2 stroke engine

Objective: To study about 2 stroke engine.

Introduction

Heat Engines: Any type of engine or machine which derives heat energy from the combustion of fuel or any other source and converts this energy into mechanical work is termed as a heat engine.

Classification of Heat Engines

• · External Combustion Engines
• · Internal Combustion Engines

External Combustion Engines: In this case, combustion of fuel takes place outside the cylinder as in case of steam engines where the heat of combustion is employed to generate steam which is used to move a piston in a cylinder. Other examples of external combustion engines are hot air engines, steam turbine and closed cycle gas turbine. These engines are generally used for driving locomotives, ships, generation of electric power etc.

Internal Combustion Engines (I.C. Engines): In this case, combustion of the fuel with oxygen of the air occurs within the cylinder of the engine. The internal combustion engines group includes engines employing mixtures of combustible gases and air, known as gas engines, those using lighter liquid fuel or spirit known as petrol engines and those using heavier liquid fuels, known as oil compression ignition or diesel engines.

Classification of I.C. Engines:

Internal combustion engines may be classified as given below:

1. According to cycle of operation:

• Two-stroke cycle engines
• Four-stroke cycle engines

2. According to cycle of combustion:

• Otto cycle engine
• Diesel cycle engine
• Dual-combustion

3. According to the fuel employed and the method of fuel supply to the engine cylinder:

• Petrol engine
• Diesel engine
• Oil, Gas engine

4. According to method of ignition:

• · Spark ignition (S.L) engine
• · Compression ignition (C.I.) engine

5. According to method of cooling the cylinder:

• Air-cooled engine
• Water-cooled engine

6. According to number of cylinders:

• Single cylinder engine
• Multi-cylinder engine

Different Parts of I.C. Engines (Parts Common to both Petrol and Diesel Engine):

1. Cylinder

2. Cylinder head

3. Piston

4. Piston rings

5. Gudgeon pin

6. Connecting rod

7. Crankshaft

8. Crank

9. Engine bearing

10. Crank case

11. Flywheel

12. Governor

13. Valves and valve operating mechanism

Parts for Petrol Engines Only:

1. Spark plugs

2. Carburetor

3. Fuel pump

Parts for Diesel Engine Only:

1. Fuel pump

2. Injector

Fig. Air Cooled I.C Engine

TWO-STROKE CYCLE ENGINES

• Two Stroke Petrol engine
• Two Stroke Diesel engine

TWO STROKE ENGINES

In 1878, a British engineer introduced a cycle which could be completed in two strokes of piston rather than four strokes as is the case with the four-stroke cycle engines.

In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh charge of fuel entering the cylinder nearly at the end of the working stroke.

A two-stroke petrol engine (used in scooters, motor cycles etc.).

The cylinder L is connected to a closed crank chamber C.C. During the upward stroke of the piston M, the gases in L are compressed and at the same time fresh air and fuel (petrol) mixture enters the crank chamber through the valve V.

When the piston moves downwards, V closes and the mixture in the crank chamber is compressed the piston is moving upwards and is compressing an explosive change which has previously been supplied to L. Ignition takes place at the end of the stroke. The piston then travels downwards due to expansion of the gases and near the end of this stroke the piston uncovers the exhaust port (E.P.) and the burnt exhaust gases escape through this port.

The transfer port (T.P.) then is uncovered immediately, and the compressed charge from the crank chamber flows into the cylinder and is deflected upwards by the hump provided on the head of the piston.

It may be noted that the incoming air-petrol mixture helps the removal of gases from the engine-cylinder; if, in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease.

The piston then again starts moving from B.D.C. to T.D.C. and the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston; thus the cycle is repeated.