Two and Four Cycle Engines

1. Two and Four Cycle Engines TRF 210

2. 2 Cycle Vs 4 Cycle • Cycle a series of events that repeat themselves • 2 cycle 2 piston strokes to complete one cycle • TDC-BDC= power stroke • BDC-TDC= compression stroke • 4 cycle 4 piston strokes to produce one power stroke

3. Advantages of a 2 cycle Engine • Simpler in construction (only 3 major moving parts piston assembly, connecting rod, and crank shaft) • Easier to maintain • Lighter in weight • Less expensive to manufacture

4. Operation of 2 Cycle Engines 3 events must take place: • Burned gasses from previous stroke must be cleared from cylinder • Fresh charge of air and gas must be brought in • Air and fuel must be compressed for ignition This is all accomplished by the opening and closing of ports in the side of the cylinder by the piston

5. Cylinder Ports • Intake: fresh air/fuel mixture flows from carburetor into crank case • Transfer: air/ fuel mixture enters cylinder • Exhaust: burned gasses leave cylinder

6. Port Actions • Power • Exhaust • Intake • Compression One power stroke every time the piston reaches TDC

8. Crank Case Compression • Changes in crank case pressure is what allow the air/fuel mixture to enter and exit the crank case • Piston as a valve • Reed valve • Rotary valve

9. Cylinder Scavenging • 2 cycle engines produce less power than 4 cycle engines because less fuel burned in the cylinder • In a 2 cycle engine the fresh air/fuel mixture is used to clean out the exhaust gases • This causes up to 25% of this fresh air/fuel mixture to be lost • Cross-scavenging: transfer and exhaust ports are opposite each other (requires a deflector) • Loop-scavenging: transfer and exhaust are 90 degrees apart

10. 4 Cycle Spark Engines • 4 cycle engines differ from two cycle engines in the way the air fuel mixture and exhaust enter and leave the cylinder • There are also more major moving parts in 4 cycle engines

11. The Four Stroke Cycle • One out of every four strokes is a power stroke • This engines requires two valves to open and to close to let the air/fuel mixture and exhaust in an out of the engine

12. The Four Stroke Cycle • Intake • Compression • Power • Exhaust Valves are opened and closed at proper times by a valve train which includes a camshaft which is driven by the engine crankshaft

14. Intake and Exhaust Valves • Intake: air/fuel mixture into cylinder • Exhaust: burned gases exit cylinder • Valves can be located beside piston L-Block) or over the top over the piston (overhead valve)

15. Operating the Valves • Valves must be opened and closed at the correct times • Each valve is kept closed by a valve spring • Below the valve stem is a lifter which moves up and down in a hole in the cylinder block

16. Operating the Valves Cont. • The lifter rest on a cam which has a high spot or lobe • The cam is located on the cam shaft there is one cam for each valve • The camshaft is driven by a gear or chain on sprockets from the crank-shaft • Cam shaft rotates with crank shaft to open and close valves

17. Comparing 2 Cycle and 4 Cycle Engines • Both use piston, cylinder, connecting rod crank, crank shaft, flywheel, and spark plug. The movement is also similar • 2 cycle valve ports in cylinder wall 2 strokes for every one power stroke • 4 cycle valves open and closed by a camshaft 4 strokes for every one power stroke

18. 4 cycle Oil sump and oil plug Oil drain and refills required Muffler installed close to exhaust valve 2 cycle Not present Oil added in fuel Muffler in middle of cylinder Identifying 2 cycle and 4 cycle engines