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Physics

Four Stroke Engine Explained: Working, Diagram, and Exam Insights

Four Stroke Engine Explained: Working, Diagram, and Exam Insights

Q: 5. Where are four-stroke engines used?

Four-stroke engines are ubiquitous. They power most cars, motorcycles, and many boats. They are also found in some generators, lawnmowers, and even smaller aircraft.

Q: 9. What is the significance of the crankshaft in a four-stroke engine?

The crankshaft is essential. It converts the reciprocating linear motion of the piston into rotational motion, which is the output power used to move a vehicle or do work. Understanding its role is crucial for grasping the mechanical advantage of the engine.

Step-by-Step Stages of the Four Stroke Engine Cycle

The topic of four-stroke engine is important in physics and helps us understand how engines power vehicles, how energy is converted, and various laws of thermodynamics and mechanics. Understanding the four-stroke engine also helps students distinguish between engine types, diagram engine parts, and apply these concepts in real-world situations.

Understanding Four-Stroke Engine

Four-stroke engine refers to an internal combustion (IC) engine that completes a power cycle using four distinct piston movements: intake, compression, power (combustion), and exhaust. This cycle is fundamental in modern automobiles, motorcycles, and small engines. It plays a vital role in topics like types of heat engines, energy conversion, and the laws of thermodynamics.

Working Principle of Four-Stroke Engine

The concept is best explained by the engine’s sequence of four operations ("strokes"):

Intake Stroke: The piston moves down as the intake valve opens, drawing a mixture of air and fuel into the cylinder (in petrol engines) or just air (in diesel engines).
Compression Stroke: The piston moves up with both valves closed, compressing the mixture for efficient combustion.
Power Stroke: The compressed mixture is ignited (by a spark plug in petrol or by heat/pressure in diesel), causing a rapid expansion that forces the piston down. This is where useful work is produced.
Exhaust Stroke: The piston moves up again with the exhaust valve open, expelling burned gases from the cylinder.

A complete four-stroke cycle requires two rotations of the crankshaft (720°), with only one power stroke per cycle. The engine repeats this cycle for continuous power generation.

Diagram and Key Parts of Four-Stroke Engine

Understanding the labelled parts is crucial for exams and real-life troubleshooting. The main components include:

Piston
Cylinder
Crankshaft
Connecting rod
Inlet (intake) valve
Exhaust valve
Spark plug (petrol engine)
Fuel injector (diesel engine)
Crankcase
Flywheel

A typical labelled diagram will show the relative positions of these parts during different strokes.

Here’s a useful table to understand four-stroke engine better:

Four-Stroke Engine Cycle Table

Stroke	What Happens	Valves Status
Intake	Air/fuel mixture drawn in	Intake: Open, Exhaust: Closed
Compression	Mixture compressed	Both closed
Power	Mixture ignited, piston forced down	Both closed
Exhaust	Burnt gases expelled	Intake: Closed, Exhaust: Open

Comparison: Two-Stroke vs Four-Stroke Engines

It’s a common exam question to distinguish between two-stroke and four-stroke engines. Here’s a comparison:

Feature	Two-Stroke Engine	Four-Stroke Engine
Number of strokes per cycle	2	4
Power stroke frequency	Every revolution	Every 2 revolutions
Oil mixing	Needed with fuel	Separate lubrication
Efficiency & pollution	Less efficient, more polluting	More efficient, less pollution
Applications	Small engines (chainsaws, boats)	Cars, motorcycles, generators

Worked Example / Practical Experiment

Let’s understand the engine cycle with an application:

1. Suppose a petrol four-stroke engine completes 2500 cycles per minute (RPM).

2. Since each cycle needs two revolutions, the crankshaft rotates at 5000 RPM.

3. If each power stroke produces 500J, the engine delivers 1,250,000J per minute.

Conclusion: This shows how engine RPM and stroke sequence affect power output in machines.

Practice Questions

Define four-stroke engine with an example.
What are the four main steps in a four-stroke cycle?
Compare two-stroke and four-stroke engines in a table.
Explain the real-world application of four-stroke engines in cars or bikes.
Draw and label a four-stroke engine diagram.

