Balancing Injectors vs Air Flow

[robert joseph dingli]

Mike writes,
> 
> This is something that just crossed my mind this morning while driving
> to work, so feel free to shoot it full of holes....
> 
> When an engine is blueprinted (and especially if it is not) the air
> flow through the manifolds and heads will not be identical for each
> cylinder. Also, injectors will not have exactly the same flow rate.
> 
> This means that when you assemble an EFI engine, you can still have a
> situation where you have lean cylinders and rich cylinders. This is my
> suggestion for balancing these effects.
>   .............
> Well how does this sound? Am I completely out to lunch? Or is this to small
> of an adjustment to be worthwhile?
>                                       Mike
The method you described is cumbersome but would obviously optimise the given
system.  Group A Touring Cars here in Australia often use exhaust temperature
as the prime tuning feedback to balance injectors.  With individual trims
they can compensate for differing airflows and injector dynamics in one hit
without swapping anything.  It does pay to choose a balanced set of injectors
to begin with, as it is mush easier to measure injector flow rates with the
injectors out of the engine.

An OEM solution is to model the delays in the engine dynamics to determine
which cylinder is producing the puff of exhaust which is currently passing
the EGO sensor.  The injectors can then be trimmed individually.
Ref "Individual Cylinder AFR Control with a Single EGO Sensor", Grizzle J,
Cook J and Dobbins K, American Control Conference 1990 pp 2881-2886.

The higher priced aftermarket ECU's allow individual cylinder trims, but 
for most aftermarket applications (which are close to OEM tolerances and tend
to run rich under load anyway) it's probably not worth the effort.

Robert
-- 
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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 344 7966                           (+613) 344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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