Pre-Ignition vs. Detonation 1

[JOJOMO]

The octane thread got me thinking about pre-ignition vs. detonation. My truck, a 93 toyota with the 22-re engine with 192k miles on it, 'pings' under what I would consider a higher load condition (in 5th gear climbing grades while trying to accellerate under mid to full throttle situations). I run mid-grade fuel and this seems to reduce the problem.

The question is how can I tell if it is pre-ignition problem, which I understand to be hot spots of carbon igniting the fuel prematurely, or, detonation, which I understand to be heat/pressure from the advancing flame front igniting the remaining af mix before the flame front reaches it causing high pressure peaks and unsmooth combustion?

Once I find this out, what are the best ways to alleviate this? Patprimmer suggested a water spray treatment into the throttle under a power run. Is that a WOT condition, or what? How much water? Is there a better way to clean the combustion chamber?

 

[btrueblood]

Pull the spark plugs and look for carbon deposits.

 

[thundair]

I have some pinging with mid grade in my Z06 and it is under low RPM while lugging around in the wrong gear.
I know there is no carbon build up but it has the need to use higher octane.

Something besides carbon build up is the timing advance being to high.

There was an article about free radicals and combustion but I can't find it right now.

This may shed some light:

http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

I don't know anything but the people that do.

 

[patprimmer]

Full power is WOT.

On a chassis dyno, I use a garden hose at a slow trickle.

If done while moving, I divert the windscreen washer bottle hose with no nozzle so it gets full washer pump output.

If it happened after a rebuild, it might be a sharp edge in the chamber or wrong reach spark plugs or a poorly fitted Helicoil in the spark plug hole.

You have summed up detonation and pre ignition correctly as I understand them.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Knap]

We should note that even cars designed to run on regular fuel might require higher octane as they age. Carbon buildup inside the cylinder can create hot spots that can initiate knock. So can malfunctioning exhaust-gas-recirculation systems that raise cylinder temperatures. Hot temperatures and exceptionally low humidity can increase an engine's octane requirements as well. High altitude reduces the demand for octane.

http://www.caranddriver.com/features/3604/regular-or-premium-page2.html

http://www.germanmotorcars.com/Detonation.htm

Apart frpm cleaning also friction reduction

http://www.bp.com/sectiongenericarticle.do?categoryId=9009092&contentId=7016863

 

[Knap]

7.7 What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than the same engine after 25,000 km. This Octane Requirement Increase (ORI) is due to the formation of a mixture of organic and inorganic deposits resulting from both the fuel and the lubricant. They reach an equilibrium amount because of flaking, however dramatic changes in driving styles can also result in dramatic changes of the equilibrium position. When the engine starts to burn more oil, the octane requirement can increase again. ORIs up to 12 are not uncommon, depending on driving style [17,19]. The deposits produce the ORI by several mechanisms:-

- they reduce the combustion chamber volume, effectively increasing the compression ratio. - they also reduce thermal conductivity, thus increasing the combustion chamber temperatures. - they catalyse undesirable pre-flame reactions that produce end gases with low autoignition temperatures.

I prefer the lsat explanation rather than hot spots.

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

 

[reidh]

Water is the best way to clean the combustion chamber without disassembling the engine.

I'm sure a garden hose/washer pump will work. We always use a two liter soda bottle filled with water. With one guy running the engine at WOT, we pour in enough water where the engine starts to cough, then back off pouring the water.

Doing this (or the hose, or the washer pump) will do wonders. Just be sure the engine is warm when you do it.

-Reidh

 

[RossABQ]

You might try some stuff called SeaFoam, sold alongside the Marvel Mystery oil. I'm always skeptical of snake oils but I have found it to be great for specific tasks.

When added to the oil about 100 miles ahead of an oil change, it will remove noticeable amouts of gum/varnish from oil control rings (the Saturn people turned me onto this, Saturns have a known problem with their oil rings pre-'99, one treatment has cut my oil consumption by 50% for the last 5,000 miles).

If you pull the spark plugs and pour a tablespoon into each cylinder and let soak/drain overnight, remove any left in the chamber by cranking with rags over the holes, then fire it up "briskly" (mucho smoke!) and drive aggressively for 15 minutes, it will burn off/flake off/get rid of noticeable amounts of build-up on the pistons. Adding it to the gas can accomplish the same thing in a much slower but less smoky fashion, and apparently also helps get rid of valve head deposits.

(I do not sell Sea Foam and have no interest in promoting it!)

