How does two stroke engine works ?

After understanding about the four stroke engine from my previous post.This time we will learn about the second type of engine i.e 2 stroke engine. The 2 stroke engine is a type of Internal combustion engine where one power cycle is completed with two strokes of the piston during only one rotation of crankshaft.

The 2 stroke engine performs all the same steps which are performed by 4 stroke engine i.e intake stroke, compression stroke, power stroke & exhaust stroke; but the 2 stroke engine performs all the 4 process(stroke) in only 2 stroke.

The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who patented his design in 1881

Application : Two-stroke engines are generally less expensive to build compared to four-stroke engines, and they are lighter and can produce a higher power-to-weight ratio. For these reasons, two-stroke engines are ideal in applications such as chainsaws, weed trimmers, outboard motors, off-road and motorcycles.Two-stroke engines are also easier to start in cold temperatures, making them ideal for use in snowmobiles

Components of two stroke engine

Main Components of Two stroke engine : The above diagram shows components of two stroke engine 

Spark plug : A spark plug delivers electric current to the combustion chamber which ignites fuel.The spark produced is used to burn the fuel in the cylinder.

Piston : Piston is the reciprocating part of the engine. It is used for transferring the energy of the expansion of gases to crankshaft using connecting rods

Connecting rod : It connects the piston and crankshaft. It transfer the power from the piston to the crankshaft.

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.

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.

Inlet port & Exhaust port : The fresh charge enters into the cylinder through inlet port and transfer the burnt gases out of the engine through exhaust port.

Counterweight : Counterweight on crankshaft are used to reduce the vibrations due to imbalances in the rotating assembly 

Counterweight are used to prevent bending of crankshaft by offsetting the weights of rod and pistons. For a crankshaft to be balanced, at any point in the assembly’s rotation, the sum of all of the forces are roughly equal to zero. If the crankshaft is not balanced, it will deform especially at pressure points and eventually fail.

In two stroke engines, there is no suction and exhaust strokes. There are only two remaining strokes the compression stroke and power stroke. These are usually called the upward stroke and downward stroke. Also, instead of valves, there are inlet and exhaust ports in two stroke cycle engines.

Working principle of two stroke engine is explained in as  shown in diagram below :

1. Upward Stroke

2. Downward Stroke

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 cylinder.

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 plug.

Two Stroke working principle

2. Downward stroke:

As soon as the combustion of the fresh charge takes place, a large amount of the pressure created due to combustion of fuel 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

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 port.

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.

The below mentioned upward and downward stroke in two stroke engine :

Animation depicting upward and downward stroke