# Topic: **Four-Stroke Internal Combustion Engine** ## Introduction The **four-stroke internal combustion engine** is the most common type of engine used in cars, motorcycles, and many other machines. It converts chemical energy from fuel into mechanical work through a series of four distinct strokes. This engine is called "four-stroke" because its piston completes four separate movements (strokes) to complete one cycle. --- ## Main Components - **Cylinder:** The hollow tube where the piston moves. - **Piston:** Moves up and down inside the cylinder. - **Crankshaft:** Converts piston's linear motion to rotational motion. - **Valves:** Control the intake of air-fuel mixture and exhaust gases. - **Spark Plug:** Ignites the air-fuel mixture (in petrol engines). --- ## The Four Strokes ### 1. **Intake Stroke** - **What happens?** The intake valve opens; the piston moves down. - **Purpose:** Draws a mixture of air (and fuel, in petrol engines) into the cylinder. - **Result:** Cylinder fills with a combustible mixture. --- ### 2. **Compression Stroke** - **What happens?** Both valves close; the piston moves up. - **Purpose:** Compresses the air-fuel mixture to a small volume. - **Result:** Increases pressure and temperature, preparing for efficient combustion. --- ### 3. **Power (Combustion) Stroke** - **What happens?** At the end of compression, the spark plug ignites the mixture (in petrol engines). In diesel engines, fuel is injected and self-ignites due to high pressure. - **Purpose:** The burning gases expand, pushing the piston down. - **Result:** This is the only stroke that produces useful work. --- ### 4. **Exhaust Stroke** - **What happens?** The exhaust valve opens; the piston moves up. - **Purpose:** Forces burnt gases out of the cylinder. - **Result:** Cylinder is cleared and ready for the next cycle. --- ## Summary Table | Stroke | Valve Status | Piston Movement | Purpose | |---------------|---------------------|-----------------|------------------------| | Intake | Intake open | Down | Draw air/fuel mixture | | Compression | All closed | Up | Compress mixture | | Power | All closed | Down | Produce power | | Exhaust | Exhaust open | Up | Expel exhaust gases | --- ## Working Principle (Cycle) The **four strokes** together form one complete cycle. For every cycle, the crankshaft rotates twice (720°), and one power stroke occurs per cylinder. This sequence ensures smooth and continuous power delivery in multi-cylinder engines. --- ## Advantages - **Higher efficiency** compared to two-stroke engines. - **Less pollution** due to separate exhaust stroke. - **Better lubrication** and durability. --- ## Applications - **Automobiles** - **Motorcycles** - **Generators** - **Lawn mowers** --- ## Diagram  --- ## Conclusion The four-stroke engine is a core technology in mechanical engineering, illustrating the conversion of chemical energy into mechanical work through an efficient and reliable process. Understanding its operation is fundamental for anyone studying engines, automotive systems, or thermodynamics.