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I had been working on a page about abnormal combustion, when I ran across an excellent article on that subject in CONTACT! Magazine.  The article was originally published in the January-February 2000 (Volume 10 Number 1) edition of CONTACT!, and was recently re-published in the EAA Experimenter.

The original author, Allen W. Cline, is an accomplished powertrain engineer, currently working at Chrysler on various state-of-the-art engines. Previously, he was a key powertrain engineer at GM, and played a major role in the design and development of the groundbreaking (for GM) Northstar DOHC, 4-Valve V8 engine.

The presentation in Mr. Cline's article is based on such a wealth of real-world experience, and is so much better than what I had put together, that I contacted both Mr. Cline and Mr. Pat Panzera (publisher of CONTACT! ) to ask for permission to put it on my website.

They both very kindly granted me permission to use the article on my website.  So, with their permission, I have taken small editorial liberties with the original, in an effort to clarify a few of the paragraphs, and to add a bit of information to the discussion.  However, I want to emphasize that the vast majority of the ideas and content in this article belong to Mr. Cline alone.

FOREWORD

In the original article by Mr. Cline, he uses the term "detonation" to describe the phenomenon that contemporary combustion experts call "auto-ignition".

Admittedly, the term "detonation" is in widespread use. However, in the pursuit of technical accuracy, I offer the following definitions.

The term "detonation" is defined as follows (from Wikipedia):

Detonation (from Latin detonare {infinitive form}, meaning 'to thunder down / forth') is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock wave propagating directly in front of it.

Detonations occur in both conventional solid and liquid explosives, and in reactive gasses, but at much lower velocities.

The velocity of detonation in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail (higher resolution).

From the writings of Dr. Andrew Randolph (a published { GM, SAE} and practical (NASCAR) expert on combustion and combustion analysis):

"Engines don’t ‘detonate’ !  'Detonation' requires either a phase change as part of the exothermic event or a diverging nozzle. Auto-ignition ahead of the flame front in SI (spark-ignition) engines is sonic, not supersonic. Therefore "detonation" is not the proper term. Either "auto-ignition" or "knock" are correct descriptors, knock referring to the audible sound emitted from sound waves traversing the combustion chamber at less than sonic velocity."

It is not my intention to nit-pick Mr. Cline's work nor to criticize it in any way. But in my meager attempts to provide technical accuracy, I have substituted the term "auto-ignition" for "detonation" throughout the following article.

INTRODUCTION

All high output engines are prone to destructive tendencies as a result of over boost, mis-fueling, mis-tuning and inadequate cooling. As the engine community pushes ever nearer to the limits of power output, they often learn that cylinder combustion processes can quickly gravitate to engine failure.

This article defines two types of engine failures:  (a) detonation and (b) pre-ignition. Those failures are insidious in nature because the root causes of those failures are often hard to recognize. This discussion is intended only as a primer about these abnormal combustion processes since whole books have been devoted to the subject.

First, let’s review normal combustion in an SI (spark-ignition) engine. In a “normal combustion” event, combustion of the fuel-air mixture begins at the spark plug and progresses across the combustion chamber, away from the plug toward the outer boundaries of the chamber.

This progression, often called the “flame front”, moves across the combustion chamber in an even, orderly 3D fashion, at a fairly constant rate of approximately 25 meters per second. The burn moves all the way across the chamber and quenches (cools) against the walls and the piston crown. Ideally, the burn SHOULD be complete, with no remaining fuel-air mixture. Unfortunately, that complete burn rarely happens.

This flame-front progression can be visualized by picturing a pebble dropped into a glass smooth pond, which produces symmetrical ripples that spread out across the surface.  Note that the mixture does not "explode"; it burns in an orderly fashion.

Engineers have been interested in the hidden details of the combustion process almost since the inception of the Otto-Cycle engine. There has been a great deal of recent progress in the field of combustion analysis, which is described in some detail on elsewhere on this site. It could be beneficial to read the Combustion Analysis page in conjunction with this article on abnormal combustion.

One of the key parameters that combustion engineers seek to quantify combustion is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at around 14 degrees after top dead center (in a naturally-aspirated engine. It will occur somewhat later in a boosted engine. Also, current combustion analysis research suggests that an earlier peak (5 to 8 degreees ATC) is actually more "optimal". {jk} ).

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 the chamber pressure peaks too soon or too late in the cycle, that cycle will not deliver the optimal amount of work.  Therefore, LPP should be 14 degrees ATDC for any (naturally-aspirated) engine.

