Thread: Fuel Pressure
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Old 09-11-2010, 06:48pm   #4
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Originally Posted by Munch View Post
That was the best explanation I have read to date, thanks. The only thing I didn't get is Fuel Injector duty cycle, but that may be because you didn't cover that, right?
I didn't.

Duty cycle is basically the % of engine revolution the injector is held open. It is the ratio of IPW (Injector pulse width-open time), and engine RPM, the time allowed before the next injector cycle begins).

In the quest to satisfy desired AFR, as engine load and RPM go up, and because IFR is fixed, the PCM commands more fuel by increasing the Injector Pulse Width (open time) to feed more fuel.

But obviously the injector can only be held open for so long, before the engine spins around, and that injector is called to fire again for the next cycle. The higher the engine RPM, the less time the injector can remain open, the higher the duty cycle becomes.

At ~100% IDC, the injector becomes static and never closes.

Injectors should ideally be sized to achieve about 80% IDC at peak RPM. Exceeding that is ok, but 100% is generally considered the absolute limit.

Over 100% means the PCM is calling for more fuel than the injector can possibly deliver, and again, although measured AFR may still be good, seeing IDC near/at/over 100% is clear indication you need more injector and should definitely upsize them.

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