By Kevin Cameron The
encountered detonation back in 1966, and I didn't know what it was.
Fortunately for me, it was a light case, and the only symptoms were
small holes being eaten into the edges of a motorcycle cylinder head's
Later, pushing to higher compression, I would generate my share of
pistons that were detonated away until their rings hung out into empty
space. I would learn to look out for the tiny, ash gray flake of
quenched aluminum on a spark plug, or in water-cooled engines, for the
sudden and otherwise unexplained rise in engine temperature. And I would
still be curious about those dusty holes, eroded into cylinder heads.
Books will tell you what Harry Ricardo learned back in 1918; that
detonation is not the same as preignition. Preignition is lighting of
the charge before the spark , by some hot object in the combustion
chamber usually the overheated center wire of a spark plug whose heat
range was too hot for the application. Preignition soon provokes
detonation, so the confusion is understandable.
Detonation, by contrast, is self ignition of some of the last parts of
the charge to burn the so-called "end gas" out at the edges of the
combustion chamber after the spark has already ignited and mostly burned
the charge. This self igniting endgas does not then burn normally, as a
flame front spread by turbulence at the usual speed of a few tens of
feet per second. This gas burns at the local speed of sound, which is
very high because the temperature is high. This form of combustion,
called detonation, forms a shock front, a sudden jump in pressure that
propagates at thousands of feet per second.
When it hits parts, it hits hard. If we hear it al all, it is as a high,
dry, irregular clicking, not unlike the reverberating sound of rocks
struck under water. Detonation's pressure front can damage bearings by
its hammering shock, but the real problem is what it does to an engine's
natural, internal insulation.
In any situation in which gases move next to solid surfaces, there is a
layer of significant thickness that remains largely stagnant because it
is attached to the surface. In internal combustion engines, this
boundary layer quite effectively shields the engine's metal internal
surfaces from direct contact with combustion gas, keeping them cooler
than they would otherwise be.
When detonation begins even light deto this boundary layer is scoured
off by the impacting shock waves, and heat transfer from hot gas to cool
metal accelerates. In only a very few detonating cycles, piston
temperatures rise dramatically, and the rest of the parts exposed to
combustion gas aren't far behind.
What is strange to many people is that as this happens, exhaust gas
temperature falls. This seems odd because people associate detonation
with heat, and heat with failure. But the fact is that as you jet an
engine down, its exhaust temperatures peak, not when the mixture becomes
lean (that is, too little fuel to react with all the oxygen in the air
charge), but when the mixture is chemically perfect. Exhaust gas
temperature falls when detonation begins because the engine's internal
insulation is destroyed; that being so, some heat that would otherwise
go out the exhaust is now being diverted into the piston, head, and
cylinder walls. Because those parts are getting hotter, the exhaust gas
Those of us who began racing before water cooling arrived tend to think
that engines get hotter the more we jet them down. With air cooling,
this seems to be true, but isn't. The engine runs cool when it's rich
because the extra fuel reduces peak flame temperature, and as we jet
down towards chemically correct mixture, the engine runs hotter and
hotter. Often, in a modified engine with high compression, this
detonation begins even before we reach correct mixture and peak flame
temperature. Then the engine really heats up. This leaves us with the
idea that leaning down the mixture raises engine temperature, in a
Now we know, from our experiences with water-cooled engines, that power,
engine temperature, and exhaust gas temperature all rise as we jet down
until we go beyond chemically correct mixture. When we do, power, engine
temperature, and exhaust gas temperature all begin to fall again. We
couldn't see this before, with air-cooling, because the power we were
making was overwhelming the engines the engine's cooling ability. But it
makes perfect sense because heat release in combustion depends upon
finding enough oxygen so that each and every hydrogen and carbon in the
fuel is completely reacted to form water and carbon dioxide. Any fuel
left over is potential chemical energy unreleased which is why running
lean makes less power. On lay well cooled engine that is not detonating,
you can jet down until it starts to slow down.
Now back to detonation. The above explanation is the common one, but it
leaves important questions unanswered. For example, why does detonating
combustion travel at the local speed of sound, and not at normal burning
speed? Why does the endgas auto ignite, rather than simply wait there
like a stand of trees in the path of a forest fire? Understanding how
this comes about helps to understand how the variables that affect
detonation generate their effects and it helps to fend off the
phenomenon that sets the upper limits on performance.
There are two basic forms of combustion, deflagration and detonation. In
deflagration, the propagation of combustion is carried out by simple
convection; the hot combustion gas heats what is ahead of it, raising
its temperature to the ignition point. Because this process of heating
what lies ahead takes time, it is relatively slow. The burning of a
quiescent gasoline air vapor is in fact slow only a foot or so per
second. Combustion in an engine cylinder is much faster than this
because of turbulence, which so wrinkles the flame front that its area
becomes hugely enlarged. This area, multiplied times the slow quiescent
combustion speed, computes out to a very large volume combustion rate.
Detonation is a different animal, and not all gaseous mixtures will
support detonation. It is a form of combustion in which the unburned
material is heated to ignition at least partly by shock compression, as
the detonation wave moves a the local speed of sound through the medium.
This has to happen very quickly, so fuels with simple molecules or those
with low stability lend themselves to this form of combustion. Now how
does the endgas ignite by itself? It does so when its temperature is
raised by any combination of effects to some critical value in the range
of 900-1000 degrees F.
