Air Flow Meter discussion (WAS: "I'm missing something...)

[Kevin _]

>-Air Flow Meter (AFM or flapper/vane sensor):  measures air flow and must
>have density correction to calculate air mass.
>-Manifold Absolute Pressure sensor (MAP):  measures manifold pressure and
>must have density plus other correction and some assumed constant values to
>*guess* at the air mass.
>-Mass Air Flow sensor (MAF or hotwire):  measures the mass of the air
>directly and does NOT require any corrections by the ECU.

Unfortunately, there is no such real thing as a physical MAF.  A hotwire 
sensing mechanism more-so measures velocity of the air than mass.  There are 
tons of corrections that go on in these sensors (standardly 2 hotwires held 
at a constant delta T with an instrumentation amp using resistance to 
reference temp) to attempt to get an output that is representative of mass 
airflow.

>-Karmon Vortex:  I believe these also measure flow and must correct for
>density.  I am a little sketchy on how these work; something about a
>honeycomb structure that creates turbulence in the air flow stream that can
>be measured (at least in the Toyotas).

The honeycomb is there for keeping the flow straight into the sensor body.  
There is a normally a triangularly shaped part before the microphones that 
upsets the flow, causing the vortecies.  This method seems to work very well 
over a broad range of airflows and normally has a frequency output.  There 
is a breakdown flowrate with these sensors after which they become erratic 
and useless.

Unfortunately, in the automotive world, all the aforementioned sensors 
besides the MAP are VERY slow.  From the airflow change to its reported 
measurement, you have a bare minimum of something like 20ms before all the 
corrections are applied.  On the hotwire, the automotive environment 
requires a very durable sensor, so a big-ole thermistor is usually used as 
the hotwire element, which only adds to its latency.  Combine that with the 
air response time of a turbo system that has 2 cubic feet of plumbing that 
is being compressed from 1:1 to 3:1 and you're in for some very 
non-representative readings during spoolup and after you get off the gas.

On the other hand, a general MAP sensor reacts in about 1ms.  This is by far 
the superior way to go for making a drivable EFI setup.  With the airflow 
measuring devices, you'd probably have to wait about 4 engine cycles to be 
able to calculate an accurate airflow input at 9k rpm (13ms/cycle, total 
wild guess of 40ms latency), where with the MAP setup you're able to catch 
it 1 cycle after the transition.  To take care of the latency issue, you 
just have to piss fuel in with throttle transition corrections.

BTW - I just joined the list earlier today, there seems to be a lot of good 
stuff here!

Kevin

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