What is injector duty cycle and how does it relate to engine load?

• A. Duty cycle is the amount of injector pulse width relative to engine rpm only.
• B. Duty cycle is the ratio of injector duty cycle to cycle time only.
• C. Duty cycle is the fraction of time an injector is open per cycle; it increases with engine load to deliver more fuel.
• D. Duty cycle is the fraction of time the fuel pump is on per minute.

Answer: (C)

Injector duty cycle is the fraction of time the injector is open during one engine cycle. It’s usually expressed as a percentage of the cycle time and is controlled by the ECU by adjusting the injector’s pulse width. As engine load increases, the engine demands more fuel to achieve the required torque, so the injector must stay open longer in each cycle. That longer on-time raises the duty cycle, delivering more fuel to match the higher air intake. At light load or idle, less fuel is needed, so the injector is open for a shorter portion of the cycle and the duty cycle is lower. This concept ties fuel delivery directly to how hard the engine is working, rather than to engine speed alone, and it’s distinct from how long the fuel pump runs.