What are the two strokes of a two-stroke engine?

Short Answer:

A two-stroke engine completes one power cycle in only two piston movements or strokes—one upward and one downward. These two strokes are called the compression stroke and the power stroke. During these two strokes, all the four operations of the engine—suction, compression, power, and exhaust—are completed simultaneously.

In the two-stroke engine, one power stroke is obtained for each revolution of the crankshaft, which makes it more powerful compared to a four-stroke engine. It is simple in design, lighter in weight, and commonly used in scooters, motorcycles, and small engines like lawnmowers.

Detailed Explanation:

Two Strokes of a Two-Stroke Engine

A two-stroke engine is a type of internal combustion engine that completes one entire power cycle in only two strokes of the piston. The main two strokes are the compression stroke and the power stroke. Unlike the four-stroke engine, where one power stroke occurs in two revolutions of the crankshaft, in a two-stroke engine, one power stroke occurs during each revolution of the crankshaft.

This means that the two-stroke engine can produce power twice as frequently as a four-stroke engine, making it more powerful for its size. However, it consumes more fuel and produces more exhaust gases. The design of a two-stroke engine is simpler because it does not use valves for gas exchange. Instead, it uses ports — namely, the inlet port, transfer port, and exhaust port.

These ports open and close automatically as the piston moves up and down, controlling the flow of fuel-air mixture and exhaust gases. Let’s understand the two main strokes in detail.

  1. Compression Stroke

The compression stroke is the first stroke in a two-stroke engine. During this stroke, the piston moves upward from the bottom dead center (BDC) to the top dead center (TDC). As the piston moves upward, it performs several actions at once:

  • The inlet port and transfer port are closed by the rising piston.
  • The exhaust port is also covered by the piston, sealing the combustion chamber.
  • The air-fuel mixture (in petrol engines) or air (in diesel engines) trapped inside the cylinder is compressed into the small space at the top.

The compression of the charge increases its temperature and pressure. Near the end of this stroke, ignition occurs:

  • In spark ignition (SI) engines, a spark plug ignites the compressed air-fuel mixture.
  • In compression ignition (CI) engines, fuel is injected into the hot compressed air, and it ignites automatically.

As the mixture burns, it generates high-pressure gases that push the piston downward, leading to the next stroke—the power stroke.

At the same time, while the piston moves upward in the cylinder, a partial vacuum is created in the crankcase (the lower part of the engine). This vacuum draws a fresh charge of air-fuel mixture into the crankcase through the inlet port. Thus, suction takes place simultaneously with compression.

  1. Power Stroke

The power stroke is the second stroke in a two-stroke engine and the only one that produces useful work. It begins right after the ignition of the compressed charge. The combustion of the air-fuel mixture generates high-pressure gases that push the piston downward from TDC to BDC.

As the piston moves downward, it performs several simultaneous functions:

  • It transfers the power produced by combustion to the crankshaft through the connecting rod.
  • Near the end of this stroke, the exhaust port opens, allowing the burnt gases to escape from the cylinder.
  • Almost immediately after, the transfer port opens, and the compressed fresh charge from the crankcase enters the cylinder.

The fresh charge helps push out the remaining exhaust gases through the exhaust port—a process known as scavenging. Scavenging is an important part of the two-stroke operation because it ensures that the cylinder is cleared of exhaust gases before the next cycle begins.

As the piston reaches the bottom of the stroke, it covers the inlet port, closing it and trapping the new charge in the crankcase. The piston then starts moving upward again, and the cycle repeats continuously.

Key Points of the Two-Stroke Cycle

  1. Two strokes per cycle: Only two movements of the piston are needed—one up and one down.
  2. Ports instead of valves: Inlet, exhaust, and transfer ports control gas flow automatically.
  3. One revolution per power stroke: Power is produced during every crankshaft revolution.
  4. Scavenging: Fresh charge helps remove exhaust gases.
  5. Compact design: Simpler and lighter compared to four-stroke engines.

Advantages of Two-Stroke Engine

  • Produces one power stroke per revolution, giving higher power output.
  • Simple in construction and easy to maintain.
  • Lightweight and compact design.
  • Cheaper to manufacture.
  • Can operate in any position (useful in small portable machines).

Disadvantages of Two-Stroke Engine

  • Lower fuel efficiency due to incomplete combustion.
  • Higher exhaust emissions and more pollution.
  • Shorter engine life due to more wear and tear.
  • Greater fuel consumption.
  • Poor lubrication as oil is mixed with fuel.
Conclusion:

A two-stroke engine performs a complete power cycle in just two piston strokes—compression and power. During these two strokes, all four operations (suction, compression, power, and exhaust) take place, making it a compact and powerful design. Though it provides high power output for small engines, it has lower fuel efficiency and higher pollution compared to four-stroke engines. Therefore, two-stroke engines are mostly used in light vehicles, small machines, and portable tools where power-to-weight ratio is more important than fuel economy.