 

[SMOKEY44211]

I have found that a properly functioning EGR will reduce the detonation threshold for a given fuel qulity.----Phil

 

[r2800]

DETONATION OCCURS AT LOWER ENGINE SPEEDS UNDER HIGH CYLINDER LOADS SUCH AS IN TOO HIGH A GEAR WITH A LOW ENGINE SPEED THE PRIME CAUSE IS SPONTANEOUS COMBUSTION OF THE END GAS.
PREIGNITION OCCURS AT HIGHER ENGINE SPEEDS AND IS DUE TO INCANDESCANT MATERIAL IN THE CYLINDER

 

[reidh]

r2800,

I respectfully disagree with your assessment of detonation vs. pre-ignition. Detonation can occur at any engine speed as can pre-ignition.

I have seen detonation only at high engine speed/load before, when the metal in the cylinder head didn't have sufficient time to cool between cycles. At high load/low speed, the cylinder head had more time to cool between cycles, and thus didn't detonate.


-Reidh

 

[romke]

what you describe (fifth gear, going uphill, with more then half throttle) sounds like low speed knock and is basically detonation. that means that locally there is sufficient heat generated to ignite the fuel air mixture. this can be cured with water injection - where the water is used to cool the air charge by vaporizing - the heat for that being taken out of the air. thus the overal temperature inside the combustion chamber is decreased to a level where no longer the conditions for autoignition are met. another possibility to "cure" it is downshifting. preignition is more linked to deposits in the combustion chamber that under higher loads tend to get "glowing hot" and therefore can act as a kind of sparkplug - unfortunately without the correct timing. depending on driving pattterns and fuel composition there can be more or less buildup of such deposits - additivated quality fuel with additives that both clean up the deposits and or bond with them to make them less prone to glowing and acting as a source of ignition can usually prevent this if used over a prolonged period.

 

[r2800]

WITHOUT RESORTING TO DEEP TECHNOLOGY AT THIS TIME MY ORIGINAL STATEMENT IS INDEED CORRECT, DETONATION IS OF A CHEMICAL NATURE WHICH HAS ITS ROOTS IN THE THERMAL INSTABILITY OF THE CHOSEN FUEL WHERE BOTH TEMPERATURE AND PRESSURE BRING ABOUT SPONTANEOUS COMBUSTION TYPICALLY IN EXCESS OF 1500 FEET PER SECOND.MIXTURE STRENGTH,TIMING AND COMPRESSION PRESSURE ALL HAVE THEIR PART TO PLAY IN THIS DESTRUCTIVE PROCESS.PREIGNITION ON THE OTHER HAND IS OF A MECHANICAL NATURE WHEREBY MATERIAL WITHIN THE COMBUSTION CHAMBER BECOMES INCANDESCENT AND CAUSES AUTO IGNITION.EACH CAN CAUSE THE OTHER TO BECOME APPARENT USUALLY HOWEVER IT IS MORE LIKELY FOR DETONATION TO LEAD TO PREIGNITION DUE TO ITS SCOURING ACTION AND METALIC DEPOSITS (IT CAN ALSO DECOKE AN ALREADY PREIGNITING COMBUSTION CHAMBER TOO)

 

[reidh]

Your original statement is correct, but incomplete as I had already stated. Your first post stated:

"DETONATION OCCURS AT LOWER ENGINE SPEEDS" is misleading because it can occur at any engine speed, which is the point of my previous post.

You also stated correctly in your second post

"MIXTURE STRENGTH,TIMING AND COMPRESSION PRESSURE ALL HAVE THEIR PART..."

So couldn't a high compression engine with poor timing detonate at a low engine speed?

My original post was trying to clear up the fact that detonation doesn't have to be speed dependent.

Reidh

 

[evelrod]

r2800
I see your a new member, sooo...I don't know what the excepted norm is in the UK, but on this side of the pond, ALL UPPER CASE COMMUNICATION INDICATES SHOUTING AND IS EXTREMELY DIFFICULT TO READ. Try to hold it to a minimum as we will all be able to 'get you point' a lot easier.

Thanks,

Rod

 

[r2800]

You are correct I am new and I am from the other side of the water and I won`t shout any more.besides I`m enjoying our discussion on combustion.We seem to be at odds,however, over the principle differences between detonation and preignition.I maintain my earlier position regarding detonation at high load/low speed condition as this creates conditions for endgas to spontaneously explode due to a slower flame front and hence more time for heat absorbtion into the endgas.Taking the case of an already detonating cylinder under a constant load it will be found that as the speed is increased, (all other conditions remaining),charge detonation will decrease due to the increased turbulence reducing heat exchange and the reduced time for exchange to occur.see next paragraph

 

[r2800]

Preignition is due to a heat source, other than the timed spark, causing autoignition and as such requires considerable heat energy such as is available at higher speeds and air/fuel quantities,hence it is of a higher speed nature.Now, Of course, the speeds and loads at which these differing phenomena occur can overlap and be changed by local ambient temperature and moisture conditions.
I still maintain my earlier position that detonation remains a high load/low speed condition and preignition is that of a higher load/higher speed regime.

