Four-stroke petrol engine — when does the compression stroke start and end? In a four-stroke cycle spark-ignition (petrol) engine, identify the ideal timing of the compression stroke relative to the piston's dead centres.

Introduction / Context:
The four fundamental strokes are suction (intake), compression, power (expansion), and exhaust. While real engines use valve lead and lag, the ideal cycle description anchors each stroke to the piston's dead centres for conceptual clarity.

Given Data / Assumptions:

• Idealized timing for teaching the basic cycle.
• Neglect of valve overlap and gas-dynamic effects.
• Conventional SI engine with throttle and spark ignition.

Concept / Approach:
In the ideal Otto cycle representation, the compression stroke begins as the piston leaves bottom dead centre (BDC) with both valves closed and ends when the piston reaches top dead centre (TDC). Heat is then added at (ideally) constant volume near TDC to initiate the power stroke. Real engines often close the intake valve slightly after BDC and may open the exhaust before BDC, but the ideal answer remains BDC to TDC for compression.

Step-by-Step Solution:
Identify compression: both valves closed; piston moves from BDC to TDC.Ideal cycle anchoring: start at BDC, end at TDC.Therefore, the correct choice is the BDC-to-TDC option.

Verification / Alternative check:
Textbook indicator diagrams show the compression line from the end of intake at BDC to TDC just before ignition.

Why Other Options Are Wrong:
Options with angles before/after BDC or TDC reflect specific valve timings but not the definition of the compression stroke itself. “Anywhere” is incorrect; stroke limits are set by piston travel.

Common Pitfalls:
Confusing valve events (intake closing after BDC) with the thermodynamic stroke definition. The stroke is still described BDC→TDC in the ideal model.

Final Answer:
Starts at bottom dead centre and ends at top dead centre