I introduce LPP now to illustrate the idea that there is a characteristic pressure buildup (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 ATDC.

Our enlarged definition of normal combustion now says that the bum is initiated with the spark plug, a nice even burn moves across the chamber, peak pressure occurs at 14 ATDC, and combustion is complete before EVO.

Confusion and a lot of questions exist with regard to the subjects of auto-ignition and pre-ignition. auto-ignition is one phenomenon that is abnormal combustion. Pre-ignition is another, different phenomenon that is abnormal combustion. These two abnormal combustion phenomena are distinctly different and can induce distinctly different failure modes.

KEY DEFINITIONS

Auto-ignition

Auto-ignition ( sometimes referred to as "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. But at some point during the burn, most probably because of the combination of heat and pressure in the chamber ahead of the flame front, the end gas in the chamber spontaneously and instantaneously combusts (“explodes”). The key point here is that auto-ignition occurs after normal combustion has been initiated with the spark plug.

Pre-ignition

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. This abnormal combustion phenomenon is completely different from auto-ignition.

AUTO-IGNITION

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.

Auto-ignition 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 during a auto-ignition event, you would see a normal pressure rise produced by a normal burn. Then, suddenly, you would see a very sharp spike when the auto-ignition occurred. That spike always occurs after the spark plug fires.

The sharp spike in pressure creates an impact-like 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 auto-ignition, occurs at approximately 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of auto-ignition is commonly called "spark knock".

This noise changes only slightly between iron and aluminum. 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 auto-ignition is not necessarily destructive. Many engines run under light levels of auto-ignition, sometimes even moderate levels. Some engines can sustain very long periods of heavy auto-ignition 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 miles. auto-ignition is not an optimum situation but it is not a guaranteed instant failure.

The higher the specific output (HP per in³) of the engine, the greater is the sensitivity to auto-ignition. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of auto-ignition without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly (sometimes within SECONDS).

Auto-ignition causes three types of failure:

1. Mechanical damage (broken ring lands)
2. Abrasion (pitting of the piston crown)
3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)

The high impact nature of the pressure spike can cause fractures. It can break the spark plug electrodes, break the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of the heads of the valves - intake or exhaust.

The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a broken second-ring land, my immediate suspicion would be auto-ignition.

Another thing auto-ignition can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of damage if auto-ignition occurs. It is a Swiss-cheesy look on a microscopic level. The mechanical pounding of auto-ignition mechanically erodes material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because combustion begins at the plug, and the flame front travels across the chamber. But before it gets to the farthest reaches of the chamber, the end gas spontaneously combusts.

That's where you will most often see the effects of the auto-ignition. You might also see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to auto-ignition by the residual heat in the valve.

In a four valve engine with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. But one can still see auto-ignition by the exhaust valves because that area is usually the hottest part of the chamber. Where the end gas is going to be hottest is where the damage, if any, will occur.

Combustion temperatures exceed 1800 degrees. If you subjected an aluminum piston to that temperature, it would just melt. The reason it doesn't melt is because of thermal inertia and because there is a boundary layer of a few molecules thick next to the piston top. This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material.

That combination of actions normally protects the piston and chamber from absorbing a destructive amount of heat. However, because the pressure spike from auto-ignition is so severe and of such short duration, it can shock the boundary layer of gas that surrounds the piston, causing that boundary layer to break apart, which allows an abnormal amount of heat to transfer into those surfaces.

Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head, transferring additional heat from the combustion chamber into the cylinder head and into the coolant.

The coolant temperature rises faster than the cooling system can handle. The coolant temperature rises; the more the coolant temperature rises, the hotter the engine components beome, therefore the end gas will be hotter, and the more it wants to detonate. The more it detonates, the more it overheats. It's a snowball effect. That's why an overheating engine wants to detonate and that's why engine auto-ignition tends to cause overheating.

Many times you will see a piston that is scuffed at the "four corners". If you look at the bottom side of a full-round piston you see the piston pin boss. If you look across each pin boss, it is solid aluminum with relatively little flexibility. It expands directly into the cylinder wall. However, the skirt of a piston is relatively flexible. If it gets hot, it can deflect. The crown of the piston is actually slightly smaller in diameter than the skirt on purpose so it doesn't contact the cylinder walls. So if, because of auto-ignition, the piston rapidly soaks up a lot of heat, the piston crown expands and drives the piston structure into the cylinder wall, causing it to scuff in four places directly across each boss. It's another dead giveaway sign of auto-ignition. Many times auto-ignition damage is just limited to this level of damage.

