EXPERIMENT NO: DATE:

STUDY OF TWO STROKE I.C.ENGINES

OBJECTIVES: After studying this practical, students are able to know

Principles, construction & working of 2-S Petrol & Diesel engine.

Introduction:In this engine, the working cycle is completed in two strokes of the piston or one revolution of the crankshaft.In case of two strokes engines, the valves are replaced by ports. Two rows of ports at different levels are cut in the cylinder walls as shown in fig. These are known as exhaust ports and transfer ports. In the case of single cylinder engines, a third row of ports is provided below the first two and these are known as inlet ports.A specific shape is given to the piston crown as shown in fig which helps to prevent loss of incoming fresh charge entering into the engine cylinder through the transfer port and helps in exhausting only burnt gases.The charging of cylinder with air fuel mixture in case of petrol engine or with air in case of diesel engine, compression of the mixture or air, expansion of gases and exhausting of the burnt gases from the cylinder are carried out in two strokes. This can be done using following two methods.By using closed crank case compression. In this method crank case works as an air pump as the piston moves up and down. The charge or air to be admitted in the cylinder is compressed in crankcase by the pumping action of underside of piston. This method is known as three channel system & used for single cylinder small power engines like scooters & motorcycles.A separate pump outside the cylinder is provided to compress the charge or air before forcing it into the cylinder. This pump is an integral part of an engine & driven by engine it self. This method of charging is used for large capacity multi-cylinder engine.

Working of 2- stroke petrol engine: It will be easier to describe the cycle beginning at the point when the piston has reached to TDC completing the compression stroke. The position of the piston at the end of compression as shown in fig. The spark is produced by spark plug as the piston reaches the TDC (Top Dead Centre). The pressure and temperature of the gases increases and the gases push the piston downwards producing power stroke, when the piston downwards producing power stroke. When the piston uncovers (opens) the exhaust port as shown in fig, during downward stroke, the expanded burnt gases leave the cylinder through the exhaust port. A little later, the piston uncovers (opens) the transfer port also as shown in fig. In this condition the crankcase is directly connected to cylinder through port. During the downward stroke of piston, the charge in crankcase is compressed by the underside of the piston to a pressure of 1-4 bar.

At this position, as shown in fig, the compressed charge (fuel & air) is transferred through the transfer port to the upper port of the cylinder. The exhaust gases are swept out with the help of fresh charge (scavenging). The piston crown shape helps in this sweeping action as well as it prevents the loss of fresh charge carried with the exhaust gases. This is continued until the piston reaches BDC position. During this stroke of piston (downward stroke) the following processes are completed.1) Power is developed by the downward movement of piston.2) The exhaust gases are removed completely from the cylinder by scavenging.3) The charge is compressed in the crankcase with the help of underside of the piston.

As the piston moves upward, it covers transfer ports stopping flow of fresh charge into the cylinder. A little later, the piston covers exhaust ports and actual compression of charge begins. This position of piston is shown in fig. The upward motion of the piston

during this stroke lowers the pressure in the crankcase below atmosphere therefore a fresh charge is admitted/induced in the crankcase through the inlet port as they are uncovered by the piston. The compression of charge is continued until the piston reaches its original position (TDC) and the cycle is completed.In this stroke of the piston, the following processes are completed.1) Partly scavenging takes place as the piston moves from BDC to position shown in fig.2) The fresh charge is sucked in the crankcase through the Carburetor.3) Compression of charge is completed as the piston moves from the position to TDC.The cycle of engine is completed within two strokes of the piston.

Working of 2- stroke diesel engine: As the piston moves down on the power stroke, it first uncovers the exhaust port, and the cylinder pressure drops to atmospheric pressure as the products of combustion come out from the cylinder. Further downward movement of the piston uncovers the transfer port (TP) and slightly compressed air enters the engine cylinder from the crankcase. Due to deflector on the top of the piston, the air will move up to the top of the cylinder and expels out the remaining exhaust gases through the exhaust port (EP). During the upward movement of the piston, first the transfer port and then the exhaust port closes. As soon as the exhaust port closes the compression of the air starts. As the piston moves up, the pressure in the crankcase decreases so that the fresh air is drawn into the crankcase through the open inlet port as shown in fig. Just before the end of compression stroke the fuel is forced under pressure in the form of fine spray into the engine cylinder through the nozzle into this hot air. At this moment the temperature of the compressed air is high enough to ignite the fuel. It suddenly increases the pressure and temperature of the products of combustion. The rate of fuel injection is such as to maintain the gas pressure constant during the combustion period. Due to increased pressure the piston is pushed down with a great force. Then the hot products of combustion expand. During expansion some of the heat energy produced is transformed into mechanical work. When the piston is near the bottom of the stroke it uncovers

exhaust port, which permits the gases to flow out of the cylinder. This completes the cycle and the engine cylinder is ready to suck the air once again.

Two stroke cycle diesel engine.

Assignment:Answer the following questions:1. What is crankcase compression? Explain?2. Write function of deflector?3. When piston moves from TDC to BDC which strokes are performed? 4. When piston moves from BDC to TDC which strokes are performed?5. Compare 4-S cycle engine & 2-S cycle engine?6. Why 4-S cycle engine is having more efficiency than 2- stroke cycle engine?7. Why 2-S cycle engine develops approximately double power than 4-S cycle engine?8. Why 4-S cycle engine requires more maintenance?

Assessment:

Participation(03)

Quiz/Viva(03)

Discipline(02)

Total(08)

Date: Signature of faculty