See also IPW? (Injector Pulse Width)
The ECU tells the fuel injectors to open up for a short pulse - roughly 1 to 20 milliseconds - for each combustion (combustion requires intake, compression, ignition, and exhaust strokes, therefore two revolutions of the crankshaft). The “duty cycle” is the ratio of the pulse time to the time between combustion events. As RPM increases, the time between combustions decreases, so even for a constant injector pulse time, the duty cycle goes up.
At 3600 RPM, the cycle time is… 3600 revolutions per minute, 1800 cycles per minute, 30 cycles per second, which works out to 33.2 milliseconds.
A 10ms pulse would then correspond to a 30% injector duty cycle. That is, the injector is open for 30% of the time between combustion events.
At 7200 RPM (slightly higher than most redlines, but it makes the math easy), the cycle time is… 7200 revolutions per minute, 3600 cycles per minute, 60 cycles per second, which works out to 16.6 milliseconds.
So, at redline, a 10ms pulse is a 60% IDC.
Why this is interesting:
At redline, the engine typically needs fuel at the highest rate. It is quite possible that, in order to get enough fuel to create the desired air/fuel ratio, the injector would need to be open for a longer period than the period between combustion events. That is, the injector duty cycle would need to be great than 100%. Of course, it is not possible for the injector to be open for more than 100% of the time, so what really happens is, the air/fuel ratio gets leaner than desired. This can facilitate detonation, which can lead to engine failure.
Many tuners try to keep IDC less than 90% or even 85% when tuning, so that some headroom is available for unusual circumstances later (such as a cold day, for example).
At 100% IDC, the injector is said to be “static,” meaning that it is no longer injecting in pulses but is instead constantly open, just spraying an uninterrupted stream of fuel. (Interestingly, at injector duty cycles greater than approximately 25%, the injector is spraying onto the back of a closed intake valve. However this is common and does not significantly affect performance.)
However, others have reported that the engine can maintain the desired AFR even as the logger indicates an IDC greater than 100%. Since an IDC greater than 100% is theoretically impossible, it is suspected that the IDC reported by the logger may not be perfectly accurate. For example, it may be computed from a pulse width time that does not include injector latency. This would allow a few percent more IDC, since latency is no longer significant for an injector that is not closing in between combustion events.