Optimal ignition advance

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Optimal ignition advance

Postby maych87 » Thu Sep 14, 2017 9:12 pm

I am new to all this and I am trying to get a full understanding of how everything works. Is optimal ignition advance set so ignition happens as far as possible before tdc on compression to allow complete burn of the air fuel mixture to happen just after tdc on the power stroke. Does that allow for maximum power? From the reading I have done if it is to far advance the end of the fuel mixture burn is at tdc or before tdc which causes knocking.


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Re: Optimal ignition advance

Postby Matt Cramer » Fri Sep 15, 2017 5:59 am

When detonation is not a factor, getting the best timing is a balancing act between having the pressure rise too fast enough on the compression stroke and not having it rise fast enough on the power stroke. Too soon and the engine has to use up some of its power to compress the burning fuel charge. Too late and the burn isn't completed in time to do useful work on the piston. Typically the best timing causes the in-cylinder pressure to reach a peak around 10 to 12 degrees after TDC.
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Re: Optimal ignition advance

Postby maych87 » Fri Sep 15, 2017 7:15 am

Maybe this is a stupid question but when setting your ignition map how do you know when the pressure is peaking around 10 to 12 degrees after tdc. I am assuming that's the point where it creates the most efficient work on the power stroke?(pushes down on the piston most efficiently)


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Re: Optimal ignition advance

Postby Dennis_Zx7r » Fri Sep 15, 2017 7:22 am

What I plan to do is locating the timing of best torque, and using the least amount of spark advance necessary to achieve. As I understand this is the "casual" way of doing it on a dyno.
I know the 10°ATDC figure for 50%MFB, but I don't think many people have cylinder pressure sensors available to measure the data.
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Re: Optimal ignition advance

Postby billr » Fri Sep 15, 2017 11:12 am

I think looking at torque is actually the best way, not a "casual" compromise at all. Reading cylinder pressure, crank angle and then doing the math to calculate the integral of the crank moment is still going to kind of an estimate. Things like friction due to side thrust on the piston, friction in bearings, and even exhaust back pressure will be variable and should be factored in. Measuring torque does all that automatically.
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Re: Optimal ignition advance

Postby Yves » Fri Sep 15, 2017 2:49 pm

billr wrote:I think looking at torque is actually the best way, not a "casual" compromise at all. Reading cylinder pressure, crank angle and then doing the math to calculate the integral of the crank moment is still going to kind of an estimate. Things like friction due to side thrust on the piston, friction in bearings, and even exhaust back pressure will be variable and should be factored in. Measuring torque does all that automatically.


Without going into debate here : from what I've been told by people that used it, in a non-detonation limited engine, the point where the ppp is, usually falls between 10 and 15° ATDC. There is some obvious variation due to the fact that the combustion never is equally fast. So a fixed torquepoint is not possible. What you see on the dyno is an average of all cylinders. Whereas what you see with pressure sensors is actual data of one cylinder.
You can see a dyno as some sort of smoothing of the data.
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Re: Optimal ignition advance

Postby maych87 » Fri Sep 15, 2017 3:59 pm

Yves wrote:
billr wrote:I think looking at torque is actually the best way, not a "casual" compromise at all. Reading cylinder pressure, crank angle and then doing the math to calculate the integral of the crank moment is still going to kind of an estimate. Things like friction due to side thrust on the piston, friction in bearings, and even exhaust back pressure will be variable and should be factored in. Measuring torque does all that automatically.


Without going into debate here : from what I've been told by people that used it, in a non-detonation limited engine, the point where the ppp is, usually falls between 10 and 15° ATDC. There is some obvious variation due to the fact that the combustion never is equally fast. So a fixed torquepoint is not possible. What you see on the dyno is an average of all cylinders. Whereas what you see with pressure sensors is actual data of one cylinder.
You can see a dyno as some sort of smoothing of the data.


