Timed mechanical fuel injection

[Mark Boxsell]

>
>A lot of aftermarket ECU's are non-sequential... meaning that they trigger
>all of the injectors every time. IOW, on a V8 you would have 4 shots of fuel
>to all of the ports per each crankshaft revolution. During each of those 4
>shots, only one cylinder has the intake valve open, the others all have
>closed or mostly closed intake valves. When doing Dyno work and comparing
>sequential to non-sequential EFI systems, with all the hardware being the
>same and the only difference being the ECU's, I have found that the top end
>power difference is so small it is not really repeatably measurable... ie.
>451 ft.lbs. at 5500 rpm compared to 453. The real advantage to sequential
>injection seems to come in the emissions department, idle quality, and low
>rpm drivability (smoothness). For emissions you have better ultimate control
>if you are able to dictate exactly when the fuel is delivered, in total, to
>the cylinder.
>
>There is another issue that rears it's ugly head here though, and that's
>injector sizing. For an injector to be able to shoot the required amount of
>fuel into the cylinder for full power operation in just one shot, as the
>intake valve is opening, and get it all in there before the valve closes, it
>has to be much larger than the injector that does the same amount of fuel
>over the course of 8 shots (for a V8) as in the case of the non-sequential
>system. Some would argue that you get better atomization out of the smaller
>injector. In addition to that, some would also argue that each little shot
>gets to hit the hot intake valve and vaporize before the next shot occurs
>and that would also be beneficial. The other part of this is that at high
>RPM full power operation, the on cycle of the injector is pretty much
>constant so it doesn't really matter anyway. It seems that you get the best
>results if you size the injectors so that they are the minimum size for the
>power required, then at full power they have to be pretty much on all the
time. 
>
>Which takes us right back to the beginning where it seems that at low RPM
>you could benefit from sequential, one shot type injection.
>
>I think the best possible system would be one that sized the injectors so
>that they would just be big enough for full on operation at full power
>(non-sequential triggering). But at low power settings trigger them
>sequentially, timed so that they start the fuel shot as soon as there is a
>positive flow after the intake valve has opened, completing the shot before
>the valve closes so air goes in last and therfore, possibly,  getting the
>most miserly milage and emmisions. Then as RPM and power levels are
>increased gradually increase the trigger count per cylinder cycle so that by
>the time you ended up at the full power end of the scale, they were just on
>all the time and JUST supplying enough fuel.
>
>For all I know, this may be what Detroit is doing now. I don't have much
>experience with OEM systems, only with the various after market programable
>one's like the Accell, Haltech, and Electromotive.
>
>-j-
>

 -j-
     Your description just about sums it up. Normally aspirated engines are
OK but with turbo engines the OEM's have problems with injector size. The
problem is the range of fuel flow they have to accomodate. Turbo rotaries
are probably the worst for this. At idle they use about as much as a
1200-1600 cc but on full boost as much as a 5000cc hence the factory using
staged injection.
If you go too big you lose the fine control needed (for emissions) at idle
and light cruise.
In Australia the turbo Commodore (factory) uses a similar system to what you
propose firing once every two revolutions at idle and light load then
changing to once every revolution. This gives them the fine control they need.
Most factory systems I know inject on a closed (hot) valve to make sure the
fuel is atomised/vaporized before the valve opens. Remember emission
vehicles run very little overlap so a race engine may like it differently.
I have been doing this stuff for ten years and my advice is give an engine
what it likes and worry about why later (if you have that luxury)!!
              regards,
                       Mark Boxsell.