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Old 03-26-2012, 05:08 AM   #72
roger 04 rt OP
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Location: Massachusetts
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Here are some questions/comments from elsewhere. I hope that it's okay to include here and that some find it interesting and useful. It adds to what poolside and I were exchanging earlier.

Originally Posted by roger 04 rt
Here's a curve of injector latency versus battery voltage
Sorry, how does this relate at the moment?
Anything past 15Vdc is pointless as our systems, bike or car (assuming it's your normal 12Vdc system) do not ever go past these voltages. Otherwise our batteries would be getting "cooked".
Any voltages below 12Vdc are irrelevant as well, as even if your bike/car would be for a short time below that voltage, once the engine is running/ the charging voltage will very quickly go above 12Vdc and head towards +13Vdc in the moment the engine get's revved above idle speed.

So our interested area of concern should be 12~15Vdc, where we are looking at around a 0.4ms window.
This latency would have a larger influence at higher rpm (at which the battery voltage/charging voltage will be higher and as such latency will be reduced again), due to the possible higher change in % angle before/after TDC.

I don't think that there is any relevance as if it would have...then I am sure the manufacturer would(uses) use a Regulator in the injector circuit to keep the voltage stable and as such have a "known" percentage of latency.

All good questions. My main reasons for showing that curve was to confirm the approximate turn on time, and to show that it varies with battery voltage. And of course I know as well as you that the higher and lower voltages on that chart are mostly irrelevant, although cranking voltages do drop, especially if you have a weak battery, I saw 11V the other day on mine (I need a new battery).

I'm going to answer your questions/comments in the context of an R11xx engine that doesn't run its best at a 14.7:1 closed loop mixture unless things are really well balanced. That's why we get our valves to less than one thousandth, and balance our throttle bodies at multiple RPMs, with a precision differential instrument. I noted earlier that we give almost no attention to fueling imbalances which are just as important to total cylinder power balance. (Nor the O2 sensor)

Your last point, if battery voltage variation was an issue the manufacturer would use a regulator, is valid. And it turns out the manufacture does regulate for battery voltage at the fuel injector. The way it is done is: the Motronic measures the battery voltage, and then looks up in a table what the "net dead time" is and adds that into the fueling calculation.

I added the curve to this thread becasue it shows just how large the "net dead time" meaning the time that the Motronic must add can be--roughly 1 mS at 13.8 volts. At idle, the injector fuel pulse is in the vicinity of 2 mS and at 3500 RPM cruise, about 3.5 mS. So the net dead time is half the idle pulse and almost 1/3 of the 3500 RPM cruise. Any net dead time difference between the injectors will have a significant difference: a 0.1 mS difference would be 10% of the fuel on time at idle, and 4% at 3500 RPM cruise.

Here is a picture that gives an even closer look at how they work.

In the picture you can see that net dead time is comprised of:
--delay to start turning on
--time to ramp up
--delay to stop
--time to ramp down

Another point: the 7 mS time is the total injector time in my example too since our Boxers fire the injectors twice per combustion cycle--half the fuel twice, with two dead time cycles. So my 3.5 mS per injection, doubled is very close to the example given above.

So my points on injector imbalance are that there can be different flow rates and different net dead times, and both are important. And for those of us trying to balance our cylinder power AIR (valves and TB) are one side of the equation and FUEL accuracy is the other. Both are equally important, UNLESS you run a mixture richer than 14.7:1. If you run at 13.8, where all the European CO potentionmeter motorcycles were set, you are almost insensitive to small fueling differences, up to several percent. Then a good TB and Valve job gets it about right.

More food for thought.
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