What is Two Stroke Engine?

Here in this article we will discuss about two stroke engine. The topics that we will cover are definition, construction, working, advantages and disadvantages and application of 2 stroke engine.

Two Stroke Engine

The two stroke engine is a reciprocating engine in which the
piston does two time movement ( i.e. TDC to BDC and BDC to TDC ) to produce a
power stroke.

What is stroke?

When the piston moves from TDC to BDC or BDC to TDC then this
movement of piston from TDC to BDC and vice versa is called one stroke.


What is Two Stroke Engine?

Main components 

1. Spark plug: Spark plug is a device which is used to
generate spark within the cylinder. The spark produced is used to burn the fuel
in the cylinder.
2. Piston: Piston is the reciprocating part of the engine.
It is used for the suction and compression of the fuel within the cylinder.
3. Cylinder: it is that part of the two stroke engine which
holds piston within it. The inlet and exhaust ports are present in the
cylinder. Cylinder head consist of the combustion chamber and the spark plug.
4. Transfer port: It is used to transfer the compressed fuel
from crankcase to the cylinder.
5. Connecting rod: It connects the piston and crankshaft. It
transfer the power from the piston to the crankshaft.
6. Crank case: It acts as the base of the engine. It
supports the crankshaft and camshaft in suitable bearings and provides the arms
for supporting the engine on the frame.
7. Crank shaft: It is that part of the engine which is used
to convert the reciprocating motion of the engine into the rotating motion with
the help of a connecting rod.
8. Exhaust port: It is used to transfer the burnt gases out
of the engine.
9. Inlet port: The fresh charge enters into the cylinder
through inlet port.
10. Balancing weight: It is the weight used for the balancing of the engine.


  • In two stroke engine the suction and exhaust strokes are
  • There are only two remaining strokes – these are the compression
    stroke and power stroke and usually called as upward stroke and downward
  • Instead of valves, the two stroke engine consists of inlet
    and exhaust ports.
  • The fresh charge enters into the cylinder through inlet port
    and burnt gases escapes out through exhaust port.
  • The burnt exhaust gases are forced out through the exhaust
    port by fresh charge which enters the cylinder nearly at the end of the working
    stroke through the inlet port.

The two strokes of a two stroke engines are described as

1. Upward stroke:

  • During upward stroke, the piston moves from BDC to TDC and
    compresses the charge (air-fuel mixture) in the combustion chamber of the
  • Because of the upward movement of the piston a partial
    vacuum is created in the crankcase and this allows the entry of the fresh
    charge into the crankcase through uncovered inlet port.
  • The exhaust port and the inlet port remains covered when the
    piston at the TDC.
  • The ignition of the fresh charge is takes place by the spark

Downward stroke:

  • As soon as the combustion of the fresh charge takes place, a
    large amount of the hot gases is produced which exerts a very high pressure
    force on the top of the piston. Due to this high pressure force, the piston
    moves downward and rotates the crankshaft and does useful work.
  • During this stroke the inlet port is covered by the piston
    and the new charge is compressed in the crankcase.
  • Further downward movement of the piston uncovers first the
    exhaust port and the transfer port and the exhaust starts through the exhaust
  • As soon as the transfer port opens, the charge through it is
    forced into the cylinder.
  • The charge strikes the deflector on the piston crown, rises
    to the top of the cylinder and pushes out most of the exhaust gases.
  • The piston is now at BDC position. The cylinder is
    completely filled with the fresh charge but it is somewhat diluted with the
    exhaust gases.
  • Finally the cycle event is then repeated. We get two strokes
    for the single revolution of the crankshaft.

Port Timing diagram for a two stroke cycle engine

What is Two Stroke Engine?

Advantages and disadvantages of two stroke engine over four
stroke engine


  • Two stroke engine gives a working stroke for each revolution
    of the crankshaft. The four stroke engine gives a working stroke for each two
    revolution of the crankshaft.
  • Power developed by the two stroke engine is twice that
    developed by the four stroke engine for the same engine speed and volume.
  • Lighter flywheel is required in two stroke engine because of
    the more turning moment on the crankshaft.
  • For the same power, a two stroke engine is more compact,
    light and requires less space than a four stroke engine, therefore is used in auto
    cycles, motorcycles and scooters.
  • It is simpler in construction and mechanism.
  • There is no valve and valve mechanism in it. The ports can
    be easily designed and covered and uncovered by the movement of the piston
  • It has high mechanical efficiency.
  • It requires fewer spare parts due to its simple design.


  • It has high fuel consumption.
  • Thermal efficiency is less than four stroke engine.
  • The charge is diluted by the burnt gases due to incomplete
  • It produces greater noise.
  • It does more consumption of the lubricating oil.
  • There is a greater wear and tear of moving parts.


  • It is used in light weight vehicles like scooters, motorcycles, mopeds etc which uses gasoline as fuel.
  • It is also used in many diesel engines, mainly industrial and marine engines, as well as in some heavy trucks and machinery

Let’s check your grasp on the topic

1. Fill in the blanks with appropriate words.
(i). The movement of piston from TDC to BDC or vice versa is known as …………………
(ii). The part of engine that produces spark is called………………………
2. Name all the main components of a 2 stroke engine.
3. Which two strokes are eliminated in these types of engines.
4. Write any two advantages and disadvantages of two stroke engine.
5. Write the function of inlet and exhaust port.

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