Richer AFR in higher gears

[GRTfast]

I have seen this on multiple cars when using my innovate LM2 data logger. AFR gets richer as the load increases. To observe this, put the car in 3rd early, ie, miss second, and keep your foot into it, watch the AFR go quite rich with that high load scenario.

I will let someone smarter than myself explain the physics behind this, however.

I have already done this, and you are right, that is exactly what happens.

[Tuner]

This phenomenon is a function of combustion chamber temperature increase as WOT time increases and also involves the "puddle", the fluid film in the manifold plenum and intake ports.

Most credible literature generated from actual laboratory research, Internal Combustion Engine textbooks, Obert's, Taylor's and others, SAE papers, etc, states that approximately 50% of the fuel enters the cylinder as vapor and 50% as liquid. Some of the liquid is entrained in the air and vapor flow as droplets, and some of it attaches to the interior walls and flows at much lower velocity than the air (like rain water on a windshield) as a fluid film moving along the walls and floor of the intake until it reaches the valve seat, at which point the sonic flow during low valve lift picks it up and the sonic shock vaporizes some of it as it enters the cylinder. The remaining liquid is vaporized by the heat of the compression stroke or the heat of combustion. The actual amount vaporized is less than 100 % but obviously will increase as combustion chamber and piston top temperature increase. The result is the A/F of the combustion increases with time.

The volume of fuel entrained in the puddle varies with the HP level. At the moment power is increased from idle or low power to WOT, the depth of the fluid film at the valve seat is the volume of the low power flow and the depth of fluid film near the carb (or injector) is that of the high power flow. Time is required for the WOT puddle volume to flow along the manifold walls until it reaches the valve seat. In other words, the puddle lags and has to play catch up until it reaches the new steady state value.

Obviously, time is required, first for the puddle to grow near the carb in the plenum, and then for the film to flow through the ports to the cylinders. As the volume of fuel entering the cylinders increases with time, during that same time the chamber temperature is increasing and vaporizing a larger percentage of the total fuel within the cylinder, so for those two reasons the A/F of the combustion becomes richer with time (actually, the combustion oxygen balance as reported by the WBO2). If enough time is spent at WOT, the actual fuel flow into the cylinder, the temperature and heat balance (assuming adequate cooling system), eventually all become steady state and A/F will remain more nearly constant.

The opposite occurs when the throttle is closed from WOT. The large amount of fuel in the WOT puddle remains in the intake system after the air is cut off, it doesn't just instantly go away, and the A/F is very rich during coasting as the WOT puddle flows to the cylinders with only idle or low power air flow.

The extreme examples of idle and WOT illustrate the lag and overrun of the puddle that occurs with each and every change in load, relative to the percentage change in load.

Pondering this should help illustrate the benefit of an exhaust heated intake manifold, as the hot manifold will more readily vaporize the fluid film off the plenum floor and runner walls.

[peejay]

I have seen this on multiple cars when using my innovate LM2 data logger. AFR gets richer as the load increases. To observe this, put the car in 3rd early, ie, miss second, and keep your foot into it, watch the AFR go quite rich with that high load scenario.

I will let someone smarter than myself explain the physics behind this, however.

Oxygen sensors are affected by temperature.

One thing that keeps getting hammered into my head is that Innovate controllers suck, and don't do temperature compensation properly. That is why they will take the same sensor that VW sticks 6" from the exhaust port, right in front of the turbo, and has live a long life, and manage to make it burn out rapidly if you don't stick it 2 feet down the pipe and use their special heat sink sensor bung.

They're inexpensive, at least.

[GRTfast]

This phenomenon is a function of combustion chamber temperature increase as WOT time increases and also involves the "puddle", the fluid film in the manifold plenum and intake ports.

Most credible literature generated from actual laboratory research, Internal Combustion Engine textbooks, Obert's, Taylor's and others, SAE papers, etc, states that approximately 50% of the fuel enters the cylinder as vapor and 50% as liquid. Some of the liquid is entrained in the air and vapor flow as droplets, and some of it attaches to the interior walls and flows at much lower velocity than the air (like rain water on a windshield) as a fluid film moving along the walls and floor of the intake until it reaches the valve seat, at which point the sonic flow during low valve lift picks it up and the sonic shock vaporizes some of it as it enters the cylinder. The remaining liquid is vaporized by the heat of the compression stroke or the heat of combustion. The actual amount vaporized is less than 100 % but obviously will increase as combustion chamber and piston top temperature increase. The result is the A/F of the combustion increases with time.