Common Mistakes to Avoid

Mixing up the sequence of the four strokes in the cycle.
Incorrectly labelling the diagram or confusing the roles of valves and spark plugs.
Believing all four-stroke engines use the same fuel or oiling system as two-stroke engines.

Real-World Applications

Four-stroke engine cycles are found in almost all modern vehicles—cars, motorcycles, buses, trucks, and even some boats and aircraft. Their efficient energy conversion and low emissions make them the main choice for transport today. With Vedantu, you learn to connect theoretical engine concepts to practical innovations in work and energy, thermodynamics, and mechanics.

In this article, we explored four-stroke engine — its meaning, working principle, comparative advantages, and applications in physics and daily life. Keep exploring such topics with Vedantu to improve your understanding and score better in Board and entrance exams.

FAQs on Four Stroke Engine Explained: Working, Diagram, and Exam Insights

1. What is a four-stroke engine?

A four-stroke engine is an internal combustion engine that converts fuel's chemical energy into mechanical energy through four distinct piston strokes: intake, compression, power, and exhaust. Each stroke plays a crucial role in the engine's power cycle.

2. How does a four-stroke engine work?

The four-stroke engine operates in a cyclical process. First, the intake stroke draws a fuel-air mixture into the cylinder. Next, the compression stroke compresses this mixture, increasing pressure and temperature. The power stroke ignites the compressed mixture, causing expansion and pushing the piston down, generating power. Finally, the exhaust stroke expels the burnt gases. This entire cycle repeats continuously.

3. What is the difference between a two-stroke and a four-stroke engine?

The main difference lies in the number of strokes per power cycle. A two-stroke engine completes a power cycle in two strokes (intake/compression and power/exhaust), while a four-stroke engine takes four strokes. Four-stroke engines are generally more efficient and produce less pollution but are less powerful than two-stroke engines for a given engine size.

4. What type of fuel does a four-stroke engine use?

Four-stroke engines can use various fuels depending on their design. Common fuels include petrol (gasoline) and diesel. The type of fuel influences the ignition method (spark ignition for petrol, compression ignition for diesel) and engine efficiency.

5. Where are four-stroke engines used?

Four-stroke engines are ubiquitous. They power most cars, motorcycles, and many boats. They are also found in some generators, lawnmowers, and even smaller aircraft.

6. Draw a four-stroke engine diagram showing all the parts?

A typical four-stroke engine diagram shows key components such as the piston, cylinder, crankshaft, connecting rod, valves (intake and exhaust), spark plug (in petrol engines), and fuel injector (in diesel engines). You should always clearly label each part in your diagram.

7. Explain the four-stroke engine cycle with a diagram?

The four-stroke cycle can be best understood with a diagram showing the piston's position in each stroke (intake, compression, power, exhaust). Each stage's valve positions, piston movement, and pressure changes should be clearly depicted. This helps visually understand the process and its impact on generating power.

8. What are some common misconceptions about four-stroke engines?

A common misconception is that all four-stroke engines use the same type of fuel or oil. Another is assuming that the piston moves at a constant speed throughout each stroke. The timing of the valves opening and closing is also crucial and should not be underestimated. Understanding these details is vital for mastering the concept.

9. What is the significance of the crankshaft in a four-stroke engine?

The crankshaft is essential. It converts the reciprocating linear motion of the piston into rotational motion, which is the output power used to move a vehicle or do work. Understanding its role is crucial for grasping the mechanical advantage of the engine.

10. How does the four-stroke engine contribute to vehicle motion?

The rotational motion produced by the crankshaft is transmitted through a transmission system to the wheels, ultimately causing vehicle motion. The power generated during the power stroke, through a series of mechanical linkages, overcomes rolling resistance and provides acceleration.

11. Explain the role of valves in a four-stroke engine.

The intake and exhaust valves control the flow of fuel-air mixture and exhaust gases into and out of the cylinder. Precise timing of their opening and closing is vital for optimizing engine efficiency and performance. They are precisely controlled by the camshaft and ensure the correct sequence of strokes.

12. What is the significance of the compression ratio in a four-stroke engine?

The compression ratio (the ratio of the cylinder volume at the beginning of the compression stroke to the volume at the end) is critical. A higher compression ratio allows for a more efficient power stroke but requires higher-quality fuel and stronger engine components. It directly affects engine efficiency and power output.