On an engine management book I am reading I found the following statement:

“There is no free lunch at the limit. If an engine has encounter its knock limit at Lambda=0.82 with 26 degrees of advance, we must either increase fueling to add ignition lead or reduce timing to find power wit a leaner mix.”

Is this completely true? Form what I understand a richer mixture will burn faster and require less timing advance. If I increase fuelling without changing anything else will that create more knock due to a faster burning mix? i.e explosion will occur before the piston reaching TDC and create knock? I understand the reduced timing part and it makes perfect sense but the add fuel part is kind of confusing. Any tuners out there that can confirm the above statement.

 

Richer mixtures burn slower.

 

Richer mixtures burn slower. Also there shouldn't be any explosions, the spark ignites a smooth burn from the spark plug out and down. Knock happens during the burn when another area of the chamber self ignites before the original flame front reaches it. It makes two flame fronts that cause a shockwave when they collide.

 

So that means richer mixtures burn faster and require more timing advance and leaner mixture burn slower and require less timing advace? Anyway I should use as much timing advacen for any mixture as knock permit right? As long as I am making power in the process.

 

No ... he said a richer mix burns slower and that means it burns slower. A richer mix:

a) is more stable allowing more advance before detonation occurs.
b) burns slower meaning peak cylinder pressure occurs later.

This is correct. Tune the fuel to where you want it, then advance timing till you encounter knock. Then back off a little for safety.

 

so whats the burn time difference between 11:1 and 12:1?

anything worth worrying about?

 

Opps, That was a typo. Its pretty clear that when he said a richer mix burns slower, that means it burns slower. For some reason I thought I read the oppsite somewhere on the book. I will check again.

 

The difference between 11.0 and 12.0 is pretty large on both my DSM and Evo. I run 12.0's daily on the Evo to give me half an AFR saftey. I run 12.0-12.5 on my DSM, it makes much more power like that. You have to reduce timing when running proper AFR rations and when you do so you increase mean brake torque.

 

Enjoy:

(Graph is from an EPA paper)

Clearly there's power to be made from running leaner if possible, especially for guys running E85.

PS I recently retuned my car to run high 11's from low 11's-high 10's that it was running (partly by design, partly because i didn't retune it when i went to my stock 8 turbo as i thought i'd get my green back quickly - wishful thinking, ha, so was running richer than i wanted) and felt a noticeable power increase. (That's on E85 and gas configured WB btw)

 

That graph is only showing the flame front speed. Ethanol and Methanol fuels are great because the cool the air going into the cylinder and actually create "artificial boost" as the Ethanol expands as it goes from liquid to gaseous state. As such Alcohol based fuels have a wider AFR range that they make great power at. Running rich on E85 isn't any big deal. I'm doing some E85 testing and that stuff is great.

 

Well yeah, this thread is about burn rate as opposed to AFR's. That's what the graph shows.

 

But you can't just look at one aspect and feel good about your tuning job. Corn Gas has a lot wider AFR band that it makes power and the timing can stay the same. If the AFR drifts too far on gasolene you have to make changes in the timing map to keep the motor from knocking or not making enough power.

 

I know I know, you're preaching to the converted here )) The thread's just not about that.

 

I just recently started using 91 oct w/ 10% ethanol. And after reading several books, decided to follow the idea of having less fuel and less timing; instead of doing what everyone else on EvoM does on 91. Its just that Im S.O.L. when it comes to trying to maximize timing. It all makes sense if you think about how it plays into the "maximum combustion pressure at 12* (not sure if thats the magic number) after top dead center.

A rich mix burns slower, requiring you to fire the plug earlier
A lean mix burns faster, requiring you to fire the plug later

There is only X amount of O2 in the combustion chamber. And you will not have enough O2 molecules to go with fuel molecules with a rich mixture. Rich best torque for a given type of fuel is what it is. Like the OP had qouted, there is no freebies. A rich mixture does not allow more timing advance, it REQUIRES more timing advance to rich peak pressure at the optimum point. Here is a link to the article on the innovate website.

http://www.innovatemotorsports.com/resources/rich.php

It explains that there is a small advantage to a cooler temps with rich mixtures, but it really doesnt justify it. So I believe that a slightly richer mixture is okay (11.8), but 11.1s are ridiculous; even on 91.

That being said, Im now running a gas WB afr of 11.5 from peak torque to redline. And planning to slowly lean it out to the upper 11s.

E100 rich best tq - .714
Gas rich best tq - .8-.85

(.825*.9)+(.714*.1) = .8139 = 11.96 on a gas WB.

I am running 2* at peak tq, 12* at redline with a sporadic knock count of 1 or so and it feels pretty darn good. I only have road tuning experience and that is fairly limited (less than 1.5 years). But I work with a bunch of engineers, and this train of thought works very well with me. I expect to give up a degree or two of timing, but I really dont beleive that it will affect power output tht much.

 

Changing the AFR only moves the peak pressure point slightly, where ignition advance moves it a degree at a time (essentially). I've found that I make more power with a richer mix (11.2 - 11.5) than a closer to perfect mix (12 - 12.2) due to the increased benefit of timing advance.

I think 15º is the magic number for cylinder pressure ... but it's kind of irrelevant on pump fuel and we also have no way to actually measure it. Most any car on pump gas will reach peak pressure after 15º since knock will be encountered well before that.

This does go full circle to the OPs original question ... there is a point of diminishing returns with timing advance and with the fuel mix. In an ideal world with no knock to worry about, you would simply set the fuel mix to .85 lambda and advance the ignition to get peak pressure at 15º ATDC and that would be the a perfect tune.

But, on pump fuel and even pump+meth we are stuck with a decision that see-saws between a proper fuel mix and less ignition advance and more ignition advance and a richer mix. Which ever option gives the most power without knock is the correct choice.