Sunday, January 10, 2010

Per-Bank Injector Grouping

So while discussing my findings from earlier this morning, I had this stray thought:  since we know what which injector flows separately, why don't we group them in a way that the Fuel Trims can account for easily?

One common trick is to put the highest flowing injector on the leanest running cylinder.  This makes sense;  we might not know how much more air that cylinder is getting, but at least we can give it some extra fuel to accommodate it better.  This is obviously not the most precise technique, but as far as rules of thumb go, it's not a bad one to practice.

This approach made me think though:  what kind of fueling are the other cylinders getting?  We might be OK on average, but it does not mean that the per cylinder fueling is anywhere near proper.  So how would we make this better, make the per cylinder fuel delivery more consistent?

We already know the flows of all injectors individually.  We don't know, and unless you stick an O2 probe in each runner, we will not know which cylinder is flowing what amount of fuel, so we can't just pick the best injector/cylinder combo and hope that airflow distribution is similarly close.  What we have however is Fuel Trims mechanism that in Closed Loop treats the engine as two banks with two separate corrective values.

So what would happen if we group our injectors accordingly not to their individual flows, but in groups of four?  The discrepancy between banks would be bigger than if we placed injectors randomly.  But the fueling within each bank would be much closer!  We cannot control fueling within a bank of cylinders, but we can control it per bank.  Thus the discrepancy among the banks will be corrected with Fuel Trims, but the discrepancy within each bank is going to be minimized, thus providing us with a consistent fueling.

Enough blabbing, let's see some numbers:




















These are three sets of SVO greentops for which I have flow sheets handy.

Some explanations are needed:
The colors in the graph are in accordance to the flow number.  The highest numbers in the set are red, the lowest are green, and the rest is somewhere in the middle accordingly.

Avg-bank are the average flows for each bank (bank defined in as the first four and the last four injectors).  They're fairly close, as random placement tends to come up with good averages.  This is however not what we want, as we'll see later.

Range is the difference in flow within a bank, how far the maximum and minimum are spread apart.

Within Bank is the percentage of how big the range is comparatively to the average.

AFR Difference is the impact that 'within bank' difference in flows has on AFR, when commanding 14.7.

As you can see, the differences in AFR within each bank can be up to 0.24.  Remember all these flow sheets data I have are already for injectors that have been cleaned, so stock injectors can be in a lot worse shape that what you see here.

Now let's take a look at the same injectors, but grouped.  On the next picture, I simply sorted the flows, and the rest is the same, but look at what difference it makes for the differences in flows within the same bank.  Since the coloring is tied to the values, and we sorted by values, now we can see the groupings of injectors with similar flows by being of similar color.  Kinda cute.




















Across all the injector sets, the AFR fueling differences dropped about 0.1.  Set 2 has one injector that flows significantly more than the rest, and that's not something with can get rid of, having only 8 injectors total, 4 per bank.
The Daniel set has nice consistent gains, bringing one of the banks to within 0.03.

So why do I care for AFR change of 0.1?  Because there is no extra cost associated with this process.  You're going to be installing injectors, you already know what they flow, so why not install them in a way that yields more consistent results?  All it takes is a simple sort and then installing them in the ordered determined by injector flows. You're getting something for nothing.  The good part is that there's a gain.  It's not a large gain, it's not going to solve all your fueling problems, but it should nudge it in the right direction.

--Marcin

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