Engine Operation Four-Stroke Cycle Objectives Describe the four-stroke

Engine Operation Four-Stroke Cycle Objectives Describe the four-stroke

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 engines

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 Combustion Pressure The crankshaft converts the

reciprocating (Up and down) motion and force of the piston into rotational motion and force (Torque). 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 airfuel 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 Check This Out! Can you see it? 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 airfuel mixture rushes in pushing any remaining exhaust gases out Four-Stroke vs. Two-Stroke

1. # of major moving parts Nine Three 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

Larger Smaller 2. Power Strokes Page 93 Fig. 5-18 Four-Stroke vs. Two-Stroke

7. Fuel and oil 8. Fuel consumption 9. Oil consumption 10. Sound 11. Operation 12. Acceleration Page 93 Fig. 5-18 No mixture required

Must be pre-mixed Fewer gallons per hour More gallons per hour Oil re-circulates & stays in engine

Oil is burned with fuel Generally quiet Louder Smoother More erratic

Slower Very quick Four-Stroke vs. Two-Stroke 13. General maintenance Greater Less

14. Initial cost Greater Less 15. Versatility of operation Limited slope

operation Unaffected at any angle of operation 16. Efficiency - hp/wt. ratio Less Greater

17. Pull starting Two crankshaft revolutions required One crankshaft revolution required 18. Flywheel

Needs heavier flywheel to carry engine through nonpower strokes Lighter flywheel Page 93 Fig. 5-18

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