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Two Stroke Engine Operation

Two Stroke Engine Operation. Automotive Technology 1 Mr. Wasacz. Objectives. The student will be able to explain the operation of a 2 stroke engine The student will be able to identify the differences between a 2 and 4 stroke engine.

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Two Stroke Engine Operation

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  1. Two Stroke Engine Operation Automotive Technology 1 Mr. Wasacz

  2. Objectives • The student will be able to explain the operation of a 2 stroke engine • The student will be able to identify the differences between a 2 and 4 stroke engine. • The students will be able to compare the lubrication systems of a 2 and 4 stroke engine

  3. Quick Reminder • Stroke is the movement of the piston from one end of the cylinder to the other • TDC to BDC • BDC to TDC

  4. Two Stroke Engine • Performs the same tasks as a 4 stroke • However, intake, compression, power, and exhaust take place simultaneously • The crankshaft only rotates ONCE, sothe cycle is complete after only 2 strokes • The design of two strokes is much more simple!

  5. Variations in Design • Cross Scavenged • Has contoured piston head to deflect air upward to prevent intake from flowing out through exhaust • Uses a reed valve or rotary valve to hold the incoming charge in the crankcase • The downward movement of the piston creates pressure in the crankcase • The Piston acts as a valve for the cylinder

  6. Cross Scavenged Diagram

  7. Variations in Design • Loop- Scavenged • Does not deflect incoming gases • Flat or Domed piston • Ports are positioned and shaped so that incoming gases swirl in the cylinder • This swirl keeps the intake gases in, and forces the exhaust gases out • May have more than one transfer port to create this swirling effect

  8. Loop-Scavenged Diagram

  9. Intake into the crankcase • As the piston moves upwards in the cylinder, low pressure in the crankcase is created • This pressure allows the higher outside atmospheric to rush into the crankcase through the carburetor • When the air moves through the carburetor, it takes a charge of gasoline and oil with it. • This newly in took charge lubricates the bearings and moving parts in the crankcase

  10. Fuel Transfer • As the piston moves downwards, it compresses the air fuel and oil in the crankcase • When it travels far enough downward, it exposes the transfer port into the cylinder • The newly compressed air, fuel, and oil then rush upwards towards the cylinder • This new charge of air, fuel, and oil also cool the hot combustion area, and push the previous exhaust gasses outward

  11. Ignition-Power • The piston travels upwards, compressing the charge to approx 1/10th its original volume • Spark ignites charge when piston reaches TDC • The ignited gasses force the piston downward

  12. Exhaust • As the piston moves downwards, the exhaust port is exposed • This allows for the removal of gases • The remainder of gases are forced out when the transfer ports are exposed • This completes one cycle of operation

  13. What it looks like all together

  14. Scavenging • When 2 strokes are properly designed, all exhaust gasses will be scavenged • This allows for a new charge to enter more rapidly for cleaner combustion • The charge needs to be held within the combustion chamber while the exhaust port is open

  15. Scavenging Con’t • Well engineered exhaust systems use the energy of sound waves to control this • As the exhaust is moving out, a megaphone shaped pipe allows the sound waves to be reflected back towards the cylinder • These reflected sound waves create back pressure for the exhaust system, and keeps the incoming charge held within the cylinder • Most 2 stroke engines will not use a straight exhaust pipe for this reason

  16. Rotary Disc Valves • The Rotary disc controls the flow of a new charge into the crankcase • Intake port is located directly in the crankcase (this allows for additional transfer ports in the engine block) • The crankshaft holds the rotary valve, and provides the rotation for it to open and close • The valve only has one cutout, allowing to to spin into position to allow for the transfer of a new charge into the cylinder

  17. Reed Valves • This also permits the intake of a charge directly into the crankcase • Reed is made of a thin flexible spring steel which is connected at one end • The Reed stop is made for thick inflexible steel. • This prevents the reed from opening too far and becoming permanently bent

  18. Reed Valves Con’t • The reed is controlled by the outside atmospheric pressure • It closes due to the springiness of the steel, and the built pressure within the crankcase • The reed valve comes in multiple designs

  19. Advantages of 2 Strokes • Have less moving parts (less parts to fail) • Simpler Design • Operates at extreme angles • Lighter construction • More power strokes per rotation

  20. Disadvantages of 2 Strokes • Eratic Operation • Smells • Burns oil • Must have premixed oil • Very Hot running

  21. Comparison of Lubrication systems • Two stroke engines are lubricated by the charge • Since the charge is what lubricates, it can operate at extreme angles • Four strokes use a pressure system or an oil slinger • If the slinger is not touching the oil, lubrication will fail. This is why it does not operate at extreme angles

  22. Comparison to Four Strokes • Look in Book!

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