Quote:
Originally Posted by lechnoid
I agree and at the same time disagree, nothing personal. You are correct on the octane rating and the flash point of the fuel. However, the slower burn prevents knock in a number of ways beyond raising the flash point. Knock can be caused by lots of different things as you're well aware. Hot carbon chunks, hot spark plug bases, bad plug wires, excessive engine heat, and many others can all lead to detonation. However, even on an engine running at normal temperatures with no carbon build up or any other flaw or defect, knock WILL still occur, albeit minor. This is a less serious example of the fuel in the combustion chamber igniting at both the piston and the plug at the same time. Picture a combustion chamber from the side, with the plug being top center. In lower octane fuel, due to the high rate of burn speed, it is possible for the center to not get burnt as fast as the sides. As the sides burn together, the now separate flames are able burn into each other, colliding and causing knock. This will produce enough of a signal for the knock sensor to respond, but not with enough voltage that the computer will care to do anything about it. A slower burn reduces the frequency of this occurring.
As for the effect on the cats, the slower burn means that by the end of the exhaust cycle, some of the fuel is still burning or is not burnt as it leaves the engine. Without tuning adjustments for the octane change, this will cause a rich condition. Running consistently rich is one of the most frequent causes of catalyst failure. Think of a muscle car running a short, wide open exhaust running on race fuel. I know I've seen flames come out the rear. There would be no flames if the fuel was fast-burning.
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OK, you are still relating the burn rate of the fuel to the Octane rating.
The burn rate of the fuel is dependant on, the temp, the pressure, the turbulence, and the mixture strength. The octane rating has no effect on the burn rate.
Lean mixtures take longer to burn, thats why we can have backfiring out the intake with lean mixtures, flame out the exhaust, and burnt exhaust valves, if the spark is not advanced enough to accomodate the slower burn.
You are confusing two different things, pre-ignition, and detonation. Pre-ignition is when a glowing carbon piece ignites the mixture before the plug fires. This won't cause knock, it just starts the burn from two points, the same as an engine with two plugs per cylinder does. If this state continues it can lead to burning a hole in the piston due to the earlier ignition and the extended heat period that the cylinder has. The octane rating has no effect on pre-ignition.
Detonation is when the plug starts the burn, and the flame front moves out across the combustion chamber, raising the radiated temperature and the pressure that the mixture, that is furthest from the plug has to endure.
If the octane rating is not high enough this end gas will spontainously combust (explode), this instainous pressure rise, causes the sound we call Knock/detonation. It is the octane rating of the fuel that allows it to resist this condition, and reduce knock. The higher the octane the less tendenacy to knock.
b3lha Quote:
Originally Posted by oab_au
...we would have to change the spark advance, with different octanes.
Isn't that exactly what the ECU does? Quote:
Yes Phil, but not because of the burn rate.
We retard the ignition to reduce the amount of time that the end gas has to endure the temp/pressure, to reduce the knock. This also reduces the torque.
Using the highest octane that the engine needs, achives the highest pressure acting on the piston, and the highest torque.