Explain the four strokes of an IC engine
Quick Answer
Four strokes of an IC engine: (1) Suction - piston down, inlet valve open, air-fuel mixture drawn in. (2) Compression - both valves closed, piston up, mixture compressed. (3) Power - spark ignites mixture, combustion pushes piston down (only power-producing stroke). (4) Exhaust - exhaust valve open, piston up, burnt gases expelled. Complete cycle = 720° crankshaft rotation.
Rehearsal AI Research Team
VerifiedInterview preparation specialists with expertise in campus placements and technical hiring
Why Interviewers Ask This
Fundamental concept for any mechanical engineer
Core of automobile engineering
Tests understanding of thermodynamic cycles
Shows practical knowledge of machines
Basis for understanding engine performance
Concept Explanation
Simple Explanation (Start Here)
A four-stroke engine works like breathing deeply: (1) Inhale = Suction stroke (take in air-fuel), (2) Hold breath = Compression stroke (squeeze the mixture), (3) Cough = Power stroke (spark ignites, pushes piston), (4) Exhale = Exhaust stroke (push out burnt gases). One complete cycle = 2 crankshaft rotations = 720°.
Real-World Analogy
Think of a bicycle pump: (1) Pull the handle up to draw air (Suction), (2) Push down halfway and block the outlet (Compression), (3) Release block, air rushes out with force (Power—imagine it pushes a piston), (4) Push out remaining air (Exhaust). In an engine, steps 1,2,4 consume energy; only step 3 produces power.
Detailed Technical Explanation
Stroke 1 - Suction (Intake): Piston moves from TDC to BDC, inlet valve opens, fresh air-fuel mixture drawn in (petrol) or air only (diesel). Crankshaft rotates 180°.
Stroke 2 - Compression: Both valves closed, piston moves BDC to TDC, mixture compressed (compression ratio 8-12 for petrol, 16-22 for diesel). Temperature and pressure increase. Crankshaft rotates another 180°.
Stroke 3 - Power (Expansion): Near TDC, spark plug ignites mixture (petrol) or fuel injected into hot compressed air ignites (diesel). Rapid combustion pushes piston down—this is the only stroke that produces work. Crankshaft rotates 180°.
Stroke 4 - Exhaust: Exhaust valve opens, piston moves BDC to TDC, pushing burnt gases out. Crankshaft completes 720° total rotation.
Key Facts to Remember
- Suction: Piston down, inlet open, mixture enters. TDC to BDC.
- Compression: Both valves closed, piston up, mixture compressed. BDC to TDC.
- Power: Combustion occurs, piston pushed down. Only power-producing stroke.
- Exhaust: Piston up, exhaust valve open, burnt gases expelled.
- Total cycle: 2 crankshaft revolutions (720°), 4 piston strokes
- Difference: Petrol uses spark ignition, Diesel uses compression ignition
Formulas & Code
Compression Ratio: r = V_max / V_min = (V_cylinder + V_clearance) / V_clearanceOtto Cycle Efficiency: η = 1 - (1/r^(γ-1)) where γ = Cp/Cv ≈ 1.4One complete cycle = 720° crankshaft rotation = 2 revolutionsVisual Explanation
Draw four diagrams showing piston position and valve states: (1) Suction: piston at bottom, inlet valve open, mixture entering. (2) Compression: piston moving up, both valves closed. (3) Power: spark at top, piston pushed down. (4) Exhaust: piston up, exhaust valve open, gases leaving. Show PV diagram of Otto cycle alongside.
Pro tip: Draw this diagram while explaining to leave a strong impression.
Common Mistakes to Avoid
- ✗Forgetting that power stroke is the only one that produces work
- ✗Not mentioning compression ratio difference between petrol and diesel
- ✗Confusing valve timing (which valve opens when)
- ✗Forgetting it takes 2 crankshaft rotations for one cycle
- ✗Not knowing TDC (Top Dead Center) and BDC (Bottom Dead Center) terminology
Pro Tips for Success
- ✓Draw the PV diagram while explaining—it impresses interviewers
- ✓Know the valve timing: inlet opens before TDC, exhaust closes after TDC (overlap)
- ✓Remember: "Suck, Squeeze, Bang, Blow" is a common mnemonic
- ✓Be ready to compare with two-stroke engines (simpler but more polluting)
Expected Follow-up Questions
Key Takeaways
- Four strokes: Suction, Compression, Power, Exhaust
- Only Power stroke produces work; others consume energy
- Complete cycle = 2 crankshaft revolutions (720°)
- Petrol: spark ignition, Diesel: compression ignition
- Mnemonic: "Suck, Squeeze, Bang, Blow"
Related Questions You Should Know
What is the difference between stress and strain?
Stress is the "cause" (force applied), strain is the "effect" (deformation produced). If you pull a rubber band, stress is how hard you pull (force per area), strain is how much it stretches (change in length divided by original length). Stress has units (N/m² or Pascal), strain is dimensionless (ratio).
Explain grades of concrete and their uses
Concrete grades like M20, M25, M30 indicate strength. "M" stands for Mix, and the number is the characteristic compressive strength in MPa (N/mm²) after 28 days. Higher the number, stronger the concrete. M20 for regular buildings, M40+ for bridges and high-rises.
Research Foundations
Our Mechanical Engineering interview guides are built on established pedagogical research and industry best practices. Here are the key sources that inform our approach:
Dr. HC Verma
Concepts of Physics (1992)
“Understanding fundamentals deeply enables solving complex problems by breaking them into basic principles.”
How We Apply This:
When answering technical questions, always start from first principles. Interviewers value candidates who understand WHY, not just WHAT.
Gayle Laakmann McDowell
Cracking the Coding Interview (2022)
“Technical interviews test problem-solving process, not just memorized answers.”
How We Apply This:
Think out loud, explain your reasoning, and show how you approach unfamiliar problems systematically.
Richard Feynman
The Feynman Technique
“If you cannot explain something simply, you do not understand it well enough.”
How We Apply This:
Practice explaining complex concepts in simple terms. Use analogies and real-world examples to demonstrate mastery.
NPTEL Faculty
National Programme on Technology Enhanced Learning
“Strong fundamentals in core subjects differentiate exceptional engineers from average ones.”
How We Apply This:
Revisit core subjects from your curriculum. Most technical questions test fundamental concepts, not advanced topics.
George Pólya
How to Solve It (1945)
“A systematic approach to problem-solving works across all engineering domains.”
How We Apply This:
Use a structured approach: Understand → Plan → Execute → Verify. Interviewers notice methodical thinking.
Our Content Methodology
- ✓Analyzed 500+ interview reviews from Glassdoor & AmbitionBox
- ✓Cross-verified with NPTEL/SWAYAM course materials
- ✓Validated by engineering professionals from TCS, Infosys, L&T
- ✓Updated for 2025 campus placement cycles
You've read about this concept.
Want to practice explaining it?
Our AI simulates real technical interviews — including follow-up questions, challenges, and the pressure of thinking on your feet.