Z

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by ViribusUnits:
<strong>Finaly, if the AFM measures mass, why did Bosch chose to include a air intake temp sensor?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">The air flowing through the AFM in the L-Jetronic system moves a flap against spring tension, and the flap is connected to a a potentiometer that varies it's electrical resistance as the flap moves. The air flowing through a CIS system moves a flap against spring tension, and that flap is connected to the mechanism that mechanically controls fuel flow through the injectors. There's no temperature sensor or electronics used to calculate air mass on the CIS system. If you've ever seen the fuel curve of a CIS system on a dyno, you know that it works pretty well at keeping a consistent mixture, without an air temperature sensor. That's because the spring tensioned flap measure air mass. The usual reason air temperature sensors are used is to control ignition timing, and not calculate air mass. If the intake air temperature gets too hot, the ignition timing is retarded to help prevent detonation. On at least the S4 and later cars, the intake temperature sensor is also one of the things that controls the radiator fans. Systems using a MAP sensor have to use an intake air temperature to calculate air mass. It has to be done on those systems because the only other input that they'd have otherwise would be the air pressure, which changes with temperature.

As to how well a flap type system might work at measuring air mass at different pressures, I've seen a CIS flap system work pretty well at maintaining the air/fuel ratio on a car that was making around 9 psi of supercharged boost. The mixture was kept very consistent with the air mass under boost up to 5,500 RPM. The only reason it started to get leaner after that was because the injectors were too small. Even at the 5,500 RPM there was plenty of boost to show that the air mass under boost was easily able to be measured by the spring tensioned flap.

mark kibort

Viribus,

Checked out the Ljet Bosch site, (nice informtion page) but, the altitude sensor was use on "some vehicles operated at high altitudes" . not all 928s have altitude sensors.

again, the AFM is very adapt at controling mass air flow up to 7000feet, which is a 20% difference. (approx)

Point its, the volume stays the same for a given engine size and rpm, but the mass flow changes for air density differences,performance mods and throttle positions. This is why we are able to operate a 928 at 7000rpm without fuel air mixture changes, and 20% increase in HP (mass flow) by doing mods to the engine.

(So, same afm, all the mods, here is what happens: same pressure outside, say its sealevel. floor the gas, more molecules of air now (increased mass flow) move through the AFM, and move the flappy door proportionate to the mass flow. think of the throttle valve as a variable presure drop device. the more it is open, the more mass flow. until it is wide open, and all the other components of the intake are now the limiting factor. So, full throttle, but I open valves, I decrease pressure drop there and the entire air stream's mass flow is increased. (think of a pressure gradient starting with opening of the AFM to the piston faces)If I can change the pressure drop , it changes the entire gradient. If I had almost no pressure drops along the intake , like individual throttle bodies all stacked tied to a common AFM , then if there was an AFM, it would have even more mass flow moving through it, and the flapper valve would be deflected even more. In this example, the AFM would more than likely be the most restrictve element, and based on its own flow characteristics, could be maxed out at say 350hp when opening the entire system with a MAF may peak out at 500hp. (see principles of restrictor plates) based on my findings, 300 rear wheel hp seems to be the max limits of the AFM metering system, even though it was use on a 928 with only 200max rear wheel hp. Hmmm how did i go from 175 rear wheel hp to 300rear wheel hp changing everthing , including displacement, EXCEPT for the AFM????? because the AFM is extremely good at metering out mass flow!!!!

MK

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by ViribusUnits:
<strong>I understand that atmophereic pressure reduces with altitude.

a short look on the internet, Denver is at about 29.93" Hg, while Houston is at about 30.14" Hg. A while back, Denver was at 29.7"Hg, while Houston was at about 30.06" Hg. I'm not seeing a 20% drop.

Assumeing that the ideal gas law is even close, 1 " Hg works out to no where near 27% of the density. Maybe 2.7% and the diffrence between Denver and Houston is about .4" Hg at most.

Denver is a little over 5,000 feet up, while Houston is basicly at sea level.

