ROB wrote:Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propogates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns
I think most people have a pretty good idea of how a TFF works, but lets go through some of the details. First, think of a chamber (say a cube or a disc for simplicty) full of nice calm, still air-fuel mixture. A spark in the middle ignites, and begins to consume the mixture. Ideally, the flame front (in this case a laminar flame front, since the mixture is still) would form a spherical shell, as it progresses. Now, this flame front is propelled by a couple of forces. First, the mixture in the wake of the flame front is obviously heated by the combustion. This heat translates to an increase in pressure. This higher pressure burned gas compresses the mixture ahead of the flame front. Since the volume of the burned gases expands it helps accelerate the flame front. Think of blowing up a ballon. The compression of the end gas also raises its temperature. Flame speeds are higher in higher temperature mixtures.
For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol.
Now, this burning action is really a race. As you compress the combustible end gas, you get ever closer to the auto-ignition temperature. Auto-ignition is a process by where a series of branching chemical reactions (which mainly all have a very strong dependence on temperature) result in the combustion of a mixture, with no flame front. These reactions begin to oxide the mixture simply due to the thermal energy available from the high temperature. Now don't get me wrong, this is a VERY complex series of chemical reactions, but some of the basics help give a good understanding.
The auto-ignition is very dependent on the time history of the mixture. If you hold the mixture at a low temperature, it may not auto-ignite for a long time. Raise the temperature, and it ignites sooner. So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time, and hence the temp. of the end gas is higher for longer.
If the end gas does auto-ignite before the TFF reaches it, or before the relatively cool cylinder wall quenches the flame, the end gas auto-ignites, or detonates. Now, if one corner of this chamber is the last to receive the flame front, and indeed does detonate, what is the result? The auto-igniting mixture essentially 'explodes' and sends a pressure wave across the chamber at the local speed of sound in the cylinder. This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound. "
140air wrote:Detonation in an engine occurs when unburned gasses reach their autoignition temperature including the fact that the autoignition temperature becomes lower for gasolines as they break down under heat and time. Therefore, most of the conditions mentioned by other posters can lead to or "cause" detonation. The MAIN cause is specific to the engine. The following is largely from CF Taylor, Ricardo and others.
Tendency to detonate, general factors:
1. Low Fuel Octane Number 1. High Fuel Octane Number
2. Timing advanced 2. Timing retarded
3. Air density higher 3. Air density lower
4. Humidity lower 4. Humidity higher
5. Intake temp higher 5. Intake colder
6. Coolant temp higher 6. Coolant colder
7. Antifreeze used (high coolant temp) 7. EGR used at part throttle
8. Engine load increased 8. Engine load reduced
9. Mixture stoichiometric or lean 9. Mixture rich or extra rich, also water injection
1. Not Compact 1. Compact
2. Domed piston 2. Flat or dished piston; more compact, unobstructed
3. Low swirl 3. High swirl, circular better than tumble
4. Plug far from center 4. Plug centralized
5. No squish turbulence 5. High squish, especially aimed at plug
6. End gasses in hot spots 6. End gasses in cool areas; plug closer to exhaust valve
7. Low stroke/bore ratio 7. High stroke/bore ratio, smaller chamber diameter, shorter burn distance
8. High compression ratio 8. Low compression ratio
1. Low RPM 1. High RPM; more turbulence, shorter burn time
Some of the above characteristics work together and some are mutually antagonistic.
David Redszus wrote:Actually, squish velocity begins with piston motion and usually peaks about 7-11 deg BTC and ATC. For an engine that produces 35 m/s at 9 deg BTC, it will produce about 60% of that velocity at 20 deg BTC.
Ahh, this is problematic. It depends on how you define "squish". Classically, squish motion is a high speed jet produced by closing the space between the piston and chamber surfaces where the clearance is very tight. What is the difference in charge motion during the initial upward piston motion for a cylinder that has "squish" as opposed to one that does NOT have "squish"? How can we call one "squish motion" at this point? At what point does the charge motion begin to differ significantly (and it differs profoundly between the two cases close to TDC)? This implies squish only comes into play relatively high in the piston's travel.
You bring up a good point. When do we have squish and when do we not?
All upward piston motion produces charge motion as the piston outer edge scrapes the air from the cylinder wall and moves it inward. As the piston approaches the cylinder head, the diminishing vertical clearance produces an increased gas motion velocity which we have found convenient to call "squish."
Considering an engine with a given rpm, bore and holding squish area constant while varying the clearance we find:
at 20 BTC = 23m/s, peak squish = 90m/s
at 20 BTC = 16m/s, peak squish = 29m/s
at 20 BTC = 8.6m/s, peak squish = 10m/s
at 20 BTC = 5.4m/s, peak squish = 5.6m/s
As can be seen, as squish clearance is increased, not only does the peak velocity decrease, the velocity curve becomes flatter. At some point it becomes an horizontal straight line.
Domed pistons in a hemi chamber are an example of large squish clearances and resulting low squish velocities, unless the piston fits snuggly into the hemi dome.
http://www.engineering.leeds.ac.uk/iets ... tion.shtml[/quote']Dr. Robin Tuluie Ph.D. wrote:Chemical Soup: The Mystery of Detonation
By Dr. Robin Tuluie Ph.D.
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. Fuel 1 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. Fuel 2 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.
Abraham, J. et al., 1985, "A Discussion of Turbulent Flame Structure in Premixed Charges", SAE paper 850345 Blizzard, N.C. and Keck, J.C., 1974, "Exp. and Theo. Investigation of Turbulent Burning Model for Internal Combustion Engines", SAE paper 740191 Chin et al., 1990, "Diagnostics and Modeling of Combustion in Internal Combustion Engines," JSME, Tokyo, p. 81-86 Goudin, et al., 1987, "An Application of Fractals to Modeling of Premixed Turbulent Flames", Combustion and Flame 68, p.249-266
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