Some engines, such as liquid cooled 2-stroke engines found in snowmobiles, watercraft and motorcycles, have a very common auto-ignition failure mode. What typically happens is that when auto-ignition occurs the piston expands excessively, scuffs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches away an area of the piston (and maybe the wall), and the engine just quits.

In the shop someone looks at the melted result and says, "pre-ignition damage". No, it's auto-ignition damage. auto-ignition caused the piston to scuff and this snowballed into loss of compression and hot gas escaping by the rings that caused the melting. Once again, auto-ignition is a source of confusion and it is very difficult, sometimes, to pin down what happened, but in terms of damage caused by auto-ignition, this is another typical sign.

While some of these examples may seem rather tedious I mention them because a "scuffed piston" is often blamed on other factors, and auto-ignition (the real cuase) is overlooked. A scuffed piston may be an indicator of a much more serious problem which may manifest itself the next time with more serious results.

In the same vein, an engine running at full throttle may be happy due to a rich WOT air/fuel ratio. Throttling back to part throttle may cause the mixture to become leaner, and auto-ignition may now occur. The piston overheats and scuffs, the engine fails but the postmortem doesn't consider auto-ignition because the failure didn't happen at WOT.

I want to reinforce the fact that the auto-ignition 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 the bore. You still have high pressure in the chamber because combustion is still in process. The pressure is pushing the piston down as it's supposed to, and superimposed on that you get a brief spike that rings the engine.

Causes

Auto-ignition is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, 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 auto-ignition. The octane rating of the fuel is really nothing magic. Octane is the ability to resist spontaneous ignition. It is determined empirically in a special test engine. To test a particular fuel formulation, you run the engine on that fuel and dynamically alter the compression ratio to determine the compression ratio at which the fuel detonates.  That result is then compared to what happens with the “standard” fuel (iso-octane). That's the octane rating of the fuel. A given fuel can have an inherently higher octane quality, or can contain a variety of additives that produce a higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance, auto-ignition could result and cause the types of failures previously discussed.

Production engines are optimized for the type or grade of fuel that the marketplace desires or offers. Engine designers use the term called MBT (Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. For example, let's pick a specific engine operating point, 4000 RPM, WOT, 98 kPa MAP. At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power.

Now our engine was initially designed for premium fuel and was calibrated for 20 degrees of spark advance. Suppose we put regular fuel in the engine and it spark knocks at 20 degrees? We back off the timing down to 10 degrees to get the auto-ignition to stop. It doesn't detonate any more, but with 10 degrees of spark retard, the engine is not optimized anymore. The engine now suffers about a 5-6 percent loss in torque output. That's an unacceptable situation. To optimize for regular fuel engine designers will lower the compression ratio to allow an increase in the spark advance to MBT. The result, typically, is only a 1-2 percent torque loss by lowering the compression. This is a better trade-off. Engine test data determines how much compression an engine can have and run at the optimum spark advance.

For emphasis, the design compression ratio is adjusted to maximize efficiency/power on the available fuel. Many times in the aftermarket the opposite occurs. A compression ratio is "picked" and the end user tries to find good enough fuel and/or retards the spark to live with the situation...or suffers engine damage due to auto-ignition.

Another thing you can do is increase the burn rate of the combustion chamber. That is why with modem engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to auto-ignition it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate.

If, however, you have a chamber design that burns very slowly, like a mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC. Because the optimum 14 degrees after top dead center (LPP) hasn't changed the chamber has far more opportunity to detonate as it is being acted upon by heat and pressure. If we have a fast burn chamber, with 15 degrees of spark advance, we've reduced our window for auto-ignition to occur considerably. It's a mechanical phenomenon. That's one of the goals of having a fast burn chamber because it is resistant to auto-ignition.

There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build-up, the air pump becomes more efficient. Pumping losses are minimized. In other words, as the piston moves towards top dead center the pressure, hence the temperature, of the fuel/air mixture increases. If you light the fire at 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If you light the spark 20 degrees before top dead center, it's only acting against it for 20. The engine becomes more mechanically efficient.

