Talk:Engine knocking

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Until someone actually quantifies the impulse associated with the shock waves associated with the collision of flame fronts, I will continue to regard all such talk as a load of nonsense. The energy associated with those shock waves cannot possibly be anywhere near as great as the total amount of energy released by the combustion of the fuel. In fact, if you stop and think about it for a few minutes, you will likely find yourself doubting that the energy associated with these shock waves could be more than a few percent of the total energy released by the combustion. Besides that, what exactly does it mean to say that two flame fronts collide with each other? It would seem that they burn each other out. When two pressure waves collide, they each bounce off the other, but you don't end up with something that is greater than the sum of the parts. All in all, there is ample reason to regard all this talk about flame fronts colliding and shock waves, etc., as all just a load of nonsense. What makes a whole lot more sense, and what also happens to be inordinately simpler, is that if the temperature at any location within the combustion chamber reaches the necessary threshold, ignition will occur spontaneously at that point and will spread from there, with the net effect being that the combustible mixture throughout the combustion chamber burns in the manner of spontaneous combustion, in which case all of the energy is released in a very short period of time, i.e., the impulse is orders of magnitude greater than it is with normal combustion, where combustion proceeds in an orderly way from the spark plug and no part of the mixture ignites until that "flame front" reaches that point. It is thus MANIFEST that all this talk about colliding flame fronts and shock waves is not the least bit necessary to explin the phenomenon, in addition to it being nonsensical from the standpoint of a simple energy perspective as I already explained. All in all, I for one find all this talk about colliding flame fronts, shock waves, and the speed of sound to amount to a whole lot of specious garbage. As for pre-detonation vs. pre-ignition or whatever, that also has been turned into something far more complex than it actually is. If the metal reaches a temperature high enough to ignite the combustible mixture before the spark plug fires, it will ignite the combustible mixture before the spark plug fires, duh. If that happens, the absolute pressure and temperature early in the power stroke will be much greater than is intended, which will greatly increase the chance of spontaneous combustion. —Preceding unsigned comment added by Princesscheetah (talkcontribs) 21:45, 26 April 2008 (UTC)

Time to quote one of the experts in the field..

"Knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous ignition of a portion of the end-gas-the fuel, air, residual gas, mixture ahead of the propagating flame occurs. When this abnormal combustion process takes place, there is an extremely rapid release of much of the chemical energy in the end-gas, causing very high local pressures and the propagation of pressure waves of substantial amplitude across the combustion chamber. Surface ignition is ignition of the fuelair mixture by a hot spot on the combustion chamber walls such as an overheated valve or spark plug, or glowing combustion chamber deposit: i.e., by any means other than the normal spark discharge. It can occur before the occurrence of the spark (preignition) or after (postignition). Following surface ignition, a turbulent flame develops at each surface-ignition location and starts to propagate across the chamber in an analogous manner to what occurs with normal spark ignition. Because the spontaneous ignition phenomenon that causes knock is governed by the temperature and pressure history of the end gas, and therefore by the phasing and rate of development of the flame, various combinations of these two phenomena-surface ignition and knock- can occur."


"With the chamber geometry'typical' of most engines where the flame propagates toward the cylinder wall, the damage is confined to the thin crescent-shaped regIon on the opposite side of the chamber to the spark plug, where one expects the end-gas to be located. A shock wave propagates from the outer edge of this high-pressure end-gas region across the chamber at supersonic velocity, and an expansion wave propagates into the highpressure region toward the near wall. The presence of such a shock wave has been observed photographically. The shock wave and expansion wave reflect off the walls of the chamber, eventually producing standing waves. Usually these standing waves are due to transverse gas vibration and are of substantial amplitude." -J. Heywood-Internal Combustion Engine Fundamentals-

For a more complete explanation with photographs read the Heywood book and also read "The internal combustion engine"-C.F.Taylor, Pioneer in this area and "The high speed internal combustion engine-Ricardo.

