Sonreir said:
Caveat #2 - Detonation and Preignition
One of the effects of additional compression is additional heat. This heat is not only what provides the increase in power, but it can also cause two other issues. These issues are detonation and preignition. Both of these problems will destroy a motor in short order (especially preignition) and so neither are acceptable.
Detonation is the spontaneous combustion of the remaining fuel/air mix after the normal combustion process is nearing completion. This is caused through the heat and pressure initiated during the combustion process and as both heat and pressure rise, it will get to a point that the molecules within the mix are pounding into one another so violently that they ignite, themselves. Death by detonation usually results in broken rings or ring lands.
Preignition differs from detonation in that it's not so much as a spontaneous combustion of the mixture. Preignition is a begin to the combustion prior to ignition from the spark plug. Preignition usually occurs when a part or parts of the combustion chamber heat up too much. This can be anything from an excess of carbon deposits (not usually an issue on a freshly assembled engine), damage to the exhaust valve, or an overheated spark plug. What happens in this case is that whatever causes the preignition has heated up to a point where it actually starts the combustion of the fuel/air mix before the spark plug fires. This causes cylinder pressures to rise too early (sometimes when the piston is still approaching TDC) and so peak cylinder pressures occur too early in the cycle. This causes greatly increased stress on engine components and will usually kill an engine a lot earlier than detonation will. Engine failure due to preignition will almost always result from holes in a piston.
Though there are many ways to combat detonation and preignition, those will be saved for a later post. For now, just be aware they can be potential problems and the most common method of dealing with these issues is to use high octane fuel. Consider premium gas to be the only acceptable fuel for a high-compression engine. Better to push the bike home than fill it with regular. Also, it's important to note that high compression engines are MUCH LESS tolerant to running lean than the stock factory offering. Most stock engines will run all day long on a lean mix, but a high compression engine at WOT will blow up right in your face as soon as the float bowls start to get even a little shallow.
Caveat #3 - Combustion Speeds and Timing
This particular issue can be hit or miss depending on how you've achieved your increase in compression, but it's unlikely you'll be able to avoid these effects all together.
First up, it should be known that increasing compression increases the combustion speed of the mixture within the cylinder. This is generally considered a good thing. <snip>
I'll admit I haven't read every word of every post on every page so maybe this got addressed… but I missed it if it got discussed.
In your description of what detonation is, you left out the concept of a delay period. this gets important later so let me ramble a bit here. The mixture in the cylinder is exposed to high temp and pressure. It can only resist this temp and pressure for a given amount of time before it detonates, as in , there isn't a given combination where it explodes, there is a given amount of time for various conditions before it explodes.
A common misconception I see is people thinking that detonation occurs because the flame front moves too fast. this isn't true. Detonation is sudden, everything goes BANG rather than burns. This creates a shock wave that bounces around in the cylinder and makes the knocking noise which identifies the condition. it also knocks the boundary layer off the piston, which removes the insulating properties of the boundary layer and lets the piston heat up, which weakens it which leads to damage. There is no flame front when a cylinder detonates I used to have a text that had pictures of detonation happening. you could see the flame, then BOOM. nothing.
Which was all supposed to tie in to something I saw in another post - that high octane gas burns slower than low octane gas.
My challenge is for someone to post a technical reference, that means not Hot Rod magazine or Wikipedia, etc, that claims there is a significant difference in the flame speeds of high and low octane pump gasoline, or that flame speed is the method used to control detonation.
Here's the thing. Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period. You want the mix to burn as fast. Hence all the other mods get get "more complete combustion" like dual plugs, swirl in the intake charge, MSD ignitions, that kind of stuff. It's all designed to burn the mix faster. You want to burn it all before the delay period expires and it goes boom. Why would it make sense to make the gas itself burn slower? A higher octane gasoline resists autoignition due to high temp and pressure longer than a lower octane gas does. it gives you more time to burn the mix but does not burn it slower. You want to reduce detonation? Lower the temp, lower the pressure, burn the mix FASTER, or make the gas more resistant to autoignition.
To address a few misconceptions I see (not necessarily here) Slow flame speeds do not prevent detonation, they make it worse. Fast flame speeds do not cause detonation, they reduce it. Multiple flame fronts coming together do not cause detonation.
Why should anyone believe me? No reason. So here are some sources I found earlier (and posted on another site) concerning this. Read the references, note the sources, and think about it. Make your own decisions.
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I've seen this basic write up on a couple different race fuel supplier sites.
http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel-
Octane number is not related to flame (burn) speed either. Variations in octane quality are independent of flame speed. There are some high octane gasolines in the marketplace with fast flame speeds and some with slow flame speeds. It depends on how they are put together. We prefer fast flame speeds because we know that a properly tuned engine will make more power on this type of gasoline than one that has a slower flame speed.
Another link from a fuel supplier
http://www.pinux-products.com/octane-rating/
pinux products said:
It should be noted that octane rating does not relate to the energy content of the fuel (see heating value), nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel’s resistance to autoignition. It is for this reason that one highly branched form, or isomer, of octane (2,2,4-trimethylpentane) has (by definition) an octane rating of 100, whereas n-octane (see octane), which has a linear arrangement of the 8 carbon atoms, has an octane rating of -10, even though the two fuels have exactly the same chemical formula and virtually identical heating values and flame speeds.
<snip>
Octane rating has no direct impact on the deflagration (burn) of the air/fuel mixture in the combustion chamber. Other properties of gasoline and engine design account for the manner at which deflagration takes place. In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating. Deflagration is the type of combustion that constitutes the normal burn. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not deflagration characteristics.
Check out Section 6.3 of this university paper:
http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html
The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:-
The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.
The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane.
Sunoco Race Fuels says this:
http://www.racegas.com/article/10
Sunoco said:
Naturally aspirated race motors with large combustion chambers spinning at high RPMs really like high-octane, fast burning fuels. They need the octane to prevent uncontrolled combustion, and they need a fast-burning fuel so that the flame front can span the large bore of the combustion chamber quickly. <snip>You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!
Any opinion can be supported by links to internet sites. But it also makes no sense that a slower burning mixture would be better for controlling detonation. Flame fronts, even fast ones, are a controlled burn, detonation is an explosion.
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No flames intended here, I've just seen the slow burning gas claim so often and no one ever questions why we would do so many mods to make the mixture burn faster, then put a slow burning mixture in there.
opcorn:
