Two Stroke Engine Updated


Two stroke internal combustion (IC) engines have found widespread applications throughout the world whenever a simple, robust engine is required with a very high power to weight ratio. This type of engine is typically petrol/gasoline fired (spark ignition engine) and is used for small applications e.g. lawn mowers, motorbikes, leaf blowers etc. Two stroke engines require only two strokes per combustion cycle whereas four stroke engines require four strokes per combustion cycle.

Small Two Stroke Engine

Two Stroke Engine Components

A two stroke engine consists of the following parts:

  • Air Fuel Mixture Suction Port – the air fuel mixture is drawn into the crankcase through the suction port. A reed valve installed within the suction port acts as a non-return valve (one-way valve) in order to control the flow of the air fuel mixture.
  • Transfer Port – the compressed air fuel mixture is transferred from the crankcase to the combustion space through the transfer port.
  • Exhaust Gas Port – exhaust gas is discharged from the combustion space through the exhaust gas port.

Two Stroke Engine Components

  • Crankcase – houses the internal parts of the engine. The air fuel mixture is compressed within the crankcase prior to entering the transfer port.
  • Piston – travels between top dead centre (TDC) and bottom dead centre (BDC) linearly (in a straight line). A thin film of oil between the piston rings and cylinder liner separates the combustion space from the crankcase.

TDC and BDC Shown

  • Cylinder Liner – where combustion occurs. The cylinder liner is also known as the combustion chamber.

Four Stroke Engine Cylinder Liner

  • Spark Plug – used to ignite the air fuel mixture. Gasoline/petrol engines employ spark plugs and are known as spark ignition enginesDiesel engines do not employ spark plugs and are known as compression ignition engines.

Spark Plug

  • Crankshaft – a shaft used to convert the linear reciprocating motion of the piston to rotary motion.

Crankshaft with Labels

  • Crank Web – used to store energy and reduce engine vibration.
  • Connecting Rod – connects the piston to the crankshaft. To be specific, there is no connection between the connecting rod and crankshaft because plain metal bearings and lubrication oil separate the two components. The connecting rod is also known as the con rod.

Crankshaft and Connecting Rod Lubrication

  • Gudgeon Pin – connects the connecting rod to the piston. The gudgeon pin is also known as the piston pin.

How Internal Combustion (IC) Engines Work

Four stroke and two stroke engines are both types of internal combustion (IC) engine. All IC engines must complete four main stages to complete one full combustion cycle. These stages are:

  1. Suction 
  2. Compression
  3. Ignition (Power)
  4. Exhaust

The above stages are also sometimes referred to as:

  1. Suck
  2. Squeeze
  3. Bang
  4. Blow

Four stroke engines require one full stroke (full movement between TDC to BDC, or BDC to TDC) per stage. Two stroke engines complete several stages per stroke.

How Two Stroke Engines Work

The below video is an extract from our Internal Combustion Engine Basics Online Video Course.


Suction Stage

As the piston nears bottom dead centre (BDC), it compresses the air fuel mixture within the crankcase and the transfer port is uncovered. As soon as the transfer port is uncovered, the compressed air fuel mixture flows from the crankcase into the combustion space.

The piston then begins travelling towards top dead centre (TDC) and covers the transfer port whist also uncovering the crankcase inlet port; the air fuel mixture then begins flowing from the uncovered inlet port into the crankcase.

Two Stroke Engine Suction Stage

Compression Stage

The piston continues travelling towards TDC and covers the exhaust port. The air fuel mixture within the combustion space is compressed by the piston as it moves towards TDC. Both the temperature and pressure within the combustion space increase significantly during this stage.

Two Stroke Engine Compression Stage

Ignition Stage

Shortly before TDC, a spark from a spark plug ignites the air fuel mixture. Ignition occurs and a rapid increase in pressure and temperature forces the piston back towards BDC.

Two Stroke Engine Ignition Stage

Exhaust Stage

As the piston travels towards BDC, two things occur. First, the exhaust port is uncovered and exhaust gas is discharged from the combustion space. Second, the movement of the piston towards BDC compresses the air fuel mixture within the crankcase.

The piston nears BDC and the transfer port is uncovered; the compressed air fuel mixture enters the combustion space and the cycle begins again.

Two Stroke Engine Exhaust Stage

The below video shows the two stroke combustion cycle in detail:


Two Stroke Engine Typical Applications

  • Motorbikes
  • Lawnmowers
  • Outboard Boat Engines
  • Leaf Blowers

Although two stroke engines are primarily used for small engine applications, it is worth noting that two stroke engines are also the largest engines in the world. These large two stroke engines are used on merchant navy vessels and can weigh several thousand tonnes.

Large Two Stroke Engine (piston hanging from crane)

Two Stroke Engine Advantages

  • The two stroke engine has considerably less parts (25-50% less parts) than a four stroke engine.
  • The reduction in parts gives the engine a much simpler design than a four stroke engine.
  • The weight of a two stroke engine is far less than a four stroke engine.
  • Due to the reduction in weight, the two stroke engine has a higher power to weight ratio than a four stroke engine.

Two Stroke Engine Disadvantages

  • The simpler design of the engine also leads to a reduction in efficiency compared to a four stroke engine.
  • Two strokes are generally noisier/louder than four stroke engines.

Why are two stroke engines lighter than four stroke engines?

The crankcase is full of gasoline, air and oil, so there is no need for additional lubricating oil pumps, piping or filters. There is also no need for cooling water pumps because there are no coolant passages in the cylinder head (no cooling water system). The two stroke engine design also does not require push rods or exhaust valves etc., all of this leads to a large weight reduction compared to a four stroke engine.

Why are two stroke engines less efficient than four stroke engines?

Four stroke engines have more engine parts and can better control when inlet valves and exhaust valves open and close. Controlling valve timing allows the maximum amount of energy to be extracted from the power stage prior to the exhaust stage occurring; this gives an overall increase in engine efficiency.

Fuel injection timing can be more tightly controlled with a four stroke engine compared to with a two stroke engine. The amount and duration of injection can be controlled using a cam or common rail system, which again leads to an increase in engine efficiency.

3D Model Components 

This 3D model shows all major components associated with a typical small two stroke engine, these include:

  • Piston
  • Spark Plug
  • Cylinder Liner
  • Crankcase
  • Crankweb
  • Piston Rod
  • Gudgeon Pin
  • Heat Exchanger Fins
  • Transfer Port
  • Air and Fuel Suction Port
  • Exhaust Gas Port
Additional Resources

Encyclopedia - saVRee