Basic thermodynamic cycle for petrol (spark-ignition) engines Identify the ideal cycle that best represents the operation of a conventional petrol engine.

Introduction / Context:
Ideal cycles model real engines to permit analysis of efficiency limits and parameter trends. Recognizing the correct idealization for petrol engines is a foundational skill in thermodynamics and engine theory.

Given Data / Assumptions:

• Conventional spark-ignition engine with nearly homogeneous mixture.
• Rapid heat addition approximated at constant volume (near top dead center).
• Compression and expansion taken as isentropic in the ideal model.

Concept / Approach:

The Otto cycle consists of two isentropic processes (compression and expansion) and two constant-volume heat-transfer processes (addition and rejection). This matches the idealized behavior of SI engines where combustion occurs quickly relative to piston motion, approximating heat addition at constant volume.

Step-by-Step Solution:

Verification / Alternative check:

Efficiency trend with compression ratio in the Otto cycle mirrors observed SI engine behavior (higher compression ratio generally improves efficiency until knock limits).

Why Other Options Are Wrong:

Joule/Brayton applies to continuous-flow gas turbines; Rankine to steam plants; Stirling to external-combustion regenerative engines; Dual cycle models CI engines more closely.

Common Pitfalls:

Assuming real combustion is exactly constant-volume; it is only an approximation, but sufficient for the ideal model choice.

Final Answer:

Otto cycle