great idea. so......is that an animation of you in your sig?

 

that's Jeremy Clarkson from Top Gear in his siggy

 

Did anyone read the link about how the Hot Wire MAF works?

If there is an increase in pressure at the same volume the MAF will read more dense air. If you increase pipe diameter the pressure increases as the flow will decrease in velocity. It's an inverse relationship. Even if you have a negative pressure or vacuum the bigger the pipe the less the vacuum or higher the pressure.

Think of an airplane wing. This is the same thing. Bigger pipe = slower air speed = higher pressure. On the bottom of a wing the air is moving slower so the pressure is higher. Same principal.

That's the story I'm sticking with untill someone can show me the light.

Plus read MM's post about them scaling the MAF way over what the engine is really taking in.

Cel

 

oh my bad!

 

Could someone explain what pressure has to do with it

The sensor measures airflow through a tube...there is no pressure sensor..

There is an IAT temp sensor...

The sensor measures the resistance change from mostly convection cooling of the hot wire...as the flow increases the resistance decreases on the wire...and the effective voltage to the PCM increases.

So a small tube will have more flow per unit area than a large tube with the same flow rate. ?????

 

Check out this link and the adjustable animation. Grab the little yellow squares and drag them and look at the graph below.

ANIMATION

Cel

 

correct. long narrow tubes like the intake runners increase the speed and turbulence of the air. short wide tubes are good for high rpms as they allow more air to get in the chamber in less time.

 

FWIW, I'm just as puzzled as you regarding the pipe diameter and the MAF readings being "counterintuitive".

 

Read this link (previously posted by r0tor )about how the hot wire MAF works:

http://en.wikipedia.org/wiki/Mass_flow_sensor

Pay attention to where it talks about an increase in pressure.

Cel

 

If you read the link about the MAF and then play with the pipe diameter you will see how it works. Also notice that the pressure is inversely proportional to the SQUARE of the velocity. If you go from large to really small you get a parabolic curve on the graph.

Cel

 

I see now. Thanks.

"If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow."

 

DING DING DING....That's the sentence that matters. Take that into account with Bernoulli's principle and there ya go.

Cel

 

Here's the WIKI quote

If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meter's paddle sensing element, the hot wire responds directly to air density. This sensor's capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume.

This is assuming that there is a change in Temp or Baro...which there wouldn't be...assume that the only variable that changes is the air flow...so there would be more density in the smaller tube...more " collisions" with the wire..and more cooling...and a corresponding higher MAF output in V. Larger tube would have less collisions per unit area and less cooling per flow...and therefore lower MAF voltage.

Bernoulli's is a formula to calculate pressure differences in a tube of changing sizes...we are talking about the air flow differences between 2 tubes of different sizes....which is an ideal gas law kind of relationship...

As per the quote....Of course if the temp drops, at the same flow rate there will be more molecules...and therefore more mass...

 

Another more automotive way to think about it is....

If you want to give yourself a better MAF scale for Higher than Stock flows...you go to a larger tube.....that allows the MAF to use the same voltage range....to measure the new (larger) flow...and scale the MAF accordingly.

If it was inverse relationship...you would make the MAF tube smaller to read larger flows with the same range....

You can't have it both ways....

 

There would be a change in Baro. Look at the animation. Make a tube size that's the same all the way across. Look at the pressure. Now make a different tube size. Look at the pressure.

That's my theory. Untill someone who really knows what their talking about can give me some facts to follow away from this, I'm sticking with it.

Cel

 

Think about trying to stuff 50 pellets through a small tube...and then 50 pellets through a larger tube......which one would result in more "hits" on the same sized target in the middle of the tube....

Obviously the narrow tube would have more pellets per unit area..and more "hits"...and in the MAF scenario...would have higher MAF Voltages....

The larger tube would have less "hits" per unit area...say like you were trying to stuff 40 pellets down the smaller tube.... So the MAF voltage would be lower.

If you tried to stuff 60 pellets down the larger tube...you might get the same # of hits as 50 in the smaller tube.......resulting in the same Voltage as the smaller tube saw at 50

 

You are thinking of solids. You can't think of any moving liquid or gas as a solid. The rules for solids don't apply.

If you can't wrap your head around it, do some research and find something that will explain it the way you're thinking it should work and let me know. He who asserts must prove. I've posted some ideas that are laws of physics, even if they may not apply here, that I think describe what MM was talking about. Prove me wrong. I'm all about that. That's how we all learn.

Cel

 

Higher velocity fluids have a lower pressure and lower velocity fluids have a higher pressure, as in your airfoil example. So, decreasing velocity will increase pressure but will also mean less air is passing through the small ring of the MAF sensor. There is some calculated point where you get more cooling from having more air at a lower pressure than you would with less air at a higher pressure. You would need the equations of these functions (cooling in terms of air density and cooling terms of air velocity), the calculated point would be the intersection their lines.

How it makes sense to me is that, if you lightly blow on a MAF sensor that should be fairly high pressure air but at a low flow rate. If you attach the MAF sensor to an engine's intake tube, the pressure will be lower but have a higher flow. The MAF obviously will read higher for the intake tube.

I think this shows that scaling a MAF sensor is a little more complex than just using a factor derived from Area1/Area2 and of course you have the boundary layers that affect flow too.

 

yes. ^

its the same as if you increase voltage to a fuel pump to increase the speed of it, but the amount flowed will be less

 

Where did " solids" come into anything.......this discussion is about air.

 

You mentioned 50 pellets. Pellets getting stuffed into a tube will act differently than air particles flowing though a pipe. That's all I was saying. Even thinking of air particles as those pellets doesn't work.

Guys, I'm sorry, it make perfect sense to me, but I can't explain the way it makes sense other than the way I already have.

Cel

 

I have never denied that a bigger pipe will have the ability to flow more air. It will.

But, if you keep the volume of air the same, there will be a higher measurable pressure in a larger pipe. If the MAF is scaled for a 3" pipe and you place it in a 3.25" pipe, with the same volume of air flowing through it, it will read more air flowing then there actually is because the pressure will be higher.

 

That makes sense. The increase in Stagnation pressure will increase the h value (heat transfer coefficient) of the air. How much of an increase I'm not sure. Although if the Baro sensor is in the intake tube it should account for it. If not then the PCM is making a heck of an assumption on the actual H of the air. Or the changes in H are very small regardless of large swings in the pressure and it's more just for calculating mass flow. Anyone know if the actual change in cooling of the hotwire due to density change is negligible or significant? significant to me is 0.1-0.5% change in h value in this case.

 

The boundary layer issue would come into play as well....as the tube size increases..the MAF filiment changes positions...and ends up in an area with slightly less flow than center ( where the flow will be maximum in a laminar model). So will skew the results to lower reading for actual flow

It still doesn't result in lower flow rates than a small tube...where the effective flow is much higher because of the area ( inv square relationship)

I think when looking at the slight differences in filament position...that the Area relationship will make the former unimportant

 

The difference in pressure would be proportional to the square of the difference in velocities between a stock car and this car. I think.