C H A P T E R 5. Principles of Engine Operation, Two- and Four-Stroke Engines

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2 C H A P T E R 5 Principles of Engine Operation, Two- and Four-Stroke Engines

3 Learning Objectives Explain simple engine operation. Explain why gasoline is atomized in the small engine. Describe four-stroke engine operation and explain the purpose of each stroke. Explain the concept of valve timing. Compare the lubrication system in a four-cycle engine to the system in a two-stroke engine.

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

5 Principles of Engine Operation Converts chemical energy into mechanical energy Gasoline engine is an internal combustion engine Gasoline must ignite easily and burn quickly Energy produced by burning gasoline must be controlled

6 Gasoline Must Be Atomized The more surface area exposed to air, the more vapor will be given off More vapor leads to faster burning Gasoline must be atomized Atomization Increased burning area Explosive release of heat energy

7 Elementary Engine

8 Two- and Four-Stroke Engines Engines identified by number of piston strokes required to complete one operating cycle Each stroke is either toward the rotating crankshaft or away from it Bottom dead center Top dead center Strokes identified by job they perform

9 Four-Stroke Engine Four strokes needed to complete operating cycle Intake stroke Compression stroke Power stroke Exhaust stroke Two strokes occur during each crankshaft revolution Two crankshaft revolutions complete one operating cycle

10 Four-Stroke Engine

11 Intake Stroke Piston travels downward Volume of space above piston increases Creates partial vacuum Intake valve open and exhaust valve closed

12 Intake Stroke Atmospheric pressure forces air through carburetor, through intake valve port, and into cylinder Intake valve must open and close at the correct time Incoming air-fuel mixture cools valve during engine operation

13 Compression Stroke Piston moves upward Both valves closed Mixture is compressed Force of combustion is increased

14 Power Stroke Both valves closed Air-fuel mixture ignited Burning action forces piston downward

15 Exhaust Stroke Intake valve closed and exhaust valve open Rising piston pushes exhaust gases from engine

16 Exhaust Stroke Exhaust valve Allow a streamlined flow of exhaust gases Heat must be controlled

17 Four-Stroke Engines Valve Timing Measured in degrees of crankshaft rotation Varies with different engines Valve overlap Lubrication Provided by oil in the crankcase Splash and pump systems Oil must be drained and replaced periodically

18 Two-Stroke Engine Two strokes occur during each revolution of crankshaft Advantages over four-stroke engine Simpler in design Smaller Lighter Adequate lubrication at extreme angles

19 Two-Stroke Engine Cross-scavenged Contoured piston head prevents air-fuel charge from going out exhaust port Use reed valves or rotary valve Loop-scavenged Flat or slightly domed piston head Transfer ports cause incoming air to swirl

20 Two-Stroke Engine (Kohler Co.)

21 Two-Stroke Engine Operation (Rupp Industries, Inc.)

22 Intake into Crankcase Piston moving upward Crankcase pressure drops Intake port exposed Intake air through carburetor pulls fuel and oil into crankcase

23 Ignition-Power Piston moving upward Compresses air-fuel charge Spark ignites air-fuel mixture Piston driven downward Creates crankshaft motion

24 Exhaust Piston moving downward Exhaust port exposed Exhaust gases expelled Complete exhausting occurs when transfer ports are opened New charge rushes in

25 Fuel Transfer Piston moving downward Air-fuel charge in crankcase compressed Transfer port opened Compressed charge rushes through port

26 Four-Stoke Engine vs. Two-Stroke Engine

27 Review Why is a gasoline engine considered an internal combustion engine? Because gasoline is combined with air and burned inside the engine.

28 Review What is atomization and why is gasoline atomized? Atomization involves breaking gasoline up into tiny droplets and mixing it with air. Gasoline is atomized to produce the rapid burning required in an engine.

29 Review What are the four-strokes in a four-stroke cycle? Intake stroke Power stroke Compression stroke Exhaust stroke

30 Review What is valve overlap? Valve overlap is a condition in which both valves are open at the same time.

31 Review Why do two-stroke engines receive adequate lubrication even when operated at extreme angles? Because it receives its lubrication as fuel mixed with oil is passed through the engine.

32 Review How many crankshaft revolutions are required to complete a two-stroke cycle? One

33 Review What advantages does a two-stroke engine have over a four-stroke engine? A two-stroke engine: is simpler in design than a four-stroke engine is smaller and lighter than a four-stroke engine of equivalent horsepower will receive adequate lubrication even when operated at extreme angles