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By Dr. Robin
Tuluie, Ph.D.
Since the previous Wrenching With Rob, Chemical
Soup: The Meaning of Gasoline we've been besieged with questions and
comments regarding the combustion process occurring in an engine. In particular,
the discussion focused on the problem of detonation, commonly referred to as
"knock," which is a very serious and detrimental problem when it
occurs - usually the pressures exerted onto the piston top during detonation are
much larger (but of a shorter duration, like a pressure spike) than the mean
combustion pressure. Nevertheless they are very detrimental to engine life, as
the continual high shock loading of the piston, rod, crankshaft and bearings is
quite destructive.
Detonation is the result of an amplification of pressure waves, such as sound
waves, occurring during the combustion process when the piston is near top dead
center (TDC). The actual "knocking" or "ringing" sound of
detonation is due to these pressure waves pounding against the insides of the
combustion chamber and the piston top, and is not due to 'colliding flame fronts'
or 'flame fronts hitting the piston or combustion chamber walls.'
Let's look in some detail at how detonation can occur during
the combustion process: First, a pressure wave, which is generated during the
initial ignition at the plug tip, races through the unburned air-fuel mix ahead
of the flame front. 
Typical flame front speeds for a gasoline/air mixture are on the order of 40 to
50 cm/s (centimeters per second), which is very slow compared to the speed of
sound, which is on the order of 300 m/s. In actuality, the true speed of the
outwards propagating flame front is considerably higher due to the turbulence of
the mixture. Basically, the "flame" is carried outwards by all the
little eddies, swirls and flow patterns of the turbulence resident in the
air-fuel mix. This model of combustion is called the "eddy burning
model" (Blizzard & Keck, 1974).
Additionally, the genus of the flame front surface - that is the degree of 'wrinkling'
- which usually has a fractal nature (you know, those weird, seemingly random
yet oddly patterned computer drawings), is increased greatly by turbulence,
which leads to an increased surface area of the flame front. This increase in
surface area is then able to burn more mixture since more mixture is exposed to
the larger flame front surface. This model of combustion is called the "fractal
burning model" (Goudin, F.C. et al. 1987, Abraham et al. 1985). The effects
of this are observed in so-called "Schlieren pictures," which are
high-speed photographs taken though a quartz window of a specially modified
combustion chamber (Fig. 1, above).
Schlieren pictures show the various stages of the combustion process, in
particular the highly wrinkled and turbulent nature of the flame front
propagation (initially called the flame 'kernel'). A higher degree of turbulence,
and hence a higher "effective" flame front propagation velocity can be
achieved with a so-called squish band combustion chamber design. 
Sometimes a swirl-type of induction process, in which the incoming mixture is
rotating quickly, will achieve the same goal of increasing the burn rate of the
mixture.
As a general rule-of-thumb the pressure rise in the combustion chamber during
the combustion phase is typically 20-30 PSI per degree of crankshaft rotation.
Once the pressure rises faster than about 35 PSI/degree, the engine will run
very roughly due to the mechanical vibration of the engine components caused by
too great of a pressure rise. Sometimes, the pressure wave can be strong enough
to cause a self ignition of the fuel, where free radicals (e.g. hydroxyl or
other molecules with similar open O-H chains) in the fuel promote this self
ignition by the pressure wave. However, this can still occur even without the
presence of free radicals; it just won't be quite as likely to happen. This is
why high octane fuels, with fewer of these active radicals, can resist
detonation better. However, even high octane fuel can detonate - not because of
too many free radicals - but because the drastic increase in cylinder pressure
has increased the local temperature (and molecular speed) so high that it has
reached the ignition temperature of the fuel. This ignition temperature is
actually somewhat lower than that of the main hydrocarbon chain of the fuel
itself because of the creation of additional radicals resulting from the
break-up of the fuel's hydrocarbon chains in intermolecular collisions.
Detonation usually happens first at the pressure wave's points of
amplification, such as at the edges of the piston crown where reflecting
pressure waves from the piston or combustion chamber walls can constructively
recombine - this is called constructive interference to yield a very high local
pressure. If the speed at which this pressure build-up to detonation occurs is
greater than the speed at which the mixture burns, the pressure waves from both
the initial ignition at the plug and the pressure waves coming from the problem
spots (e.g. the edges of the piston crown, etc.) will set off immediate
explosions, rather than combustion, of the mixture across the combustion chamber,
leading to further pressure waves and even more havoc. Whenever these colliding
pressure fronts meet, their destructive power is unleashed on the engine parts,
often leading to a mechanical destruction of the motor. The pinging sound of
detonation is just these pressure waves pounding against the insides of the
combustion chamber and piston top. Piston tops, ring lands and rod bearings are
especially exposed to damage from detonation. In addition, these pressure fronts
(or shock waves) can sweep away the unburned boundary layer (see figure 2 above)
of air-fuel mix near the metal surfaces in the combustion chamber.
The boundary layer is a thin layer of fuel-air mix just above the metal
surfaces of the combustion chamber (see figure 2, above). Physical principles (aptly
called boundary conditions) require that under normal circumstances (i.e.
equilibrium combustion, which means "nice, slow and thermally well
transmitted") this boundary layer stays close to the metal surfaces. It
usually is quite thin, maybe a fraction of a millimeter to a millimeter thick.
This boundary layer will not burn even when reached by the flame front because
it is in thermal contact with the cool metal, whose temperature is always well
below the ignition temperature of the fuel-air mix.
Only under the extreme conditions of detonation can this boundary layer be
"swept away" by the high-pressure shock front that occurs during
detonation. In that case, during these "far from equilibrium" process
of the pressure-induced shock wave entering the boundary layer, the physical
principles allured to above (the boundary conditions) will be effectively
violated. The degree of violation will depend on (a) the pressure fluctuation
caused by the shock front and (b) the adhesive and cohesive strength of the
boundary layer. These boundary layers of air-fuel mix remain unburned during the
normal combustion process due to their close proximity to the cool metal
surfaces and act as an insulating layer and prevent a direct exposure of metal
to the flame. Since pressure waves created during detonation can sweep away
these unburned boundary layers of air-fuel mix, they leave parts of the piston
top and combustion chamber exposed to the flame front. This, in turn, causes an
immediate rise in the temperature of these parts, often leading to direct
failure or at least to engine overheating.
Scientists and engineers have recently begun to understand combustion in much
greater detail thanks to very ambitious computer simulations that model every
detail of the combustion process (Chin et al. 1990). Basically, a complete
computer model includes a solution to the thermodynamical problem, that is a
solution to the conservation equations and equation of state, as well as a mass
burning rate and heat transfer model. In addition, a separate code (called a
chemical kinetics code) models the chemical processes which occur during
combustion and sometimes juggles several thousand different chemical species,
some in vanishingly small concentrations! Needless to say these codes require
huge amounts of memory and CPU time that only the largest supercomputers in the
world can provide. They are far beyond the reach of the private individual and
usually only employed by large research institutions or major car manufactures.
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