Two Stroke Engine

A two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a four-stroke engine that requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In this article, we will learn about the two-stroke engine in detail.

Table of Contents

Construction of A Two-Stroke Engine

Two Stroke Engine

Piston โ€“ The piston transfers the expanding force of gases to the mechanical rotation of the crankshaft through a connecting rod.

Crankshaft โ€“ It converts the reciprocating motion to rotational motion.

Connecting Rod – It transfers motion from a piston to the crankshaft and acts as a lever arm.

Flywheel โ€“ It is a mechanical device that is used to store energy.

Spark Plug โ€“ It delivers electric current to the combustion chamber and in turn ignites the air-fuel mixture leading to abrupt expansion of gases.

Counter Weight – Counterweight on the crankshaft is used to reduce the vibrations due to imbalances in the rotating assembly.

Inlet and Outlet Ports โ€“ These ports allow fresh air with fuel to enter and exit from the cylinder.

Two-Stroke Engine Cycle

Down Stroke

The piston moves from TDC (Top-Dead-Center) to BDC (Bottom-Dead-Center) letting the fresh air enter the combustion chamber. The fresh air-fuel mixture gets into the combustion chamber through the crankcase. In this stroke, the crankshaft makes the rotation of 1800.

Up Stroke

The piston is pushed from BDC to TDC. As a result, the fuel-air mixture gets compressed and the spark plug ignites the mixture. The mixture expands and the piston is pushed down. The inlet port is open during the upstroke. While the inlet port is opened, the mixture gets sucked inside the crankcase. When the mixture is pushed up into the combustion chamber during the previous upstroke, a partial vacuum is created as no mixture is left behind in the crankcase. This mixture is ready to go into the combustion chamber during downstroke but remains in the crankcase until the piston goes up to TDC. In this stroke, the crankshaft makes the rotation of 1800.

From the 2nd downstroke onwards the exhaust gases get expelled out from one side while a fresh mixture enters into the combustion chamber simultaneously due to a partial vacuum created in the combustion chamber after the removal of exhaust gases. This is the beauty of the engine. Both things happen at the same time which makes it a 2-stroke engine.

The exhaust gases are expelled from the 2nd downstroke onwards from one side while simultaneously a fresh mixture of air and fuel is injected into the combustion chamber due to the partial vacuum created in the combustion chamber after the removal of exhaust gases.

Applications of Two-Stroke Engine

  • Two-stroke engines are preferred when mechanical simplicity, lightweight, and high power-to-weight ratio are design priorities.
  • They are lubricated by the traditional method of mixing oil into the fuel, they can be worked within any orientation as they do not have a reservoir dependent on gravity. This makes them desirable for their use in handheld tools such as chainsaws.
  • Two-stroke engines are found in small scales propulsion applications such as motorcycles, Mopeds, and dirt bikes.


Watch the video and learn how thermodynamic processes are applied to heat engines.

Frequently Asked Questions โ€“ FAQs


What is an engine?

An engine is a machine that is predominantly used for converting any form of energy into motion and mechanical force.

What is a one-stroke engine?

A one-stroke engine only needs one 1-stroke piston system or one power stroke for rotating the output shaft constantly or completing a full cycle.

What is a two-stroke engine?

A two-stroke engine is a version of a combustion engine that finishes one power cycle with two strokes of the piston through the course of only a single crankshaft revolution.

What is a crankshaft?

A crankshaft transforms the reciprocating motion into rotational motion.

What is a connecting rod?

Connecting rod transfers motion from a piston to the crankshaft and behaves like a lever arm.

What is a flywheel?

Flywheel mechanical tool that is employed to store energy.
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