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Offline rsss396

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detonation
« on: August 26, 2013, 12:02:28 PM »
By Kevin Cameron The Cellar Dweller

I first 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 squish band.

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
becomes colder.

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
straightline relationship.

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.
Anyone looking for a great builder I highly recommend the following.
For CP products dealers I would recommend:
Arlan at LED(site sponsor), Pete Schemberger at Hybrid Engineering, Mat Shearer at Shearer Custom Pipes, Dennis Packard at Packard Racing, and Nate McCoy of McCoys Peformance.

Other great builders I also would recommend: Neil Prichard, Jerry Hall, Bubba Ramsey and James Dodge.

Offline rsss396

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detonation
« Reply #1 on: August 26, 2013, 12:03:02 PM »
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 heated.

(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.

To Subscribe to DYNOTECH call 716.344.1313
Anyone looking for a great builder I highly recommend the following.
For CP products dealers I would recommend:
Arlan at LED(site sponsor), Pete Schemberger at Hybrid Engineering, Mat Shearer at Shearer Custom Pipes, Dennis Packard at Packard Racing, and Nate McCoy of McCoys Peformance.

Other great builders I also would recommend: Neil Prichard, Jerry Hall, Bubba Ramsey and James Dodge.