Detonation and Pre-Ignition

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Engine Basics: Detonation and Pre-Ignition

by Allen W. Cline

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This article was originally published in the January-February 2000 (Volume 0 !umber "

edition of Contact(maga#ine$ This maga#ine is published bi-monthly by the Aeronautics

%ducation %nterprises (A%%"& an Ari#ona nonprofit organi#ation$ Contact(maga#ine has

granted us permission to reproduce it here$ For subscription information& please see the last

pageof this article$

All high output engines are prone to destructi'e tendencies as a result of o'er boost&

misfueling& mis-tuning and inadeuate cooling$ The engine community pushes e'er nearer to

the limits of power output$ As they often learn cylinder chamber combustion processes can

uic)ly gra'itate to engine failure$ This article defines two types of engine failures& detonation

and pre-ignition& that are as insidious in nature to users as they are hard to recogni#e and

detect$ This discussion is intended only as a primer about these combustion processes since

whole boo)s ha'e been de'oted to the sub*ect$

First& let us re'iew normal combustion$ +t is the burning of a fuel and air mi,ture charge in the

combustion chamber$ +t should burn in a steady& e'en fashion across the chamber& originating

at the spar) plug and progressing across the chamber in a three dimensional fashion$ imilar

to a pebble in a glass smooth pond with the ripples spreading out& the flame front should

progress in an orderly fashion$ The burn mo'es all the way across the chamber and & uenches

(cools" against the walls and the piston crown$ The burn should be complete with no remaining

fuel-air mi,ture$ !ote that the mi,ture does not .e,plode. but burns in an orderly fashion$

There is another factor that engineers loo) for to uantify combustion$ +t is called .location of

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pea) pressure (/"$. +t is measured by an in-cylinder pressure transducer$ +deally& the /

should occur at 1 degrees after top dead center$ epending on the chamber design and the

burn rate& if one would initiate the spar) at its optimum timing (20 degrees 3T4& for

e,ample" the burn would progress through the chamber and reach /& or pea) pressure at 1

degrees after top dead center$ / is a mechanical factor *ust as an engine is a mechanical

de'ice$ The piston can only go up and down so fast$ +f you pea) the pressure too soon or too

late in the cycle& you won5t ha'e optimum wor)$ Therefore& / is always 1 degrees AT4 for

any engine$

+ introduce / now to illustrate the idea that there is a characteristic pressure buildup

(compression and combustion" and decay (piston downward mo'ement and e,haust 'al'e

opening" during the combustion process that can be considered .normal. if it is smooth&

controlled and its pea) occurs at 1 degrees AT4$

6ur enlarged definition of normal combustion now says that the charge7bum is initiated with

the spar) plug& a nice e'en burn mo'es across the chamber& combustion is completed and

pea) pressure occurs at at 1 AT4$

4onfusion and a lot of uestions e,ist as to detonationand pre)ignition$ ometimes you

hear mista)en terms li)e .pre-detonation.$ etonation is one phenomenon that is abnormal

combustion$ re-ignition is another phenomenon that is abnormal combustion$ The two& as we

will tal) about& are somewhat related but are two distinctly different phenomenon and can

induce distinctly different failure modes$

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Engine Basics: Detonation and Pre-Ignitionby Allen W. Cline

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-e. Definitions

Detonation

etonation is the spontaneous combustion of the end-gas (remaining fuel7air mi,ture"

in the chamber$ +t always occurs after normal combustion is initiated by the spar)

plug$ The initial combustion at the spar) plug is followed by a normal combustion

burn$ For some reason& li)ely heat and pressure& the end gas in the chamber

spontaneously combusts$ The )ey point here is that detonation occurs after you ha'e

initiated the normal combustion with the spar) plug$

Pre)ignition

re-ignition is defined as the ignition of the mi,ture prior to the spar) plug firing$

Anytime something causes the mi,ture in the chamber to ignite priorto the spar)

plug e'ent it is classified as pre-ignition$ The two are completely different and

abnormal phenomenon$

Detonation

;nburned end gas& under increasing pressure and heat (from the normal progressi'e 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 lac)s sufficient octane

rating to withstand this combination of heat and pressure$

etonation causes a 'ery high& 'ery sharp pressure spi)e in the combustion chamber but it is

of a 'ery short duration$ +f you loo) at a pressure trace of the combustion chamber process&

you would see the normal burn as a normal pressure rise& then all of a sudden you would see

a 'ery sharp spi)e when the detonation occurred$ That spi)e always occurs after the spar)

plug fires$ The sharp spi)e in pressure creates a force in the combustion chamber$ +t causes

the structure of the engine to ring& or resonate& much as if it were hit by a hammer$

ert#$ o

the pinging you hear is actually the structure of the engine reacting to the pressure spi)es$
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4/19

