Air–Fuel Mixture Terminology – What Is the Stoichiometric Ratio? Choose the best definition of the stoichiometric ratio used in internal combustion engines.

Introduction / Context:
The stoichiometric air–fuel ratio is fundamental for engine calibration and emissions. It is the proportion at which exactly enough oxygen is supplied to completely oxidize the fuel, leaving no excess air and no unburnt fuel. For gasoline, this is approximately 14.7–15:1 by mass; for diesel it is much higher but engines usually operate lean of stoichiometric.

Given Data / Assumptions:

• Definition is mass based unless explicitly stated otherwise.
• Complete combustion to CO2 and H2O is assumed.
• No allowance for dissociation or residuals in the simplified definition.

Concept / Approach:

Combustion chemical equations relate fuel molecules to required oxygen molecules. Because fuel metering and emissions limits are typically treated on a mass basis, the conventional definition of stoichiometric ratio in engines is by weight. Volumetric ratios can vary widely with temperature and pressure and are less standard for defining stoichiometry in engine practice.

Step-by-Step Solution:

Verification / Alternative check:

Closed-loop control with lambda sensors targets lambda = 1, corresponding to the mass-based stoichiometric ratio for the given fuel. This operational practice confirms the mass-based definition.

Why Other Options Are Wrong:

Volume-based statements are not standard for stoichiometric definition in engines. Maximum efficiency and maximum power typically occur at mixtures different from stoichiometric. Cold start enrichment is a separate strategy and not a definition.

Common Pitfalls:

Mixing up lambda (relative ratio) with the absolute AFR; confusing stoichiometric with best economy or best power mixtures.

Final Answer:

chemically correct air–fuel ratio by weight