Previously I wrote:
The detailed record shows one very short period of pressure reversal, and the amplitude is quite small. I very much doubt there is any significant fuel escaping from the nozzle bleed.
The data is on my laptop, which is at the office today. I'll try to post a plot later.
So much for memory. Here's a data sample from 10500 ft @ WOT, a likely setup for LOP cruise. I have not yet grasped a connection to fuel flow, but it describes the pressures.
You're looking at the difference between bleed air supply pressure (here in a turbo rail and shroud) and the intake port at the primer fitting. Green is bleed pressure exceeding port pressure, while red is port pressure exceed bleed pressure. Put another way, red has the potential to blow fuel out the nozzle bleed hole.
The 23 BTDC notation is where I used a second channel to put a timing marker on the plot...it's #1 plug firing. There are 720 crank degrees between 23 BTDC markers; at 2400 RPM each division represents 0.00133 seconds.
With the throttle wide open, there are three
periods of pressure reversal, not one as I remembered this AM. The periods are short. The longest averages about 3.5 divisions (0.00466 sec, 67 crank degrees), just after the intake valve closes on the compression stroke. All three added together come to 0.0117 seconds, or about 168 degrees of crank rotation. 168/720 means there is the potential for reverse flow about 23% of the time. Hmmm.
Here is the effect of throttling at 6500 feet. Assuming the same forward airspeed, bleed pressure would remain the same, while the partially closed throttle plate reduces pressure in the intake port. DeltaP shifts about 1" Hg in the desired direction, but the time periods (in seconds or crank degrees) don't change much.