Two Stroke Engines

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Two stroke internal combustion engine is known for its simplicity of design, high power output, low weight, low cost and ease of maintenance. The two stroke engine which has been the mainstay of the two wheelers and some of the four wheelers, as well as portable power tools since long, is gradually being phased out due to its higher emission levels. However, it is worth a look.


  • Credit for the invention of the nascent version of a two stroke engine goes to a Scottish engineer, Sir Dugald Clerk. His engine, developed around 1881, was primitive and used a compressor to pump in air-fuel mixture into the cylinder. These engines were further developed and sold by the Detroit Diesel Company for larger trucks and locomotives.
  • Joseph Day could well be the inventor of what is the precursor of modern two stroke engines. In 1889 he worked on a two stroke engine model that used flaps in place of valves.
  • Frederick Cock, an employee of Joseph Day, developed the modification to the piston that is the foundation of all two stroke engines.

Although it was Alfred Angas Scott, who is attributed a first practical two-stroke engine which was used in making water-cooled twin-cylinder motorcycles.



The basic principle of operation of a two stroke engine remains the same, but the variants have been developed relating to how fuel is introduced to the cylinder and how exhaust gases are sifted.

  • Piston port: This is the basic two stroke design used in most smaller engines. The piston has a skirt in which there are inlet ports and exhaust ports. The crankcase is sealed and draws in the fuel-air mixture through the carburetor, some of which goes to the top of the piston and out of the exhaust pipe
  • Reed valve: Reed valve, two-stroke engines are an improvement over previous engines as they make use of check valves in the intake side of the piston port. Fuel efficiency and power is improved in this design
  • Rotary inlet valve: The crankcase carries a slotted disk that serves as the intake valve, covering and uncovering a port in the crankcase. A variation is the use of two cylindrical parts with ports. Another type of the rotary inlet valve engine features a closely machined crank disc in the crankcase that serves as a cutout, linked to a timing mechanism. The timing in such engines is asymmetrical leading to better performance.
  • Cross-flow scavenged: in this engine the intake and exhaust ports are in opposition at 180 degrees to each other in the cylinder. The piston head is shaped to have a deflector design on top to direct fuel mixture to the top and push exhaust gases down to the exhaust. Due to deficiencies in design this type of engine did not prove popular.
  • Loop scavenged: this design makes use of specifically shaped ports to control flow of fuel to the cylinder. The advantage of this design is that it prevents part of fuel air mixture from going directly to the exhaust pipe. Most modern engines use this design and Japanese motorcycles with the loop scavenged engines becoming popular.
  • Uniflow scavenged: This engine design allows fuel and air mixture to enter at one end and exhaust at the other end with the piston acting as the controller working in conjunction with an exhaust valve.
  • Power valve: Valves are located at the exhaust port. They can close the top of the port to change timing or volume of exhaust resulting in engine with better low speed performance.
  • Direct injection: A fuel injection system is used in place of the carburetor to inject fuel into the cylinder resulting in better fuel efficiency and reduced emissions. Engines may use a high pressure injection system or low pressure type. The fuel does not pass through the crankcase in this type of engine.
  • Diesel engine: Two stroke loop scavenged diesel engines are loop scavenged types where the in-take and exhaust are controlled by piston ports. The uniflow engine sucks in air through the scavenge port and exhausts burnt gas through a poppet valve. Mainly used in marine applications such engines can run in reverse by adjusting cams.

Main Parts of a Two Stroke Engine:

A two stroke engine is simpler in construction compared to a four stroke engine and does away with the complicated timing belt and overhead camshaft driving the valves.

The main parts:

  • Carburetor assembly, throttle barrel and fuel needle valve
  • Crankshaft, crankshaft end, crankshaft inlet port, crankcase, counterbalance, crankshaft pin
  • Cylinder, cylinder head with a spark plug
  • Piston, piston pin, connecting rod

The shaft may be connected to a gear box through a clutch assembly. 

Working of Two-Stroke

Power is generated in a two stroke when the piston twice moves vertically. The two stroke cycle goes like this:

  • Fuel mixes with air in the carburetor. When the piston moves up in the cylinder it creates a vacuum in the crankcase that draws in the fuel air mixture through the crankshaft inlet port into the crankcase.
  • The downward stroke of the piston closes the port and increases pressure of fuel air mixture in the crankcase.
  • At the bottom of the downward stroke the piston travels past the intake port, allowing the compressed air-fuel mixture to enter the cylinder. At the same time the exhaust gases are expelled from the exhaust pipe along with some part of the fresh air-fuel mixture.
  • The piston moves up in the cylinder and compresses the air-fuel mixture in the domed cavity at the top.
  • When it reaches the topmost point the spark plug fires and ignites the air-fuel mixture while air-fuel mix is being drawn into the crankcase at the same time.
  • The resulting explosion of gases pushes piston down into the cylinder. This results in compression of air and fuel mix in the crankcase.
  • The downward movement transmits power through the crankshaft to the gear and to the wheels. 



Two stroke engines are small, lightweight and more powerful. They have fewer moving parts, cost less and also cost less to maintain. From small power tools to giant ships, two stroke engines can provide high power to weight ratio along with simplicity of operation.

  • Throttle response is fast, a reason why they became so popular in motorcycles.
  • There are disadvantages as well in that lubricating oil is mixed with fuel and the engine does not completely burn the fuel resulting in higher pollution and reduced fuel efficiency.
  • Two stroke engines have a shorter life span and the piston-cylinder wear and tear is high necessitating frequent replacement.

One of the biggest challenge is to improve lubrication by separating fuel from lube oil and improve upon the separation of intake and exhaust gases to deliver higher fuel efficiency.


  • Clean spark plugs regularly to prevent the oil or fuel depositing over it.
  • Clean carburetor and the “points”. Adjust or replace them to ensure ignition and fuel flow are maintained.
  • Check for exhaust smoke type and quality which to the piston ring wear or improper carburetion.

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