Engine management sensors — the oxygen (O₂) sensor provides feedback to the control unit primarily about which combustion parameter for closed-loop mixture regulation?

Introduction / Context:
Modern engines use sensors to control mixture, spark, and emissions. The oxygen sensor, mounted in the exhaust, is a key element of closed-loop fuel control. By measuring oxygen content in exhaust gases, the controller adjusts injector pulse width to maintain stoichiometry for three-way catalyst efficiency, or targets richer or leaner mixtures as needed for power or economy.

Given Data / Assumptions:

• Closed-loop operation with a narrowband or wideband oxygen sensor.
• Electronic control unit capable of interpreting sensor voltage or lambda signal.
• Catalyst system that benefits from stoichiometric combustion around lambda equal to one.

Concept / Approach:

The oxygen sensor output is directly related to lambda, which is the ratio of actual to stoichiometric air-fuel ratio. A narrowband sensor toggles rapidly around lambda equal to one, while a wideband sensor quantifies deviation above or below stoichiometry. The control unit uses this signal to correct fuel delivery, thereby controlling the actual air-fuel ratio. It does not measure intake air temperature or flow speed directly, nor does it indicate exhaust volume; those are handled by other sensors such as intake air temperature, mass air flow, and manifold pressure sensors.

Step-by-Step Solution:

Verification / Alternative check:

Control strategies in OEM documentation describe short-term and long-term fuel trim derived from oxygen sensor feedback, confirming that air-fuel ratio is the parameter being controlled.

Why Other Options Are Wrong:

Air temperature and air flow speed come from separate intake sensors. Exhaust gas volume is not directly used for mixture control and is influenced by many variables.

Common Pitfalls:

Assuming sensor voltage indicates absolute oxygen concentration without context; confusing narrowband switching behavior with wideband proportional output.

Final Answer:

air-fuel ratio