Sensor Questions?

Clinton L. Corbin CCORBIN at INTEL7.intel.com
Sat Feb 3 03:36:25 GMT 1996


>"Oh man, now I'm confused.  MAF isn't good as the basis for fuel injection 
>alone, since the amount of air past the MAF sensor isn't necessarily the 
>amount of air into the engine.  Remember the plenum stores air, so a sudden 
>increase in the amount of required air will draw a vacuum on the plenum before
>the MAF actually sees the increase  in airflow."

Quick question: what exactly is going to cause this "sudden increase in the
amount of required air" that is going to draw a vacuum?  About the only thing
that I can think of would be going down a steep hill and then closing the
throttle.  At this point, you need a leaner mixture anyway.  While air does
have mass, this will primarily cause a delay from when you open the throttle.

>You are absolutely right. That's whys carburetors had pump shots. EFI systems
>(even SD based) have acceleration enrichment strategies that are a function of
>throttle transients. That's one of the main reasons for using a TPS(throttle 
>position sensor) on hot wire MAF engines. SD systems are NOT "perfectly 
>capable" of learning about engine mods which affect volumetric efficiency and 
>never will be because of the way they work. You have to teach them. Part 
>throttle learning from O2 sensors doesn't work "perfect

Definitely agree on the need for an acceleration enrichment.   And the need for
a TPS sensor.  Only a do believe that both MAF (mass air flow) and SD (speed,
density) systems need them.
   
>I'm not discarding SD based systems outright. They work great if you have 
>the time to map a system out, and they are less expensive. It just seemed to 
>me that they have recently been touted as being capable of doing things things
>they simply cannot do as well as a MAF system. If you have the time and 
>facilities for mapping and engine's air consumption, good for you. Most of us
>do not.

Just remember that a MAF system also has to be calibrated to the engine it is
on.  The MAF give out a voltage (or frequency) depending on how much air is
going through it.  It does not tell you "currently, there is x Kg/sec flowing
through me".  How long you need to keep the injectors open is also dependent on
the size of the injectors (and the number).  Both systems must be calibrated. 
But I do agree that the MAF system will take less time.  Just my two cents
worth, so all those in opposition, fire away!

Clint



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