Two Stroke SI and CI Engines

Two Stroke SI and CI Engines

1. I.C. Engines 2 Stroke Petrol & Diesel Engines 1 Dr. S. VIJAYA BHASKAR Professor in Mechanical Engineering Sreenidhi Institute of Science & Technology, Hyderabad

2. Working Principle of IC Engines All IC Engines must have the following FOUR EVENTS/Operations: Suction Compression Expansion / Power and Exhaust

3. Two Stroke Cycle Petrol Engine A two-stroke cycle-petrol engine was devised by Duglad Clerk in 1880. In this Cycle, the suction, compression, expansion and exhaust takes place during two strokes of the piston. It means that there is one working stroke after every revolution of the crank shaft. A two stroke engine has ports instead of valves X

4. Working Principle of 2 Stroke Engine TWO stroke engine rotates only 360  or ONE revolution of crank shaft rotation Suction and Exhaust strokes are two unproductive strokes in any engine But these operations are must in order to operate the engine By combining productive and unproductive strokes in ONE revolution of the crank shaft or 2 strokes of the engine in order to obtain one power stroke

5. Operation of 2-Stroke Petrol Engine 5 INLET   EXHAUST

6. Suction Stage In this stage, the piston, while going down towards BDC, uncovers both the transfer port and the exhaust port. The fresh fuel-air mixture flows into the engine cylinder from the crank case, as shown in the figure below.

7. Compression Stage: In this stage, the piston, while moving up, first covers the transfer port and then exhausts port. After that the fuel is compressed as the piston moves upwards as shown in the figure. In this stage, the inlet port opens and fresh fuel-air mixture enters into the crank case.

8. P i s t on u n c o v e rs t ra n s fe r po rt B e g i nni ng o f th e f ir s t s tr o ke D u ri ng fi r st st r oke

9. Expansion Stage : Shortly before this piston reaches the TDC (during compression stroke), the charge is ignited with the help of a spark plug. It suddenly increases the pressure and temperature of the products of combustion. But the volume, practically, remains constant. Due to rise in the pressure, the piston is pushed downwards with a great force as shown in the figure. The hot burnt gases expand due to high speed of the piston. During this expansion, some of the heat energy produced is transformed into mechanical work.

10. Exhaust Stage In this stage, the exhaust port is opened as the piston moves downwards. The products of combustion, from the engine cylinder are exhausted through the exhaust port into the atmosphere, as shown in the Figure. This completes the cycle and the engine cylinder is ready to suck the charge again.

11. T ra n s fe r p o rt cove r e d C o mp r e s s ion c o mm e n ce d

12. 13 TWO STROKE ENGINE - CYCLE OF OPERATIONS Air charge simultaneously inducted into the crank case through spring loaded inlet valve, as the pressure in the crank case drops due to the upward motion of the piston during the compression stroke. After the compression and ignition, the expansion follows in the usual way. During the expansion stroke, the charge in the crank case is compressed. Near the end of the expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure drops to atmospheric pressure, as the combustion products leave the cylinder.

13. 14 Further movement of the piston uncovers the transfer ports, thus permitting the slightly compressed charge in the crank case to enter the engine cylinder. The top of the piston has usually a projection to deflect the fresh charge towards the top of the cylinder before flowing through the exhaust ports. This has the dual purpose of Scavenging the upper part of the cylinder of the combustion products and Preventing the fresh charge from flowing directly towards the exhaust ports. TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.)

14. 15 Same objective can be achieved without piston deflector through proper shaping of the transfer port. During the upward motion of the piston from BDC, the transfer ports close first and then the exhaust ports close when the compression of the charge begins and the cycle is repeated. TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.)

15. 2-Stroke Petrol Engine 16

16. 2-Stroke Petrol Engine 17

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19. 2-Stroke Diesel (CI) Engine 20

20. Essentially Required 2 Strokes 21

21. As per Text Book…. 22

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24. First Stroke 25

25. First Stroke in 2-Stroke CI Engine 26 Figure 2.1 shows one of the simplest two-stroke engines, viz., the crankcase scavenged engine As the piston starts moving from its BDC to TDC position, it closes the transfer port and the exhaust port . The air which is already there in the cylinder is compressed. At the same time with the upward movement of the piston, vacuum is created in the crank case . As soon as the inlet port is uncovered the fresh air is sucked in the crank case. The charging is continued until the crank case and the space in the cylinder beneath the piston in filled with the air.

26. First Stroke in 2-Stroke CI Engine 27 Figure 2.1 shows one of the simplest two-stroke engines, viz., the crankcase scavenged engine As the piston starts moving from its BDC to TDC position, it closes the transfer port and the exhaust port . The air which is already there in the cylinder is compressed. At the same time with the upward movement of the piston, vacuum is created in the crank case . As soon as the inlet port is uncovered the fresh air is sucked in the crank case. The charging is continued until the crank case and the space in the cylinder beneath the piston in filled with the air.

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28. Second Stroke in 2-Stroke CI Engine 29 Slightly before the completion of the compression stroke a very fine spray of diesel is injected into the compressed air (which is at a very high temperature). The fuel ignites spontaneously due to high pressure and temperature in combustion chamber Pressure is exerted on the crown of the piston due to the combustion of the charge and the piston is pushed in the downward direction towards BDC to produce some useful power.

29. SECOND STROKE 30

30. Second Stroke in 2-Stroke CI Engine (Cond..) 31 The downward movement of the piston will first close the inlet port and then it will compress the air already sucked in the crank case . Just at the end of power stroke, the piston uncovers the exhaust port and the transfer port simultaneously. The expanded gases start escaping through the exhaust port and at the same time the fresh air which is already compressed in the crank case, rushes into the cylinder through the transfer port and thus the cycle is repeated again

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32. S No Four Stroke Engine Two Stroke Engine 1. The thermodynamic cycle is completed in four strokes of the piston or in two revolutions of crank shaft or 720 ° of crank angle. The thermodynamic cycle is completed in two strokes of the piston or in one revolution of the crank shaft or 360 of crank angle 2. One power stroke is obtained in every two revolution of crank shaft / 4- strokes of the Piston. One power stroke is obtained in each revolution of crank shaft. 3. Because of above, turning moment is not so uniform and hence a heavier flywheel is needed. Because of above, turning moment is more uniform and hence a lighter flywheel can be used. 4. Power produced for same size of engine is less. In order to produce same power the engine is heavier and bulkier. Power produced for same size of engine is twice, or for same power the engine is lighter and more compact .

33. S.No Four Stroke Engine Two Stroke Engine 5. Lesser cooling and lubrication requirements. Lower rate of wear and tear. Greater cooling and lubrication requirements. Higher rate of wear and tear. 6. It has valves and valve actuating mechanisms for opening and closing of the intake and exhaust valves. It has no valves but only ports 7. Because of comparatively higher weight and complicated valve mechanism, the initial cost of the engine is more. Because of light weight and simplicity due to the absence of valve actuating mechanism, initial cost of the engine is less. 8. Volumetric efficiency is more due to more time for induction . Volumetric efficiency is low due to lesser time for induction. 9. Thermal efficiency is higher, part load efficiency is better. Thermal efficiency is lower, part load efficiency is poor. 10. Application : Cars, Buses, Trucks, Tractors, Aero planes and Power generation etc., Application : Mopeds, Scooters, Motorcycles, Hand sprayers etc.,