The attachments are those you sent on 11/18/2009.

I'm working on my own engine control and some questions arose as I
referred to the attached data.
The pressure tap data from Rotor 1 and Rotor 2 do not appear 180 degrees
out of phase, and the phase difference varies with rpm. Could this be
from different lengths between the two lines from ports to sensors?
In the photo "Apex-oil-hole-data.jpg",  the sensor plotted by the yellow
line appears to be saturating at 4.1 volts. Is that sensor in the apex
oil hole, or is it in the intake ports?

It would appear that in order to prevent pressure waves from saturating
the sensor, it needs either to have a range considerably above one
atmosphere or the volume of the line from the pressure tap to the sensor
needs to be high enough to damp out the variations and provide an
average pressure.

What model sensor is used to provide the red and blue plots?
Just an observation, but it would seem that the peak pressure at the
location you are recommending for the pressure tap should be a valid
indicator of the air charge. In order to have a volumetric efficiency of
120%, it would seem that the pressure would have to reach 120% of
ambient, and would be seen by the sensor connected there. If so, then
the MAP sensor peak output would account for VE, or be closely
proportional to it. An op amp configured to output the highest voltage
applied to it could then be used to provide an output proportional to
air charge.


Victor Roberts


Victor,
The technique of using a very small hole (orifice) in the pressure sense port produces a "pneumatic low pass filter" when combined with the volume of the connecting tubing downstream of the orifice.
This low pass filter will cause a phase shift in the pressure waveform in the tubing.  The amount of phase shift will be changed by the volume of the tubing involved.  More tubing = more internal volume = longer phase delay.

This MAY be what you are seeing.  Try making the two tubes connecting the orifice(s) to the pressure
sensors the same length and see if the phase difference goes away.

To get a better idea what is happening you need to look at the analog signal directly from the pressure sensor(s).  Once it is sampled by the A to D converter it can be distorted by aliasing (sampling too seldom).  A rule of thumb for quick and dirty design is to sample the signal at least 5 times the HIGHEST frequency in the raw signal.  This allows the computer to process the samples without producing totally false results.

It should be obvious, therefore, that filtering out any UNNEEDED higher frequency information from the pressure signal is a very good idea.  Any real design will need either a very fast sampling speed (expensive) OR will filter the analog signal before it is sampled.

Unneeded, in your case, is anything faster than the EFI system needs to track changing throttle position and engine speed.  My guess is that anything over about 10Hz in the pressure signal is not useful for adjusting fuel flow.  That implies that, if you see the individual pressure pulses as the intake ports open and close, you have much higher frequency information than you need OR WANT!

There are several undergraduate Electrical Engineering courses that discuss the possible evils of sampled data systems and much math has been developed to deal with the issue.  In my experience, simply filtering out the unneeded higher frequency information and then sampling 5x faster than the highest is enough to prevent really bad results.
Good luck with the project.

Craig Taylor