Mapping fuel load against injector pressure differential?

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Drwaffles
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Mapping fuel load against injector pressure differential?

Postby Drwaffles » Fri Mar 08, 2019 6:43 pm

Need a sanity check please..

On a dead head setup with boost, would this be worthwhile?

Originally I had MAP load axis and under Injector flow rate I just plugged in the two injector flow rates at 40 and 55psi. Worked fine, but for finer adjustments would using IPD as the base fuel axis help? (Presumably I'd have to fix the flow rate at a given value)

My logic is that if fuel pressure stays steady then it's acting just like a MAP axis, but as you start to loose that pressure difference (either from MAP increase or fuel pressure drop) it'll account for that by reducing the IPD and moving to a new cell increasing flow.
Realistically were just trying to keep the mass of fuel consistent with the mass of air coming in.

Thoughts?

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blakebonkofsky
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Re: Mapping fuel load against injector pressure differential?

Postby blakebonkofsky » Sat Mar 09, 2019 8:18 pm

Why not just scale the injector flow rate against IPD? That's what I do on cars fitted with a fuel pressure sensor, and it works very well as long as you know the flow rate of the injectors at various pressures. We took one car and cranked the base fuel pressure up from 60 to 90psi and the tune was dead on. No reason it shouldn't work in a non-referenced application.

Or just set the fuel pressure as "Fixed" instead of manifold referenced. I believe this would accomplish the same thing but without having to fill in flow data.

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Drwaffles
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Re: Mapping fuel load against injector pressure differential?

Postby Drwaffles » Sun Mar 10, 2019 12:45 pm

I pretty much had that setup, mentioned in the post I had MAP load and FP Based flow rate in my original post :)

I had the injectors tested at 40IPD and 55IPD, was 510cc vs 610cc
I'm not sure that the flow rate change at 100% duty will translate down to the lower end of the pulse width.
15 psi change may be say 20% fuel mass change at 100%duty, but may be less at lower duty levels. Temperature of the fuel may change that flow rate also.

This is not reduce a variable of that pressure differential, and potentially account for non linear flow rate without knowing the low PW compensation values.

It's already set to fixed :)
I'm just trying to reduce as many variables as possible!

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Drwaffles
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Re: Mapping fuel load against injector pressure differential?

Postby Drwaffles » Tue Mar 12, 2019 8:31 pm

Worked ok, but less consistent than MAP with the injector flow rate base.
Suspect it doesn't work with the air mass calculations as well.


Back to MAP and trying to figure out how I test the FPD scaling on the injector.

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HaltechMatthew
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Re: Mapping fuel load against injector pressure differential?

Postby HaltechMatthew » Wed Mar 13, 2019 9:38 am

Change the pressure. Move flow up or down until close. Move on to tuning and enjoy driving the car. 8)

This will be like one of these internet dead time arguments. You will spend forever and a day profiling the injector for every possible, duty, pressure, fluid medium, and planet alignment you can think of, and then another variable will come along and skew it slightly anyway.

Eg..How does the localised MAP at the actual injector pintle react to the open and closing of the valve, and if you change injector timing does it change this and therefore the flow out of the injector? MAP is not a static thing, it varies throughout the inlet valve event and so does then the actual flow of the injector throughout the inlet cycle. Fuel pressure regulators that are linked to MAP are also not very fast reacting and you will most certainly see this delay when you monitor a fuel pressure sensor.

There are so many subtle variables that I would say to cut your losses, get it as close as you can, and move on to other things. The end result is that even at an OEM level with almost unlimited engineering time and budgets they still need to have short, (sometimes medium), and long term learning to cover for the unmeasureable.

Otherwise in a perfect world you would measure the air, send in the fuel, and it would be absolutely pin perfect, but in reality it never is as perfect as you would think from the drivers seat.

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Drwaffles
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Re: Mapping fuel load against injector pressure differential?

Postby Drwaffles » Sat Mar 23, 2019 1:02 pm

Yeah, I've since learnt to accept its a case of "near enough is good enough"
Especially when calculating mass.. lol. Otherwise you have to compensate for humidity, air temp, fuel temp, altitude, ambient pressure, head/valve temperature.. Realistically AFR being .2 either side of target isn't going to do much, it'd just be nice lol.


I'll run a pair of hose clamps on the fuel line to drop the fuel flow and make sure it doesn't deviate too far.
Cheers Matt.


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