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At what crank angle should cylinder pressure peak for max work?

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Old 06-17-2017, 01:10 PM   #1
DAVE396LT1
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Default At what crank angle should cylinder pressure peak for max work?

Piston reaches "0" speed at TDC and BDC, and you have maximum piston speed at 90 degrees past TDC.

I can't seem to find a reference for where you want the cylinder pressure to peak for ideal maximum power production (ie: best ignition timing).

Maybe there's fancy calculus involved in it because the pressure graph isn't symmetrical, but there still must be some angle over which it's centered.

When you look at graphs it seems to be somewhere in the 12-20 degree range, but that's pretty wide.

Help me hardware people, I'm a software guy!

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Old 06-18-2017, 05:25 PM   #2
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Different RPMs will change the advancement, remember the centrifical advance of mechanically timed ignition. As the RPM increased the ignition advanced accordingly. I would think the computer also advances the spark according to a multiple number of inputs. Why are you concerned?
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Old 06-18-2017, 06:11 PM   #3
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Quote:
Originally Posted by Larry/car View Post
Different RPMs will change the advancement, remember the centrifical advance of mechanically timed ignition. As the RPM increased the ignition advanced accordingly. I would think the computer also advances the spark according to a multiple number of inputs. Why are you concerned?
The optimal angle is constant (I think - assuming the plot of pressure is symmetrical but I don't know for sure that it is). The amount of advance needed to hit it accurately changes with conditions, that's all. I'm not asking how much advance is required to hit the optimal, I'm asking where the optimal is.

I think it's somewhere around 23 degrees ATDC, but:

a) First I want to confirm where optimal is,
b) Then I want to understand why it's wherever it is

That way I'll have my ducks in a row next time I need to explain why advancing timing (can) cool the engine, but I'd like to have this fact squared away in advance (no pun!).

Sure lots of opinions on chrome wheels and automatic transmissions on this forum compared to the amount of physics help :-)

This article makes it sounds like 16 degrees is ideal, and the amount of lead varies to hit it:
https://www.jcmmachineandcoatings.co...ng-combustion/

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Old 06-19-2017, 02:05 PM   #4
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Here's a chart that shows it at 15 degrees, but there's no explanation. I know that maximum leverage is at 90 degrees, but what's magic about 15?



I found a different source that says you're trying to target 15-20 ATDC, but again it doesn't explain why:



Best explanation I can find so far says that point is indeed constant for a given engine, but that it varies based on engine geometry:

"Many experiments have shown that the optimum position for this pressure peak is about 15 to 20 degrees after TDC. The exact location of the optimum pressure peak is actually independent of engine load or RPM, but dependent on engine geometry."

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Old 06-19-2017, 04:38 PM   #5
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I've actually measured this on a diesel truck making 650ish about 15 years ago using in cylinder pressure transducers. We settled on 4-5 degrees ATDC as the ideal peak. Fuel takes time to burn and peaking too late means a lot of that force is wasted. Too early is hell on hard parts.
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Old 06-19-2017, 05:07 PM   #6
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Quote:
Originally Posted by Suns_PSD View Post
I've actually measured this on a diesel truck making 650ish about 15 years ago using in cylinder pressure transducers. We settled on 4-5 degrees ATDC as the ideal peak. Fuel takes time to burn and peaking too late means a lot of that force is wasted. Too early is hell on hard parts.
Since diesel is burning slower than pertol, the ideal point of peak pressure is earlier in diesel than in petrol engines.
For a small block chevy I read the ideal point to be 15°.

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Old 06-19-2017, 05:28 PM   #7
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I thought of that, and you certainly might be correct. However, that old diesel only spun up to 3200 rpm or something. I dunno.
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Old 06-19-2017, 08:35 PM   #8
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Typically, for most spark ignition homogeneous charge engines, crank angle location of peak pressure at maximum torque is about 8 degrees after Top Dead Center. This is driven from typical combustion duration of 30 to 45 degrees.

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Old 06-20-2017, 04:16 AM   #9
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Quote:
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I thought of that, and you certainly might be correct. However, that old diesel only spun up to 3200 rpm or something. I dunno.


The slow burning rate is the reason why even modern diesel engines don't go beyond app. 5000 rpm.
There is simply not enough time to burn the diesel efficiently.
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