustion before the spark plug arcs. Under those conditions most engines will be running at least 25 or 30 degrees of spark advance before top dead center so there is no reason to suspect the mixture would auto-ignite at that point. The mixture is still relatively cool and not totally compressed. Not even close to conditions to cause auto-ignition so raising octane ratings of the fuel will not help in the least to prevent preignition. If preignition occurs it would have to happen well before the normal 25 to 30 degrees of spark advance to cause any damage. If the spark is firing at 30 degrees then preignition initiating at 45 degrees BTDC is not likely to cause problems since it is so close to the normal spark advance. I think what happens in most cases is that if there is a glowing spark plug tip or glowing carbon deposit (possible but unlikely) it lights the mixture when the piston is near bottom dead center just starting the compression stroke WELL before compression builds up. This is actually when the mixture is very easy to ignite so if there is an incidental ignition source it is easy to picture preignition happening when the piston is 180 to 90 degrees BTDC and on the way up increasing compression. Ignition from any source at that point would cause terrific damage due to the large compression loads and heat being driven into the piston for a relatively long period of time.

warsw...thanks....I missed the discussions, too!!... Detonation is the spontaneous combustion of the end gas in the chamber. The spark plug ignites the mixture and it burns from the spark plug outward along an expanding flame front. It is a relatively orderly, progressive burn across the chamber, not a instantaneous complete explosion of the entire mixture. Given the orderly burn it is easy to perceive that the mixture at the very edges of the chamber, farthest from the spark plug is sitting there waiting for the flame front to get there, meanwhile getting heated and further compressed. Heat pours into the end gases from surrounding metal (combustion chamber and piston crown) and heat from compression raises the end gas temperature as the normal burn increases pressure in the chamber. The temperature of the mixture going into the engine could start the end gas at a higher temperature if the day is very hot. At some point the end gas in the chamber gets so hot it spontaneously combusts. Detonation. The pressure in the chamber is increasing steadily as the normal burn progresses and then spikes tremendously when the end gas suddenly auto-ignites. That sudden spike in presusre is just like a hammer blow hitting the walls of the chamber causing the ringing or pinging or knocking noise. The octane rating of the fuel is simply the fuel's ability to resist spontaneous combustion under those conditions. It could be because of lead added to the fuel, higher octane concentration in the fuel, additives such as alcohols or other oxygenates, etc. Lots of ways to increase the octane rating of the fuel to resist detonation. Spraying water into the mixture will delay the onset of detonation as the water molecules in the chamber soak up heat thus cooling the end gas to keep it from auto-igniting. That is why an engine will detonate easier on a dry day as opposed to a humid day. Thus, why one would want to consider premium when blasting across Death Valley in the summer when regular has worked fine for the precedeing cool part of the riding.

 

[jestal]

I've been involved with a number of pre-ignition tests to make sure that engines had the correct heat range of spark plug and to determine just how much "cushion" in the heat range rating of the recommended plug there was. I would say that in most all reasonable cases the spark plug center electrode ends up being the ignition source for preignition. The combustion chamber pressure and temperature at full load just drives too much heat into the spark plug tip and it cannot reject enough heat due to its heat range (or heat rejection) rating so it starts to glow, ionize the air gap between the plug tip and the ground electrode and, almost immediately, cause pre-ignition.

I have seen very rare cases when an engine with mega miles on it that had very heavy carbon deposits in the combustion chamber was put on a dyno and IMMEDIATELY run up to max RPM and full load cuasing significant pre-ignition damage to the engine even though the spark plug heat range was correct....i.e..plenty cold. Some of the excess carbon material in the chamber started to glow under load and was the ignition source. This is why one cannot say that the ONLY pre-ignition source is the plug tip......never say never. But, to make this sort of thing happen you would have to contrive the circumstances and really TRY to fail an engine this way. It is just not very likely at all to occur naturally. Even in a car or bike, if the engine WAS run slow/moderately enough to build that excess of carbon in the chambers you would be hard pressed to suddenly put the engine under full load and hold it there without some load transition to clean some of the carbon out. Plus it would not likely be hot and under immediate full load. If you tried you might be able to recreate this situation but I would not bet on it. Bet on the plug being too hot.