There are a lot of reasons for fast burn chambers but one nice thing about them is that they become more resistant to auto-ignition. A real world example is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1 compression ratio. It was a premium fuel engine. For the 2000 model year, we revised the combustion chamber, achieved faster bum. We designed it to operate on regular fuel and we only had to lower the compression ratio .3 to only 10:1 to make it work. Normally, on a given engine (if you didn't change the combustion chamber design) to go from premium to regular fuel, it will typically drop one point in compression ratio: With our example, you would expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1 in order to work on regular. Because of the faster burn chamber, we only had to drop to 10:1. That 10:1 figure is still a high compression ratio, with the attendant high mechanical efficiency, and yet we can operate it at optimum spark advance on regular fuel. That is one example of spark advance in terms of technology. A lot of that was achieved through computational fluid dynamics analysis of the combustion chamber to improve the swirl and tumble and the mixture motion in the chamber to enhance the bum rate.

Chamber Design

One of the characteristic chambers that people are familiar with is the Chrysler Hemi. The engine had a chamber that was like a half of a baseball (Hemispherical in nature and in nomenclature, too). The two valves were on either side of the chamber with the spark plug at the very top. The charge burned downward across the chamber. That approach worked fairly well in passenger car engines but racing versions of the Hemi had problems. Because the chamber was so big and the bores were so large, the chamber volume also was large; it was difficult to get the compression ratio high. Racers put a dome on the piston to increase the compression ratio. If you took that approach to the extreme to achieve a 13:1 or 14:1 compression ratio in the engine, the pistons had a very tall dome. The piston dome almost mimicked the shape of the head's combustion chamber with the piston at top dead center. One could call the remaining volume "the skin of the orange." When ignited the charge burned very slowly, like the ripples in a pond, covering the distance to the block cylinder wall. As a result of the chamber design, those engines required a tremendous amount of spark advance, about 40-45 degrees. With that much spark advance auto-ignition was a serious possibility if not fed high octane fuel. Hemis tended to be very sensitive to tuning. As often happened, one would keep advancing the spark, get more power and all of a sudden the engine would detonate. Because they were high output engines, turning at high RPM, things would happen suddenly.

Hemi racing engines would typically knock the ring land off, get blow by, torch the piston and fall apart. No one then understood why. We now know that the Hemi design is at the worst end of the spectrum for a combustion chamber. A nice compact chamber is best; that's why the four valve pent roof style chambers are so popular. The flatter the chamber, the smaller the closed volume of the chamber, the less dome you need in the piston. We can get inherently high compression ratios with a flat top piston with a very nice bum pattern right in the combustion chamber, with very short distances, with very good mixture motion - a very efficient chamber.

Look at a Northstar or most of the 4 valve type engines - all with flat top pistons, very compact combustion chambers, very narrow valve angles and there is no need for a dome that impedes the burn to raise the compression ratio to 10:1.

Auto-ignition Indicators

The best indication of auto-ignition is the pinging sound that cars, particularly old models, make at low speeds and under load. It is very difficult to hear the sound in well insulated luxury interiors of today's cars. The sounds made by an engine running straight pipes or by a turning propeller, for example, can easily mask the characteristic ping. The point is that you honestly don't know that auto-ignition is going on. In some cases, the engine may smoke but not as a rule. Broken piston ring lands are the most typical result of auto-ignition but are usually not spotted. If the engine has detonated visual signs like broken spark plug porcelains or broken ground electrodes are dead giveaways and call for further examination or engine disassembly.

It is also very difficult to sense auto-ignition while an engine is running in a remote and insulated dyno test cell. One technique seems almost elementary but, believe it or not, it is employed in some of the highest priced dyno cells in the world. We refer to it as the "Tin Ear". You might think of it as a simple stethoscope applied to the engine block.

We run an ordinary rubber hose from engine in the cell to the dyno operator room. To amplify the engine sounds we just stick the end of the hose through the bottom of a Styrofoam cup and listen in! It is common for ride-test engineers to use this method on development cars particularly if there is a suspicion out on the road borderline auto-ignition is occurring. Try it on your engine; you will be amazed at how well you can hear the different engine noises.

The other technique is a little more subtle but usable if one pays close attention to EGT (Exhaust Gas Temperature). auto-ignition will actually cause EGTs to drop. This behavior has fooled a lot of people because they will watch the EGT and think that it is in a low enough range to be safe, but the only reason it is low is because the engine is detonating.

The only way you know what is actually happening is to be very familiar with your specific engine EGT readings as calibrations and probe locations vary. If, for example, you normally run 1500 degrees at a given MAP setting and you suddenly see 1125 after picking up a fresh load of fuel, you should be alert to possible or incipient auto-ignition.