In a running engine, air is drawn in at some ambient temperature, and
this air then begins to pick up heat from the hot internal engine
surfaces it contacts. Finally it enters the actual cylinder, where is it
heated by even hotter surfaces. Trapped there, it is heated again by the
process of compression.
In this heating process, some little discussed chemical reactions begin
to occur in the fuel. Called preflame reactions, these take the form of
slow, partial breakdown of the least durable types of fuel molecule.
Fuel hydrocarbons have three basic forms; straight carbon chains,
branched chains, and ring structures. Temperature is a measure of
average molecular activity, but there are always some gas molecules
moving significantly faster than the others. These faster moving
molecules hit and break some of the less durable fuel molecules,
dislodging some of their more weakly bonded hydrogen atoms. This
released hydrogen is very reactive (normally hydrogenous travel in
bonded pairs, but his is atomic hydrogen) and instantly pairs with an
oxygen from the air to form what is called a radical, a chemical
fragment that is highly reactive because if contains and unpaired
electron. Its attraction for the missing electron is so great that it
can snap one out of other chemical species it happens to collide with,
thereby breaking it down as well.
At some point in the compression stroke, the spark ignites the mixture
and combustion begins. The burned gases, being very hot, expand against
the still unburned charge, compressing it outward into the squish band.
This compression rapidly heats the unburned charge even more,
accelerating the preflame reactions in it. As a rule of thumb, the rate
of chemical reaction doubles every seventeen degrees F. All this while,
the population of reactive molecular fragments radicals is increasing
in the unburned endgas. If this process of heating takes long enough,
and reaches a temperature high enough, this population of radicals
becomes great enough that its own reaction rate one radical creating
more and more through further reactions accelerates into outright
combustion. This is autoignition.
Now why does this heated, chemically changed endgas detonate instead of
simply burning? The fuel in the endgas is no longer ordinary gasoline.
The preflame reaction that have taken place in it have changed it into a
violent explosive much like a mixture of hydrogen and air, or acetylene
and oxygen. It is in a hair-trigger state, being filled with reactive
fragments from preflame reactions. When it autoignites spontaneously,
combustion accelerates almost instantly because the material is so
easily ignited now. The combustion front accelerates to the local speed
of sound, igniting the material it passes through simply by suddenly
raising its temperature, through the shock wave it has now become.
STOPPING THE SHOW
Anything that contributes to lowering the temperature that the endgas
reaches will make detonation less likely. Anything that slows the
process of conversion from normal gasoline into a sensitive explosive,
will make detonation less likely. Anything that speeds up combustion, so
that is it completed before the conditions needed for detonation can
develop fully, will make deto less likely.
Therefore the following will work;
(1) Lower intake temperature
(2) Lower throttle position, lower volumetric efficiency, or reduced
turbo boost the less mixture that enters the cylinder, the less it is
heated by compression.
(3) Lower intake pipe, crankcase, and/or cylinder, piston, or head
temperatures. This year's Yamaha 250cc road race engine, for instance,
has a copper cylinder head insert to conduct combustion heat away
faster, resulting in a lower combustion chamber surface temperature.
(4) Lower compression ratio. The less you squeeze it, the less it is
(5) A more breakdown resistant fuel, such as toluene or isooctane. If
straight chain molecules are not present, the fuel will not be broken
down so rapidly by preflame reactions.
(6) A negative catalyst something that will either pin down active
radicals or convert them into something harmless. Tetraethyl lead, MMT,
or other antiknock compounds are the medicine.
(7) Retarded timing shortens the time during which proknock reactions
can take place.
(8) Incylinder turbulence or anything else that will speed up combustion
(faster burning fuel such as benzene). This works by completing
combustion before the time bomb of preflame reactions cooks long enough
to cause autoignition.
(9) Higher engine rpm This simply shortens the time during which the
mixture is held at high temp. In Honda experiments in the 1960's, they
found that an engine's octane requirements began to decrease steadily
over 12,000 rpm, and were under 60 octane up near 20,000. In a more
accessible example, note that engines knock when they are "lugged" run
at low rpm, wide open throttle and stop knocking promptly when you
shift down a gear and let the engine rev up more. This stops deto by not
allowing enough time for the reactions that cause it.
(10) Redesigning troublesome exhaust pipes. Some pipes give great
numbers on the dyno, but can't be used because they cause seizures. They
either simply overcharge the engine in some narrow rpm band (pushing it
into detonation just as too much turbo boost would do), or back pump
mixture from the header pipe that has picked up too much heat (this is
why nobody heat wraps header pipes anymore).
(11) Avoiding excessive backpressure. Exhaust pipes always create back
pressure, but the more there is, the higher the fraction of hot exhaust
gas that will be unable to leave the cylinder during exhaust. Its heat,
added to the fresh charge that next enters the cylinder, may push the
engine over the line into detonation. Sometimes a one or two millimeter
reduction in tailpipe ID will get you a couple of extra horsepower, but
it may also push enough extra heat into the charge to make the engine
detonate after a few seconds.
The number of ways of playing footsie with detonation is endless, but
nothing works every time. This guarantees that we will never be bored,
and will never run out of seized pistons.
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