Excellent reading material part 2

[PassionRE]

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 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.

 

 

 

 

 

 

 

 

 

 

 

 

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