If we can read whats been ACTUALLY discovered LONG AGO we wont have this endless folktales and old mechanics tales to deal with. Colliding flame fronts INDEED!?--=Motorhead (talk) 01:38, 27 April 2008 (UTC)

I deleted this para: "Knock is detonation within the cylinder. Uncontrolled combustion. It is not the same as pinging/pinking which is pre-ignition.". As far as I'm aware (25 years of experience with engines aware, that is), this is wrong - preignition and pinking ARE the same thing. It's also pinKing, not pinGing, though perhaps in some parts of the world it's called that? - I suppose the sound is a sort of ping GRAHAMUK 06:35, 3 Aug 2003 (UTC)

I clarified the difference between knocking and pre-ignition, and added some external links. In the USA at least, engine knocking is also called "pinging" (or just "knock"), but I've never heard it called "pinking". -- Arteitle 05:03, 17 Sep 2003 (UTC)

It's "pinking" in the UK. As my car used to suffer from it I'd describe the sound as a sort of tinkling sound.

Exile 16:08, 20 Jan 2005 (UTC)

Haha, "pinking." I'm surprised it's not called something like "twinkling" as in perhaps "my car was twinkling after I sucked on a fag" 23:07, 9 April 2007 (UTC)

what it is[edit]

Pre-ignition is the normal meaning of this, isn't it? Note that true detonation is a supersonic process. Many times people say that something detonates, but it doesn't really. Note that "supersonic" would refer to the compressed fuel-air mixture, so you can't call the result supersonic just for exceeding the speed of sound in sea-level air. AlbertCahalan 06:23, 7 Jun 2005 (UTC)

[Gibberish. Detonation is a sonic process, not supersonic - a reaction progated by a shock wave which moves at the local speed of sound. Someone please explain to me how a shock wave moves faster than the local speed of sound. Forget the speed of sound in air at sea level, unless you are detonating air at sea level. MikeD] —Preceding unsigned comment added by (talk) 19:30, 1 September 2009 (UTC)

Not true at all - detonation IS a supersonic process, it is actually defined as such! As for your question how a shock wave moves faster than the local speed of sound, there is the concept of a "chemical wave". These can be faster than the local speed of sound and this is exactly what happens in a detonating material.

--Felix Tritschler (talk) 19:05, 16 November 2013 (UTC)

Pre-ignition is the igniting of the fuel by a source other than the actual spark. It does not follow necessarily that detonation will occur because of it. Detonation refers to the creation of a high pressure wave that traverses the combustion chamber potentially causing damage. This pressure wave is akin to the wave of a sonic boom.--=Motorhead 04:42, 12 Jun 2005 (UTC)

It's not really detonation unless that pressure wave travels faster than the speed of sound in that compressed fuel-air mixture. Given the high compression and the density of fuel, I would expect the speed of sound to be relatively high. I'd really like to see some proof that detonation occurs, because it's a fairly wild claim and because people so often casually call things "detonation". AlbertCahalan 02:20, 14 Jun 2005 (UTC)

The term detonation, as applied to internal combustion engines, is not new. (Not something made up by a couple of mechanics over a case of beer hehe!) Since the first works of Sir Harry Ricardo (Knighted for his work in this very area) in 1922 to the present there have been numerous research papers written on it. Probably the most studied area of engine performance since detonation limits maximum performance. The term “detonation is used to describe the very high pressure shock waves traveling at supersonic (compared to immediately local conditions) velocities (which is the only way you can create a shock wave). These waves cause real life broken parts and are observable on pressure traces taken from real engines. Your own ears will tell you something nasty is going on in the engine. For further proof read:

“The internal combustion engine in theory and practice” Vol 2 ch.2 “Detonation”[1] by C.F. Taylor (The M.I.T. press) for an authoritative, detailed report from a founding researcher on the topic.

There are numerous other sources if you are interested.

Asside from the symantics of it all, the term "Detonation" is the defacto standard term used to decribe this phenomenon in the field of internal combustion engine engineering.

--=Motorhead 04:26, 15 Jun 2005 (UTC)

The statement that "detonation" is a standard term is highly debatable. Detonation has a very specific meaning in reacting flow, and engine knock does not fit under this description. I suggest that the application of the term "detonation" to engine knock is obsolete and should be removed to avoid confusion between knock auto-ignition processes and true detonation phonomena.

Also, the book by Taylor that is cited is quite dated, I think that this 1985 edition is a just a reprint of the 1961 edition. The current "Bible" of engine engineering is:

J.B. Heywood, Internal Combustion Engine Fundamentals, New York: McGraw Hill, 1988.

