Excellent reading part 3

[PassionRE]

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.