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Understanding AFR

Discussion and questions related to the course Understand AFR

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To verify my understanding of calculating Injector Duty Cycle which is IDC=(Required Flow/Total Flow)*100. Do we use the required fuel flow (lbs/foot) calculation (Fuel Flow = Mass per Cylinder or Mass per # of cylinders/AFR) and the total flow which would be what the injector flow capacity? Also are the required flow and total flow per cylinder or the whole system in the IDC equation? Also what should the units be on required flow and total flow to use in the IDC formula so it works in the Pulse Width equation.


First, I haven't seen the "Understanding AFR" course.

So I can't talk about its content.

Injector duty cycle (like any duty cycle) is an electrical value.

Even if it's related to the actual flow of the injector, it has nothing to do with it.

IDC = injector signal / engine cycle time * 100

Let say you have an engine running at a frequency of 3000rpm.

That give you a total engine cycle time of 40ms.

If you are interested by the math:

-1 minute = 60000 milliseconds

-engine cycle = 2 revolutions


T = (1/3000)*2*60000 or 120000/3000 = 40

If you have an injector pulse width (signal) of 10ms, that give you a duty cycle of 25% (10/40*100)

Based on that pulse width length, if you have the specs of your injectors (offset and max theoretical flow) you can calculate your actual flow (that's what a modern VE based ECU do for you)

But 25% IDC is NOT 25% of the total flow of the injector.

The required flow is a totally different topic.