--- title: "Four-stroke SI engine working and components" subject: "Internal Combustion Engines" chapter: 2 key: "semester_6/internal_combustion_engines:four-stroke-si-engine-working-components" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/four-stroke-si-engine-working-components site: BCT Engineering --- # Four-stroke SI engine working and components Internal Combustion Engines · Chapter 2: Engine construction and operation - Asked in: year not given, 6 marks - Video: [4-Stroke & 2-Stroke Engine | Its Parts & Working Explained](https://www.youtube.com/watch?v=8dAbcbAJRw8) — The Engineers Post - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/four-stroke-si-engine-working-components - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-2 ## Question Explain the working of a four-stroke spark-ignition engine with a neat sketch of the strokes. List the major engine components and state the function of each. ## Short answer Suction: inlet valve open, piston descends, air-fuel mixture enters. Compression: both valves closed, piston rises, mixture compressed, spark near TDC. Power: burning mixture expands, forcing piston down. Exhaust: exhaust valve opens, piston rises, expelling gases. Main components: cylinder block, head, piston, rings, connecting rod, crankshaft, flywheel, camshaft, valves, sump, gaskets. ## Answer A four-stroke SI engine completes one power cycle in four piston strokes, i.e. two crankshaft revolutions (720 degrees). ```text 1 SUCTION 2 COMPRESSION 3 POWER 4 EXHAUST IV open both closed both closed EV open | |v| | | | | | | |* | | | | |^| |_ | |_| |_ | |_ | |_ | |_ | |_ | | | | | ||| ||| | | piston down piston up piston down piston up mixture in mixture spark, burnt gas compressed expansion out ``` 1. **Suction:** the piston moves from TDC to BDC, the inlet valve is open and a petrol-air mixture from the carburettor or injector enters. 2. **Compression:** both valves are closed, the piston rises and compresses the mixture (r = 8-10). Near the end the spark plug ignites it. 3. **Power (expansion):** the burning gas pushes the piston down. This is the only stroke giving work. 4. **Exhaust:** the exhaust valve opens and the rising piston pushes out the burnt gases. #### Major components and functions | Component | Function | |---|---| | Cylinder block and liner | Holds the cylinder, guides the piston | | Cylinder head | Closes the cylinder; holds valves, spark plug, combustion chamber | | Piston | Receives gas force and transmits it to the connecting rod | | Piston rings | Seal the gas, control oil film, conduct heat to the wall | | Gudgeon pin | Connects piston to the small end of the connecting rod | | Connecting rod | Transmits the piston force to the crankshaft | | Crankshaft | Converts the reciprocating motion into rotary motion and delivers torque | | Flywheel | Stores energy, smooths speed fluctuation | | Camshaft, valves | Open and close the inlet and exhaust ports at the correct time | | Crankcase / sump | Encloses the crankshaft, stores lubricating oil | | Gaskets | Seal joints against gas, oil and water leakage | Materials: cast iron or aluminium alloy for block and piston, forged steel for the crankshaft and connecting rod. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A four-stroke engine completes one power cycle in how many crankshaft revolutions? A. Two (720 degrees) B. One (360 degrees) C. Four D. Half Answer: A. Four strokes take two revolutions. (Key point: Cycle duration) 2. Which stroke produces work, and when is the charge ignited? A. Compression stroke; ignition at BDC B. Power stroke; ignition near the end of compression C. Power stroke; ignition at the start of suction D. Exhaust stroke; ignition at TDC after power Answer: B. The spark fires near the end of compression and the burning gas pushes the piston down in the power stroke, the only work stroke. (Key point: Ignition and work stroke) 3. What are the valve positions during compression and power strokes? A. Inlet open, exhaust closed B. Exhaust open, inlet closed C. Both closed D. Both open Answer: C. Both valves stay closed so that the charge can be compressed and the gas can expand. (Key point: Valve state during compression) 4. What is the function of the flywheel? A. Seals the gas B. Converts reciprocating to rotary motion C. Stores lubricating oil D. Stores energy and smooths speed fluctuation Answer: D. Since only one stroke in four gives work, the flywheel stores energy to keep speed steady. (Key point: Flywheel function) ## Related in Internal Combustion Engines - [Two-stroke and four-stroke engine comparison](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Atwo-stroke-versus-four-stroke-engine) — Internal Combustion Engines, chapter 2 - [Valve timing diagram of four-stroke engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Avalve-timing-diagram-four-stroke) — Internal Combustion Engines, chapter 4 - [Engine displacement and mean piston speed](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-displacement-piston-speed-numerical) — Internal Combustion Engines, chapter 2 - [Stages of SI combustion and flame propagation](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Astages-combustion-si-flame-propagation) — Internal Combustion Engines, chapter 5 - [Knocking in SI engines and its control](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aknocking-in-si-engines-control) — Internal Combustion Engines, chapter 5 - [Battery and magneto ignition systems](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Abattery-coil-magneto-ignition-systems) — Internal Combustion Engines, chapter 5 ## Related in other subjects - [Spark ignition vs compression ignition engine](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aspark-vs-compression-ignition) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Battery ignition system and spark timing calculation](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abattery-ignition-spark-numerical) — Automotive Technology, chapter 3 - [Two-stroke versus four-stroke engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atwo-stroke-vs-four-stroke) — Automotive Technology, chapter 1 - [Knock and pre-ignition in SI engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aknock-and-pre-ignition-si-engine) — Automotive Technology, chapter 3