Another popular book is:

R. Stone, Introduction to Internal Combustion Engines, 3rd Ed., Warrendale, Pa: Society of Automotive Engineers, 1999.

-- 17:35, 10 August 2005 (UTC)

There is no "bible" in existence when it comes to engines. Saying Taylor is dated is a little like saying Newton is dated. Not to mention that Heywood refers extensively to Taylor in his book. The current Taylor edition is updated not just reprinted. The term detonation is used extensively in the body of literature on engines. Should Wikipedia take it upon itself to try to change the terminology? Not in my opinion.--=Motorhead 20:32, 11 August 2005 (UTC)

Just be aware that the term detonation is inaccurate when applied to knock. Detonation involves a shockwave propagation sustained by chemical heat release, which is not generally descriptive of engine knock processes. I will say that the term detonation is no longer used commonly in the technical literature (like SAE), but the term is still used more outside of the literature as a legacy. Should Wikipedia use incorrect terminology because of historical use? If you like.

Tongue-in-cheek,I will say that very few engineers pull Newton's Principia off the shelf to calculate a F=ma (I do of course, and only the original Latin for me!). Taylor is an excellent text, I have it on my shelf, along with Heywood, Stone, Furgueson, and Obert - another excellent text written at the same time as Taylor's first edition. Still, great advances in the scientific understanding of combustion and knock have come in the last 30 years, and these are more reflected in other texts.

-- 02:17, 12 August 2005 (UTC)

You write detonation involves a shockwave propagation sustained by chemical heat release, which is exactly what happens when the engine knocks. The fuel-air mixture detonates, explodes, at supersonic speeds, releasing the energy that can break rods and bearings. Alga 19:46, 2 May 2007 (UTC)

I believe the general description of "knock" refers to the sound: engines subject to "knock" make a lot of noise. Traditionally defined, "knock" is brought about by ignition of the fuel/air charge prior to TDC of the piston, however that happens. Whenever it happens, rapidly expanding, burning charge encounters a piston that is still moving toward TDC as a function of the momentum imparted by the crank and flywheel. This causes a confrontation between the rising piston/crank/flywheel assembly and the expanding charge wishing to drive the piston down; hence lots of things are driven into each other when they don't want to be, and noise and wear and damage result. Knocking, or pinging/pinking, can commonly happen in a spark-ignited crank piston engine when the timing is too far advanced, even though detonation does not occur.

"Detonation" essentially refers to things burning really fast, such that there is no flame front, or essentially no flame front. It happens in HCCI, and in "engine run-on" when ignition in a very hot crank-piston engine is turned off, especially when running on low-octane fuel. HCCI engines are often referred to as "knock engines" although they are actually not so, according to the definition above. That is, HCCI engines really do detonate, in that pretty much all of the micro-droplets of fuel in a homogeneous mixture combust nearly simultaneously, which is more than can be said for either SI or CIDI engines. But they do not "knock" unless this detonation happens prior to TDC. Actually, though, recall that knock as defined above really just means making a lot of noise. According to this definition, crank-piston HCCI engines also "knock," because, even when spontaneous ignition happens exactly at TDC (which it hardly ever does, by the way), the high peak pressures and energy release rates of HCCI tend to hammer the piston against connecting rod and crank, even when they are already moving downwards. They are simply not moving downward "fast enough," such that the piston is driven downward faster than it would "like" to be, thus again creating lots of wear and making lots of noise.

So get rid of the connecting rod and crank. Now, if the thing still detonates, which it will, on an HCCI model, as long as the charge is homogeneous and the compression pressure is sufficient to initiate spontaneous combustion, you have an engine running on detonation alone, without the phenomenon of "knock." I.e., run a double-acting free-piston HCCI engine and there is no such thing as knock, but lots of such thing as detonation. Run it as rich as you want-- as rich as stoich will let you. It doesn't make lots of noise, because there are no connecting rods and no crank to oppose the piston motion. It wants to move extremely rapidly in a direction opposite to that which caused the compression; OK, go for it. There's nothing stopping you. No noise, no knock. Total detonation. It has been built in the past, and it worked exactly as described. It is presently being built again, and it will once again work exactly as described. It's exactly the same thing as a pulse-detonation jet engine, only smarter and incomparably cheaper and more reliable. So there. No references; don't need 'em. References are for people who don't have confidence in what they are talking about. I am not one of those people. 04:30, 24 December 2005

Your ( statement...