 

[warsw]

warsw...thanks....I missed the discussions, too!!... Detonation is the spontaneous combustion of the end gas in the chamber. The spark plug ignites the mixture and it burns from the spark plug outward along an expanding flame front. It is a relatively orderly, progressive burn across the chamber, not a instantaneous complete explosion of the entire mixture. Given the orderly burn it is easy to perceive that the mixture at the very edges of the chamber, farthest from the spark plug is sitting there waiting for the flame front to get there, meanwhile getting heated and further compressed. Heat pours into the end gases from surrounding metal (combustion chamber and piston crown) and heat from compression raises the end gas temperature as the normal burn increases pressure in the chamber. The temperature of the mixture going into the engine could start the end gas at a higher temperature if the day is very hot. At some point the end gas in the chamber gets so hot it spontaneously combusts. Detonation. The pressure in the chamber is increasing steadily as the normal burn progresses and then spikes tremendously when the end gas suddenly auto-ignites. That sudden spike in presusre is just like a hammer blow hitting the walls of the chamber causing the ringing or pinging or knocking noise. The octane rating of the fuel is simply the fuel's ability to resist spontaneous combustion under those conditions. It could be because of lead added to the fuel, higher octane concentration in the fuel, additives such as alcohols or other oxygenates, etc. Lots of ways to increase the octane rating of the fuel to resist detonation. Spraying water into the mixture will delay the onset of detonation as the water molecules in the chamber soak up heat thus cooling the end gas to keep it from auto-igniting. That is why an engine will detonate easier on a dry day as opposed to a humid day. Thus, why one would want to consider premium when blasting across Death Valley in the summer when regular has worked fine for the precedeing cool part of the riding.

Well explained. Thanks.

 

[jestal]

OK....here is the fly in the ointment and where the lines between detonation and pre-ignition and octane get really confused.

There is such a thing as detonation induced pre-ignition. Todate, the posts have described detonation and preignition as two separate occurances...and they most often are. BUT (you knew it was coming) suppose an engine was under heavy load, hot, high RPM and detonation started. The spontaneous combustion of the end gas in the chamber causes the characteristic high pressure spike. The pressure spike disrupts the boundary layer of gas that keeps the combustion chamber components isolated from the direct heat of the combustion. As a result the spark plug tip starts to soak up heat, to the point that the plug tip overheats (due to detonation) and initiates pre-ignition. The detonation and pre-ignition are still two separate abnormal combustion phenomenon but the detonation caused conditions leading to pre-ignition. In this case, increasing the octane rating of the fuel WOULD eliminate the chance of pre-ignition. Not directly, but because the increased octane rating would elminate or minimize the detonation that caused the plug to overheat and induce pre-ignition.

Ultimately, detonation induced pre-ignition is probably more common that most people might realize. Detonation itself is destructive but an engine will live much longer under heavy detonation compared to no time when pre-ignition occurs. Anytime detonation occurs the spark plug tip (along with everything in the chamber) will start to heat up. If their is inadequate "cushion" in the spark plug heat range it could overheat and then enduce pre-ignition. That is why cold plugs are always recommended when tuning changes are made, boost is changed, nitrous is introduced, etc....detonation is easy to incur under those conditions leading to the plug overheating and inducting preignition. The engine will live thru the detonation but then hole the piston about 3 nanoseconds after pre-ignition occurs.

Increasing the octane rating will not directly reduce the tendency for pre-ignition itself. Increasing the octane rating will decrease the likely hood of detonation that COULD progress into pre-ignition if it is heavy and continous and the plug is not cold enough.

So...conwest was right....but he didn't know it....or why......

