Discussion and questions related to the course Link G4 Software Tutorial
Has anyone done this?
Or a GP PWM using differential fuel pressure as an axis, similar to 'passive close loop' boost setup?
There's no facility for actual closed loop low pressure fuel pump control, however your idea of using differential pressure as an axis could work. I haven't done it myself so can't really give first hand experience. This isn't something that would give you accurate control to +/- 1 psi for example though so I'm not sure it's strictly necessary. It's the fuel pressure regulator that's responsible for controlling the fuel pressure so provided you make sure the pump can always provide a slight excess of fuel (more than the engine is using), you should get accurate pressure control.
Thanks Andre. As some background...
I have 2 walbro 460's in tank, running in parallel via a SSR, which is working well.
I am looking for a strategy to minimise fuel flow without compromising delivery. Open Loop FP Speed against injector duty is working well, as in I am not experiencing any pressure drop, but the values in the table ramping up the pump duty are guessed. I have no idea how much over required fuel flow I am at any given injector duty.
How do I tighten the values in order to safely reduce flow volume? Or is the only way to just reduce them and check logs for pressure drop?
My thoughts were (hoping) that leaving the fuel pressure reg set to control differential pressure (which is at least in my instance very stable FPR2000), any drop in differential pressure would be directly caused by lack of pump flow (pump duty too low). This moves the active cell away from base and increases pump speed. Likewise, if pump speed is too much, differential pressure creeps up and moves active cell right and decreases pump duty.
I suppose that modelled fuel increases injector duty to account for pressure drop, which would move the normal open loop fp speed further to the right, increasing pump speed.
I'd be inclined to just make small drops in your FP speed control table until you start seeing a drop in differential pressure. At that point you know your flow is marginal and you can increase the duty cycle again. As you've noted, the differential pressure is accounted for in the modelled fuel equation so the ECU can account for some amount of pressure variation. Of course to make the model as stable and accurate as possible you'd want the differential pressure to remain as close to target as possible.
You could also put a temporary Tee/valve into your return line, crack it open into a jug when car is under load on dyno and you will quickly see how much excess flow you have. I know - safe as F**K!, but simple and effective.
Really I would not want to run a performance engine with only just enough fuel pump speed to maintain pressure - I would want to see at least 1L/m excess coming down the return line so you have a bit of headroom to cover for situations like a failed alternator or blocking filter etc,
Thanks Adam, I have spent too much time on this as it is. FP Speed is working well, I'm just going to stick with that!
Was interested to see if anyone else had tried anything different.
Just as an addition to this thread, I generally work on a 25% over supply of fuel when sizing a fuel system to ensure that the flow is never marginal and the regulator can control pressure effectively. Possibly slight overkill but I feel it's better safe than sorry.