"Traditionally defined, "knock" is brought about by ignition of the fuel/air charge prior to TDC of the piston, however that happens"

Not so , in all practical spark ignition engine applications, ignition is ALWAYS before TDC. This is required to allow sufficient time for combustion and as a rule of thumb half the combustion will be before TDC and half after for a system with optimal timing.

Thus there is no “confrontation” between the gas and piston.

On your statement…

“No references; don't need 'em. References are for people who don't have confidence in what they are talking about. I am not one of those people”

The point of references is to allow others to follow the reasoning without having to draw pictures and to add credibility by showing that research done supports the fact in question. You may have all the confidence in the world but without facts its just annother baseless opinion. You learned whatever you know ( or think you know) somewhere. Where? Share it with us.--=Motorhead 16:26, 30 December 2005 (UTC)

This article is a bit off. Heywood is the right reference to go to, there are more recent sources, but they're really mostly regurgitating the foundations or moving away from the thermochemistry. It seems there's some confusion over the source of knock vs. symptoms: Ping, run-on, rumble are all symptomatic not causal. Pre-ignition: what it isn't- an cause of knock, what it is- when knock occurs in the timing cycle Causes of knock: surface ignition, spark knock (generally a pressure/temperature on the mix effect) Causes of surface ignition: sharp edges in cylinders (cracks, scratches, bridges, plug effects, etc), carbon deposits (heat transfer from is greater than heat transfer to causing more rapid mix heating), floating debris (generally carbonacious), there's probably a few other miscellaneous effects

Bottom line knock is any ignition which takes place in the uncombusted mix outside of the spark ignition flame front. This may be from multiple sources (ignitions/flame fronts) or a single ignition source.

9.6 Abnormal Combustion: Knock and Surface Ignition, pg. 450-478, Internal Combustion Engine Fundamentals, Heywood, 1988

I'm just an engineer ;)! I was looking for sources and just found this article a bit off. —Preceding unsigned comment added by (talk) 01:23, 9 April 2008 (UTC)

Article on aviation fuels in a 1951 issue of Flight mentioning that Harry Ricardo was the first to observe and describe 'detonation' in engines during experiments between 1916 and 1919, here: [2]
BTW, it is called 'detonation' because the fuel/air mixture explodes violently within the cylinder rather than burning evenly as it should. The explosions then eventually cause damage to the piston crown. it's referred-to as 'knocking' due to the sound the engine makes when detonation occurs.— Preceding unsigned comment added by (talk) 22:49, 15 March 2016 (UTC)

Merge proposals[edit]

I saw the suggestion on merging this article with Engine Knocking. If to merge means also move, ie the original article willo ceases to exist then I am against this. Detonation is not unique to engine knocking at all. The Detonation process is also core to the Pulse Detonation Engine [no cylinders] among many other things which is why I found it. Had it been merged with Engine Knocking I doubtfully would have come across it. I would suggest keeping the original article and continue to add to it from the detonation science theoretical point of view but additionally incorporate whatever is needed in the Engine Knocking Article.

Just my 2 cents ? Any thoughts ? Steve Mew Sunday, 5 February 2006, 07:07:19 GMT

Actually, the proposal is for the Detonation internal combustion engine article to be eliminated and moved in this article on Engine knocking. I don't know enough about the other article to comment. Samw 22:09, 5 February 2006 (UTC)

Whichever way they are merged they should be because they deal with exactly the same topic under different names. Kind of inconvenient for readers to have 2 different articles on the same subject.--=Motorhead 02:02, 6 February 2006 (UTC)

Flame front[edit]

What is it? This needs to be explained or linked to in the article. 22:58, 25 March 2007 (UTC)

"Flame front" refers simply to the border between what is already on fire and what is not yet on fire.--=Motorhead 00:15, 10 April 2007 (UTC)

The front is the fire. In front is the unburnt charge, behind is the burnt charge. The burnt charge cools down due to radiating through the flame front or later by wall cooling. The front is heated by the reaction to 2200 K. Compare this to the exhaust temperature (after adiabatic decompression and wall cooling) of 1000 K.

Arnero (talk) 12:41, 7 April 2009 (UTC)

the reality of detonation[edit]

detonation occurs when the flame fronts pressure wave ignites the air fuel mixture instead of the heat of the front itself causing the burn to advance at the local speed of sound rather than the relatively benign rate of 50-100 feet per second these multiple pockets and colliding flame fronts you speak of are fictional —Preceding unsigned comment added by (talk) 04:26, 16 October 2007 (UTC)

Detonation: The last poster sheds the most light on the subject so far. Good one.

