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just getting into tuning and trying to get my head around the logic.
my question is how does the ecu determine fuel quantity?
you have fuel volume maps, injection pressure maps, and duration of injection maps. if i increase injection pressure, thats going to increase volume of fuel injected and if i increase duration the same applies, so what command does the ecu choose to follow
Most use an acceleration deceleration calculation. Specifically in the case of a Duramax you will have a "Balance rate". This is at an idle and it uses the acceleration from the detonation of diesel fuel to calculate how fast the piston accelerated to figure its base fuel used. If the cylinder in question did not accelerate fast enough it will inject more fuel to get it going faster and vise versa. As injectors wear you will see these balance rates go further from a "ZERO". At specific plus or minus calculations based on what year the truck is (max high and low varies with years) it will let the tech determine the need for replacement injectors. Hope that helps.
I'm more thinking in terms of the fuel control maps.
When I increase the commanded mg of fuel in my main injection map, this obviously is going to require either a longer PW or higher injection pressure to accomplish.
If I don't adjust those maps as well, will I actually see any more fuel injected?
you dont need to increase your pw, thats adjusted in the ficm. EBP, timing, Fuel Quality, torque limit, ICP is all going to add more fuel.
sorry shoulda said, this is common rail
Common rail stuff works like this, there is a user demanded fuel amount. Typically modern stuff is in MG/stroke and older stuff is volume or millimeters cubed, aka MM3. Modern is MG/stroke because you mix pounds of air with pounds of fuel, there for the fuel amount already being injected based on weight makes more sense then trying to do another calculation on top of that. Since you're asking about common rail ford stuff I'll explain the 6.7's way for looking at fueling. It has a torque based system. This starts in your Driver demanded engine torque, this lays out accelerator pedal vs RPM and in the graph is desired torque amount asked for by the driver. That gives you a torque number that is then looked up in the torque to fuel quantity map, which is based on RPM and torque amount. That fuel amount is MG/stroke and is now going to go through a lambda check based on the current airflow through the engine. If you have enough available air it will then lookup the current desired rail pressure, achieve that rail pressure if not already met, then look at the Injector PW map at that rail pressure and fuel volume to produce X MG/stroke. If you increase rail pressure and do nothing else all you're going to do is change how long the injector is open for, but you'll be at the same amount of fuel and almost the same power output. If you want more fuel increase the desired torque amount, or the fuel amount in the torque based fueling, or a combination of the two. OR increase the injector PW table, but know this will affect the actual lambda and fuel milage readout as it's now outputting a inaccurate fuel amount. A good running truck uses a combination of all three in the right areas.
Perfect, that's the info I was after. Makes sense.
Thank you sir