Your going to want page 24 and page 27. You'll probably have to enlarge the photo. Notice the optional altitude compensater. Why? An L-jet is a L-jet.

Finaly, if the AFM measures mass, why did Bosch chose to include a air intake temp sensor?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">

mark kibort

Z,
Very good!. Yes, the CIS is very simular to how an AFM works but instead of fuel metering being controled by a fuel plunger, the AFM uses a potentiometer. principles are the same, and both are extremely good!!!
MAF is just better, but also can be tough to diagnos if you have any problems.

MK

dr bob

OK--

Using the weather channel barometer reading is deceptive, as the reading is normalized for altitude. Imagine that!

Here in Colorado Springs today, I'm correcting gas turbine performance instruments for altitude to the tune of about -2psi. So figger that ~2psi/14.7psia at sea level, and then consider that old perfect gas law when you "calculate" your air density differences. Temps here are running from the teens overnight to almost 70f in the heat of the afternoon. I do correct the GT performance based on pressure on the suction side of the air filters. Don't forget that temps for the equation are K not F to see the effect.

Headed back to the hotel for some sleep...

Steve Cattaneo

Not all CIS systems are mechanical. On CIS with a 02 sensor, the fuel is controlled electronically by a frequency valve and control unit. The frequency valve will enrich and lean the fuel mixture by controlling fuel quantity.

<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

ViribusUnits

Now why the heck would they "normalize" the reading for altitude? What, we normal people don't know enough to know that higher altitudes should have lower presures?

I hate it when people start trying to idoit proof stuff. They make it even harder to tell what they're trying to say/do. Like correcting the barometric pressure for altitude.

All right, I'll go back and look at it again.

I still can't belive they would do something stupid like that. Why???????

Lets see, 2 psi works out to about a 13% change in pressure, which since we're assumeing constant tempeture, that works out to about a 13% change in density. Yep, that is deffently sigificant! If the AFM doesn't take that into effect, there would be a problem. Now, to figure out how it does it. Humm...

Question, how would you go about modeling the AFM mathameticaly? So far, I've always assumed it acts like an orifice, w/ a varable size, controled by the diffrence in pressure on each side of the orifice. Eighter I'm looking at it wrong, or I can't add. Both have been known to happen. Rarely, but they can happen...

IIRC, F would be best changed into Rankin, rather than Kelvin. Course I hate imperial units as much as the next guy. Well, probably more...

mark kibort

I know, this air flow stuff can be quite "Un-intuitive" . so, now you see that the altitude is significant (sorry for the numbers all over the map, i was going off memory of different tests we did at different altitudes)
basically, its 3% per 1000feet. if 29.92 is at sea level, then its
24.90 at 5k (down 17%)
23.98 at 6k 20%
23.09 at 7k 23%
20.58 at 10k feet and thats 32%

Since Bosch said the AFM adjusts almost perfectly at 10k feet, but the US wanted more fuel air mixture regulation, they added the temp sensor, and optional barometric sensor. If the AFM didnt measure mass flow, imagine, how bad the air fuel ratios would be at altitude!! imagine your fuel air ratio being at around 13.5 stock, and then going to 10:1 if you took a drive to the mountains at 7k/feet. You would be spittin'rich . Since that doesnt happen, the AFM is awesome in measureing the mass flow that passes over the AFM's flapper door. remember its just a wing with a pivot. (more acurately, a wings flap, so use that for your modeling)

cheers,

Mk

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by ViribusUnits:
<strong>Now why the heck would they "normalize" the reading for altitude? What, we normal people don't know enough to know that higher altitudes should have lower presures?

I hate it when people start trying to idoit proof stuff. They make it even harder to tell what they're trying to say/do. Like correcting the barometric pressure for altitude.

All right, I'll go back and look at it again.

I still can't belive they would do something stupid like that. Why???????

Lets see, 2 psi works out to about a 13% change in pressure, which since we're assumeing constant tempeture, that works out to about a 13% change in density. Yep, that is deffently sigificant! If the AFM doesn't take that into effect, there would be a problem. Now, to figure out how it does it. Humm...

