Air-standard cycle efficiency – dominant parameter For idealized air-standard analyses of internal combustion engine cycles, the thermal efficiency depends primarily on which of the following?

Introduction / Context:
Air-standard cycle models (Otto, Diesel, Dual) provide insight into how design parameters influence theoretical efficiency. Among many variables, compression ratio is the key determinant of thermal efficiency in these idealized cycles.

Given Data / Assumptions:

• Air-standard assumptions: working fluid is air with constant properties.
• Comparing ideal thermal efficiencies, not real-engine indicated or brake efficiencies.
• Same cycle type unless specified otherwise.

Concept / Approach:
For the Otto cycle, efficiency = 1 − 1/(r^(γ−1)), directly showing dependence on compression ratio r. Diesel and dual cycles also exhibit strong efficiency increases with r, though with additional dependence on cut-off ratio. Fuel type and speed do not appear explicitly in the ideal efficiency expressions.

Step-by-Step Solution:
Write the ideal Otto efficiency form: η = 1 − 1/(r^(γ−1)).Observe r as the controlling parameter; fuel identity and speed are absent.Therefore, select “Compression ratio.”

Verification / Alternative check:
Plotting η versus r for γ ≈ 1.4 shows strong monotonic increase; similar trends hold for Diesel/Dual with their respective parameters.

Why Other Options Are Wrong:

• Fuel type and speed affect real engines but not the idealized air-standard thermal efficiency.
• Chamber color and ambient humidity are irrelevant in the ideal model.

Common Pitfalls:
Confusing real-world knock limits (which cap allowable compression ratio) with the ideal mathematical dependence.

Final Answer:
Compression ratio