ZFuel

[Captain Obvious]

However, I would like to understand the physics behind it.

I'm no expert on the topic, but I believe the flowmeter style used in the L-jet is more akin to an anemometer in that it measures more "flow velocity" than "flow volume". Of course, if you know the cross sectional area and the velocity, you can calculate volumetric flow per unit time.

I'm assuming this is the same thing you found out with your research and why you said "For a compressible fluid, it's all about the speed." Is that the case?

[Hodgimus-maximus]

The 280z AFM is actually vane meter sensor also known as a Volumetric Airflow sensor. It doesn't measure mass or density directly. it's closest mesurment would be drag force which is what actually moves the flap of the air flow meter which is dependent on fluid density. that movement is conveted into voltage drop by the potentiometer connected to the vane which is calabrated to a certain actual mass air flow. It's also good to note that the temp please a decent roll in determining the mass of a fluid. At least that's what Wikipedia says and from my ChemE background it makes sence. But it's a crapy way to measure air speed. Mass flow sensor - Wikipedia, the free encyclopedia I also found this web too http://www.sdsefi.com/techtheo.htm

Edited September 30, 2013 by Hodgimus-maximus

[Captain Obvious]

The 280z AFM is actually vane meter sensor also known as a Volumetric Airflow sensor.

Actually it traces directly back to what is called a "Pressure Plate Anemometer".

See:

pdf doc about instrumentation

Anemometers

More about instrumentation

[superlen]

Captain,

Yes, you and I are thinking alike. I do equate the AFM as a velocity measuring device. Great picture,BTW I'll have to investigate those links.

Hodgimus-maximus,

Yes, temp does play a major roll in the mass of the air, but from my research it doesn't change the drag force on the vane tremendously. Since the air is a compressible fluid, the speed of the fluid is the primary initiator of the drag and overshadows the mass. I couldn't wrap my head around this very well until after some research I realized the "compressibility" of the fluid is what makes it this way.

Then as Captain pointed out the speed gets turned to volume by using the cross sectional area of the AFM. The cross section won't change of course, so I really don't have to worry about that math or calculating any of that. I will simply use the voltage output as an indicator of volume flow. Now that we have an accurate measure of volume/time we definitely must use the air temperature as you noted to calculate the mass/time.

Len

[Hodgimus-maximus]

I would not like to solve that equation because the cross sectional area of the depends on the speed of the air flowing threw it. :cross-eye:

Edited September 30, 2013 by Hodgimus-maximus

[Captain Obvious]

I do equate the AFM as a velocity measuring device. Great picture,BTW I'll have to investigate those links.

High-tech back in 1450 as originally invented by Leon Battista Alberti! There isn't any technical info in those articles, just history and a pic or two. The pic I linked to above is Robert Hooke's anemometer from the mid-1600's. And yes, it's the same Hooke as "Hookes Law" (the spring guy).

About the cross sectional area, it does change as the vane lifts up. I have absolutely no idea how linear the pot output is however. One of the sources I found indicated that it was intentionally non-linear and was in fact a logarithmic output.

If it were me, I would put it in series with a known transfer function mass flow meter and pump some air through it at varying rates and characterize it. I wouldn't try to predict the output... I would cheat and test it.

[superlen]

Hodg, I thought about the cross section changing, but I think that only affects the area right near the flap. So, the air right there speeds up, but a few inches later is slows back down to an average airflow for the round duct size. Still looks iffy doesn't it? Regardless, I'm not going to try and model it in software. I'm perfectly fine just calibrating the output and going forward.

Captain,

Yes, it's purposely non-linear to give more resolution down in the idle/off-idle area.

I too think that putting in series with a MAF is a great idea. I had been thinking I would tune with Speed Density first and then use it to calibrate the AFM, but the MAF would be less work & dyno time. MAF was going to be the last sensor I worried with, but maybe I should make it first. Much easier than building a flow bench which I thought about as well. Time to walk the local yard & surf Ebay.

FYI. I have written the firmware to actually calculates air mass flow for any/all meters that are enabled. For instance, I could get the grams/sec flowing from all the following algorithms every cycle.

1. AFM

2. Speed Density

3. MAF

4. Alpha-N

The theory being that if the AFM started giving strange readings, I could revert to Speed-Denisty on the fly, like a limp mode. Most all OEM ecu's do this. If MAF goes south, they revert to Speed -Density or even Alpha-N. With the frankenMaf spliced in between the filter and AFM & an replacement linear TPS I could drive around with the test car with FOUR redundant air mass sensors. hahaha.

Lenny

[superlen]

This looks like a great test candidate for MAF. It even has the Bosch style connector, so if one wanted to go ahead and replace their AFM with a MAF, the harness would still look "stockish".

New Maxima J30 I30 Q45 Mass Air Flow Sensor MAF Meter 2268031U00 2268031U05 | eBay

Edited October 2, 2013 by superlen

[superlen]

Just another quick update. I've been sidetracked with other projects, but I did get the raw circuit boards in this week & will try and get time to populate them next week. Pics as well to follow.

Lenny

[superlen]

Here's a quick picture of the HellFire ECU. It passed it's first test. It fits in the stock ECU frame perfectly. All mounting hole locations were spot on and the stock connector attaches as it should with no interference. I do have one resistor that is a little close to the frame, but it's acceptable. It's also an optional part for someone running more than six injectors, so stock configurations it won't even be loaded.

For inquiring minds, here's some technical details:

1. It's a four layer board with internal power and ground planes, not a cheap simple 2-layer. The internal planes are further split into multiple analog and digital planes to keep *ALL* the analog signals above/below their respective planes. This should give me us extremely clean analog signals from the external sensors. I'm extremely anal about routing mixed analog/digital and for many pcb designs I do it's pretty critical. For the signals on this board it's a fair amount of overkill, but it's nice to know that the signals that hits the a/d converters are going to be as clean as possible.

2. All the optional cam/crank trigger inputs are in the top right section of the pcb. They are isolated of course with their own filtered supply, and I even added guard rings to the ttl signals as they made there way to the CPU input capture pins.

3. Injector driver circuits and optional ignition drivers are all along the bottom edge. Again, all are highly isolated. The same drivers will run the external fuel pump relay & some additional general purpose inputs/outputs that someone may need. (thinking in the future for turbo/nos/shift light, ect. ect)

4. USB communication, & optional blue tooth module & accelerometer module are near the top middle. The USB connection will pigtail out with a simple cable to cabin & a female B connector. You can let it hide up over the odometer reset cable/hood latch cable or extend it to somewhere else like the glove compartment or console.

Now, all I need is more time...

This shows the HellFire on the bottom with my test harness connected and the stock ECU above it.

For comparison here's the entire stock ECU guts. There are actually two circuit boards in the stock design with a ribbon cable connecting them.

[Captain Obvious]

Lenny, Awesome. Can't wait to see what happens when you get one assembled and powered.

I see in the pic you've got the board edge connector installed. Did you harvest that from a stock ECU, or did the part numbers we talked about earlier pan out? I worry sometimes about large low volume parts like that... They say they have stock, but they're in the dark dusty part of the stockroom and they only think they have stock.

BTW - Enjoy the smoke from prototype #1! :devious:

[FastWoman]

NICE!

I'm getting rather excited! With the Bluetooth capabilities, are you going to have an app for that?

I would love to help you beta-test in my Z. It's not the freshest engine in the world (about 170k), but it it's straight, with the exception of the wonky ECU.