This noise of detonation is commonly called spar) )noc)$ This noise changes only slightly

between iron and aluminum$ This noise or 'ibration is what a )noc) sensor pic)s up$ The

)noc) sensors are tuned to =100 hert# and they will pic) up that spar) )noc)$ +ncidentally& the

)noc)ing or pinging sound is not the result of .two flame fronts meeting. as is often stated$

Although this clash does generate a spi)e the noise you sense comes from the 'ibration of the

engine structure reacting to the pressure spi)e$

6ne thing to understand is that detonation is not necessarily destructi'e$ ?any engines run

under light le'els of detonation& e'en moderate le'els$ ome engines can sustain 'ery long

periods of hea'y detonation without incurring any damage$ +f you5'e dri'en a car that has a lot

of spar) ad'ance on the freeway& you5ll hear it pinging$ +t can run that way for thousands and

thousands of miles$ etonation is not necessarily destructi'e$ +t5s not an optimum situation

but it is not a guaranteed instant failure$ The higher the specific output (>7in@" of the engine&

the greater the sensiti'ity to detonation$ An engine that is ma)ing 0$: >7in@ or less can

sustain moderate le'els of detonation without any damage but an engine that is ma)ing $:

>7in@& if it detonates& it will probably be damaged fairly uic)ly& here + mean within minutes$

etonation causes three types of failureB

$ ?echanical damage (bro)en ring lands"

2$ Abrasion (pitting of the piston crown"

@$ 6'erheating (scuffed piston s)irts due to e,cess heat input or high coolant

temperatures"

The high impact nature of the spi)e can cause fractures it can brea) the spar) plug

electrodes& the porcelain around the plug& cause a clean fracture of the ring land and can

actually cause fracture of 'al'es-inta)e or e,haust$ The piston ring land& either top or second

depending on the piston design& is susceptible to fracture type failures$ +f + were to loo) at a

piston with a second bro)en ring land& my immediate suspicion would be detonation$

Another thing detonation can cause is a sandblasted appearance to the top of the piston$ The

piston near the perimeter will typically ha'e that )ind of loo) if detonation occurs$ +t is a

swiss-cheesy loo) on a microscopic basis$ The detonation& the mechanical pounding& actually

mechanically erodes or fatigues material out of the piston$ Cou can typically e,pect to see that

sanded loo) in the part of the chamber most distant from the spar) plug& because if you thin)

about it& you would ignite the flame front at the plug& it would tra'el across the chamber


5/19

before it got to the farthest reaches of the chamber where the end gas spontaneously

combusted$ That5s where you will see the effects of the detonation you might see it at the

hottest part of the chamber in some engines& possibly by the e,haust 'al'es$ +n that case the

end gas was heated to detonation by the residual heat in the 'al'e$

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+n a four 'al'e engine with a pent roof chamber with a spar) plug in the center& the chamber is

fairly uniform in distance around the spar) plug$ 3ut one may still may see detonation by the

e,haust 'al'es because that area is usually the hottest part of the chamber$ Dhere the end

gas is going to be hottest is where the damage& if any& will occur$

3ecause this pressure spi)e is 'ery se'ere and of 'ery short duration& it can actually shoc) the

boundary layer of gas that surrounds the piston$ 4ombustion temperatures e,ceed 800

degrees$ +f you sub*ected an aluminum piston to that temperature& it would *ust melt$ The

reason it doesn5t melt is because of thermal inertia and because there is a boundary layer of a

few molecules thic) ne,t to the piston top$ This thin layer isolates the flame and causes it to

be uenched as the flame approaches this relati'ely cold material$ That combination of actions

normally protects the piston and chamber from absorbing that much heat$ >owe'er& under

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8/19

greatly and the end gas spontaneously combusts$ 3ac)ing off the spar) timing will stop the

detonation$ The octane rating of the fuel is really nothing magic$ 6ctane is the ability to resist

detonation$ +t is determined empirically in a special running test engine where you run the

fuel& determine the compression ratio that it detonates at and compare that to a standard fuel&