Let me start by saying that combustion in an IC engine is wickedly complex. I suppose we are still learning. I doubt we will have much input here from those that are studying it in labs, but perhaps.

Because it is so complex the descriptions here are all partially correct, however, even what I have to say should be taken with skepticism.

No one here has mentioned the idea of adiabatic compression bringing the fuel air mixture above the ignition temperature, (You know Diesel Cycle.) When the fuel air mix rises above the ignition temperature the entire mix burns at extreme speed, you know detonation.

How does the temperature of the fuel air mix get compressed adiabatically above the ignition temperature? Good question! One method is raising the compression ratio, higher pressures equals higher temperatures. Another way is for the starting temperature to be higher. That is why hot days seem to equate to more detonation. Absorbing heat before the compression stroke is another. That is the main reason racers used to block off the intake heating channels. They didn't care how long it took for warm up.

Another way is for the remaining gasses in the combustion chamber to be compressed by the burnt and expanding gasses from the flame front. This would be normal combustion that converts to detonation as the remaining fuel air rises above the ignition temperature and spontaneously explodes to a finish.

It is also possible to increase the maximum pressure and hence temperature by supercharging, thus increasing the chance of detonation.

It is my understanding that the Diesel Cycle originally was based on the process of detonation. It may not use that process now. However, the ideal Diesel Cycle includes a change of temperature and pressure from low to high with zero volume change. That is the very description of "detonation", supersonic speed of combustion, or more accurately "explosion". That may be an old view and newer engines may use a different approach. Note: No Diesel Engine follows the Diesel Cycle perfectly, which may also be a failure in my description.

Because the Diesel works by detonation, supercharging/turbocharging works extremely well. Now, you all know why Diesels are usually turbocharged, detonation, where gasoline engines aren't, detonation.

Pre-ignition, to me, just the sound of it, seems to mean "combustion before the ignition pulse". I could be wrong but, that seems to mean that something is initiating combustion before the spark.

From what I can tell, pre-ignition can come from many sources, detonation, if the temperature/pressure is above the ignition temperature of the fuel/air mix before the spark, that could happen. Glowing spark plugs, glowing chunks of carbon, hot engine parts, false sparks from cross feeding of ignition signals...etc... Ignition may initiate from several sources, rather than just one. Airplane engines have two spark plugs so, multiple sources is not always a bad thing.

Knock, Ping, Pinging, and Pinking: Are descriptions of the sounds that the above could make. Any of the above, since only a very trained ear could distinguish them, if at all.

Note: When normal combustion converts to detonation, the pressure peak happens before it should, causing a "ping" or "knock" sound very similar to pre-ignition, since the pressure wave comes too early.

So not all engine knock is detonation, and not all is pre-ignition, but some are both.

PS, I don't think "pre-ignition" can apply to a Diesel Engine.

Eric Norby —Preceding comment was added at 05:36, 4 November 2007 (UTC)

"Knock" and "Ping"[edit]

As I have been taught, knock and ping are actually two different things. Although they are somewhat related in this topic they do occur at different stages of the cycle.

Knocking can occur under pre-ignition due to poor quality fuel, carbon deposits, etc. or due to improper timing (advanced), many reasons have been mentioned earlier in this discussion. The 'knock' comes from the piston being forced down on its up-stroke by the pre-ignition or the spark firing too soon, banging the piston down while it's trying to move up and against the cylinder wall, rattling the works all the way to the crank. This opposition of forces is what makes it louder and more pronounced.

Pinging occurs from post ignition, after the piston reaches TDC and begins its down-stroke. Caused by poorly ignited fuel resulting in an extended or delayed ignition, too much fuel resulting in an extended combustion cycle or improper timing (retarded), Etc.. Basically anything that could cause an undesirable ignition after TDC.

Although the two are both ignition problems whereas the air fuel mixture is ignited either too soon or too late, or burns too long. But the difference being that the 'Knock' is usually much more audible than the 'Ping' since the piston is already on its way down. You can actually have ping without the knock, or knock without the ping. You could even have a ping and not know it, but chances are you'd hear a knock!

