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wasn't sure where to put this, so OT seems acceptable for now...
when i get bored, i tend to think a lot about things that would make normal people go "why bother?"....
so, i started thinking about GM's various injector firing schemes over the years... came across information that is contrary to what most people are told, though i've had this verified by a few sources now:
now, i've heard of this for quite some time, and it does make sense, so i thought i'd run with it. grabbed cam specs for a 3x00(not sure where, so it may or may not be 100% accurate), and generated this in CAD:
now, this is assuming the 3x00 cam has 262* of duration on the intake lobe. note that i have it positioned so the "graph" starts with cylinder 1 intake opening. not necessarily accurate, but will work for this demo. note the rings, cylinder 1 is the largest, then goes further in cylinder by cylinder down to #6. an 8 or 4 cylinder can be generated using the same method with more or less messyness, but we're 60V6, so it would be wrong to use something else.
now, the red portions of the graph are when the intake valve is open, green when closed. now, we want the injectors firing when the valve is closed for any 1 cylinder, something easily done with SFI, just need to know when the valve closes and start pumping fuel. with MPFI, a compromise must be made...
using double-fire MPFI as an example we have this:
assuming the injectors fire at the top, cylinders 2, 3 and 4 have closed intake valves, with 5 just about to close. the injectors will fire again at cylinder #4, at that point 5, 6 and 1 are closed with 2 just about to close. with this method, all cylinders will have 1 injection event where the injectors at least start spraying against a closed valve, if not finishing, if the pulse is short enough.
now, we look at single-fire MPFI, which i had THOUGHT was a good idea until now...
only 1 injection event, let's assume it happens at the top of the chart. only 2, 3 and 4 will happen against a closed valve. 1, 5 and 6 will always be shooting(initially at least) with an open valve.
so, now i'm pretty much turned off of the idea of single-fire MPFI, but now let's compare double-fire MPFI to SFI: both are shooting against closed valves in all cylinders, with rising fuel demands making the SFI "advantage" less noticable, since if enough fuel is required, both will be shooting with open valves. because of this, SFI has an advantage at lower RPMs, but how significant is it?
grabbed a log from the MC, at a hot, closed loop idle at 600RPM, i was seeing a PW of 2.6mS(keep in mind, this is SFI). at 600RPM, 2.6mS translates to the crank revolving 9.36*(1.3% injector duty cycle) between the injector opening and closing. if we look at a new chart:
we see a couple of new lines. there are two up top and one on the bottomside. the furthest right on the top represents the time the injectors are open in SFI mode (only showing 1 for sake of clarity, not all 6), the line to the left of it and the bottom line showing double-fire MPFI mode injector opening time(since they're fired twice per 4 stroke cycle).
now, the SFI line is 4.68* away from the top line, not 9.36, since this chart displays 720* of crank revolution, and the double fire MPFI is half of that, 2.34*.
make of it what you will, but from what i've just simulated, SFI has damn near no advantage over double-fire MPFI in terms of being able to shoot against a closed intake valve, since double-fire assures that at least one of the injections per 4 stroke cycle in each cylinder will be against a closed valve.
so, half rant, half information.
while it would be nice to have injectors fire against the intake valve as soon as it closes, it's just not that critical, really. and once you start getting into situations with longer pulse widths, or cams with more duration, the SFI "advantage" is completely non-exisitant. if you can get something to work better with one or the other, it's not the hardware/method, it's calibration.
when i get bored, i tend to think a lot about things that would make normal people go "why bother?"....
so, i started thinking about GM's various injector firing schemes over the years... came across information that is contrary to what most people are told, though i've had this verified by a few sources now:
SFI intentionally fires the injector at a _closed_ intake valve (once for every 2 revolutions, synchronized to precede the intake event on the respective cylinder) - that's what you want. The "hot" intake valve improves vaporization of the fuel as it "waits" to be drawn into the cylinder. An open intake valve allows gas flow that interferes with the spray pattern and does not provide the best fuel vaporization
now, this is assuming the 3x00 cam has 262* of duration on the intake lobe. note that i have it positioned so the "graph" starts with cylinder 1 intake opening. not necessarily accurate, but will work for this demo. note the rings, cylinder 1 is the largest, then goes further in cylinder by cylinder down to #6. an 8 or 4 cylinder can be generated using the same method with more or less messyness, but we're 60V6, so it would be wrong to use something else.
now, the red portions of the graph are when the intake valve is open, green when closed. now, we want the injectors firing when the valve is closed for any 1 cylinder, something easily done with SFI, just need to know when the valve closes and start pumping fuel. with MPFI, a compromise must be made...
using double-fire MPFI as an example we have this:
assuming the injectors fire at the top, cylinders 2, 3 and 4 have closed intake valves, with 5 just about to close. the injectors will fire again at cylinder #4, at that point 5, 6 and 1 are closed with 2 just about to close. with this method, all cylinders will have 1 injection event where the injectors at least start spraying against a closed valve, if not finishing, if the pulse is short enough.
now, we look at single-fire MPFI, which i had THOUGHT was a good idea until now...
only 1 injection event, let's assume it happens at the top of the chart. only 2, 3 and 4 will happen against a closed valve. 1, 5 and 6 will always be shooting(initially at least) with an open valve.
so, now i'm pretty much turned off of the idea of single-fire MPFI, but now let's compare double-fire MPFI to SFI: both are shooting against closed valves in all cylinders, with rising fuel demands making the SFI "advantage" less noticable, since if enough fuel is required, both will be shooting with open valves. because of this, SFI has an advantage at lower RPMs, but how significant is it?
grabbed a log from the MC, at a hot, closed loop idle at 600RPM, i was seeing a PW of 2.6mS(keep in mind, this is SFI). at 600RPM, 2.6mS translates to the crank revolving 9.36*(1.3% injector duty cycle) between the injector opening and closing. if we look at a new chart:
we see a couple of new lines. there are two up top and one on the bottomside. the furthest right on the top represents the time the injectors are open in SFI mode (only showing 1 for sake of clarity, not all 6), the line to the left of it and the bottom line showing double-fire MPFI mode injector opening time(since they're fired twice per 4 stroke cycle).
now, the SFI line is 4.68* away from the top line, not 9.36, since this chart displays 720* of crank revolution, and the double fire MPFI is half of that, 2.34*.
make of it what you will, but from what i've just simulated, SFI has damn near no advantage over double-fire MPFI in terms of being able to shoot against a closed intake valve, since double-fire assures that at least one of the injections per 4 stroke cycle in each cylinder will be against a closed valve.
so, half rant, half information.
while it would be nice to have injectors fire against the intake valve as soon as it closes, it's just not that critical, really. and once you start getting into situations with longer pulse widths, or cams with more duration, the SFI "advantage" is completely non-exisitant. if you can get something to work better with one or the other, it's not the hardware/method, it's calibration.
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