That5s the octane rating of the fuel$ A fuel can ha'e a 'ariety of additi'es or ha'e higher

octane uality$ For instance& alcohol as fuel has a much better octane rating *ust because it

cools the mi,ture significantly due to the e,tra amount of liuid being used$ +f the fuel you got

was of a lower octane rating than that demanded by the engine5s compression ratio and spar)

ad'ance detonation could result and cause the types of failures pre'iously discussed$

roduction engines are optimi#ed for the type or grade of fuel that the mar)etplace desires or

offers$ %ngine designers use the term called ?3T ( ?inimum spar) for 3est Torue" for

efficiency and ma,imum power it is desirable to operate at ?3T at all times$ For e,ample&

let5s pic) a specific engine operating point& 1000


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rate$

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Chamer Design

6ne of the characteristic chambers that people are familiar with is the 4hrysler >emi$ The

engine had a chamber that was li)e a half of a baseball$ >emispherical in nature and in

nomenclature& too$ The two 'al'es were on either side of the chamber with the spar) plug at

the 'ery top$ The charge burned downward across the chamber$ That approach wor)ed fairly

well in passenger car engines but racing 'ersions of the >emi had problems$ 3ecause the

chamber was so big and the bores were so large& the chamber 'olume also was large it was

difficult to get the compression ratio high$


11/19

about 10-1: degrees$ Dith that much spar) ad'ance detonation was a serious possibility if not

fed high octane fuel$ >emis tended to be 'ery sensiti'e to tuning$ As often happened& one

would )eep ad'ancing the spar)& get more power and all of a sudden the engine would

detonate& 3ecause they were high output engines& turning at high emi racing engines would typically )noc) the ring land off& get blow by& torch the piston and

fall apart$ !o one then understood why$ De now )now that the >emi design is at the worst

end of the spectrum for a combustion chamber$ A nice compact chamber is best that5s why

the four 'al'e pent roof style chambers are so popular$ The flatter the chamber& the smaller

the closed 'olume of the chamber& the less dome you need in the piston$ De can get

inherently high compression ratios with a flat top piston with a 'ery nice bum pattern right in

the combustion chamber& with 'ery short distances& with 'ery good mi,ture motion - a 'ery

efficient chamber$

/oo) at a !orthstar or most of the 1 'al'e type engines - all with flat top pistons& 'ery

compact combustion chambers& 'ery narrow 'al'e angles and there is no need for a dome that

impedes the burn to raise the compression ratio to 0B$

Detonation 2ndicators

The best indication of detonation is the pinging sound that cars& particularly old models& ma)e

at low speeds and under load$ +t is 'ery difficult to hear the sound in well insulated lu,ury

interiors of today5s cars$ An unmuffled engine running straight pipes or a propeller turning can

easily mas) the characteristic ping$ The point is that you honestly don5t )now that detonation

is going on$ +n some cases& the engine may smo)e but not as a rule$ 3ro)en piston ring lands

are the most typical result of detonation but are usually not spotted$ +f the engine has

detonated 'isual signs li)e bro)en spar) plug porcelains or bro)en ground electrodes are dead

gi'eaways and call for further e,amination or engine disassembly$

+t is also 'ery difficult to sense detonation while an engine is running in an remote and

insulated dyno test cell$ 6ne techniue seems almost elementary but& belie'e it or not& it is

employed in some of the highest priced dyno cells in the world$ De refer to it as the .Tin %ar.$

Cou might thin) of it as a simple stethoscope applied to the engine bloc)$ De run a ordinary

rubber hose from the dyno operator area ne,t to the engine$ To amplify the engine sounds we

*ust stic) the end of the hose through the bottom of a tyrofoam cup and listen inG +t is


12/19

common for ride test engineers to use this method on de'elopment cars particularly if there is

a suspicion out on the road borderline detonation is occurring$ Try it on your engine you will

be ama#ed at how well you can hear the different engine noises$

The other techniueis a little more subtle but usable if attention is paid to %HT (%,haust Has

Temperature"$ etonation will actually cause %HTs to drop$ This beha'ior has fooled a lot of

people because they will watch the %HT and thin) that it is in a low enough range to be safe&

the only reason it is low is because the engine is detonating$

The only way you )now what is actually happening is to be 'ery familiar with your specific

engine %HT readings as calibrations and probe locations 'ary$ +f& for e,ample& you normally

run :00 degrees at a gi'en ?A setting and you suddenly see 2: after pic)ing up a fresh

load of fuel you should be alert to possible or incipient detonation$ Any drop from normal %HT

should be reason for concern$ ;sing the .Tin %ar. during the early test stage and watching the