Any drop from normal EGT should be reason for concern. Unless you have a sophisticated combustion analysis system (beginning at several hundred thousand dollars), about the only ways to identify auto-ignition are listening with your ear without any augmentation, using the "Tin Ear" during the early test stage and watching the EGT very carefully. The good thing is, most engines will live with a fairly high level of auto-ignition for some period of time. It is not an instantaneous type failure.

PRE-IGNITION

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. Except for direct-injection engines, the fresh charge entering the combustion chamber on the intake stroke is A COMBUSTIBLE MIXTURE. The fresh charge has nearly filled the cylinder as the piston approaches BDC / Intake. At BDC, the piston next reverses direction and soon after, the intake valve closes and the piston begins to compress the charge.

Since the spark voltage requirements to light the charge increase in proportion to the in-cylinder pressure, it is much easier to ignite the fresh fuel/air mixture when the cylinder pressure is low - in the vicinity of IVC (Intake Valve Closing). 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.

Remember, when the spark plug ignites the mixture and a sharp pressure spike occurs after that, it is auto-ignition. 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 auto-ignition. 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, and when it occurs you only know about it after the fact. It causes a catastrophic failure very quickly because the heat and pressure are so intense.

An engine can live with auto-ignition occurring for considerable periods of time, relatively speaking. There are no engines that will live for any period of time when pre-ignition occurs. 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.

When people see broken ring lands they often mistakenly blame it on pre-ignition and overlook the hammering from auto-ignition that caused the problem.

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.

The typical pre-ignition indicator, of course, would be the hole in the piston. This occurs because, in trying to compress the already-burned mixture, the parts soak up a tremendous amount of heat very quickly.

 The only parts that survive are the ones that have a high thermal inertia, like the cylinder head or cylinder wall. Aluminum pistons have a low thermal inertia (they are relatively low mass, and aluminum soaks up heat very rapidly). The crown of the piston is relatively thin, so if it gets very hot, it can't reject the heat and the material weakens dramatically. At the same time, it has tremendous pressure loads against it. The result is a hole in the middle of the piston where the crown is weakest.

I want to emphasis that when most people think of pre-ignition they generally accept the fact that the charge was ignited before the spark plug fires. However, I believe they limit their thinking to 5-10 degrees before the spark plug fires. You have to really accept that the most likely point for pre-ignition to occur is around 160 degrees BTDC (depending on the IVC timing), some 140 degrees before the spark plug would have fired because that's the point (if there is a glowing ember in the chamber) when it's most likely to be ignited.

We are experiencing 140-160 degrees of bumed very hot gas being compressed, that would normally be relatively cool and at a relatively low temperature. A piston will only take a few revolutions of that distress before it fails. As for auto-ignition, it can get hammered on for seconds, minutes, or hours d epending on the output of the engine and the load, before any damage occurs. Pre-ignition damage is almost instantaneous.

When the piston crown temperature rises rapidly, there usually is not enough time for the heat to transfer down to the skirt and expand and cause it to scuff. It just melts the center right out of the piston.

The only way to control pre-ignition is to keep any pre-ignition sources at bay. Spark plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.

Passenger car engine designers face a dilemma. Spark plugs must cold start at -40 degrees F. (which calls for hot plugs that resist fouling) yet be capable of extended WOT operation (which calls for cold plugs and maximum heat transfer to the cylinder head).

Here is how spark plug effectiveness or "pre-ignition" testing is done at WOT. Plug tip / gap temperature is measured with a blocking diode, and a small battery, connected through a milliammeter, applies a voltage to the spark plug terminal. The secondary voltage cannot come backwards up the wire because the large blocking diode prevents it.

As the spark plug tip heats up, it tends to ionize the gap and small levels of current will flow from the battery as indicated by the milliammeter. The engine is run under load and the gauges are closely watched. Through experience, technicians learn what to expect from the gauges. Typically, very light activity, just a few milliamps of current, is observed across the spark plug gap. In instances where the spark plug tip / gap gets hot enough to act as an ignition source the current suddenly jumps off scale. When that happens, instant power reduction is necessary to avoid major engine damage.

Back in the 80s, running engines that made half a horsepower per cubic inch, we could artificially and safely induce pre-ignition by using too hot of a plug and leaning out the mixture. We could determine how close we were by watching the gauges and had plenty of time (seconds) to power down, before any damage occurred.

