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Old 12-31-2017, 05:46 PM
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Tom Vaught Tom Vaught is offline
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Ok, Lunch is over so back to the second part of the Topic (DYNO TESTING vs Air/Fuel Readings)

Article starts here:
So what really happened with this engine? We never changed a main jet and yet the AFR went from lambda 0.83 to 0.87 (12.2 to 12.8 on a standard gas scale) Thats a 4% change in fueling with actually changing the fueling. When I changed the IFR size it didnt change the lambda reading so it wasn't that change that did it. So what did change?

People are taught by every Wideband manufacturer in the world that they are accurate tools for reading the AFR. And yet here is an example of not actually doing anything that actually has bearing on the AFR, AND YET IT CHANGED.

ANSWER:
What happened was the engine became "RUN IN". (Broken In") And the timing was adjusted for best power.

So why does that result in a different reading?

Its all to do with the species of gasses output from the chamber. You have to look at the percentage of each species to understand why. As a burn progresses the chemicals produced go through stages until the final outcome is for a perfect burn just water and CO2. Our engines don't get to that stage but they get to the stage of producing water and CO and CO2 and a little bit of NOx and having some O2 left over. The point to consider is that it takes time to go through the stages to get to the final result.

The most important stage is the preparation prior to ignition. When that is not right, as in this engine, the flame kernel doesn't grow fast enough and the result is a lower cylinder pressure at an equivalent crank position to a good kernel. The flame kernel is a growth of laminar conditions to around 20mm diameter, then the conditions change to turbulent flame and the speed of burn is much faster. What you have to know is its droplet size and the vaporization that influences the speed of both laminar and turbulent flames. So when an engine has large droplets its needs a lot of heat to vaporize them in the time available.

(AND HERE IS WHERE A PROPER CARBURETOR CALIBRATION COMES INTO PLAY -TV)

When I say to you guys that a particular engine hasn't got enough vaporization that is what I'm commenting on. How it looks inside the cylinder is like this, the AFR at ignition time is leaner than the average of the cylinder, lets say the average is 12.8 but at ignition time the AFR around the plug is AFR 14, the flame is going to grow slow and some of the flames energy is going to used in vaporizing the fuel thats not gas yet. So the flame kernel doesn't generate enough heat and you dont get a TAN plug, you also run the risk of extinguishing the flame if turbulence is poorly directed.

(AND HERE IS WHERE A GOOD HEAD DESIGN COMES INTO PLAY -TV)

The kernel uses up about 10% of the mixture then the conditions transition to turbulent. Once the flame is turbulent it cant be extinguished. However the conditions toward the remainder of the chamber ahead of the flame kernel front are still under vaporized, therefor requiring energy from the advancing flame front to vporize mixture ahead of the front, not an efficient way to go. It's the burn angle duration between 10 and 90% that matters. Good burns are in the range of 15 to 25 degrees. Burns of engine with poor vaporization can be as long as 45 degrees. THAT'S WHY some engines make best power at timings like 38 to 45 degrees compared to another engine that has best power at 28 degrees.

The burn time determines the peak cylinder pressure. Its the peak pressure that is one of the principle determining factors for the production of CO into CO2. If its not high enough you wont get the conversion underway so the exhaust contains high values of CO. If the CO is produced with a high oxygen consumption then the exhaust stream will be low in O2.

Now a WB theorist will tell you that the WB will use the unused O2 to convert the CO into CO2 etc and complete the burn to perfection and report a result. Well that's correct but the reality of this type of burn is that the O2 is used to make excess CO and forgo the conversion to CO2 because it just didn't get hot enough for long enough. This type of burn is far from correct as the theory books only want to consider.

There isn't the normal correct balance of O2 to CO of a rich burn. The CO2 levels are low and the WB catalyst needs the CO2 to break it apart and get some O2 to reform the CO to CO2, the reaction of which is O2 neutral. So the WB has to get the O2 from the outside air and that causes it to report a high O2 requirement thus richer than truth.

I fail to understand why the industry doesn't educate people to correct this anomaly. The only manufacturer who is game to come close but they dont realize the reason why is Innovate, their system goes into error 8. But they fail to list this cause in their error reasoning help chart.

SO THE REALITY OF THE DISCUSSION (TV)

Article starts:

I've found that Widebands report erroneous readings when the gas in the exhaust is high in CO and also high in O2. That is an unnatural situation. HC levels are not a problem to a WB, its the CO that is the trouble maker. The problem is without a gas bench you have no way of knowing if the reading is correct or not. A gas stream of high CO and high O2 is a dangerous one, it means the engine is not making its power potential and it means there are going to be excessively lean areas in the chamber. So when you look at the plugs and they are not a dark enough tan for the AFR your running you can pretty well assume the engine isnt preparing the fuel enough prior to ignition.

This is the reason why we use Gas Analysers at the same time we use Air/Fuel Meters and Exhaust Temp Sensors when testing on the Dyno.

So whatever you do don't tune off the WB, tune off the plug. The engine of this thread is not a bad one, its pretty good actually because I've seen WB's up to 1 or even slightly over ratios out of truth.

You can't put nitrous on an engine with that sort of issues, well people do but they rely on the fuel to get them through.
I've run engines in perfect combustion scenarios with correct gas balances and even with perfect combustion as in zero CO and O2 and HC and only CO2 and I've seen how plugs look.
I have experience at ranging the AFR around the plug from the plug color. I can tell you this issue is the most common thing I see.

So that finishes his post. Some really good info there. So purely counting on a Air/Fuel meter without having knowledge of how to read a plug might get you into a lot of issues with the engine and the Holley Carb Calibration

Tom V.

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