Quote:
Originally Posted by mid_life_crisis
Isn't the shorter length of time offset somewhat by the violence of the expansion of fuel under that much pressure being released into an open space? Wouldn't that help with the mixing
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Yes, if the rail pressure is high enough, and the spray targeting is appropriate for the injection timing. Raising the rail pressure increases parasitic losses though. Even good spray targeting has a range of injection timing where it won't significantly impinge on the cylinder wall on piston top.
Quote:
Originally Posted by Kodename47
Having gone an re-read all the old posts on here, there must be a reason why running a 20:80 split was chosen. I have seen little benefit of leaning heavily on the DI on a NA car. I have taken Mike's advice though on my car, running up to 6ms DI and then having it pegged there until redline seems to.work nicely.
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Remember that knock is a bigger problem at low speed and high load, and that's where the most DI percentage is being used. At low speed there is more time for the gases at the end of the combustion chamber to auto-ignite.
Quote:
Originally Posted by mid_life_crisis
I can see that. Increasing the PI pulse length would increase the amount of fuel per injector cycle. I wish I had a better understanding of how the ECU tries to meet the percentage target. With all those other tables to reference, what exactly is it doing by way of reading what table and then physically activating injectors, cycling pump valves, etc. What does it take to satisfy it that the target percentage was met? If you change a pulse length, does it take that into account?
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Solenoid injectors have a linear and non-linear range of operation. On modern PFI with saturated injectors (voltage controlled) it's simpler because the current and voltage control are simple compared to the old "peak and hold" injectors from the 80s which used a burst of current to open those clunky 80s needle valves and then a lower current to keep it going. The old peak and hold control lives on in solenoid direct injection, but it's more complex (see below).
So basically on modern PFI there's a bunch of stuff in there to translate pulsewidth into mass flow, based on the characteristics of the injector and on the environmental conditions (air temperature, water temperature, etc).
The direct injection is more complicated because they are run with peak-and-hold current control and booster voltage. They also are much trickier to control at low pulsewidths (typically under 1 millisecond). You have to have an injector flow vs pulsewidth model in the code with a bunch of look up tables populated by bench tests and engine dynos for the most part. The basic intellectual property is owned by the supplier and then adapted to the application.
With a lower flow DI injector, you flow less and don't need low pulsewidths so much. You spend less time in the "ballistic" non linear range, where the valve opens and closes like you are lobbing a Hail Mary pass at the end of the Super Bowl.
So when the DI injector is being driven, you have 4 basic phases:
1) booster voltage phase -- a bunch of current and voltage applied to open the injector quickly
2) high current holding phase - current dithers at a high level as the injector reaches full open
3) lower current holding phase - the majority of the on the time when long pulsewidths are used. the split between high and low current hold has effects on injection noise and behavior of the spray pattern, especially with respect to emissions
4) closing phase - sort of the inverse of the boosting phase
So the ECU signal is the pulsewidth, yes, but there are all these separate sub-operations going on within that pulsewidth that are in separate modules of code with a bunch of lookup tables and settings controlling them. The fuel mass model can calculate flow taking into account all this complexity.