Four-stroke petrol engine: expansion (power) stroke timing In a conventional four-stroke spark-ignition engine, the power or expansion stroke ideally begins near top dead centre after combustion and continues as the piston travels toward bottom dead centre. Which statement best represents this expansion period?

Introduction / Context:
The expansion (power) stroke in a four-stroke spark-ignition engine converts the high-pressure, high-temperature gases produced by combustion into useful mechanical work. Understanding when the expansion begins and ends on the crank angle diagram is fundamental to valve-timing, combustion phasing, and performance analysis.

Given Data / Assumptions:

• Four-stroke petrol (spark-ignition) engine with conventional valve timing.
• Combustion is initiated before top dead centre (spark advance), but the useful expansion predominantly occurs after top dead centre.
• Textbook idealization treats valve events as instantaneous for clarity.

Concept / Approach:

In the ideal four-stroke cycle, combustion is assumed to occur rapidly around top dead centre (TDC). The power stroke therefore begins at TDC and continues as the piston moves toward bottom dead centre (BDC). Real engines introduce small departures: spark occurs before TDC for proper phasing, and the exhaust valve opens slightly before BDC to reduce pumping work during blowdown. Nonetheless, the conceptual description aligns with TDC-to-BDC for the expansion stroke.

Step-by-Step Solution:

Verification / Alternative check:

Valve-timing diagrams show ignition advance before TDC and exhaust valve opening before BDC. Despite these optimizations, the expansion stroke is conventionally defined as the piston’s motion from TDC to BDC when cylinder pressure is doing positive work on the piston.

Why Other Options Are Wrong:

Option (b) suggests expansion starts before TDC, which is incorrect; pressure may rise due to combustion, but expansion stroke formally begins at or just after TDC.Option (c) fixes specific angles that are not standard nor universally correct.Option (d) ignores definite kinematic limits set by TDC and BDC.Option (e) reverses the direction of the power stroke.

Common Pitfalls:

Confusing the start of combustion (spark advance before TDC) with the start of the expansion stroke; also assuming exhaust always opens exactly at BDC in reality. Ideal definitions are used for conceptual clarity.

Final Answer:

starts at top dead centre and ends at bottom dead centre