Internal Combustion Engine: A Deep Dive into Two-Stroke Engines and Valve Timing Diagrams

 

We are diving deeper into the fascinating world of internal combustion engines, and our focus will be on the two-stroke engine. Whether you're an engineering enthusiast, a student, or just curious about how engines work, this post will guide you through the basic principles of the two-stroke engine and its valve timing diagrams. Let’s get started and explore how this powerful and efficient engine operates!

The two-stroke internal combustion engine (IC engine) is a type of engine that completes its power cycle in two strokes of the piston (one up and one down), compared to the four strokes of a typical four-stroke engine. This makes the two-stroke engine simpler, lighter, and more powerful relative to its size. Two-stroke engines are commonly used in motorcycles, chainsaws, lawnmowers, and small marine engines.

Working Principle of a Two-Stroke IC Engine

Schematic Representation of Two Stroke Engine

In a two-stroke engine, the entire combustion cycle (intake, compression, power, and exhaust) is completed in just two piston strokes:

Compression Stroke (Upward Stroke)

• As the piston moves upward in the cylinder, it compresses the air-fuel mixture inside the combustion chamber.
• While the piston moves upward, a fresh air-fuel mixture is drawn into the crankcase through the intake port from the carburetor (in carbureted engines).
• Near the end of the compression stroke, ignition occurs. A spark plug (in gasoline engines) ignites the compressed air-fuel mixture.
• As the air-fuel mixture ignites, it causes combustion, and the expanding gases push the piston downward.

Power Stroke (Downward Stroke)

• The combustion of the air-fuel mixture generates hot gases, creating high pressure that forces the piston downward, producing the power stroke.
• As the piston moves downwards, it uncovers the exhaust port, allowing the exhaust gases from the previous combustion cycle to escape from the cylinder.
• Simultaneously, the downward motion of the piston compresses the fresh air-fuel mixture in the crankcase.
• As the piston moves down further, it uncovers the transfer port, allowing the compressed air-fuel mixture from the crankcase to flow into the combustion chamber. This process is called scavenging, where the fresh charge pushes the remaining exhaust gases out through the exhaust port.

Two Main Phases in a Two-Stroke Engine

Compression Phase

• Intake of a fresh air-fuel mixture into the crankcase.
• Compression of the mixture in the combustion chamber as the piston moves up.
• Ignition of the compressed mixture near the end of the compression stroke.

Power Phase

• Combustion drives the piston down, generating power.
• Exhaust gases exit the cylinder via the exhaust port.
• Scavenging introduces fresh air-fuel mixture from the crankcase into the combustion chamber.

Ports and Timing in a Two-Stroke Engine

Unlike four-stroke engines, two-stroke engines do not have valves. Instead, they use ports that are uncovered by the piston at specific points during its movement:

Exhaust Port: 

Allows exhaust gases to escape after combustion.

Transfer Port: 

Transfers the fresh air-fuel mixture from the crankcase to the combustion chamber.

Intake Port: 

Allows the fresh air-fuel mixture to enter the crankcase.

Typical Valve Timings as per Angle Rotation of Crank

Advantages of Two-Stroke Engines

• Simpler Design: Fewer components (no valves, camshaft) make two-stroke engines lighter and simpler to manufacture.
• Higher Power-to-Weight Ratio: Since a power stroke occurs every two strokes, two-stroke engines can generate more power relative to their size and weight compared to four-stroke engines.
• Compact and Lightweight: Due to fewer moving parts, two-stroke engines are more compact and lighter, making them ideal for smaller vehicles and machines.
• Easier Maintenance: With fewer components, maintenance is easier and less frequent.

Disadvantages of Two-Stroke Engines

• Less Efficient: Two-stroke engines are less fuel-efficient due to incomplete combustion and loss of fuel through the exhaust.
• Higher Emissions: The scavenging process can result in unburned fuel mixing with the exhaust gases, leading to higher emissions of pollutants.
• Shorter Lifespan: Due to higher wear and tear (since power is produced on every stroke), two-stroke engines generally have a shorter lifespan compared to four-stroke engines.
• Increased Oil Consumption: Two-stroke engines require a mix of oil with fuel to lubricate the engine, leading to higher oil consumption and increased operational costs.

Summary of the Two-Stroke Cycle

• Compression Stroke (Upward): The piston compresses the air-fuel mixture in the cylinder, while new fuel is drawn into the crankcase.
• Power Stroke (Downward): Combustion forces the piston down, driving the engine. Exhaust gases exit, and fresh air-fuel mixture is transferred into the cylinder.

Two-stroke engines are efficient for small, lightweight applications, but their environmental impact and efficiency drawbacks have led to a decline in use for larger applications like automobiles, where four-stroke engines are more common.