Absolute Fuel Rail Pressure Settings

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R3VM3UP
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Absolute Fuel Rail Pressure Settings

Post by R3VM3UP » Sat Oct 26, 2019 6:17 pm

I'm looking for some clarification on how to properly set fuel rail pressure for a vehicle that does not have a manifold vacuum referenced fuel pressure regulator. The vehicle is an NB Miata, which has a pressure regulator at the fuel tank that provides a constant 60 PSI (413.7 kPa) of pressure W.R.T. barometric pressure. Assuming a typical barometric pressure of 14.7 PSI or 101 kPa around sea level, that results in a constant absolute fuel rail pressure of 515kPa.

Of course what we actually care about for calculating injector flow rate is the DELTA pressure across the fuel injector, I.E. (fuel rail absolute pressure)-(intake manifold absolute pressure). On a typical return style fuel system the delta pressure across the injector is held constant at all operating conditions because the fuel pressure regulator is referenced to manifold pressure via vacuum line. On a system like the NB miata with a fixed absolute pressure, the delta pressure across the injector (and therefore the injector flow rate) will change as intake manifold pressure changes, and this change in flow rate needs to be accounted for in the control software.

It appears that newer releases of the RusEFI software may have this capability built in already but while helping a friend of mine with his NB turbo build we have been having trouble getting this to work correctly. In the fuel rail part of the base engine settings there is a true/false option for 'Absolute Fuel Pressure'. Presumably this should be set to 'true' for a system like the one in the NB Miata. I would assume fuel pressure should then be set to 515 kPa, which is the constant absolute fuel pressure. The other key setting would be the fuel injector flow rate in cc/min in the injection settings. My assumption would be that this should be set to the flow rate of the injector at a delta pressure across the injector of (absolute rail pressure)-(baro pressure), so in the case of the NB Miata that would mean 60 PSI or 413.7 kPa. None of this is explicitly mentioned in the documentation so I'm hoping someone here can help.

However, we tried these settings and the result is that we had to put unreasonably high values in the VE table at idle in order to hit the target AFR, so either something is wrong with our settings, or the logic to dynamically calculate injector flow rate on an absolute rail pressure system has not been implemented in the software. In fact, I tried searching through the source code to find where this calculation was computed, and I could not seem to find it. The closest I could find was the variable called 'injector.flow' but I could not find how that is defined. Could someone point me to where the injector flow rate is computed?

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russian
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Re: Absolute Fuel Rail Pressure Settings

Post by russian » Sat Oct 26, 2019 8:14 pm

We do not have such a setting - this is known issue https://github.com/rusefi/rusefi/issues/954

Good news is that flow error is only 16% max with natural aspirated and pretty much your VE table would account for that. Only place where it matters is acceleration enrichment - but again, only 16% error on NB2.
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Re: Absolute Fuel Rail Pressure Settings

Post by 960 » Sat Oct 26, 2019 11:54 pm


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Re: Absolute Fuel Rail Pressure Settings

Post by R3VM3UP » Wed Oct 30, 2019 12:22 am

Yes, that is a good reference link. I'm familiar with the math and actually put together a spreadsheet to calculate it, it's fairly simple. I think this could be easily added as a multiplier to the injector flow rate in the code. Default value if absolute rail pressure is not selected would be 1, so the math would be unchanged for a return style fuel system. If absolute rail pressure was selected a multiplier would be calculated based on delta pressure across the injector by referencing the calibrated fuel rail pressure and the MAP at the current operating conditions.

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Re: Absolute Fuel Rail Pressure Settings

Post by russian » Wed Oct 30, 2019 12:39 am

one day someone might make code change. when and who is an open question :(
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Re: Absolute Fuel Rail Pressure Settings

Post by R3VM3UP » Wed Oct 30, 2019 1:13 am

Okay, maybe I need to try my hand at getting things set up to compile so I can give it a try myself.

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