New member intro

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

Steve and all,

>>This is true, but only at steady-state.  During (air flow) transients a value 
>>for volumetric efficiency is still needed if you are trying to account for 
>>first-order intake manifold dynamics.
>> 
>>Tony

>Interesting -- can you elaborate further?  Suction through the MAF sensor 
>"seems" pretty steady, although I imagine there is quite a lot of turbulence 
>further down in the manifold.  What dynamics do you need to account for?

I'm sorry I can't elaborate in full detail. I'm working on these types of
issues for a large company and must maintain an appropriate degree of 
confidentiality.  I can however discuss what is already public information
(e.g. SAE papers, ASME Journals).

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.

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.

At steady flows, the dynamics are irrelevant, obviously.  During transients 
however, the time constant, and therefore the volumetric efficiency, comes 
into play.  So even though a mass flow sensor measures the air flow directly, 
volumetric efficiency may still be needed.

It's true a mass flow sensor measures the actual flow into the manifold, but
we are most likely more interested in the flow OUT of the manifold.  Both 
mass flow and speed density systems have advantages.  I'd be interested in 
people's opinions on and experiences with speed density systems.


Tony