TBI with TPI ecm???

[Ludis Langens]

Bruce Plecan wrote:
> 
> Dunno, how you can say "won't get any fuel".   The fuel is just being spit
> into a plenum anyway.  In theory firing them twice as often would be better
> for atomization, yes.  But, there is a whole lot of vaporization going on
> anyway.

Lets assume the 2.5 L4 single injector TBI engine.  On this engine, the
TBI feeds into a small common area which then immediately splits into
four seperate runners to the ports.  Below is a diagram of when each
cylinder is receiving air+fuel and when the TBI injector is firing. 
(View this in a fixed width font.)  Lets also assume a light throttle
condition where the injector duty cycle is low.  Further, lets assume
the ECM is firing the injector in middle of each intake stroke:

1111111111111111111111__________________________________________________
__________________3333333333333333333333________________________________
____________________________________4444444444444444444444______________
2222__________________________________________________222222222222222222
_______Fuel______________Fuel______________Fuel______________Fuel_______

>From the above diagram, each squirt of fuel should be immediately swept
toward a different cylinder.  The runners to the non-intake cylinders
should have little or no air flow, hence they won't get the majority of
the fuel.  Now lets look at a diagram for a batch fired port injected four:

1111111111111111111111__________________________________________________
__________________3333333333333333333333________________________________
____________________________________4444444444444444444444______________
2222__________________________________________________222222222222222222
FuelFuel____________________________FuelFuel____________________________

Because of the port injection, each cylinder gets the same amount of
fuel although the timing is different.  For cylinders 1 and 4, half of
the fuel waits around on the intake valve and the rest is injected
during the intake stroke.  Cylinders 2 and 3 have almost all the fuel
waiting on the valve.

Now consider what happens when a TBI injector is fired this way.  Most
of the first injection goes to cylinder 1, with a small portion getting
into cylinder 2.  Similarly, most of the second squirt goes to cylinder
4.  Thus, 1 and 4 run rich, while 2 and 3 are lean.  Now lets open the
throttle causing a larger duty cycle:

1111111111111111111111__________________________________________________
__________________3333333333333333333333________________________________
____________________________________4444444444444444444444______________
2222__________________________________________________222222222222222222
FuelFuelFuelFuel____________________FuelFuelFuelFuel____________________

In this case, all the extra fuel goes to 1 and 4 making them even
richer.  Meanwhile, 2 and 3 are relatively even leaner.  Now lets back
off the original duty cycle and assume the ECM is firing the injector
only once per every other crank revolution:

1111111111111111111111__________________________________________________
__________________3333333333333333333333________________________________
____________________________________4444444444444444444444______________
2222__________________________________________________222222222222222222
FuelFuelFuelFuel________________________________________________________

In this case, 1 gets most of the fuel, 2 gets a little, and 3 and 4 get
essentially no fuel.  That's what I mean by "won't get any fuel".

>   I've run some 5.7 TBIs with both a 730 (8D), and the 749 (using the 8D),
> and while not perfect, for an as installed calibration, they weren't at all
> bad.  Yes, work was need but there was worked needed for the 747 too.

Was the 5.7 running on all eight cylinders, or could it have been
running on just four?  Perhaps the more complex runner configuration of
the "dual plane 2bbl" intake manifold mixed the fuel around better.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/


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