238/242

 

Right, the 0% humidity will have more O2 at the same IAT and MAP. What I am saying is (for speed density) all the pcm can see is IAT and MAP so it can't tell the difference between the 0%/100kpa/50 degree air and the 100%/100kpa/50 degree air. Since it will add the same amount of fuel for either batch of air, that's why i'm saying it can't compensate.

I guess maybe another way to say what i'm trying to say is: humid air requires a different air/fuel ratio than dry air because it has a different air/o2 ratio. That may not be true in practice, but at least for stoichiometric reaction it would be.

As for the maf, i guess the humid air would register as less airflow on the maf, but i'm thinking it would undercompensate because the difference in heat transfer between the humid air and dry air would be less than the difference in o2 content. I could be wrong, but it would be a coincidence if the humidity affected heat transfer exactly the same percentage as o2 content.

Oh and I agree in either case the difference is so small that we're splitting hairs.

 

It worries me when you guys use the word "airflow" when talking about a MAF. It's really measuring mass flow.

H2O molecules have mass too so as far as where the MAF will read more or less mass with more humid air I think it depends on other factors.

 

Talk about a curve ball - we're suddenly back on topic!

P-Mack - I see your point.

However, I still believe the PCM is making the necessary adjustments for humitidy whether you are in SD or MAF operation. Reason I believe so is because the PCM does sample barometric pressure.

If you have the EFILive scanner, or someone that can let you borrow theirs, I have two logs with similar temperature conditions, on two different days, with very different humidity between those two days. In both logs I was logging gm.baro and there is quite a notable difference.

One of the things the PCM does to adjust fueling is calculate how far off current air density is from the VE tables referenced density. May seem like splitting hairs, which you and I may be, but the PCM has to do this so a car running in Houston, San Antonio, Phoenix, Hawaii, Denver, etc... will run reliably.

 

Agreed, H2O does have mass, but the the molecular weight of dry air is heavier than water. Humid air should register as less mass flow than dry air.

 

I agree that humid weather should register as less air mass by the MAF. The PCM should also see high O2 values which would confirm the MAf readings.

 

I guess it would depend on the specific heat and the density of humid air vs dry air mostly.

As for airflow vs massflow, i don't see the big deal. I guess i am using "airflow" to be short for "air massflow". Water vapor is a component of air, and "airflow" encompasses massflow of N2, O2, CO2, H20 (vapor), etc. It's just semantics.

 

Although I guess airflow could be taken to mean volume flow, as in cubic feet per minute. In that case I guess it would be better to use "massflow" for clarity.

 

Dry air is 1.28kg/m^3
Water is 1000kg/m^3
1.28kg > 1000kg?
I guess I have to disagree with that (but I doubt that's what you meant).
I don't see how that follows. Humid air (with relative humidity of 100%) contains 0.023 kg of water. It's less than 2% of the mass passing the MAF. My understanding of how a MAF works is that it doesn't care what is traversing it, atoms, molecules, it's all mass to the MAF.

Let's say you stuck an argon tank in your car, plumbed it into the airbox and cranked it open. Pretend 1/2 of the gas passing the MAF is argon. The MAF will report the mass entering the engine, the PCM would think that twice as much O2 is coming in as really is but the O2's will show things are rich as hell.

When the humidity changes, the STFTs change, maybe the LTFTs change a little and everything is ok.

Now if you're saying as far as what the MAF reports, humid air has less O2 than dry air, I agree. But we've established that the difference is small, right?
That's exactly the point I was trying to make. A MAF is neat because when you know the mass of the air entering the engine, you can directly calculate the mass of fuel you need to add to have (in theory anyway) chemically perfect combustion.

When I see "airflow" I think of volume per time unit. In that case, you need other information (i.e. air temperature and barometric pressure) to figure out how much O2 your're dealing with.

I think my deal is I had a chemistry teacher in high school who beat UNITS into my head. Which reminds me of my favorite fortune cookie, "An inch of gold is not equal to an inch of time."

(This is a great thread!)

 

the LS1 and LS2 PCM does not use a relative VE table like older code did, the main VE table is absolute and is not referenced to any temperature or pressure. It assumes density varies linearly with pressure (MAP) and temperature (manifold air temperature).

 

Atomic mass of O is ~16; molecular mass of O2 is ~32;
Atomic mass of H is ~1; molecular mass of H2O is ~18;

However...
density of air is ~1.28kg/m^3;
density of liquid water is ~1000kg/m^3;
density of vapour water at STP is....?

(molecular mass does not directly determine density).



Yes, I agree, very interesting thread;
Btw: I do free car work for relatives, friends, and neighbours only; I just have them pay for parts and fluids.

 

guys, remember the basic construction of the MAF... it is sending out a frequency based on how much the wire is being cooled. The relative cooling of the wire is all that matters.

Neither the MAF or the PCM know what is cooling the wire, they make an assumption. This assumption is reflected in the Frequency(hz) to Airflow(g/sec) lookup.

The two biggest assumptions the MAF makes are:

1. The air that is passing over the wire is representative of all the air passing thru the MAF (laminar flow).

2. the flow is unidirectional

Unfortunately these ideal conditions are not present in the engine intake and are the primary sources of error.

 

I think I understand what you are saying, and if I do, I agree...

What do you mean by "absolute"?

 

i mean the PCM doesn't look at Baro and then calulate the density of the air based on temperature etc. and then look at the VE table to determine "what % of ideal cylinder loading am i getting?"

The VE table stores an absolute (but scaled) reference to the cylinder loading (g/cyl) which is scaled based on the current MAP and temperature.

The native GM units of the table are grams.Kelvin/kPa. If you run all this thru the ideal gas equations you get exactly the same result as if you had used a traditional % based approach, just this way is a much faster calculation for the PCM. Its a non-intuitive approach but many optimizations seem that way.

Chris...

 

Wouldn't frequency(MAF) be a meaure of time? There are AIT, and MAP to help the PCM understand the conditions the engine is experiencing along with the O2's to inform the PCM how well it adjusted.

While I argue that the stock MAF calibration should be sufficient for any engine modifications, I don't believe the MAF itself is accurate enough to inform the PCM of exactly how much air is entering the engine.

 

Staying sidetracked here....

Chris,
It's hard for me to back up my theories having never seen the PCM code, but I do not agree 100% that the PCM is not calculating varients in air density. I draw this conclusion based off what I can see in my tune, barometric pressure update tables & offset factors, and based off my logs.

Barometric pressure isn't sampled as often as temp and MAP. This leads me to believe the barometric offsets would be accounted for else where in the code.

Also, within my logs, I can see significant differences in IPW's when the baro values change from day to day (when comparing logs with similar temperatures (+-2*F), time of day, and route).

 

i concur! yesterday evening i had trims on idle at -2, and 15 mins later they were at -10. why? that of course changed IPW, which would be nice to have lean on idle, as the gas prices are starting to go past $3/gallon... i feel so european again

 

It does calculate air density using the ideal gas law, pressure and temperature. The maf takes density into account by measuring massflow instead of volume/time. I don't see where you think Chris said it doesn't.

I think it is possible that ve can change with baro in the form of exhaust back pressure, which isn't taken into account in the ve calculations.

 

MAP and BARO are both pressure, atmospheric changes are reflected in the MAP reading. Baro is not needed.

 

I think I got it. Just need some time to digest.