Well, I'm not so sure some of the assertions in this post are correct. Lean mixtures burn slower? I always thought they burned faster-- hence why lean mixtures are more likely to detonate.

Maybe we should back up and ask the question of what determines speed of combustion? Do we want fast or slow? How does ratio affect this?

Speed of combustion is greatly determined by combustion space volume and the availability of oxygen to support combustion. Think about it this way: when the spark fires, the combustion event begins in the plug gap. The ionization in the gap ignites the vapor in the spark path and immediately next to it. As each little vapor molecule ignites, it ignites the molecule next to it, and so the "flame front" travels across the cylinder.

A richer mixture has a couple problems. First, the higher "thermal mass" of the charge will require heat to change the temperature high enough for combustion. Second, the increase in fuel ratio makes condensation more likely, and hence you'll get more "wet out" like action. This isn't much a concern at part throttle because engine vacuum vaporizes the fuel, and air velocity is low (hence, less wet out), but at WOT this is more of a concern.

Lean is better if you can swing it, but as others have mentioned, a couple points of ratio in certain regions is likely to increase risk but not give any more HP as a result.

jh

 

Pretty good posts Hohn.
EGTs are good indicators, but not real accurate. They can be skewed terribly by ignition timing.
The best, most accurate method of measuring combustion efficiency, and that after all is what it boils down to, is to measure dynamic cylinder pressure (key word DYNAMIC) and combustion chamber temperature. Many hi end engine dynos have this cabability, everybody else doesnt.
So, how do we determine combustion chamber temperature? With a 5 gas, of course. Everybody should have one. You'd throw your wideband in the trash.
The production of NOX is directly related to combustion temperature.
Monitoring NOX, and the rest of the exhaust gases, as well as A/F, paints as complete a picture of the combustion process as you can get, outside of directly measuring the dynamic cylinder pressure and temperature.

 

I totally missed your reference to five gas. I've never heard of such a unit, but your reference to NOx is all I need to know to infer what it does.

I know some test outfits have gone so far as to contruct a synthetic ruby block to see what's going on during combustion, then they use one of those 4000fps cameras to slow it down.

I'd agree in a heartbeat that 5-gas is better than EGT, but how many people have access to that vs a regular type-K?

Would certainly be nice to have, though!
Justin

 

Low engine speed is when fuel drop out would be the biggest concern, at W.O.T. (300+ mph.) intake air speeds, fuel atomization is your least concern. And I tend to disagree with the first part of your statement about a lean mixture burning "faster" than a rich mixture. Petroleum fuel, like any other combustible, has a burn rate. In the case of an internal combustion engine, that burn rate can be slightly altered through engine design, but it is basically constant. The detonation heard under a lean condition is created from an erratic flame front, not a faster one.

 

Agreed. The burn rate of any fuel should be fairly constant. Not really effected by mixture. Also not to be confused with volatility. Which does not indicate the burn rate.

 

You sound like a fairly educated and experienced person. How long have you been in the business? Or have you just read up?
Cant believe someone that sounds like he has your experience has never heard of a 5 gas analyzer. Its mostly used for emissions testing, but is the ultimate tool for any type of engine diagnostics, and a superior way of tuning.
As for EGTs, you're basically testing using data (the EGTs) that is 'after the fact'. And while they're good indicators, they are slightly flawed, and are not a true representation of the combustion cycle. They can be skewed by nothing more than their placement in the exhaust stream. If you move one from a straight part of the pipe, to a bend, the temps can vary greatly.
They do have their place. I used to tune a 1500 HP top sportsman car (nitrous), using a racepak and EGTs.
Also timing has a big effect on EGTs. Retarding the timing will make your EGTs shoot through the roof, while at the same time greatly reducing NOX levels.
NOX is the ONLY indicator of combustion temperature that we can monitor using any type of equipment outside of actual combustion temp and pressure sensors. And the only way to do this is using a 5 gas analyzer. And with the same piece of equipment, you can get a detailed picture of everything else. Left over 02, HCs, production of CO and C02.
Screw a wideband (I use one of those too) 5 gas analysis is where its at.

 

Could you make an example? Like combination of exhaust gasses versus power?
What is a good / bad combination?

 

Not really a combination, but as an example, you want to minimize your oxygen, and your HCs (hydrocarbons) which indicates close to complete combustion.
While maximizing your C02.
NOX also should be kept to a minimum. specifically under 2000 PPM (parts per million) Preferably in the 1000 PPM range. The higher the NOX count, the hotter the combustion temperature. 2500 PPM is the magic number. At that level you're pingin, even if you dont hear it.
Additionally, using a gas analyzer to tune idle mixtures again is FAR superior to using a wideband. Widebands are skewed by the excess 02 when trying to idle tune a cam with a bunch of overlap.
Using a 5 gas, you can idle tune while monitoring exact levels of oxygen and hydrocarbons. Again, trying to minimize both.
As an example, a cam with approx 10 inches of vacuum at idle will typically run in the 2000 PPM HCs, and about 3-5% 02. BUT, without the A/F changing much at all on a wideband, can be as high as 3000 PPM and about the same in oxygen. Using the 5 gas you can lean on it, and reduce the HC emissions as low as possible. You'll never clean em up all the way, but I have done a bunch of cars this way where I've reduced the tailpipe emissions at idle by 30-50%.

 

Lean mixtures do burn slower. Flame travel is fastest around 12:1 AFR with gasoline.

"Average flame speed in the combustion chamber of a SI engine is a function of air-fuel ratio for gasoline type fuels. Lean air fuel mixtures have slower flame speeds with maximum speed occuring when slightly rich at eq ratio of ~ 1.2"

from "Kinetics of Nitric oxide formation in high pressure flames" & "Engineering fundamentals of the internal combustion engine"

 

That sounds interesting.. Where did you get that?

