Variable Ignition Timing Based on Knock Sensors

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Originally Posted By: bbhero
Yes knock resistance.... AKI... But what causes the knock???? Ignition too quickly right??? Or no?? What does octane really do??? Slow down or create resistance to ignition... Does that not have a relationship to how fast something ignites?? It does I think.. maybe. Most regular people think of volatility in terms of how easliy something or some substance can catch fire or blow up. Water is not as volatile as gasoline. But in your definition of volatility it would be the other way around. Which would be correct. Strange in a way but true if taken to a very technical level.
You see ................ War and Peace
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Knock is the unwanted early spontaneous combustion of an air/fuel mixture (because of enough pressure present in the combustion chamber and way before the spark ignition point has been reached and triggered) once the inlet valve has been closed (volatility totally irrelevant at this stage because the mixture has already been prepared and its only sitting there waiting to be ignited and the resulting pressure/heat to be used)
 
You added something that makes a whole lot of sense... "In the absence of spark ignition"... That is what octane does truly help with. Now that makes a whole lot of sense to me. I understand that. Question remains what causes too early ignition?? Hot Carbon deposits left behind?? Why do some cars recommend or even require 91+ octane?? Isn't that still related to say regular timed ignition for those vehicles?? With spark induced ignition?? Why does it matter if those vehicles use 93 or 87?? It shouldn't matter then correct??
 
Originally Posted By: bbhero
You added something that makes a whole lot of sense... "In the absence of spark ignition"... That is what octane does truly help with. Now that makes a whole lot of sense to me. I understand that. Question remains what causes too early ignition?? Hot Carbon deposits left behind?? Why do some cars recommend or even require 91+ octane?? Isn't that still related to say regular timed ignition for those vehicles?? With spark induced ignition?? Why does it matter if those vehicles use 93 or 87?? It shouldn't matter then correct??


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Question remains what causes too early ignition??
............... simple - too much pressure (read compression ratio and think multiply atmosphere pressure by that ratio because of combustion chamber size)

Hot carbon can start to glow ............... its presence can result in an increased compression ratio (smaller combustion chamber) and given ideal conditions it can start to glow in the combustion chamber causing ignition soon as the right fuel/air mixture is present - even before inlet valve closure.

Most manufacturers spec a specific fuel quality (RON/MON) because the engineering effort involved in making an engine cope with any old garbage fuel efficiently is simply too much in the eyes of the engineering staff and the bean counters will have a fit
 
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I think all that makes a whole lot of sense. Olas and you did a very good job explaining everything.
 
Originally Posted By: bbhero
Isn't 87 octane more volatile than 93?? Would the ECU actually advance timing for lower grade 87 due to it being more volatile?? And pull back timing for 93 being that it is less volatile?? Therefore the ECU would take more advantage of the 93 by pulling back timing and actually compensate for 87 being advancing the timing.


No

No

No

Think of knock as compression ignition before It is lit by spark. 87 is more prone to this than 93 and burns slightly faster

93 lets you advance the timing because it burns slightly slower so you can light the fire a little earlier for optimum peak pressure on the power stroke.

87 has to be lit a little later because it burns a little faster AND it will more readily auto ignite. If you light it too soon you just added heat and pressure and instead of a smooth flame front in the cylinder as fuel burns, you can get a second front from a portion of the fuel auto igniting from heat and pressure. When these two flame fronts meet, you get knock.
 
I was just hypothesizing there Captain. That's all. Nothing more.

I am not giving you a hard time here. I was just "thinking out loud" so to speak.
 
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Spark knock occurs after the spark plug has ignited the fuel/air mix.
The flame front has traveled only part way through the combustion chamber.
The remaining unburned fuel/air is undergoing high neat and compression.
This pocket ignites essentially all at once, causing a sudden spike in chamber pressure.
This all happens in a few milliseconds or less.
When the spark is advanced before TDC the piston is moving up as the flame front starts to travel through the chamber.
With more advance there's more piston motion to raise the pressure of the unburned fraction, and more tendency to knock.
Pre-ignition occurs before the spark, so ig timing has no direct effect on it.
 
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Originally Posted By: bbhero
Isn't 87 octane more volatile than 93?? Would the ECU actually advance timing for lower grade 87 due to it being more volatile?? And pull back timing for 93 being that it is less volatile?? Therefore the ECU would take more advantage of the 93 by pulling back timing and actually compensate for 87 being advancing the timing.


Volatility and octane rating are unrelated. Case in point, butane, IIRC, has a relatively high octane rating, though it is by definition highly volatile, being a gas at room temperature. (I mean an ACTUAL gas, unrelated to US usage of "gas" for automotive spirit, which is a liquid)
 
Originally Posted By: LvR
Given enough pressure just about any fuel will spontaneously combust in ideal conditions (read possible knock condition if it happens at an unwanted position of the piston travel) and its exactly this phenomena that timing optimization needs to prevent. (Note that most engines actually make use of the fuel mixture's ability to combust at certain pressures created by and after the initial spark ignition point in order to improve flame speed propagation and burn efficiency in the combustion chamber)

Like this? https://en.wikipedia.org/wiki/Fire_piston
 
I use to tune with SCT software for Fords back in the early 2000s. I would set up the retard to be a big number so it would be really noticeable. Then start bumping up the timing to see when it knocked. Adjust it from there. But the time to bring timing back to your goal number is so many cycles without any knock. So it's almost immediate that the timing comes back with higher octane gas.

Usually you set the timing high say 50 degrees or so. The ECU takes it from there. But there are also special settings like idle and cold start among others. There are tables that have peak advance or retard per load and rpm.

Timing is also used for idle speed control and cruise control.
 
Originally Posted By: Ducked
Volatility and octane rating are unrelated. Case in point, butane, IIRC, has a relatively high octane rating, though it is by definition highly volatile, being a gas at room temperature. (I mean an ACTUAL gas, unrelated to US usage of "gas" for automotive spirit, which is a liquid)

As such, in countries where vehicles running LPG / Propane are common, the butane used is actually mixed with another gas to help reduce consumption by a degree.
 
Originally Posted By: SHOZ
I use to tune with SCT software for Fords back in the early 2000s. I would set up the retard to be a big number so it would be really noticeable. Then start bumping up the timing to see when it knocked. Adjust it from there. But the time to bring timing back to your goal number is so many cycles without any knock. So it's almost immediate that the timing comes back with higher octane gas.

Usually you set the timing high say 50 degrees or so. The ECU takes it from there. But there are also special settings like idle and cold start among others. There are tables that have peak advance or retard per load and rpm.

Timing is also used for idle speed control and cruise control.


Good information. Thanks.
 
I found some old screen shots for the timing tables and KS advance and retard rate for my '97 Taurus SHO 3.4L V8.

This appears to be a WOT standing start run through 3rd gear, rpm on the left.

rpm-and-spark.jpg


And here is the same run with spark on the left.

spark-and-rpm.jpg


And here is the primary timing table.

base-timing-table.jpg


Here i one from the turbo charged 2010 SHO. Not the diffrence in load values on the left due to boosted intake pressure.

2010-sho-timing-table.jpg


The KS advance rate '97.

ks-advance-rate.jpg


And KS retard rate, This appears to be rather restrained. I use to sometimes put large values in here so that when it knocked you would really notice. But it takes too long to recover.

ks-retard-rate.jpg


This is a MBT timing table. Something Ford used to set the very base timing for peak torque.

mbt-spark-table-1998-sho.jpg
 
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