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Old 03-04-2009, 08:45 PM   #1
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fuel pressure delta sensor, and upping fuel pressure for boost

Hey all,

I'm in the middle of Banish's book, and came across something.

I was just wondering, since the Focus has a delta fuel pressure sensor (that measures rail pressure MINUS manifold pressure), why is it recommended to increase rail pressure 1psi for each psi of boost?

As I understand it, since it is a delta sensor this is already compensated for... When you tell the PCM to target 40psi it will target 40psi pressure drop even under boost. Due to it being a delta sensor you dont need to target 50psi in order to get 40psi across the injectors at 10psi of boost... If you target 50psi at 10psi of boost, you will get an absolute rail pressure of 60psi, and a delta pressure across the injectors of 50psi.

Discuss.


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Old 03-08-2009, 12:14 AM   #2
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bump?

I just read that he's going to cover it more in chapter 10, im on 7 and have a lot of work to do this week. so i'll update when i get there. But i believe he said it has to do with compensation for flow rates at different pressures. But that wouldnt make sense, again the fact that its a DELTA sensor means its already compensated!
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Old 03-18-2009, 08:09 PM   #3
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bump again?

Just need to know the reason people add 1psi fuel pressure for each psi of boost.
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Old 03-18-2009, 08:16 PM   #4
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You add 1 psi of fuel psi for each psi of boost because the boost (manifold psi ) is working against the incoming fuel psi from the injector so 1-1 is needed to overcome the manifold psi

I wish the fuel rail psi sensor read boost !

Read all the books you want , then take them and throw most of it in the trash as you will soon find out

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Old 03-18-2009, 08:17 PM   #5
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Now explain why that is the case when the fuel rail pressure sensor is a DELTA pressure sensor?

It already takes that fact into account! It reads the difference between the rail and the manifold, no matter if its vac or boost.

By your argument you need to change fuel pressure at idle to 30 because there is more vacuum in the manifold...
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Old 03-18-2009, 11:54 PM   #6
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Delta sensors read high side pressure, and low side pressure, then gives only the difference. If they were using 2 pressure transducers an then calculating the difference, then i could see them using using one as a monitor for a setpoint..... I have a hard time believing they use a delta sensor for fuel pressure setpoints.... Wait a sec, i re-read what u said last. A true delta sensor measures 2 of the same thing, ie... air pressure or liquid pressures. Not both, makes no sence for what ur saying about both rail pressure and manifold pressure. And not to mention, i've never seen an engine come through my work that had a fuel delta sensor, but we use them if they ask us to. Only delta sensor i see on production engines are dpf delta transducers that measure how much soot accumulates on the dpf, then it calculates it to run a regen....
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Old 03-18-2009, 11:58 PM   #7
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The sensor on the end of the zetec rail has a vac line coming off of it, so I'm convinced its a delta sensor.

Add to this the fact that the DMR used for datalogging is "pressure drop across injectors", meaning the delta pressure between the manifold and the rail.

Also keep in mind the zetec fuel system is returnless, which I think would change things as you're thinking of them.

and fwiw the EGR uses the "delta pressure feedback exhaust" sensor.
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Old 03-19-2009, 12:24 AM   #8
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A rubber vacuum line with fuel in it? Like i said, all delta transducer ive seen have 2 lines of the same source running to it. If it only has one, then its a regular pressure transducer and they are calculating the pressure drop somehow. I can only see pressure drop across injectors as being a calculated, not an actual. As for egr delta, that too is a calculated number. You dont have any pressure lines coming from you exhaust, correct? Only the vacuum line that actuates the valve, and manifold pressure would come into play with the egr along with speed vs load. During testing they measure everything, they get the data they need, then figure out what "normal sensors" are reading..... Then they come ip with a calculation to calculate what certain things "should" be from certain sensor readings.
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Old 03-19-2009, 12:31 AM   #9
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The EGR's DPFE sensor uses two venturi tubes in the EGR pipe. Yes there are a lot of things coming into the calculation for EGR duty cycle, but I was just pointing out another system that uses delta pressure.

Here is a pic of the focus rail pressure sensor. You can see the oring that goes in the rail on the right, the nipple for the fuel line on the left, and the port for the vac line coming up in the middle.

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Old 03-19-2009, 01:02 AM   #10
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That to me looks like a pressure sensor that uses vacuum to allow more or less fuel, or essentually alters pressure to the rail. How i see it, fuel coming to the sensor, which a connecter to the ecu sees what the pressure is, and the vacuum line changes flow/pressure depending on the load, to the rail.

All deltas ive seen:
Fuel, measures pressure in to the rail, and pressure out or back to tank. The difference or drop= delta.

Pressure, measures the pressure drop accross a filter/catalyst. One line measures the pressure on front or high side, then one on the back or low side. The sensor tells you only the difference or say -4 kpa drop. Meaning front side is 6 kpa, rear is 2 kpa.

Forgive me, but ive been stuck with 2010+ diesel engines lately, but..... Delta is differance between/across something. I have been doing automotive testing for about 5 years and never seen delta measurement between vacuum and fuel. Only one or the other.
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