Timed mechanical fuel injection

[Johnny]

At 12:57 PM 11/20/95 +1100, you wrote:

> Sure, go for it Carter. I've a '70 Porsche 911 with a bosch mechanical
>injection system and dread the day I ever have to replace the thing.
>It has the same rack and variable displacement plunger mechanism.
>Those pumps aren't cheap I hear. So any news on part(just control the rack)
>or full conversion to electronic control would be welcome.
> I'd also like to hear your(or any one else's) experiences on the effect of
>injection timing on the torque curve. Normal setup for the 911 is for the
>timed end of injection to occur at about 40degATDC on the intake stroke.
>The begining of injection is varied by a 3-dimensional cam according to
>the fuel requirements(throttle pos,rpm,air density-barometric cell). However
>I discovered that the previous owner's mechanic had set it up 180deg out.
>ie it was injecting on a closed intake valve during the combustion cycle.
>It ran hard, accelerated well, good low rpm torque, felt noisey and harsh,
>cruised roughly at low throttle settings - lot of transmission noise-like
>the engine was slapping it at every combustion and the motor ran cool.
> When I changed the timing to standard the motor smoothed right out, was
>running way too rich, bottom end torque felt lacking. Even after correcting
>the mixture low rpm torque still didn't feel as strong as before. The motor
>ran slightly hotter, cruised very smoothly and quietly without the
>transmission noise. Fuel consumption was initially higher and only equalled
>the 180deg case after mixture correction.
> My thoughts are that (and I could be totally wrong): 
>	for the 40deg case:
>	- at low rpm the corresponding low intake air
>velocity results in poor vapourization of the tiny injected droplets,
>giving slower combustion- longer flame duration and more cylinder wall
>exposure to the flame-causing the higher operating temp. 
>	for the 180deg case:
>	- the droplets have the whole combustion and exhaust cycles in which
>to vapourize to some extent on the hot closed intake valve before being
>drawn into the engine. A lot may be lost to wetting on the relatively cool
>intake walls, but are compensated for by richening the mixture. Richening will
>also help throttle response to reduce leaning from rapid throttle opening.
>The highly vapourized pocket of fuel burns rapidly-giving a short flame
>duration and less heat loss to the engine.
>	So if anyone has dyno figures on torque vs injection timing I'd love
>to hear from them. Does anybody inject both on the closed valve during
>combustion and during the intake cycle? 

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-