 

[evelrod]

I agree that preign. is caused by something 'glowing' that shouldn't be, but in my experience with the very high CR engines that I am using (>14:1) I have found it happens at low to mid range light or heavy load (due in this case by my using the wrong head gasket). On the other hand, I find that on at least one occasion on a dyno run, we had a big problem crop up with destructive detonation under heavy load at 5000 to 5500 rpm (engine redline was 10,000). We corrected the problem with an ign. 'recurve' and 'rejet'(actually we had the wrong paperwork for this engine to start with). At least one of the first 'restrictor plate' Ford NASCAR ~17:1 CR engines that I watched (clandestinely, I might add)) on a Superflow had about a 1000+rpm area around midrange where the throttle had to be backed off until the engine got up into the 7000+rpm range due to heavy detonation.

Bottom line, I have seen preignition at low to mid range and detonation at mid to high range. I'm not saying that it 'cannot' occur at other points in the normal operating range, just that I have not seen it, except in diesel engines, of course...they seem to thrive on detonation.;-)

Rod

 

[patprimmer]

Detonation is most likely around peak VE all other factors being equal. Peak VE is around peak torque.

Not much heat transfers to the charge, most of the heat is generated in the gas by compression.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[crysta1c1ear]

Detonation is most likely around peak VE all other factors being equal.

But other factors aren't equal in my opinion. Firstly, let me say I'd like to replace the word detonation by knock in most of the previous postings.

Let's say that normal combustions has a flame front propagated by chemical reaction, and let's say that a faster mode of combustion is possible where the flame front is created by a mixture autoigniting due to increased temperature and pressure from the combustion that has already occured.

So let's say that in normal combustion the pressure wave front precedes the flame front, and in detonation the pressure wave front is the flame front.

Now, in an engine that gets closer and closer to its limits, what is going to happen? You might expect that there is a sudden transition from normal combustion to detonation. Thank goodness that isn't the case. The pressure from combustion is distributed by gases expanding. This is what causes the wave of pressure towards the edge of the cylinders. But when the expanding gas reaches the edge of the cylinders there is nowhere else for it to go and it bounces back.

Without having sufficient pressure for detonation the higher pressures achieved near the cylinder walls, where the pressure can no longer dissipate sideways, causes autoignition of gas near the cylinder wall, and a second flame front can start. So now there are two normal chemically propageted flame fronts, approaching each other, one say expanding outwards from a central spark plug, and another moving inwards from around the cylinder walls.

Let's call that knocking. Knocking occurs then, when conditions in the mixture are approaching the conditions required for actual detonation of the gas. Knocking then, is not actual detonation - thank goodness, otherwise it would be time to get a new engine.

Detonation is most likely around peak VE all other factors being equal. ?? I assume the train of thought here is that more air in means more pressure and autoignition of the mixture occurs more readily. We tend to think of pressure as something fundamental, due to it being easy to measure, but let's split it up: pressures can be higher when there is more gas physically present - a density thing; and when gases are hotter - a temperature thing. Higher volumetric efficiency is saying there are more gas molecules physically present in the cylinders. They aren't especially hotter; there's just more of them.
Now let's assume there exists an autoignition temperature. The higher volumetric efficiency then hasn't made the denser air particulary more prone to autoignition.

So what would make a mixture prone to autoignition and thus potentially make an engine more prone to knock?


Low Speed Knock
Well, the obvious way to get a gas closer to its autoignition temperture is to heat it. Let's say best BSFC is around 2000 RPM. Why is the engine less efficient at lower RPM? Heat is lost to the cylinder walls, as each cycle takes longer and the heat lost per cycle is therefore larger. A consequence of this is that the incoming fresh air in the mixture is heated more and the mixture is closer to its autoignition temperature. So more mixture heating time at low RPM causes low speed knock.


High Speed Knock
Let's say some engine redlines around 7500 RPM. What's stopping it just going faster and faster with more air and fuel giving more power until it breaks? Airflow. Suppose the size of the exhaust valve is restricting how much exhaust gas can actually get past it. Higher RPM doesn't increase flow and power as only so much gas can get past the valve. Some hot exhaust gas starts staying in the cylinder. Again, we can be heating the incoming mixture and creating conditions where end gases exceed their autoignition temperatures and knocking occurs.

Summary
There is nice good airflow around peak VE and knocking occurs when air in the cylinders is heating up for one reason (staying in there too long) or another (not getting out properly). I will now stick my neck out on the chopping board and say :
detonation is unlikely around peak VE all other factors being equal.