MAF and SD (was re: Re: New member intro)

[tsakiris at ed8200.ped.pto.ford.com]

|Hmmmmm..... that large company wouldn't be ford, now would it?

Oh dear.  I've been found out.  Yes, I work for Ford Motor Company, as is 
obvious from my e-mail address.



|| The intake manifold is a dynamic system.  It can be analyzed as a simple 
|| control volume, with inflows and outflows of air.  The air that flows out is
||not the same as the air that flows in.  The inflow is measured by the sensor.
|| The outflow can be approximated from the conditions of the air inside the 
|| manifold.  The difference is flows is what accounts for changes in manifold
|| pressure.

|Duh, that makes sense.  But it still seems like you'd have more problems with 
|SD.  Simply driving to a different altitude would change the density of the 
|air, and throw off the calculation.  Or would it?  Maybe I'm missing something 
|wrt SD.

Speed density systems that I know of include a barometric pressure sensor.  I
don't really know who the audience is here yet, so if my comments are to 
generalized or simplified I apologize.



|| In the simplest of models, the dynamics can be approximated as a first-order
|| lag (i.e. a differential equation for pressure can be written which is 
|| first-order).  The time constant in such an equation may include a term 
|| representing volumetric efficiency.

|Geez, I hate differential equations :-).  I assume what you mean is that if 
|you suddenly accelerate or decelerate the engine, the change in airflow won't 
|be seen immediately at the MAF sensor due to the fact that the manifold takes 
|some time to respond.  I can see that, but can you elaborate on how 
|significant that really is?  Since manifolds are designed to have little 
|restriction it would seem that their time constant would also be small, 
|especially when compared to the max delta rpm you are likely to see.

The time constant in the equation ranges from about 10 to about 300 
milliseconds (over a variety of engines and engine speeds).  It's more a matter
of size (manifold volume vs. engine displacement volume) than flow restriction.

You're correct on the engine speed effect.  Engine speed changes relatively 
slowly, due to the inertia of the engine.  Manifold pressure is affected by  
the inflow of air to the manifold also though, and this can be changed quite 
drastically in a very short time simply by moving the throttle plate.



For everyone out there interested in more details, the following published
papers might be interested reading.

SAE 880561 - Frequency Domain Characterization of Mass Flow Sensors, 
             by W. C. Follmer, 1988.

FISITA -     Adaptive Transient Air-Fuel Ratio Control to Minimize Gasoline
             Engine Emissions, by Beaumont, Noble, and Scarisbrick, 1992.

SAE 900616 - Mean Value Modelling of Spark Ignition Engines, by Hendricks
             and Sorenson, 1990.

 
And just one more comment, if you're interest is purely to increase power, most
of this is irrelevant.  Just run rich.  Manufacturers are trying to solve many, 
many other problems (e.g. emissions, driveability issues, cost, and fuel 
consumption).


Tony