With contemporary engines (like the Northstar ) which make over 1 HP per cubic inch, at 6000 RPM, if the needles move from nominal, you just failed the engine. It's that quick. When you disassemble the engine, you'll find definite evidence of damage. It might be just melted spark plugs. But pre-ignition happens that quickly in high output engines. There is very little time to react.

If cold starts and plug fouling are not major concerns, then the answer it to run very cold spark plugs. A typical case of very cold plug application is a NASCAR type engine. Because the prime pre-ignition source is eliminated, engine tuners can lean out the mixture (some) for maximum fuel economy and add a lot of spark advance for power and even risk some levels of auto-ignition. Those plugs, however, are terrible for cold starting and emissions and they would foul up while you were idling around town. But for running at full throttle at 9500 RPM, they function fine. They eliminate a variable that could induce pre-ignition.

Engine developers run very cold spark plugs to avoid the risk of getting into pre-ignition during engine mapping of air / fuel and spark advance.  However, calibrating production engines requires that we have much hotter spark plugs for cold startability and fouling resistance. To avoid pre-ignition, we compensate by making sure the fuel/air calibration is rich enough to keep the spark plugs cool at high loads and at high temperatures, so that they don't induce pre-ignition.

Again, consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But after about 20 seconds of sustained load, the ECU enriches the ratio to about 10:1. That is done to keep the spark plugs cool, as well as the piston crowns cool. That richness is necessary if you are running under continuous WOT load. A slight penalty in horsepower and fuel economy is the result.

To get the maximum acceleration out of the engine, you can actually lean it out, but under sustained full load, it has to go back to rich. Higher specific output engines are much more sensitive to pre-ignition damage because they are turning more RPM, they are generating a lot more heat and they are burning more fuel. Plugs have a tendency to get hot at that high specific output and reaction time to damage is minimal.

A carburetor set up for a drag racer would never work on a NASCAR or stock car engine because it would overheat and cause pre-ignition. But on the drag strip for 8 or 10 seconds, pre-ignition never has time to occur, so dragsters can get away with it. Differences in tuning for those two different types of engine applications are dramatic. That's why a drag race engine would make a poor choice for an aircraft engine.

Muddy Water......

There is a situation called auto-ignition induced pre-ignition. I don't want to sound like double speak here but it does happen. Imagine an engine under heavy load starting to detonate. auto-ignition continues for a long period of time. The plug heats up because the pressure spikes break down the protective boundary layer of gas surrounding the electrodes. The plug temperature suddenly starts to elevate unnaturally, to the point where it becomes a glow plug and induces pre-ignition. When the engine fails, I categorize that result as "auto-ignition induced pre-ignition." There would not have been any danger of pre-ignition if the auto-ignition had not occurred. Damage attributed to both auto-ignition and pre-ignition would be evident.

Typically, that is what we see in passenger car engines. The engines will typically live for long periods of time under auto-ignition. In fact, we actually run a lot of piston tests where we run the engine at the torque peak, induce moderate levels of auto-ignition deliberately. Based on our resulting production design, the piston should pass those tests without any problem; the pistons should be robust enough to survive. If, however, under circumstances due to overheating or poor fuel, the spark plug tip overheats and induces pre-ignition, it's obviously not going to survive. If we see a failure, it probably is a auto-ignition induced pre-ignition situation.

I would urge any experimenter to be cautious using automotive based engines in other applications. In general, engines producing 0.5 HP/in3 (typical air-cooled aircraft engines) can be forgiving (for example, during leaning to peak EGT, etc.). But at 1.0 HP/in3 (very typical of many high performance automotive conversions) the window for calibration induced engine damage is much less forgiving. Start out rich, retarded and with cold plugs and watch the EGTs!

Hopefully this discussion will serve as a thought starter. I welcome any communication on this subject. Every application is unique so beware of blanket statements as many variables affect these processes.

FOOTNOTE:

During a discussion with Mr. Cline, he mentioned a case in which the heads from a 2000 Northstar were ported by someone seeking improved flow. Although they did achieve higher gross flow numbers, the swirl and tumble characteristics that the OEM intake ports imparted to the incoming charge were totally destroyed, and although there was more "flow", the engine made less power than before, and it was on the verge of rich-misfire at the AFR needed to keep the pistons from melting out of the engine.

Obviously mixture motion is importamt, combustion is tricky, and the OEM engineers actually DO know stuff.

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