Maybe another dumb question. In a knock sensor system as far as what I can understand it advances the timing until it knocks then it backs it off. Is that how it is done on a dyno as well? Will backing of the advance slightly from knock give you that 10 to 15 degrees after tdc and maximum torque?


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Re: Optimal ignition advance

Postby billr » Fri Sep 15, 2017 4:56 pm

No, tuning by knock will not give you the best timing. If the engine is "knock limited", that procedure can protect the engine from harm, but more power can be made using a different fuel, different AFR, water injection, several other options. If it is not knock limited, then that procedure may actually cause the engine to produce less power than it could (with the the same fuel, AFR, etc) by setting timing using torque as the criteria.

A product plug here: I just finished reading through the DIY book by Matt C. and Jerry G; this subject is covered well in that book. Read it. I'm not new to EFI or cars, by any means, but still found the book to be interesting with good reference material handy. For me, the descriptions of various other OEM and aftermarket systems was especially interesting as I never have had experience with any of them. Maybe the book will help me follow along on some threads where specific dizzy/crank wheel configs seem to be troublesome.
Last edited by billr on Fri Sep 15, 2017 8:20 pm, edited 1 time in total.
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Re: Optimal ignition advance

Postby maych87 » Fri Sep 15, 2017 7:54 pm

billr wrote:No, tuning by knock will not give you the best timing. If the engine is "knock limited", that procedure can protect the engine from harm, but more power can be made using a different fuel, different AFR, water injection, several other options. If it is not knock limited, then that procedure may actually cause the engine to produce less power than it could (with the the same fuel, AFR, etc) by setting timing using torque as the criteria.

A product plug here: I just finished reading through the DIY book by Matt C. and Jerry G; this subject if covered well in that book. Read it. I'm not new to EFI or cars, by any means, but still found the book to be interesting with good reference material handy. For me, the descriptions of various other OEM and aftermarket systems was especially interesting as I never have had experience with any of them. Maybe the book will help me follow along on some threads where specific dizzy/crank wheel configs seem to be troublesome.


Thank you for the suggestion


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Re: Optimal ignition advance

Postby Yves » Sat Sep 16, 2017 7:13 am

maych87 wrote:
Yves wrote:
billr wrote:I think looking at torque is actually the best way, not a "casual" compromise at all. Reading cylinder pressure, crank angle and then doing the math to calculate the integral of the crank moment is still going to kind of an estimate. Things like friction due to side thrust on the piston, friction in bearings, and even exhaust back pressure will be variable and should be factored in. Measuring torque does all that automatically.


Without going into debate here : from what I've been told by people that used it, in a non-detonation limited engine, the point where the ppp is, usually falls between 10 and 15° ATDC. There is some obvious variation due to the fact that the combustion never is equally fast. So a fixed torquepoint is not possible. What you see on the dyno is an average of all cylinders. Whereas what you see with pressure sensors is actual data of one cylinder.
You can see a dyno as some sort of smoothing of the data.


Maybe another dumb question. In a knock sensor system as far as what I can understand it advances the timing until it knocks then it backs it off. Is that how it is done on a dyno as well? Will backing of the advance slightly from knock give you that 10 to 15 degrees after tdc and maximum torque?


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As Bill stated, this approach would not yield MBT. It could be that with your engine and settings, you reach MBT with a PPP of maybe even 45° ATDC. With 20° you might have knock already occuring, but still below the detectable threshold.
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Re: Optimal ignition advance

Postby Dennis_Zx7r » Sat Sep 16, 2017 8:00 am

FYI some data taken with a cylinder pressure sensor taken from a PhD dissertation I recently read about this.
X-axis shows degree crank. 360 equals TDC in this case. Y-axis is proportional to the torque output.
Ign_indWork.png
Ign_indWork.png (205.14 KiB) Viewed 201 times

Ignition timing on the X-axis in deg BTDC. On the Y-axis the average of the indicated peak pressure in deg ATDC is displayed.
pMax.png
pMax.png (7.22 KiB) Viewed 201 times
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