Mr. Banish, am I understanding your book correctly/timing advance and high octane ?

[angrygoat]

So everyone seems to think that running higher octane fuels ALLOW you run more timing. Now Im not sure if its a misnomer but if I understand what I have read, when running a higher octane fuel you HAVE to advance the timing. The advantage to running higher octane fuels is they release more heat during the combustion process and there by increase the pressure that is exerted on the piston. However, the chemical bonds in higher octane fuels are stronger than that of pump gas and therefore it takes longer to break those bonds. The increase in time it takes to break these bonds is why the timing advance is needed to get the peak cylinder pressure to occur in that 15-17 ATDC window. Is that about right?

[bigwillys58]

i too am intrested in this! my understanding was that higher octane "resisted heat" or resisting spark knock allowing you to run more timing and/or compression... i really need to git my ass in gear and git that book...

[angrygoat]

It is an awesome book. Higher octane does resist spark knock because it burns slower...again if I understand correctly. hopefully Greg will chime in and clear this up for us

[angrygoat]

anyone else care to chime while waiting for Greg

[Mez]

It is an awesome book. Higher octane does resist spark knock because it burns slower...again if I understand correctly. hopefully Greg will chime in and clear this up for us

Here is the definition of Octane per Wikiopedia:

OCTANE RATING - A measure of a gasoline's resistance to exploding too early in the engine cycle, which causes knocking. The higher the rating, the lower the chance of premature ignition.

Notice it did not say anything about burning slower.

[angrygoat]

True, but it also doesn't say how it resists knock.


From Greg's book:
"Fuels with a higher octane number have a stronger concentrations of these more robust molecules versus the more easily broken longer chains. Thus, they require more energy to break these bonds,, and have more resistance to knock. An interesting side note is that the energy required to break these bonds comes in the form of heat during combustion. Heat transfer from one molecule to the next requires a small amount of time. Even though this is a very small amount of time, more total transfer means more time for it to happen. Adding more heat to the molecules in order to separate their ions means more time is required. This means that the burn rate is slightly slower for high octane fuels. This becomes important later when we choose spark advance timing."

[S2H]

True, but it also doesn't say how it resists knock.


From Greg's book:
"Fuels with a higher octane number have a stronger concentrations of these more robust molecules versus the more easily broken longer chains. Thus, they require more energy to break these bonds,, and have more resistance to knock. An interesting side note is that the energy required to break these bonds comes in the form of heat during combustion. Heat transfer from one molecule to the next requires a small amount of time. Even though this is a very small amount of time, more total transfer means more time for it to happen. Adding more heat to the molecules in order to separate their ions means more time is required. This means that the burn rate is slightly slower for high octane fuels. This becomes important later when we choose spark advance timing."

your resistance to knock in in the very definition....knock is a predetonation...
heat alone can cause predetonation...
hell... a diesel engine is based on detonation
when you get knock from timing its because you added to much timing advance and your fuel is igniting before TDC...
advance timing meant to make it happen sooner...retard timing means to make it happen later...
if you get knock...then you have the spark so far advance that combustion is happening before top dead center....counter productive and extrememly stressful on your engine
think of it like passing a heavy rolling object(in my world its usually a road case full of gear...)
if somebody shoves it towards me and I get in front of it.....its extremely had to stop it and then restart...
instead..if I just let it pass me slightly and then go about continuing to push it...life is a lot easier..

same thing happen in our engines...
fire spark too son and combustion happen before top dead center...and we are trying to get in the way of motion.... if we fire after top dead center then we get the nice add to our already existing motion....

now ...knowing that..because of the "slower burn"..it resists knock becasue it takes longer to combust...which means we have to start our spark sooner....so you can ad more timing without it causing knock....

this is basic engine dynamics

[Redline MS]

I can tell you Greg is out of town until tonight so I would expect a response later tonight at best.

