Damn! I don't have to go to Webster's very often, but "Stoichiometric" sent me diving for the Google button. So, I now know that Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burned completely.
I am old school simple... but I learned that spark knock, AKA Pinging, was caused by the cylinder(s) firing off time... specifically BTDC. As the piston, mechanically locked by the rod, rod end bearings etc., rose towards TDC, the timing circuits fired the Fuel/Air mixture in the combustion chamber at precisely the right moment to ADD the power of that piston's power stroke to the overall shaft horsepower. Should the combustion happen at the wrong time, say, Before Top Dead Center... the power generated by the power stroke is counter productive for the period of time that the cylinder is trying to drive it's respective crankshaft journal in the opposite direction that it is going. Even though at 5,000 rpm the amount of time is very small... it is still enough to slap all the slack out of the mechanical connections. The Wrist Pin, lower Rod Bearings, Valve Train and possibly the mains all feel the power going in the reverse direction. Depending on the wear and available wiggle room, that slap can be loud enough to be heard as knocking or pinging.
What causes the mis-timed combustion? I was taught that, barring actual mis-adjustment of the timing itself... it can be caused by carbon deposits in the combustion chamber, holding enough heat to cause spontaneous combustion (dieseling) leading to the mis-timed combustion. Or, the fuel/air mixture itself can be off, leading to a mis-timed and thereby stoichiometric (<-- HA! <G>) combustion due to pressure driven volatility and early combustion (or dieseling) as the piston nears TDC.
I was taught that Octane is merely a measurement of how combustible the fuel is, flash point wise. Higher octane yeilds higher flash point, which in turn increases the heat the fuel air mixture will tolerate before spontaneously combusting. The higher the flash point, the more compression you can engineer into the motor. Stroke, bore and piston head geometry are what yields higher output... not the fuel octane. The higher octane just lets you play with ways to make the power stroke punch more without the mixture firing when you don't want it to... it will fire precisely when you ignite it, not before and not after.
Pretty simple description, but that's my understanding of the process. Anyone that can correct or add to my understanding of the power generation cycle is welcomed to correct and/or add away. But based on my understanding, both of the process and Riku's problem, I don't see why the thread is going in the direction that it is. All the really cool but esoteric discussions about octane and the like is REALLY interesting, but it doesn't address the problem as I understand it. Spark Knock, in my experience, is a problem when you are accelerating and all the situations I described above come into play. Why would it knock when going from an idle to "Tapping the throttle"? It doesn't fit the basic scenario for spark knock in a classic sense. I am under the impression that something is mechanically wrong in the power train, and it has nothing to do with Octane or anything else. But that's just my humble opinion <G>.
Falcon
I am old school simple... but I learned that spark knock, AKA Pinging, was caused by the cylinder(s) firing off time... specifically BTDC. As the piston, mechanically locked by the rod, rod end bearings etc., rose towards TDC, the timing circuits fired the Fuel/Air mixture in the combustion chamber at precisely the right moment to ADD the power of that piston's power stroke to the overall shaft horsepower. Should the combustion happen at the wrong time, say, Before Top Dead Center... the power generated by the power stroke is counter productive for the period of time that the cylinder is trying to drive it's respective crankshaft journal in the opposite direction that it is going. Even though at 5,000 rpm the amount of time is very small... it is still enough to slap all the slack out of the mechanical connections. The Wrist Pin, lower Rod Bearings, Valve Train and possibly the mains all feel the power going in the reverse direction. Depending on the wear and available wiggle room, that slap can be loud enough to be heard as knocking or pinging.
What causes the mis-timed combustion? I was taught that, barring actual mis-adjustment of the timing itself... it can be caused by carbon deposits in the combustion chamber, holding enough heat to cause spontaneous combustion (dieseling) leading to the mis-timed combustion. Or, the fuel/air mixture itself can be off, leading to a mis-timed and thereby stoichiometric (<-- HA! <G>) combustion due to pressure driven volatility and early combustion (or dieseling) as the piston nears TDC.
I was taught that Octane is merely a measurement of how combustible the fuel is, flash point wise. Higher octane yeilds higher flash point, which in turn increases the heat the fuel air mixture will tolerate before spontaneously combusting. The higher the flash point, the more compression you can engineer into the motor. Stroke, bore and piston head geometry are what yields higher output... not the fuel octane. The higher octane just lets you play with ways to make the power stroke punch more without the mixture firing when you don't want it to... it will fire precisely when you ignite it, not before and not after.
Pretty simple description, but that's my understanding of the process. Anyone that can correct or add to my understanding of the power generation cycle is welcomed to correct and/or add away. But based on my understanding, both of the process and Riku's problem, I don't see why the thread is going in the direction that it is. All the really cool but esoteric discussions about octane and the like is REALLY interesting, but it doesn't address the problem as I understand it. Spark Knock, in my experience, is a problem when you are accelerating and all the situations I described above come into play. Why would it knock when going from an idle to "Tapping the throttle"? It doesn't fit the basic scenario for spark knock in a classic sense. I am under the impression that something is mechanically wrong in the power train, and it has nothing to do with Octane or anything else. But that's just my humble opinion <G>.
Falcon