%HT 'ery carefully& other than *ust plain listening with your ear without any augmentation& is

the only way to identify detonation$ The good thing is& most engines will li'e with a fairly high

le'el of detonation for some period of time$ +t is not an instantaneous type failure$

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13/19

Pre)2gnition

The definition of pre-ignition is the ignition of the fuel7air charge prior to the spar) plug firing$

re-ignition caused by some other ignition source such as an o'erheated spar) plug tip&carbon deposits in the combustion chamber and& rarely& a burned e,haust 'al'e all act as a

glow plug to ignite the charge$

Ieep in mind the following seuence when analy#ing pre-ignition$ The charge enters the

combustion chamber as the piston reaches 34 for inta)e the piston ne,t re'erses direction

and starts to compress the charge$ ince the spar) 'oltage reuirements to light the charge

increase in proportion with the amount of charge compression almost anything can ignite the

proper fuel7air mi,ture at 34GG 34 or before is the easiest time to light that mi,ture$ +t

becomes progressi'ely more difficult as the pressure starts to build$

A glowing spot somewhere in the chamber is the most li)ely point for pre-ignition to occur$ +t

is 'ery concei'able that if you ha'e something glowing& li)e a spar) plug tip or a carbon

ember& it could ignite the charge while the piston is 'ery early in the compression sto)e$ The

result is understandable for the entire compression stro)e& or a great portion of it& the engine

is trying to compress a hot mass of e,panded gas$ That ob'iously puts tremendous load on

the engine and adds tremendous heat into its parts$ ubstantial damage occurs 'ery uic)ly$

Cou can5t hear it because there is no rapid pressure rise$ This all occurs well before the spar)

plug fires$


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17/19

that air fuel ratio is richened up by the 4? to about 0B$ That is done to )eep the spar)

plugs cool& as well as the piston crowns cool$ That richness is necessary if you are running

under continuous D6T load$ A slight penalty in horsepower and fuel economy is the result$ To

get the ma,imum acceleration out of the engine& you can actually lean it out& but under full

load& it has to go bac) to rich$ >igher specific output engines are much more sensiti'e to pre-

ignition damage because they are turning more Articles> Engine> Detonation> Page 5


Engine Basics: Detonation and Pre-Ignition

by Allen W. Cline

Page 5of 8

+ pre,ious

Mudd. 6ater

There is a situation called detonation induced pre-ignition$ + don5t want to sound li)e double

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spea) here but it does happen$ +magine an engine under hea'y load starting to detonate$

etonation continues for a long period of time$ The plug heats up because the pressure spi)es

brea) down the protecti'e boundary layer of gas surrounding the electrodes$ The plug

temperature suddenly starts to ele'ate unnaturally& to the point when it becomes a glow plug

and induces pre-ignition$ Dhen the engine fails& + categon5#e that result as .detonation

induced pre-ignition$. There would not ha'e been any danger of pre-ignition if the detonation

had not occurred$ amage attributed to both detonation and pre-ignition would be e'ident$

Typically& that is what we see in passenger car engines$ The engines will typically li'e for long

periods of time under detonation$ +n fact& we actually run a lot of piston tests where we run

the engine at the torue pea)& induce moderate le'els of detonation deliberately$ 3ased on our

resulting production design& the piston should pass those tests without any problem the

pistons should be robust enough to sur'i'e$ +f& howe'er& under circumstances due to

o'erheating or poor fuel& the spar) plug tip o'erheats and induces pre-ignition& it5s ob'iously

not going to sur'i'e$ +f we see a failure& it probably is a detonation induced pre-ignition

situation$

+ would urge any e,perimenter to be cautious using automoti'e based engines in other

applications$ +n general& engines producing $: >7in@ (typical air-cooled aircraft engines" can

be forgi'ing (as leaning to pea) %HT& etc$"$ 3ut at $0 >7in@ ('ery typical of many high

performance automoti'e con'ersions" the window for calibration induced engine damage is

much less forgi'ing$ tart out rich& retarded and with cold plugs and watch the %HTsG

>opefully this discussion will ser'e as a thought starter$ + welcome any communication on this

sub*ect$ %'ery application is uniue so beware of blan)et statements as many 'ariables affect

these processes$

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Detonation and Pre-Ignition

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