The volume of fuel entrained in the puddle varies with the HP level. At the moment power is increased from idle or low power to WOT, the depth of the fluid film at the valve seat is the volume of the low power flow and the depth of fluid film near the carb (or injector) is that of the high power flow. Time is required for the WOT puddle volume to flow along the manifold walls until it reaches the valve seat. In other words, the puddle lags and has to play catch up until it reaches the new steady state value.

Obviously, time is required, first for the puddle to grow near the carb in the plenum, and then for the film to flow through the ports to the cylinders. As the volume of fuel entering the cylinders increases with time, during that same time the chamber temperature is increasing and vaporizing a larger percentage of the total fuel within the cylinder, so for those two reasons the A/F of the combustion becomes richer with time (actually, the combustion oxygen balance as reported by the WBO2). If enough time is spent at WOT, the actual fuel flow into the cylinder, the temperature and heat balance (assuming adequate cooling system), eventually all become steady state and A/F will remain more nearly constant.

The opposite occurs when the throttle is closed from WOT. The large amount of fuel in the WOT puddle remains in the intake system after the air is cut off, it doesn't just instantly go away, and the A/F is very rich during coasting as the WOT puddle flows to the cylinders with only idle or low power air flow.

The extreme examples of idle and WOT illustrate the lag and overrun of the puddle that occurs with each and every change in load, relative to the percentage change in load.

Pondering this should help illustrate the benefit of an exhaust heated intake manifold, as the hot manifold will more readily vaporize the fluid film off the plenum floor and runner walls.

Exactly the answer I was looking for. Thank you!

[GRTfast]

Cool looking car. I bet that thing is a blast to drive with a 4 speed.

It is fun. Here are a few better pics.

[BobbyB]

Really nice!

[F-BIRD'88]

Reversion.....
The carb venturi will flow fuel with airflow in both directions.
This is the result of reversion.

Further on reversion... The 02 sensor in the exhaust can see raw air coming back up the exhaust pipe (reversion) especially if when the end of the exhaust pipe is too near the 02 sensor location. This makes the AfR read leaner at lower rpm loads than higher rpm loads.

Further magnified in any all exhaust leaks at ex joint gaskets including at the cylinder ports..

[F-BIRD'88]

Carb metering blocks main metering emulsion is set up wrong.

[gruntguru]

Given the results of your various tests - none of the explanations above make sense (except perhaps issues with the wideband readings - perhaps confirm with some plug cuts).

Perhaps you have a g-force inertial problem with the carb. High g acceleration in low gears causing fuel surge away from the fuel circuit? Or high G closing a power valve?

I'm tending to think its the WB AFR.

[GRTfast]

Given the results of your various tests - none of the explanations above make sense (except perhaps issues with the wideband readings - perhaps confirm with some plug cuts).

Perhaps you have a g-force inertial problem with the carb. High g acceleration in low gears causing fuel surge away from the fuel circuit? Or high G closing a power valve?

I'm tending to think its the WB AFR.

Plug chop is a good idea. I could do one in second, one in third, and one in fourth. The car is running great, so this is all really just to satisfy my curiosity and learn. As always, "simple" things aren't so simple when you dig in to the minutia.

[GRTfast]

Reversion.....
The carb venturi will flow fuel with airflow in both directions.
This is the result of reversion.

Further on reversion... The 02 sensor in the exhaust can see raw air coming back up the exhaust pipe (reversion) especially if when the end of the exhaust pipe is too near the 02 sensor location. This makes the AfR read leaner at lower rpm loads than higher rpm loads.

Further magnified in any all exhaust leaks at ex joint gaskets including at the cylinder ports..

There are no exhaust leaks, the sensor is further from the exhaust exit than the recommended minimum, and I can't see how any significant reversion is happening in the intake in the meat of the powerband, especially with the dual plenums completely isolated. Thanks for pointing out every flaw you can think of though. It's a miracle my car even runs...

Just kidding, I appreciate the opinions. Keep 'em coming if you think of anything else. FWIW, I have seen this issue in carbed motorcycles as well, in combinations that aren't as mismatched as my engine combo.