Question, how would you go about modeling the AFM mathameticaly? So far, I've always assumed it acts like an orifice, w/ a varable size, controled by the diffrence in pressure on each side of the orifice. Eighter I'm looking at it wrong, or I can't add. Both have been known to happen. Rarely, but they can happen...

IIRC, F would be best changed into Rankin, rather than Kelvin. Course I hate imperial units as much as the next guy. Well, probably more...</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">

ViribusUnits

OK

Here's what I know. For an orifice*:

dP = .5*r*v^2

dP = diffrence in pressure on each side
r = density
v = velosity of the gas through the orifice.

Pa (kg/(m*s^2))) = .5*(kg/m^3)*(m/s)^2 **

OK, lets see how that would react with an increaseing density. The pressure changes, and turbulance should be so slight that it should be mostly true for the air we're working with.

If I increase the density, and hold pressure constant, the velosity decreases to half it's square. The bigger density changes, the farther the velosity gets from it's linear value. So, if I'm doing this right, the farther the density gets from it's set value, the father off the velosity is going to be.

And since:*

V = A*v

V = volume
A = area
v = velosity

or

v = V/A

Putting the two equations together:

dP = .5r(V/A)^2

Since the dp is right at constant, for a given flap reading, (read a given area) when we increase density, the volume must come down to 1/2 it's square. At small changes from the adjusted density value, the AFM will still be pretty accurate. The more the density changes, the more inaccurate it should be.

All right, now I understand how it works, and why the altitude sensor and probably the temp sensor are optional. The AFM is not a highly precise Mass Air Sensor. Instead, at close to the adjusted density, it's very close to the needed correction; however, the farther off the density gets, the more innaccurate the reading is. Simple, crude, and effective. And it becomes clear why the hot wire is better.

Does this seem to make sence?

*I pulled this from my sophmore level physics book.

**See the SI units canncled.

Rich9928p

The L-Jet has a base CO adjustment. Once that is properly adjusted for your local altitude, the MAF works just fine.

It probably is much better that an carburetor metering jet for altitude changes.

The LH MAF used with the LH-Jetronic is the best, because it measures mass directly! It has no mechanical issues, only problems with keeping the filament clean and intact.

BTW, LH stands for Luft Heiß (air hot in German).

ViribusUnits

Hay, I just did something kinda neat too.

Since I know that:

r = M/V

then

M = rV

r = density
M = mass
V = Volume

I plug it to the final equation from my last post and solve for M:

M = (2*dPa^2)/V

So yea, the AFM doesn't exactly measure the mass, but gets close enough for this game.

mark kibort

the AFM measures mass as accurately as a MAF up to 7k feet. In fact in tests to 10k feet (32% change in density) it was accurate. US emissions wanted even closer regulation, hence the temp sensor and optional altitude sensor.
PV=NRT
mk

Fletcher

Hey Mark don't the S4 cars use a differant injection system than our 83-84 S cars. A mass air flow sensor system. I just wanted to be clear about what my car has.

P.S. I have a great shot of you with Derek Bell from laguna last year. give me your email and I can send the pics from the race...

ViribusUnits

Something I noticed after going over hose and wireing of the L-jet and K-jet again.

The fuel rail pressure is controled by the engine's vacume.

Would this allow both systems to make a much more accurate represention of the mixture control, or does it just compensate for the vacume pressure?

Hum... Complex all of a sudden!

Jim V

VU, the K-jet (CIS) doesn't have a fuel rail but rather individual tubes to each injector, the only vac control is done thru the control pressure regulator which allows full load enrichment. Also the control pressure regulator has warm up enrichment built in as well as altitude compensation on some models, I can't recall if the 928 units have the alt. comp. feature or not.

On the L-jet-as I understand it- the regulator is a rising rate type controlled by vaccuum signal from the engine. These are similar to the aftermarket adjustable regulators except their is no adjustment.

ViribusUnits

Thanks.

That pretty much explains that.

Now, on to problem 2!