How Motor Works: Two stroke Engine

Two stroke engine is the very basic type of engine with very few parts and simple in construction.There are no complex valve train mechanics which makes it very easy to understand and simple. There is one power stroke for every revolution of crankshaft so it theoretically produce twice the power as would be produced by a four stroke Internal combustion engine.

Terminology:

Stroke:

Stroke refers to movement of piston in the engine, either from Top dead centre(TDC) to bottom dead centre(BDC) or vice versa

Top Dead centre(TDC):

The top most surface of the cylinder is called Top dead centre

Bottom Dead centre(BDC):

The bottom most surface of the cylinder is called bottom dead centre.

The working Explained:

To understand any process even the complex mathematical models, visualization is very important so please try to visualize the process and refer the video above. Its very simple to understand.

The First Stroke :

The Piston is at bottom dead centre (BDC).
The transfer port which is on the left side of the engine is open.
The exhaust port which is on the right side of the engine is open.
The Crankcase is filled with the already compressed air (from previous cycle).
The compressed air from crankcase is rushing to the cylinder of the engine as transfer port is open.
The air rushing from transfer port is scavenging the exhaust gas from previous cycle which is in the cylinder at this moment.
Now the Piston moves from BDC towards Top Dead Center (TDC) it closes the transfer port and exhaust port.
It opens the inlet port and sucks the air-fuel mixture into the crankcase
The moving piston compresses the air inside the cylinder.
When the piston reaches TDC the first stroke is complete

The second Stroke:

The piston is at TDC.
The air inside the combustion chamber i.e. the cylinder is highly compressed.
The spark plug ignites and the fire starts
There is a sudden raise in pressure and the the high pressure applies thrust on the top of piston
The piston starts moving towards BDC
It covers the inlet port and
As it moves towards BDC it compresses the air in the crankcase
When it reaches the BDC the second stroke is complete.

This whole process repeats itself again and again.

Advantages of two stroke engines:

Two-stroke engines do not have valves, simplifying their construction.
Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
Two-stroke engines are lighter, and cost less to manufacture.
Two stroke cycle engine is more compact,requires less space and lighter in weight than four stroke cycle engine for same output or power.Power developed per kilogram of engine weight is higher in two stroke cycle engine.That's why it is more suitable for motor bike and scooters.
The two stroke cycle engine gives one working stroke for each revolution of crankshaft.Hence theoretically the power developed for the same engine
speed and cylinder volume is twice is that of four stroke cycle engine which gives one working stroke for every two revolution of the crankshaft.
Lighter flywheel can be used in two stroke cycle engine as their is one working stroke for each revolution of crankshaft.
The design and construction is much simpler than four stroke engine.
There is no suctionand exhaust valve used in two stroke cycle engine hence no need of cams,camshaft and rockers etc. e.g. the complex valve train mechanism, resulting in higher mechanical efficiency.
Two stroke cycle engine gives less torsional oscillations.
Less spare parts are required.
A two stroke cycle engine can be easily reversed if it is of valveless type.
Due to simplicity in construction and fewer parts,two stroke cycle engine is easy for maintenance.Hence reduce the initial and maintenance cost also.

Despite of so many advantages the two stroke engines are being phased out, the main

Disadvantages of two stroke cycle Engine are :

Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
Two-stroke engines produce more pollution from

The combustion of the oil in the gas.
The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.