[Diy_efi] Q regarding placement of a pressure sensor

[Grant Beaty]

Hmm, and if you've got a pressure sensor before and after the metering area,
you could calculate airflow at any inlet pressure, so you could use the MAF
after the turbo and IC. Pressure transducers aren't effected by small
amounts of oil are they?

Bob, have you actually measured the pressure drop across your current MAS? A
DSM friend of mine thought his car lost power with the more restictive 2G
MAS.

Grant

-----Original Message-----
From: Chris Conlon <synchris at speakeasy.org>
To: diy_efi at diy-efi.org <diy_efi at diy-efi.org>
Date: Monday, July 15, 2002 4:24 PM
Subject: Re: [Diy_efi] Q regarding placement of a pressure sensor


>At 10:31 AM 7/15/02 -0700, Bob Moon wrote:
>
>>  This leads to my question:  why not run a pressure
>>xducer before the turbo (say, in the stock MAF
>>location) and just read the vacuum that is created as
>>the turbo/engine inhales the air?  This would provide
>>a reading from -15 to 0 psi (the further negative, the
>>more air is flowing)
>
>This could be done, but it is probably not a great idea.
>You run into 2 basic problems.
>
>To start with you need some kind of geometry that will
>generate a pressure drop as flow rises. If there is no
>pressure drop, you can't measure the pressure drop to
>estimate flow. Ideally you'd like to keep the pressure
>drop low, so you're not choking the engine, but then you
>need very sensitive pressure transducers, noise becomes
>more of a problem, hysteresis becomes a problem, and so
>forth. So from a design point of view there are competing
>goals, one to keep the pressure drop low to maximize power,
>the other to make it higher to maximise resolution and SNR.
>
>The second problem compounds the first. The 2 common
>airflow measuring geometries (pitot tube and venturi)
>both generate a pressure signal proportional to the
>square of the airflow. This makes your resolution worse
>in the low airflow areas than in the high airflow areas,
>which is pretty much the opposite of what many other
>systems strive for. (AFMs and MAFs tend to have more
>resolution in low airflow areas than in high, so that
>idling, tip in and light cruise can be dialed in well.)
>This provides more of an incentive to use a high pressure
>drop, to regain resolution on the low flow end.
>
>There is probably some exception but my crude understanding
>of aerodynamics is that most any fixed geometry will generate
>a pressure drop proportional to the square of the airflow
>speed, at least within some range of Reynolds numbers.
>This is the basic principle behind pitot tubes, venturis
>and orifice airflow meters.
>
>Flap type AFMs use a variable geometry and so the pressure
>drop is roughly flat over the design airflow range. But then
>you need to measure flap opening as well... sounds like you
>just built an AFM.
>
>Someone clever could probably build a variable sprung
>geometry which resulted in a pressure vs. flow curve that
>was less than N^2 and closer to N^1. That might work.
>
>You might consider other types of variable geometries,
>such as a larger venturi blocked with a butterfly which opens
>at higher loads. Basically you're building a carb here,
>and so lots of what has been done with carbs will be
>applicable.
>
>If you can find a precision, very low noise pressure sensor
>that reads in a very low pressure range (say 0-1psi) you could
>probably make a system like this work, if not super well.
>
>You might also get away with 2 venturis in parallel, one much
>larger than the other, but with the smaller one towards the
>center of the flow and the larger to the outside. If you sized
>them right you'd get a usable reading from the large one just
>about the time the small one was going nonlinear. It might
>take more than 2 different sizes to cover the whole flow range
>needed, especially on a high boost turbo engine.
>
>Don't forget to compensate for airflow density at the inlet.
>(BAP and ambient temp will allow for this.)
>
>
>Along these lines I've generated flow curves for a couple
>Bosch style flapper AFMs with a crude flowbench, using sharp
>edge orifices and a low range pressure sensor. I needed to use
>several different sized orifices to cover the AFM's full range
>in sufficient resolution, even allowing up to ~2.5psi drop
>across the orifice. I would think your current Karmann meter
>does not generate anywhere near 2.5 psi of drop. Certainly
>the design is capable of doing that, anyway.
>
>Picking one AFM at random, I see that the highest airflow it
>can measure is ~120 times the lowest it can measure. Let's
>round down to 100:1 since the factory design doubtless includes
>some margin beyond what is really needed. An airflow range of
>100:1 will give a pressure drop range of 10000:1. That's
>between 13 and 14 bits of a/d. You should go to 16 to have
>some margin to deal with noise. That by itself is doable, but
>the basic problem here is that you need a pressure sensor
>ranged for (say) 0-1psi, which is about 1 in 10000 parts
>accurate, or 0.01% accuracy. If you find one even close to
>that anywhere close to cheap - let me know! (I'm dead serious
>here, I'd love to have one.)
>
>If you can come up with a geometry such that pressure drop
>is linear in airflow, you only need ~1% accuracy which is
>vastly more doable.
>
>AFMs and the MAFs I've seen deal with this by compressing
>the higher airflow ranges into much smaller signal changes than
>the lower airflow ranges, basically companding the signal
>so you get more effective resolution down low where you need it.
>
>
>   Chris C.
>
>
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