EXPERIMENT NO: DATE:

STUDY OF FOUR STROKE I.C.ENGINES

Objectives: At the end of this experiment students are able to know

Principle, construction & working of 4-S Petrol & Diesel engine.

Introduction:Any I.C.Engine works on particular cycle and in any I.C.Engines a cycle comprises of four basic operations viz. suction, compression, expansion and exhaust. If all these four basic operations are performed / completed in four strokes of piston or two revolutions of crankshaft, the engine is called four stroke engines. It may be petrol engine or diesel engine, depending upon type of fuel used. In this there will be one power stroke in two revolution of crankshaft / four strokes of piston. Valves are used to induct the charges into the cylinder and to push out the exhaust

gases from the cylinder. Valves are actuated by cam & camshaft mechanism. To open valve, the moment is

transferred in this sequence: crankshaft-transmission gear-cam shaft-cam-follower-push rod-rocker arm.

Overall efficiency of 4-Stroke engines is more than 2-Stroke engines. The speed of camshaft is half than crankshaft speed. (e.g. if engine is running at 1000

RPM, camshaft speed will be 500 RPM.)Now we will study the basic four operations executed in cylinder to complete the cycle.

Working of 4-Stroke Petrol Engine:The working cycle of the engine is completed in four strokes or in two revolutions of crank and petrol is used as fuel.

Suction stroke: The piston is at the top most position (TDC) and is ready to move down wards BDC. The inlet valve is open and exhaust valve is closed. As the piston moves downwards, a fresh charge of fuel and air mixture enters the cylinder through inlet valve due to vacuum created. This continues until piston reaches BDC. At this position inlet valve closes. This downward movement of piston is known as suction stroke & crank rotates by 180 during this period.

Compression stroke: During this stroke, both valves (inlet and exhaust) are closed and piston moves upward and compresses the charge enclosed in the cylinder. The pressure and temperature of mixture increases and volume decreases continuously, during this process. As the piston reaches, the top dead centre (TDC position), the mixture is ignited by an electric spark. The burning of mixture is more or less instantaneous and pressure & temperature of gases increases while the volume remains constant.

Power stroke or Expansion stroke: During the expansion stroke, both the valves remain closed. The high pressure and temperature gases push the piston downwards and the gas pressure gradually decreases. During this stroke, piston moves from TDC to BDC. This stroke is known as power stroke, as work is done during this stroke. The exhaust valve opens as the piston reaches BDC position and pressure falls suddenly to atmospheric pressure at constant volume.

Exhaust stroke: During upward motion of the piston the exhaust valve is open and inlet valve is closed. The piston moves up in cylinder pushing out the burnt gases through the exhaust valve. As the piston reaches the TDC, again inlet valve opens and fresh charge is taken in during next downward movement of the piston and cycle is repeated.

Working of 4-Stroke Diesel Engine: All working operations such as suction, compression, expansion and exhaust are completed in 4-strokes of piston or two revolutions of crankshaft and diesel is used as fuel. Therefore it is known as 4-stroke diesel engine. The liquid fuel like diesel, which cannot be vaporized, is injected into the engine cylinder in the form of fine spray with the help of fuel pump and injector. The working of the 4-stroke diesel engine is described as given below.

Suction Stroke:The inlet valve remains open during this operation. The air is taken in through the inlet valve as the piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC).

Compression Stroke:During this stroke, the inlet and exhaust valves remain closed. The piston moves from BDC to TDC and compresses the air, taken in during suction stroke, with an increase in pressure and temperature and decrease in volume.

Expansion Stroke:Just before completing the compression stroke, the fuel injector opens and injects the diesel fuel in the form of fine spray inside the cylinder. The injected fuel starts burning due to the high temperature inside the cylinder, as compression ratio of diesel engine is high. The supply of fuel is cut-off after some part of the expansion stroke, and the hot gases expand pushing the piston towards BDC. During this stroke, the piston moves from TDC to BDC. At the end of the stroke exhaust valve opens and the pressure inside the cylinder falls to atmospheric pressure.

Exhaust Stroke:The hot gases in the cylinder are driven out through exhaust valve from the cylinder as the piston move from BDC to TDC. The exhaust valve is closed at the end of the stroke. Again the inlet valve opens and the same operations are repeated.

Assignment:

1. Write position of valve during each stroke of 4-S cycle engine?

2. If crankshaft rotates at 1500 rpm then at what speed cam shaft rotates? Why?

3. How working of 4-stroke petrol engine and diesel engine differs?

4. Why suction, compression and exhaust strokes are called idle strokes?

Assessment:

Participation(03)

Quiz/Viva(03)

Discipline(02)

Total(08)

Date: Signature of faculty