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Difficulty: HardFuel and Air Intake Systems

A technician evaluates an engine's combustion efficiency. Which statement accurately describes the ideal stoichiometric air-fuel ratio of 14.7:1 for standard gasoline engines?

  1. A
    It represents 14.7 parts of fuel to 1 part of air by weight to maximize peak horsepower.
  2. It represents 14.7 parts of air to 1 part of fuel by weight for chemically complete combustion.Answer
  3. C
    It represents 14.7 parts of air to 1 part of fuel by volume measured in the intake manifold.
  4. D
    It represents a rich fuel mixture with surplus gasoline to cool internal cylinder temperatures.

Answer

It represents 14.7 parts of air to 1 part of fuel by weight for chemically complete combustion.
In gasoline internal combustion engines, the ideal stoichiometric air-fuel ratio is 14.7:1 by weight (mass). At this exact ratio, all fuel molecules react chemically with all oxygen molecules in the intake air, resulting in complete combustion and optimal catalytic converter operation.

Step-by-Step Solution

1
Identify the chemical definition of stoichiometric combustion for gasoline engines.
Stoichiometry is the exact chemical proportion where fuel and oxygen burn completely without leaving excess fuel or unreacted oxygen.
Understanding the fundamental definition establishes that 14.7:1 targets complete combustion rather than an intentionally rich or lean condition.
2
Analyze the units of measurement used for automotive air-fuel ratios.
Automotive air-fuel ratios are calculated by weight (mass), because atmospheric air density fluctuates with ambient temperature and altitude.
Volume is not a reliable measurement for air mass entering the intake manifold.
3
Determine the proportion of air to fuel.
A ratio of 14.7:1 denotes 14.7 pounds (or grams) of air for every 1.0 pound (or gram) of gasoline.
The larger number always represents the intake air mass in standard internal combustion ratings.

Key Concept

Stoichiometric Air-Fuel Ratio Principles
Estimated Time:1m 0s
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Topics

Fuel and Air Intake Systems

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