Rich mixtures are actually more prone to knock but its all a balance.. It takes temperature, pressure, and some ignition delay. What I mean buy that little statement that everyone already knows is it is possible to have a car on the dyno at 12:1 with no knock, lean it out to 13:1 and have knock just because pressure and temperature have changed..

 

Audible detonation is created when the piston "crashes" into an erratic flame front. A lean condition amplifies this occurance due to the fact that less fuel in the chamber means more heat, which leads to a pre-ignition condition. Due to the higher heat in the combustion area, and more oxygen, the mixture is ignited much easier than with a heavier, wetter fuel charge. In this lean situation, any source of ignition can light the charge, casting flash, sharp edges, high heat range spark plugs, etc., and since the ignition is not controlled, neither is the flame front.

 

I read a lot, but apparently not enough, LOL! The reason why I wanted to join this forum was that there were so many knowlegeable folks here. Yes, I'm hoping to build an LSx car, but I'm also just hoping to learn a bunch.

That means I'll sometime say things in my ignorance that are wrong, or sometimes just stir the pot a bit to try to scare out the truth.


As for experience--well, I've wrenched a lot on my old Mopars, and then on my CTD. NOthing that would even come close to holding a candle to what the pros on here do.

I apologize if I come off as more educated than I am, playing the poser. I really just have a huge passion for hi-po engines and want to learn all I can.

Justin

 

One way to think of an "erratic flame front" is one that propagates from more than one source. Ideally, it would begin at the plug gap, and propagate radially from there until the entire chamber is covered. This produces a fairly linear rise in cylinder pressure.

But if the flame propates from multiple sources at the same time, you can have a rapid spike in pressure that can "knock" even if there isn't any autoignition, per se. You see this all the time with overadvanced timing.

Let me digress a second and illustrate this with a subwoofer analogy. Sound sources radiate energy as sound waves, which are nothing more than pressure variances transmitted through the air (which is a compressible and elastic fluid medium). If you point two subwooofers at each other and they are operating perfectly in phase, then there will be a spike in volume (pressure) observed by someone standing along the centerline between the two speakers.

Now, in the combustion chamber, you can have a similar situation if the pressure of combustion propagates from more than one source. Waves don't audibly "crash" into each other, but their pressuress are additive, depending on the source of radiation (establishing a kind of "phase relationship", though different than a sound wave).

To preserve the quickest possible burn that's propagating evenly, a couple things are important. First is mixture homogeneity-- we want to eliminate rich and lean spots in the chamber. Second is complete vaporization- no condensation or wet out. Finally, we can speed up the propagation be using tight quench that generates a shock wave at TDC that ensures vaporization and can speed flame travel faster than it will go on its own.

I know of multiple builds done with 4V Honda engines running 14:1 and up compression on pump gas with no detonation at all-- such is the efficiency of the chamber/piston design. This is with pretty mild cams, too! Cranking compression pressure would be 250+, defying all we've ever been told about what can be done. But it's being done, and done all the time.

So I suspect that with a more modern piston design, we could reap similar benefits in an LS.

The combustion chamber is composed of two parts-- the piston crown and the cut out in the cylinder head. WHY aren't these designed together? IMO, a piston crown should be almost a mirror image of the combustion chamber, set up to bias the combustion event to the exhaust side of the head.

In weapons design, it's well known that the power of a weapon is directly related to how high a pressure you can contain before the weapon ruptures. That's why an exploding LP cylinder is more violent than a baloon popping.

Similarly, ever effort should be made to sustain a high peak pressure at or near TDC, as this will extract more power from a given amount of charge.

Gee, who would have ever thought that reading up on nuclear weapons design would be relevant to my car hobby??


Justin

 

Exactly.....

 

Thats not the entire situation though. Pre-ignition is when it clashes (only) with the piston. In detonation you are hearing the two flame fronts collide along with other things.

HORN just outlined this nicely.

 

I think HOHN may be a bit confused when it comes to practical application of relative engine theory. I won't for one minute detract from his apperent study of physics, he sounds knowledgeable, but I doubt he has actually built too many engines. As far as flame fronts colliding, I agree, the problem is Identifying and correcting the ignition source for the additional flame fronts. As HOHN said, ideally it starts at the spark plug, but that is not the case with detonation/pre-ignition. I guess this all started when, to me, it looked like HOHN was saying that fuel drop out is most noticeable at higher intake velocities, and that just simply is not the case. Now if I misread, or misunderstood his statement, then I appologize.

 

I didn't mean to imply the fuel drop out was highest at highest intake velocities-- just that under vacuum conditions, fuel vaporizes more readily and wet out at low throttle openings is a non-issue.

JH

 

One of the reasons I had to initiate this thread is the discrepancy between the so called optimal 12.8-13.0 AFR I read on almost every North American forum and what tuners do here (Europe).

Our tuning shops set the AFR at around 13.5, no matter if NA or SC engines (maybe 13.0 if over 10 PSI charge air).

On this (or other) forum the common accepted optimal AFR for a FI engine is 11-12 depending on the boost level.

Why this difference?

I personally prefer the North American settings YOU GUYS produced my car and you probably better know how it works

 

13.5:1 A/F, in addition with compressed air (FI) leads to a very hot combustion temperature. And, that can lead to detonation. Or even if it doesnt detonate, it can super heat the crown of the piston, and if they are hypereutectics (stock) thats big problems.

 

Isn't the fuel different in Switzerland vs. USA?

Christian