#3 - Detonation, Hypereutectic Pistons, Timing....

[DAMN YANKEE]

There are essentially two classes of stock Gen2 Vipers out there to my mind, those with “hypereutectic” pistons and those without them. In 2000, Dodge wanted to add some additional performance to their new millennium Viper as well as higher efficiency and environmental standards. Among other things, the engineers decided to add new Hypereutectic pistons to all models. Those who bought Vipers in the production years 2000 and 20001 have hypereutectic pistons.

We read about hypereutectic (sometimes referred to as cast) pistons in the VCA threads all the time. These threads usually refer to concerns by Viper owners of those years as it relates to the limitations of cast pistons, especially in forced induction engines. These are real concerns and we need to understand why.

So what are hypereutectic pistons? Simply put they are cast pistons as opposed to being forged. Almost all pistons have aluminum in them. By being cast, hypereutectic pistons are able to have a much higher amount of silicon in the alloy. This silicon greatly increases the thermal stability and, as a result, these piston’s dimensional tolerances can be much tighter (most cast piston have a skirt clearance of approximately .0007 to .0009, forged about .005 to .007). Their expansion rates are much smaller. When cold, these pistons are much tighter than forged pistons (especially at the skirt and towards the top where the rings are), therefore they rock less, their rings seal better and they have less blow-by. They are lighter and quieter as well. And all that silicon makes them harder (unlike higher aluminum content in forged pistons) so that they don’t “scuff up” the cylinder walls when cold and in a low oil start-up. But, all that goodness comes at a price, and that price is brittleness. Hypereutectic are much more brittle than low silicon forged pistons.

Now, in a stock Viper, these pistons are well within operational spec and all the really superb aspects of hypereutectic pistons comes into play. The 2000 and 2001 Vipers have great pistons, no doubt. But, when we begin to modify our Vipers we have to recognize the very real limitations of the stock hypereutectic pistons. Simply put, if you are going to go over 700hp, you won’t be doing it with hypereutectic pistons and anything over 650 hp will require real tuning precision if you use hypereutectic pistons. Are hypereutectic pistons inherently weak or of inferior quality or used to cut costs, etc, etc? Not at all, they are a superior piston when used in the right application. Can you supercharge a Viper with hypereutectic pistons? Heck yea, mine is and its fabulous! If I have them and I run a supercharger do I need to constantly be on gaurd and worried? No and no, proper boost (5lbs no W/M or 6.5 lbs with W/M), proper A/F, proper timing/tune and you are going to have a great, stable rig!

What’s the danger? Detonation. What’s that? Lots of people talk about it, few people understand it. Detonation is simply one of two kinds of combustion:

Good Combustion (Deflagration):
In an internal combustion engine, we provide the right mixture of fuel and air. That mixture provides enough air and enough fuel to burn completely. That fuel is ignited in the cylinder by the spark plug just before the piston comes all the way up in its compression cycle to Top Dead Center (TDC). In other words, we are igniting the proper air mix in the last stages of compression. When all is well, the spark plug fires, creating a tiny ignition flame the size of the gap and, believe it or not, in the first few milliseconds that flame struggles to survive, then it spreads across the face of the compressed air/fuel and triggers a uniform, consistent fuel burn that carries over TDC and down through the power stroke. The burn is controlled and pressures rise smoothly and just as the piston starts its downward movement. Nice. People, when we talk about “ignition timing”, this is it! Where does the spark plug fire in the pistons cycle….. “Retarding” or “Advancing” the timing! There it is!

Bad Combustion (Detonation)
You can probably guess this, right? In detonation there is no uniform flame front moving efficiently through the air/fuel mix. Rather, due to a number of very identifiable and resolvable issues the unburned air/fuel mixture is compressed and instantly ignited creating enormous pressures. Simply put, we have an explosive, instantaneous explosion near the top of the piston stroke, too early, with a massive pressure wave. More than enough to blow up a hypereutectic piston. One can see, if we were to delay or “retard” the spark until the piston is farther through its cycle we could, inpart overcome detonation (piston already moving down through the stroke, etc.)

Lastly, we need to know that Detonation is NOT “pre-ignition” or “dieseling.” That is when forces other than the spark plug is firing off the fuel/air mixture. That’s why you can turn off your key on some cars and the engine still runs….poorly.

Forged pistons, on the other hand, can take this kind of abuse (though good timing is good timing) and keep on going. We should also note that a number of superb houses produce
remarkable new hypereutectic pistons for massive engines....very different than those found in those earlier Vipers!

What can we do about Detonation?

1. Better Timing (retarding)
2. Higher Octane and a
3. More perfect Air/Fuel
4. Cooler Fuel/Air incoming mix (W/M)
5. Cooler Plugs
6. Lower compression

and more....

 

[dansauto]

"Simply put, we have an explosive, instantaneous explosion near the top of the piston stroke, too early, with a massive pressure wave"

Not exactly, detonation is the spontaneous combustion of the remaining gas in the chamber, it occurs AFTER the normal combustion occurs not before. The heat and pressure ignite the remaining air/fuel in the chamber.

 

[DAMN YANKEE]

Man, I don't whether I should just sit down and shut up as you are the first person to even leave a comment.....

dansauto, is there the first point ignition of the initial flame kernel?...yes.
Are the posterior piston crown fractal detonations and pressure waves occuring simultaneously and meeting the pressure band from the kernel reaction? yes.
Does it matter....not to most readers, but man do I appreciate somebody taking the time to contribute.
My original key word was..as you quoted "Simply put"...I wanted to keep this simple and let people have a well founded sense of confidence as they
approach tuning. As everybody at every level of skill can always learn more, here is a good write up from Dr. Robin Tuluie Ph.D.

I'll jump into the piece and start where you picked up on the process...

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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. (this is where you can say "I told you so!" )

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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.

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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. (This is where I say "it takes both conditions" ). Whenever these colliding pressure fronts meet (as in happening together, instantaneously!), 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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Whatever the reasons, SImply put...its bad motor mojo!


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[plumcrazy]

DY, keep typing. im reading and learning. but not replying much at all.

thanx

 

[RMBSRT]

Thanks for the info...very interesting and helpful..

 

[dansauto]

I orginally thought detonation happend before the spark, and that is not that case, I just wanted to point that out.

 

[SYNFULL]

Great stuff. I am learning a lot from you.

 

[Fatboy 18]

Very good post, Thanks

MARK


2000 Creampuff owner

 

[ViperGeorge]

Ok, now I have a headache. Wow is this stuff complex or what?