Engine Operation Four-Stroke Cycle

Objectives Describe the four-stroke cycle engine operation and explain the purpose of each stroke Explain the concept of valve timing

Engine Operation Fuel is burned inside the engine’s combustion chamber to produce heat Heat causes expansion of gases in the engine Expansion in the combustion chamber produces pressure Engine components convert pressure into rotating motion

Engine Operation

Result of Combustion Combustion Pressure

Conversion of Motion The crankshaft converts the reciprocating (Up and down) motion and force of the piston into rotational motion and force (Torque). Combustion Pressure

Piston Travel (TDC, BDC) Top dead center (TDC) piston is at its highest point in the cylinder Bottom dead center (BDC) piston is at its lowest point in the cylinder Piston stroke distance the piston slides up or down from TDC to BDC

Piston Travel (TDC, BDC)

Four-Stroke Cycle Requires four piston strokes to complete one cycle Every four strokes, the engine produces one power stroke Two complete crankshaft rotations are required to complete the four-stroke cycle (720o) One complete camshaft rotation is required as well (360o)

Intake Stroke Purpose: Draw fuel and air into the engine Piston slides down and forms a low pressure area in the cylinder Atmospheric pressure pushes the air-fuel mixture into the cylinder

Intake Stroke Intake valve open Exhaust valve closed Piston moving down Crankshaft rotates 180o Camshaft rotates 90o

Compression Stroke Purpose: Compresses (squeezes) the air-fuel mixture, making it more combustible

Compression Stroke Both valves are closed Piston moving up Crankshaft rotates to 360o Camshaft rotates to 180o

Power Stroke Purpose: Burns the air-fuel mixture and pushes the piston down with tremendous force (nearly 2 tons!) Spark plug fires, igniting the air-fuel mixture Pressure forms on the top of the piston Piston is forced down, rotating the crankshaft

Power Stroke Both valves are closed Piston moving down Crankshaft rotates to 540o Camshaft rotates to 270o

Exhaust Stroke Purpose: Removes the burned gases from the cylinder

Exhaust Stroke Exhaust valve opens Intake valve closed Piston moving up Crankshaft rotates to 720o Camshaft rotates to 360o

Four-Stroke Cycle – All Together

Strokes & Valve Timing

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Engine Operation Two-Stroke Cycle

Objectives Describe two-stroke cycle engine operation and explain the principles of two-cycle operation List the advantages and disadvantages of two-stroke and four-stroke engines

Intake into the Crankcase As the piston moves upwards, crankcase pressure drops Atmospheric pressure causes air-fuel/oil mixture to flow in While in the crankcase, the oil sticks to parts providing lubrication

Fuel Transfer As the piston moves downward, it compresses the mixture in the crankcase The pressure forces the mixture up through the transfer port

Ignition-Power The piston travels upwards compressing the mixture The spark plug fires when the piston is just before TDC

Exhaust The piston moves down to expose the exhaust port, exhaust gases rush out Intake bypass port is exposed and new air-fuel mixture rushes in pushing any remaining exhaust gases out

Four-Stroke vs. Two-Stroke 1. # of major moving parts Nine Three 2. Power Strokes One every two revolutions One every revolution 3. Running temperature Cooler Hotter 4. Overall size Larger Smaller 5. Weight Heavier Lighter 6. Bore size equal Hp Page 93 Fig. 5-18

Four-Stroke vs. Two-Stroke 7. Fuel and oil No mixture required Must be pre-mixed 8. Fuel consumption Fewer gallons per hour More gallons per hour 9. Oil consumption Oil re-circulates & stays in engine Oil is burned with fuel 10. Sound Generally quiet Louder 11. Operation Smoother More erratic 12. Acceleration Slower Very quick Page 93 Fig. 5-18

Four-Stroke vs. Two-Stroke 13. General maintenance Greater Less 14. Initial cost 15. Versatility of operation Limited slope operation Unaffected at any angle of operation 16. Efficiency - hp/wt. ratio 17. Pull starting Two crankshaft revolutions required One crankshaft revolution required 18. Flywheel Needs heavier flywheel to carry engine through non-power strokes Lighter flywheel Page 93 Fig. 5-18