The two terms when used properly actually do better define which part of the cycle is malfunctioning.

--DP67 (talk/contribs) 23:28, 4 December 2007 (UTC)

Preflame reactions, knock sensor, and the German version[edit]

[3], be sure to read the discussion (ion sensor).

I think it is better than this English article. It seems that I have to look in all languages again (sometimes French or Spain link to very good images). They mention tail gas.

[4] is as bad as the English article.

Maybe we should merge Automatic Performance Control into the present article and make it a section called knock sensor.

Nice images on [5].

Can we link to glow plug engine. I read it is a small catalytic converter. I still do not understand why a glow plug doesn't ignite the charge at the intake stroke. An why does Klopfen_(Verbrennungsmotor) mention soot as source for knock? It sounds similar to this controlled self ignition stuff based on exhaust recirculation.

Has anybody a reference which proves that this is detonation? Everything I read said that the combustion of the tail gas is somewhat faster than the original flame front, but they could not prove that it is faster than the speed of sound in that hot turbulent charge.

Nice images about the pressure fluctuations due to knock and many references on: [6]. Be sure to inherit in this article that the mean pressure is only marginally higher, but that knock is fast enough to be in resonance with cavity modes.

And what about self ignition after the flame front? We should mention that this is believed to be a measurement artifact of the very old (1940) measurements or so.

Arnero (talk) 14:19, 5 April 2009 (UTC)

>you need to do it encyclopædically rather than damaging the structu I only see a structured layout. But I am glad to read your reversion reasons in the history. As no one seems to be going to copy edit this article, I will just add my plans into this talk subsection and when I am retired in 100 years or so take a day to rewrite the article.

The ignition of the tail gas is due to temperature and pressure. Not sure why pressure is so important, but measurements hint at that direction. The turbulence in the chamber leads to pressure fluctuations. Sorta like in a freak wave somewhere in the volume of the rest charge a critical mass may get at high temperature and pressure and ignite the charge. Combustion is at least fast enough that the air cannot leave this area of the cylinder due to inertia. Therefor the combustion is faster there. Literature talks about negative temperature gradients, as to why the fast combustion is mostly triggered by self ignition and not by the flame front reaching the area. It seems that at high temperature the combustion somehow fizzles out. As said above: Pressure is important. This is the only knock prevention technique which directly prevents ignition: Low compression ratio (or throttle).

And about this Dieseling. This stems from one of the layman's references I think. As seen with the glow plug engine, Dieseling is not generally a bad thing. In real Diesel the ignition occurs shortly after mixing. As the ignition does not occur at the intake stroke, Dieseling is a very bad name.

Knock sensor and glow plug can only work because weak knock is no detonation.

Arnero (talk) 12:28, 7 April 2009 (UTC)

I just grabbed my edits out of the history to keep them in context:

Preflame chemical reactions are important for engine knock, sources:

When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat (about 400 °C) and pressure (above 50 bar) for a certain duration (1 ms) it is converted into a chemical cocktail which detonates already at 1000 °C, while the original mixture needs more than 2000 °C. Therefor a cold air intake is used. The mixture is cooled by the cylinder walls and the piston, which have a temperature of 100 °C (far below 400 °C). Liquid fuel droplets are sprayed into the cylinder for evaporation cooling since gasoline boils below 100 °C. Small intake valves are used to create turbulence to equalize the temperature and prevent hot spots or pockets. The turbulence also later speeds up the flame front to burn all mixture before the above mentioned certain duration passes. A central spark plug and a small bore radius reduce the distance the front has to travel. Any pockets in the piston for the valves or for two-stroke scavenging or for stratified combustion are omitted. The exhaust valves benefit from sodium cooling and still are so hot that they need to be placed close to the spark plug. Never is unburnt air fuel mixture allowed to leave through the exhaust valve and burn on its outer size and aggravate the cooling problem. It may be noted that a central glow plug creates ideal conditions for knock in the center of the combustion chamber without experiencing any damage, but unfortunately also heats the rest of the cylinder.