Howard

[eficalibrator]

First and foremost high octane fuels resist knock better than low octane fuels. This is the definition of octane number.

The other observation I've had is that when going from 93 octane to a much higher octane fuel like C16, I've seen a lot of engines that respond at part and full throttle as if they were under-sparked if no calibration changes are made. A/F is the same, but EGTs are high and torque is off. Think about this for a second. Where must the fuel be when it's burning if this is the case? Most likely, not entirely in the cylinder. If we started the burn at the same time (same spark calibration, right?), then the reason some of the combustion is happening late is that it's not burning at the same rate as our reference fuel. Increase the timing a bit, and we get back to our reference torque and EGTs. If the engine was knock limited before, we can also now go a little further into previously uncharted territory and run closer to MBT at WOT, even under boost.

Going from 87 to 93 usually isn't enough to see any drastic effect on burn rate, but going up to race gas can do it. As always, it's dependent upon the engine combination and not all engines will have a noticeable effect.

[angrygoat]

your resistance to knock in in the very definition....knock is a predetonation...
heat alone can cause predetonation...
hell... a diesel engine is based on detonation
when you get knock from timing its because you added to much timing advance and your fuel is igniting before TDC...
advance timing meant to make it happen sooner...retard timing means to make it happen later...
if you get knock...then you have the spark so far advance that combustion is happening before top dead center....counter productive and extrememly stressful on your engine
think of it like passing a heavy rolling object(in my world its usually a road case full of gear...)
if somebody shoves it towards me and I get in front of it.....its extremely had to stop it and then restart...
instead..if I just let it pass me slightly and then go about continuing to push it...life is a lot easier..

same thing happen in our engines...
fire spark too son and combustion happen before top dead center...and we are trying to get in the way of motion.... if we fire after top dead center then we get the nice add to our already existing motion....

now ...knowing that..because of the "slower burn"..it resists knock becasue it takes longer to combust...which means we have to start our spark sooner....so you can ad more timing without it causing knock....

this is basic engine dynamics

Im not sure if you meant that literally but I have not seen timing in referrence to ATDC rather than BTDC. I have read that you want peak cylinder pressure to occur just after TDC but if you dont start the combustion process before TDC, the piston will just be running away from the combustion mixture.

On the subject of knock, I always understood knock to be an uncontrolled ignition of the mixture where as normal ignition is a controlled flame front that moves across the cylinder. There is pre-ignition where the mixture ignites before the spark plug fires and there is detonation where a portion of the mixture ignites before the flame front reaches that portion of the mixture. With either one of these, I would think you would not see after TDC. I would think that even though you want peak cylinder pressure around 15* ATDC and given that it takes time for the gasses to expand to reach peak cylinder pressure, that the mixture would have been burned already; given the correct timing.

Thoughts?

[S2H]

Im not sure if you meant that literally but I have not seen timing in referrence to ATDC rather than BTDC. I have read that you want peak cylinder pressure to occur just after TDC but if you dont start the combustion process before TDC, the piston will just be running away from the combustion mixture.

On the subject of knock, I always understood knock to be an uncontrolled ignition of the mixture where as normal ignition is a controlled flame front that moves across the cylinder. There is pre-ignition where the mixture ignites before the spark plug fires and there is detonation where a portion of the mixture ignites before the flame front reaches that portion of the mixture. With either one of these, I would think you would not see after TDC. I would think that even though you want peak cylinder pressure around 15* ATDC and given that it takes time for the gasses to expand to reach peak cylinder pressure, that the mixture would have been burned already; given the correct timing.

Thoughts?

yes...Timing Advance = before top dead center....
if you have negative #'s then you are starting to fire After top dead center

I ddnt say spark meant combustion happened exactly then..it takes time to develop..which is why we need timing further advanced at higher RPM's...faster piston speed means we need to fire spark sooner to get the flame to happen at the right time....


heres a good wiki article...
http://en.wikipedia.org/wiki/Ignition_timing