[John Wallace]

I had a vacuum gauge fitted when I had the 750 on it, when I would go to WOT it would drop to almost zero, then climb to 2 inches at red line, this is why I went to a bigger carb. Keep in mind, the issue I am talking about in this thread is only at WOT. I have the carb working super sweet in cruising, idle, part throttle, etc..

Probably time for a vacuum operated secondary type carb?
It would only open the secondary as needed?
Probably too much of a swing in vacuum to keep the mixture in range?
Also the power valve is probably doing some funky stuff?

One of those things where one has to be there to see what actually is happening.

[GRTfast]

I had a vacuum gauge fitted when I had the 750 on it, when I would go to WOT it would drop to almost zero, then climb to 2 inches at red line, this is why I went to a bigger carb. Keep in mind, the issue I am talking about in this thread is only at WOT. I have the carb working super sweet in cruising, idle, part throttle, etc..

Probably time for a vacuum operated secondary type carb?
It would only open the secondary as needed?
Probably too much of a swing in vacuum to keep the mixture in range?
Also the power valve is probably doing some funky stuff?

One of those things where one has to be there to see what actually is happening.

Yeah I’m good with the carb it has now. Maybe if it was a race car and I was trying to extract every fraction of ET, would I keep changing out parts, but it’s just a street toy. Idles well (lumpy, sounds mean), cruises well, roasts the tires in first and second, flattens eyeballs anything past half throttle. I just like to understand why things do what they do. As a M.E., I have a decent knowledge of physics and complex system behavior, but grasping the subtle nuance of carburetor behavior to the extent that you can know what is going on purely from the behavior is as much an experience thing as it is a physics knowledge thing.

[BobbyB]

Can you post more pictures of your car? Show build pictures if you have some please.

[Mattax]

I've also seen that on a logger in lower gears.

Thanks Tuner!

This phenomenon is a function of combustion chamber temperature increase as WOT time increases and also involves the "puddle", the fluid film in the manifold plenum and intake ports.

Most credible literature generated from actual laboratory research, Internal Combustion Engine textbooks, Obert's, Taylor's and others, SAE papers, etc, states that approximately 50% of the fuel enters the cylinder as vapor and 50% as liquid. Some of the liquid is entrained in the air and vapor flow as droplets, and some of it attaches to the interior walls and flows at much lower velocity than the air (like rain water on a windshield) as a fluid film moving along the walls and floor of the intake until it reaches the valve seat, at which point the sonic flow during low valve lift picks it up and the sonic shock vaporizes some of it as it enters the cylinder. The remaining liquid is vaporized by the heat of the compression stroke or the heat of combustion. The actual amount vaporized is less than 100 % but obviously will increase as combustion chamber and piston top temperature increase. The result is the A/F of the combustion increases with time.

The volume of fuel entrained in the puddle varies with the HP level. At the moment power is increased from idle or low power to WOT, the depth of the fluid film at the valve seat is the volume of the low power flow and the depth of fluid film near the carb (or injector) is that of the high power flow. Time is required for the WOT puddle volume to flow along the manifold walls until it reaches the valve seat. In other words, the puddle lags and has to play catch up until it reaches the new steady state value.

Obviously, time is required, first for the puddle to grow near the carb in the plenum, and then for the film to flow through the ports to the cylinders. As the volume of fuel entering the cylinders increases with time, during that same time the chamber temperature is increasing and vaporizing a larger percentage of the total fuel within the cylinder, so for those two reasons the A/F of the combustion becomes richer with time (actually, the combustion oxygen balance as reported by the WBO2). If enough time is spent at WOT, the actual fuel flow into the cylinder, the temperature and heat balance (assuming adequate cooling system), eventually all become steady state and A/F will remain more nearly constant.

The opposite occurs when the throttle is closed from WOT. The large amount of fuel in the WOT puddle remains in the intake system after the air is cut off, it doesn't just instantly go away, and the A/F is very rich during coasting as the WOT puddle flows to the cylinders with only idle or low power air flow.

The extreme examples of idle and WOT illustrate the lag and overrun of the puddle that occurs with each and every change in load, relative to the percentage change in load.

Pondering this should help illustrate the benefit of an exhaust heated intake manifold, as the hot manifold will more readily vaporize the fluid film off the plenum floor and runner walls.