Four-stroke engines — correct sequence of the four operations Identify the correct order of strokes in a four-stroke internal-combustion engine.

Introduction / Context:
The operating sequence in a four-stroke engine defines gas exchange and thermodynamic processes and directly affects valve timing, ignition/injection control, and performance analysis.

Given Data / Assumptions:

• Conventional SI or CI four-stroke engine.
• Ideal sequence without focusing on valve overlap nuances.

Concept / Approach:
The classic order is: intake (suction) to draw in fresh charge, compression to raise temperature/pressure, expansion (power) after ignition/combustion to produce work, and exhaust to expel products. This order is fundamental to engine kinematics and thermodynamics in textbooks and practical engines.

Step-by-Step Solution:
Stroke 1: Suction (intake valve open, piston BDC→TDC direction reversed starting from TDC→BDC).Stroke 2: Compression (both valves closed, piston BDC→TDC).Stroke 3: Expansion (power) (combustion, piston TDC→BDC).Stroke 4: Exhaust (exhaust valve open, piston BDC→TDC).

Verification / Alternative check:
Valve timing diagrams and indicator diagrams universally reflect this order.

Why Other Options Are Wrong:
Any permutation that places expansion before compression or mixes suction and compression order contradicts the required sequence for trapping and burning the charge.

Common Pitfalls:
Overlooking that intake and exhaust strokes bookend the thermodynamic cycle and that compression must precede the power stroke.

Final Answer:
Suction, compression, expansion, exhaust