Ignition Timing Mechanical Advance

[Zed Head]

I think it's just colliding asymptotes.  You're asking why did "they" stop, or why is the effect there?  

I wonder if it changes with a turbocharger.  Get away from the 14.7 psi limit.

[Captain Obvious]

Using mostly jonbills words (since they were so beauteous)...

Since the speed of combustion is pretty much constant, why would there be an upper limit to the mechanical advance?

In other words,... Why did Nissan stop advancing the mechanical timing at 2500 RPM? Why not keep advancing the timing mechanically all the way to redline?

At first blush, there's no (mathematical) reason to stop. The simple math is the "first order" effect. But things like volumetric efficiency isn't simply math.

Edited March 11, 2020 by Captain Obvious
its not simply math

[Captain Obvious]

And just because everyone likes pictures... Here's what the timing would look like if you just hard peg the advance at 1.28 milliseconds BTDC. I tried to model the extended curve using the stock slope as a guideline and that's what I came up with. Mechanical only (no vacuum):

 

[Captain Obvious]

9 hours ago, cgsheen1 said:

CO, you just made my head hurt...

Mine too! Thanks for that!   Haha!!

[jonbill]

The combustion is happening both sides of TDC. With low engine rpm, most of the combustion and gas expansion happens on the downside after TDC. As engine speed goes up, in order to keep peak pressure at 22 ATDC, more and more of the gas expansion has to happen on the upside, before TDC. Somewhere around 2500 to 3000 rpm the benefits of increasing advance and the downside of increasing combustion before TDC cross over and so Nissan (and everyone else) decided to limit the mechanical advance at that engine speed.

Imagine the ignition has been advanced to 80 deg BTDC at 6000 rpm. (I'm just guessing at numbers!) combustion had to complete between 80 BTDC and 22 ATDC. That means approx 80% of combustion has happened before TDC, and that's a lot of gas pressure trying to stop the piston from coming up, and the gas is under very high pressure and any remaining fuel mixture is going to detonate. So you get serious knock and lots of heat but no more power.
Something like that anyway :)

[Captain Obvious]

Thanks again for the input. I appreciate the discussion (which is clearly academic in nature).

You said above "the speed of combustion is reasonably constant." If that's the case, then the same amount of time after a spark would develop the same pressure at the same rate regardless of the rotational speed of the engine. That's the whole point we both made above (and it's the theory behind adding centrip advance at all in the first place).

I'm saying don't think degrees. Think TIME. The problem is that "degrees" changes with rotational speed. Time does not.

If the burn takes 1.3 milliseconds to develop optimum pressure at 1000 RPM, wouldn't it take 1.3 milliseconds at 2000 RPM as well? And 3000 RPM? And 6000 RPM? Why would the burn occur faster at 6000 RPM that would necessitate backing off the advance* at that speed?

If the burn rate is constant (in TIME), then shouldn't it be the same regardless of RPM?

In other words, the fuel mixture doesn't know or care what the engine RPM is. All it knows is at what TIME it was lit off. If it burns at a constant rate you should adjust the light off to the same TIME regardless of how fast you're approaching TDC.

 

* I say "backing off" because the speed goes up, but the amount of advance does not. It's like volunteering because everyone else in the line steps back. Just a different point of reference.  

[Captain Obvious]

@AK260, you had posted a pic earlier showing the timing you're using with your 123 device...

The button at the bottom right of the screen "Edit centrifugal curve" indicates that this is what you're running for your mechanical advance. Is that correct? That pic looks much closer to what I posted above than the original stock configuration.

Is this just mechanical, or is this mechanical plus vacuum? :

[AK260]

So Captain, this is purely my mechanical advance. I have zero base advance, which does mean I have to crank the car a bit when cold but at least it’s WISIWYG and I won’t do something dumb + cranking the extra 5 - 10 secs helps to oil get round before the action starts.

 

The vac curve is a work in progress at the moment.

 

Here is the stock 240z vac curve in 123 speak ...

 

 

It is basically a recreation of the upper vac curve of this ...

 

 

Does that help?

 

[AK260]

OK - here is another rookie thought for discussion - doesn’t the dynamic compression increase with revs? Wouldn’t that heat the charge more and so easier for flame front to propagate?

[240260280]

Volumetric Efficiency must come into play.... that is the amount of air (and fuel) filling the chamber.

As the throttle opens wider,  more air  and fuel enter the chamber. Somehow this must be part of the reason why advance is limited at higher RPM WOT.

 

 

[Zed Head]

Piston speed, and combustion chamber volume, varies dramatically with crankshaft position.  If you created a plot of chamber volume with time, there would be a long stretch where it was close to its minimum, almost unchanging.

[jonbill]

 

 

 

Thanks again for the input. I appreciate the discussion (which is clearly academic in nature).

You said above "the speed of combustion is reasonably constant." If that's the case, then the same amount of time after a spark would develop the same pressure at the same rate regardless of the rotational speed of the engine. That's the whole point we both made above (and it's the theory behind adding centrip advance at all in the first place).

I'm saying don't think degrees. Think TIME. The problem is that "degrees" changes with rotational speed. Time does not.

If the burn takes 1.3 milliseconds to develop optimum pressure at 1000 RPM, wouldn't it take 1.3 milliseconds at 2000 RPM as well? And 3000 RPM? And 6000 RPM? Why would the burn occur faster at 6000 RPM that would necessitate backing off the advance* at that speed?

If the burn rate is constant (in TIME), then shouldn't it be the same regardless of RPM?

In other words, the fuel mixture doesn't know or care what the engine RPM is. All it knows is at what TIME it was lit off. If it burns at a constant rate you should adjust the light off to the same TIME regardless of how fast you're approaching TDC.

 

* I say "backing off" because the speed goes up, but the amount of advance does not. It's like volunteering because everyone else in the line steps back. Just a different point of reference.  

 

Yep, if you only pay attention to the interest of having peak gas pressure at 22 ATDC, you would keep advancing the timing forever. But there are competing interests.

First, although the combustion takes a reasonably constant time, that combustion doesn't proceed linearly. It starts slow with a small flame front and grows and gets faster.

Second, keep this in mind: gas pressure when the piston is on the way down is good. gas pressure when the piston is on the way up is bad.

At 1000rpm with 15 deg advance, the 1.3 ms of combustion takes 37 degrees of crank rotation. 15 BTDC and 22 ATDC (say). most of the burn and and nearly all the gas pressure happens when the piston has passed TDC (ATDC).

 

At say 5000 rpm, the engine is going 5x faster but the combustion still takes 1.3ms. So you need 5x more degrees of rotation to give time for that 1.3ms burn.

So we need 5x37=185 degrees for that 1.3ms. If it is to complete at 22 ATDC then it has to start at 163 degrees before TDC. So now nearly 90% of the combustion and a lot of the gas pressure happens before TDC, while the piston is still on the way up. So although peak pressure will still come at 22 ATDC, very significant pressure has happened while the piston is rising, which slows it and will cause some of the fuel mixture to explode. (detonation) so the negative impact of so much gas pressure before TDC outweighs the value of having peak pressure at 22 ATDC.

Caveat: I am guessing at the causes and science here (although the article zedhead posted seems to agree with me) . But.. I am sure it is impirical fact that there is no more power to find by having more advance than that approx 35 number, and actually you get a broken engine instead through detonation.