Batch EFI on Siamesed Port Motors

[Carter Shore]

Peter,
I face the same problem with my Mk II Spitfire., which
also has siamesed intake ports.
I think I know how to make it work at speed, but idle
and cruise is still a problem. 
Consider just a pair of siamesed cyclinders.
Assume for a moment that the aiflow is equal into both
cylinders (probably not true!). The problem is to time
the injection events to avoid intake overlap. If we
are running at 7200 RPM, then there are 60 full cycles
per second, around 17 mSec. Each intake event lasts
roughly 180 degrees, around 8.5 mSec. The intake
events of the siamesed cylinders are adjacent. So we
have around 8.5 mSec for the first cylinder, then 8.5
mSec for the next, then 17 mSec until the whole thing
repeats. If we size the injectors such that the max
amount of fuel needed can be injected during a 6 mSec
interval, then we should be able to time the injection
pulses to achive fairly equal fuel distribution to
each cylinder.
Remember that in a SEFI, we inject 1 shot of fuel for
each cylinder of air. So the mass of air sucked in
determines how much fuel should be injected each time.
And the mass of air is proportional to the Volumetric
Efficiency under that set of conditions (temperature,
RPM, air density, throttle position, etc.). We can
pretty easily choose injectors that will supply enough
fuel for each cylinder in 6 mSec at WOT and high RPM
It's a 1300 CC motor after all! Let's say we need a
max total of 50 lb/hr of fuel. On a normal SEFI that
would require 4 x 13 lb injectors. For our oddball
setup with 2 injectors, each must flow 26 lb/hr. And
we will pulse a maximum 6+6 = 12 mSec out of 17, so we
would need:
 26 x 17/12 = 37 lb/hr units.
Our problem is getting the darn thing to idle, when VE
is very low, or to run decently at part throttle
cruise. We need a dynamic range of at least 25:1. Most
injectors start to operate poorly somewhere under 1
mSec or so, so we have only around 8:1 range.
My solution? 
Two sets of different sized injectors. Each intake
tract has a high volume and a low volume injector,
both mounted along the center line, perhaps in tandem.
The injector capacities are chosen to overlap. As long
as the injectors are of the proper type (saturated vs
p/h), the ECU driver hardware should need no
modifications. 
The software however, would require some major
hacking. First off is to arrange for the oddball
injector timing. Next is to handle the transition
region between high volume and low volume injector
operation.
But the rest of the code should require few changes.
Fuel and spark tables are used the same, and load
calculations are made the same way, since the same
sensors would be used.

Comments, suggestions?

Carter Shore




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