Alternative explanation: flame front acceleration

When the flame grows it heats (by radiation) and pressurizes the surrounding unburned fuel/air mixture. If the compensation by cylinder expansion and wall cooling is too low, the flame front will get faster and faster as it travels through the ever hotter mixture and may reach the speed of sound. Then the pressure rise cannot be distributed equally through the cylinder, but is concentrated into the flame front, leading to locally high combustion temperatures and a slightly supersonic speed of the flame front. If this front hits any wall, it separates the insulating boundary layer (of cold gas) and applies temperatures far beyond the melting point of the wall material. The sudden heating leads to expansion of the surface material and thus to cracks into the bulk.

Octane_rating covers the same topic in a much better way.

skimming over the revision history I am a bit hopeless about this article. On the other hand there IS hope: "removed idiotic colliding flame reference" Arnero (talk) 18:10, 7 April 2009 (UTC)


try to give information about subjects in simple languages for genaral people —Preceding unsigned comment added by Vuradi praveen (talkcontribs) 03:15, 16 March 2010 (UTC)


Echoing what a couple of posters have observed, it is indeed called "pinking" in the UK.Captain deathbeard (talk) 08:23, 20 May 2010 (UTC)

The UK term "pinking" is onomatopoeic. — Preceding unsigned comment added by (talk) 11:28, 25 February 2017 (UTC)

Sound of Knock[edit]

Four-stroke cycle used in gasoline engines.

I think someone should add an audio file with the characteristic sound of engine knock. Also a video demonstration would be nice (perhaps a mod of this >)--MarsInSVG (talk) 19:18, 10 August 2010 (UTC)


According to "Internal combustion engine fundamentals", by John B. Heywood (1988) it is more correct to divide knock into the types spark knock and knock caused by surface ignition. Where spark knock is a knock that is caused by end gases in front of the propagating flame igniting because of the increased pressure, temperatue and density. The resulting self ignition will result in a energy release that is far faster than in the propagating flame resulting in vibrations which is heard as the knock. This type of knock is generally fixable by delaying the ignition.

Surface ignition on the other hand is presented as a combustion happening outside of the propagating flame caused by a hot spot, which might be a hot particle, overheated plugs or valves or similar. A surface ignition might result in a knock or it might not... a surface ignition happening before the ignition is what is called pre-ignition here. But the exact same thing might also happen in other parts of the cylinder after the ignition have started, without being a regular spark knock, and therefore not being fixable by delay of the ignition.

19 June 2012 — Preceding unsigned comment added by (talk) 10:55, 19 June 2012 (UTC)

Thanks for that useful information. The book you quote, by John B. Heywood, is not one of the sources cited in the article. Can you provide further details about this book? Dolphin (t) 11:28, 19 June 2012 (UTC)

Appart from what you will se on amazon/google books, I don't really know what to say. It is the book we are told to use in the course "Internal combustion engines" at University of New South Wales, Australia (together with up to date slides covering the improvements to date). The lecturer is and he uses the same definition as I presented as well (If I haven't misunderstood him completely). — Preceding unsigned comment added by (talk) 23:11, 19 June 2012 (UTC)

The reason we are always interested in the details of what document is being quoted as the source is because everything in Wikipedia that is likely to be challenged should be explicitly linked to its source document to allow independent verification. See WP:Verifiability. The way the source document is shown is by using in-line citations and by identifying reference documents. For some useful introductory guidance on this subject see WP:REFBEGIN. The principle of disclosing the source of information on Wikipedia is no different to the principle behind quoting sources and references used in writing a University assignment, honours thesis, PhD thesis etc. Dolphin (t) 23:19, 19 June 2012 (UTC)


I searched Wikipedia for "misfire" and all I found (car related) was "An engine misfire, see engine knocking." Of course this article doesn't say anything about misfires. I'm thinking a new page should be made for that, or adding a section in this one, but I am not an expert on the topic. What I understand as a "misfire" just means no combustion. Maybe that can lead to knocking or other problems, but not always.

Anyway, just wondered if anyone agrees with that, or knows of another article that talks about misfires. Autumn Wind (talk) 21:09, 6 October 2015 (UTC)

Misfires have pretty much zero in common with knocking. There's a reason why WP isn't considered a reliable source. Andy Dingley (talk) 21:15, 6 October 2015 (UTC)

Particularly ones that use methanol[edit]

In the chapter Pre-ignition there is a sentence claiming that methanol is prone to pre-ignition. However this seems nonsense, as methanol is a high octane fuel. Also the offered reference does not say anything about methanol (on page 165). —Nikolas Ojala (talk) 12:15, 17 October 2016 (UTC)

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