There is also a phenomenon known as pre-ignition induced detonation. The idea being that the mixture pre-ignites just before the spark plug would have ignited it anyway. Not too soon as to cause any damage...but soon enough that the mixture burns ahead of schedule leading to detonation just like an overadvanced spark condition might. This is a theory. I have never seen anyone actually show that it happened. One of those "could happen" sorts of things. The pre-ignition would have to occur so close to the normal spark event that it is unlikely. If pre-ignition conditions existed the mixture would take off long before the normal spark. So....preigntion induced detonation makes a nice beer discussion but is of little practical relevence.

 

[jestal]

EDIT: Does this guy know what he is talking about?

Pretty good description and information. Certainly worth the read.

 

[wpbfjr]

Jestal is an engineer with GM, he tests engines for a living. He is generally acknowledged around here as knowing his stuff from an academic and real world experience point of view....

IMO (But I think most on the forum will agree) Jestal really knows his sh*t, I know he is human and could be wrong at times but I will take his advise until he is proven wrong.

+1

.... The spark plug ignites the mixture and it burns from the spark plug outward along an expanding flame front. It is a relatively orderly, progressive burn across the chamber, not a instantaneous complete explosion of the entire mixture.....

wahoo!!!

jestal said "flame front"....

flame front is the take home point i got last big go-round bout fuel and all the finite details that make my brain numb...

thanks jestal!!!

dana

 

[C&C]

Would indexing the plugs have any beneficial effect to adding power or reducing potential to detonation? You often hear the engine tuners mention indexing the plugs to present the incoming fuel charge the most direct exposure to the spark. Also would the position of the electrode be better served by indexing to maximize the cooling off during the end of the power stroke and during the exhaust stroke?

 

[DN325Ci]

warsw...thanks....I missed the discussions, too!!... Detonation is the spontaneous combustion of the end gas in the chamber. The spark plug ignites the mixture and it burns from the spark plug outward along an expanding flame front. It is a relatively orderly, progressive burn across the chamber, not a instantaneous complete explosion of the entire mixture. Given the orderly burn it is easy to perceive that the mixture at the very edges of the chamber, farthest from the spark plug is sitting there waiting for the flame front to get there, meanwhile getting heated and further compressed. Heat pours into the end gases from surrounding metal (combustion chamber and piston crown) and heat from compression raises the end gas temperature as the normal burn increases pressure in the chamber. The temperature of the mixture going into the engine could start the end gas at a higher temperature if the day is very hot. At some point the end gas in the chamber gets so hot it spontaneously combusts. Detonation. The pressure in the chamber is increasing steadily as the normal burn progresses and then spikes tremendously when the end gas suddenly auto-ignites. That sudden spike in presusre is just like a hammer blow hitting the walls of the chamber causing the ringing or pinging or knocking noise. The octane rating of the fuel is simply the fuel's ability to resist spontaneous combustion under those conditions. It could be because of lead added to the fuel, higher octane concentration in the fuel, additives such as alcohols or other oxygenates, etc. Lots of ways to increase the octane rating of the fuel to resist detonation. Spraying water into the mixture will delay the onset of detonation as the water molecules in the chamber soak up heat thus cooling the end gas to keep it from auto-igniting. That is why an engine will detonate easier on a dry day as opposed to a humid day. Thus, why one would want to consider premium when blasting across Death Valley in the summer when regular has worked fine for the precedeing cool part of the riding.

Well explained. Thanks.

You guys are freaking me out with this love-in.

Don

 

[sprint_st]

Is premium gas purchased at Joe S. TheRagMan's discount gas and snake oil emporium really premium??? Then again how does foreign particulate level affect detonation? Where is RON + MON divided by 2 measured? Is it measured on every tank truck. What load was that truck carrying on the last run and was it fully drained? Are you sure that big gas tanker is REALLY pumping premium down that hole or is he filling all the tanks with the same stuff??? How can we be sure??? Maybe we should never venture away from gas stations we trust. Maybe, just maybe, we ought to just fill the tank and go for a ride.