Lean Mixture Spark Knock

[mechtech2]

Ok.. Why spark knock [ping] with a lean mixture?
It is the standard Mantra forever.

My conclusions:
Being lean in and of itself does not cause pinging.
It is a slower burn and has less power then stoichiometric [normal]. It has less gas molecules to 'shake hands' with in the combustion chamber.
So why can lean mixtures cause knock?
It is the cooling that is missing. Normal or rich mixtures cool a lot by fuel evaporation, and that is diminished with lean fuel mixtures. Therefore the CC temps are hotter, and causes the knock/ping.
This is why high performance turbo engines run so rich - to cool things much and get more power at the cost of excessive fuel. They can actually have black smoke out the pipe and be correct.

 

[eljefino]

As I understand it, the original Hemi intake manifold was specifically designed to condense gas vapors into droplets so they could later re-evaporate and cool things down to get more power.

Stick that in your SULEV Prius.

 

[SteveSRT8]

Jives with everything I've read and learned.

And it doesn't have to be FI for rich tuning, my Chrysler's factory tune was pig rich.

Note also that lean mix can ignite more easily in certain head designs, then burns too fast.

 

[A_Harman]

In my past work with people that work with engines, I find the concept of "lean" and "rich" is relative to whom you are talking. A racer's idea of "leaning it out" is to go from 11.5 A/F to 12.5. An OEM Calibration Engineer's idea of "leaning it out" is to go from 14.7 to 15.7 A/F. In the former case, the engine will pick up power, and be more susceptible to detonation because the flame speed will increase because the engine is still running rich, stoichiometrically speaking. In the latter case, the engine will lose power, and be less susceptible to detonation because flame speed decreases on the lean side of stoichiometric.

OE factory calibrations are stupidly rich at WOT because extra fuel is needed to protect the cats from high temperatures. Get rid of the cats, and increase the A/F from ~11 to 12.5 and watch the power increase. Watch out for detonation when you do that.

 

[morris]

there are two types of knock. detonation or preignition. they are NOT the same. but both do much damage. for a long time it was not known what the knock was. i think in the 20s one of the car markers put a thick quartz window on the chamber and could see the chamber burn VERY fast instead of burn at a controlled rate.

 

[yonyon]

As I understand it:
You don't need more gas "shaking hands" to make the combustion happen. Consider how much hotter and faster a propane or acetylene torch flame can burn with oxygen injected through the nozzle than without. A well-atomized lean mix can definitely self-ignite at lower compression ratios than a richer mix. Even when it doesn't self ignite it can be close to the point of doing so and burn too fast. Burning too fast and too hot can also generate higher surface temperatures inside the chamber and inspire preignition in that manner.

There's a reason why smoking is strictly prohibited on penalty of tackle where oxygen is being administered for medical purposes and it has nothing to do with cancer.

 

[punisher]

Detonation occurs at the tail end of the combustion event. Circumstances occur which push heat and pressure past the point where the gasoline molecule literally detonates instead of burns.

Why do lean air/fuel ratios cause detonation/pinging? In a normal stoic/slightly rich combustion there is a boundary layer of fuel/air on all of the chamber surfaces that never ignites. Besides bumping up HC emissions slightly, the boundary layer acts as insulation between the combustion gasses, and the chamber surfaces. This leads to reduced chamber temps.

Lean mixes burn slower, as well as transmit more heat to the chamber surfaces. Combine slower combustion speeds with increased IR heating and conditions for detonation arise. Any unburned mixture is a candidate for detonation. Detonation occurs in the coolest part of the chamber, and fairly far away from the plug electrode. 2 valve engines detonate on the bore edge of the intake valve. Look for detonation damage on a piston right on the ring land under the intake valve.

Propensity for detonation is inversely proportional to RPM and piston diameter.

Heck, just Wiki it.

 

[mechtech2]

Right.
We have to limit the ignition advance with big bores.
2 plugs per cyl are often used to get things under control.

 

[Shannow]

Originally Posted By: punisher
Why do lean air/fuel ratios cause detonation/pinging? In a normal stoic/slightly rich combustion there is a boundary layer of fuel/air on all of the chamber surfaces that never ignites. Besides bumping up HC emissions slightly, the boundary layer acts as insulation between the combustion gasses, and the chamber surfaces. This leads to reduced chamber temps.


You are talking quench here.

It's when the flat surfaces of the piston and the chamber are less than around 0.060" apart, and the heat transfer from the compressed gas INTO the metal is so high that the flame can't travel through the tiny gap.

If the fuel formed an insulating boundary layer, the temperatures of compression would be even higher.

The pressure and compressive heat wave travels at the local speed of sound, K*R*SQRT T in S.I. units, (T increasing with pressure isentropically), meaning that in a long chamber (open chamber in the parlance), the end gasses can autoignite like a diesel...end gas autoignition, classic detonation.

The quench band, beyond which combustion is not possible make a small, less distant end gas..."closed chamber" in the parlance.

 

[OVERKILL]

Originally Posted By: mechtech2
Right.
We have to limit the ignition advance with big bores.
2 plugs per cyl are often used to get things under control.


Yup. And this idea is hardly new.....

A number of the wooden boats I grew up around were of the dual ignition variety.

 

[punisher]

Ha, that old engine with the updraft carb reminds me of my old Ford 9N. Every couple hours or so the float would stick and the engine would stumble. I would have to get up real quick and lean over to the right and "tap" the carb with my sledge hammer to get it "fixed". Got a little overenthusiastic one time and now it has a nice bead of JB Weld holding it together. Fixed the float problem problem though.

 

[mechtech2]

Overkill -
That engine is absolutely beautiful!
Really, a work of art.
Thanks .

Yonyon -
An acetylene torch without O2 is very 'rich'. The incomplete combustion due to lack of O2 to shake hands with.

And in an engine, lean mixtures burn slower than normal or slightly rich mixures.

 

[Shannow]

Yeah, there's a common misconception in spark engines.

Lean of stoichiometric, and lean of normal running are different.

 

[morris]

I think( correct me if iam wrong) that acetylene is NOT used as engine fuel cause it self ignites at 15 psi. if I remember my welding class right.

 

[OVERKILL]

Originally Posted By: mechtech2
Overkill -
That engine is absolutely beautiful!
Really, a work of art.
Thanks .

Yonyon -
An acetylene torch without O2 is very 'rich'. The incomplete combustion due to lack of O2 to shake hands with.

And in an engine, lean mixtures burn slower than normal or slightly rich mixures.


Picture that engine, but about 4x the size, and you have the one in the boat I've had the extreme pleasure of driving whilst the owner and my grandfather drank Crown Royal and chatted on our way to Lake Muskoka. Boat is a 38' Minette launch formerly of the Eaton family and powered by the pictured engine's, big, BIG brother. Cast iron pistons as big around as dinner plates with the same dual ignition setup

Gravity oiling, but the big girl, at least in this application, has a beautiful brass drip tray instead of the plain metal one you see below the one in the craft pictured.

He has some beautiful boats