Engine Operation

1. Engine Operation Engine Components • Four-Stroke Cycle Engines • Engine Output

2. Competencies • List the components of an engine block. • Describe a cylinder head. • Explain the operation and components of a crankshaft. • Describe pistons and piston rings. • Explain the function of connecting rods and bearings.

3. Competencies • Describe the flywheel and valve train. • Compare the operation of four-stroke cycle and two-stroke cycle engines. • Describe valving systems. • Explain important features of diesel engines. • Explain measurement of engine output.

4. How it works • The internal combustion engine converts potential chemical energy into mechanical energy • Is done by heat derived from a fuel source • Approximately 30% of energy released in engine is converted into work • Remaining energy lost in form of heat & friction in engine

5. It’s Got To Hold Up • Components are designed to convert energy for maximum efficiency • They must withstand heat and stress generated inside engine during operation • Must meet size and weight requirements

6. Engine Blocks • cylinder block • cylinder bore • cooling fins • valve train components • crankcase

7. Engine Block • Main structure of engine • Supports & maintains alignments of internal & external components • Block can be produced as one-piece or two-piece unit

8. Cylinder Bore • Hole in block that aligns and directs piston during movement

9. Stroke • Stroke is the linear distance the piston travels in bore from top dead center (TDC) to bottom dead center (BDC) • TDC point in which piston is closest to cylinder head before piston moves back down cylinder • BDC point which piston is farthest from cylinder head before piston moves back up cylinder

10. Displacement • Displacement (swept volume) that a piston displaces in an engine when it travels from TDC to BDC during same piston stroke

11. Calculating Displacement • When bore and stroke are known then displacement can be found • D=.7854 * B2 * S • D = displacement (in cubic inches) • .7854 = constant • B2 = bore squared in inches • S = Stroke in inches

12. Displacement Example • An engine has a 2.5” bore & a 2” stroke. Figure out displacement: • D=.7854 * (2.5*2.5) * 2 • D = .7854 * 6.25 * 2 • D = 9.8175 • D = 9.82 cubic inches

13. One More Time!! • What is the displacement of a single-cylinder engine that has a 3.25” bore and a 3.375” stroke? • D = .7854 * (3.25*3.25) * 3.375 • D = .7854 * 10.5625 * 3.375 • D = 27.9982 • D = 28 cubic inches

14. Do That Times 2 • For multiple-cylinder engines: • Multiply the displacement of the single-cylinder by the total number of cylinders • Generally, the larger the displacement, the more power it can produce

15. Air vs. Liquid Cooled • Air cooled engines • Have cooling fins on exterior of block • Thin cast strips designed to provide efficient air circulation & head dissipation away from block into air stream • Increases surface area of block contacting ambient (existing) air for cooling efficiency • Fins cast into or bolted onto flywheel act as fan blades & give air circulation around block & head

16. Air vs. Liquid Cooled

17. Air vs. Liquid Cooled • Liquid cooled engines • Have channels (sleeves) drilled or cast into block to allow coolant to pass through engine and cool it • Similar to engine in automobile

18. Cylinder Heads • engine component fastened to the end of the cylinder block farthest from the crankshaft • head gasket is filler material

19. Crankshafts • convert linear motion of pistons to rotary motion • crankpin journal • bearing journal • counterweight • crankgear

20. Crankcase • Houses and supports crankshaft • In 4-stroke acts as oil reservoir for lubrication • May have multiple parts like a sump or crankcase cover • Sump is removable & acts like oil reservoir & provides access to internal parts • Has bearing surface for vertical shaft engines & forms lower section of engines • Horizontal shaft engines have no sump as block serves as oil reservoir • Crankcase cover provides access to internal parts in crankcase & supports crankcase

21. Crankcase Breather • Relieves crankcase pressure created by reciprocating motion of the piston during operation • When piston moves towards TDC volume in crank increases resulting in lower existing pressure in crankcase • When piston toward BDC volume in crankcase decreases generating higher than ambient pressure in crankcase

22. Crankcase Breather • Acts as a check valve allowing more air to escape than can enter crankcase • Maintains pressure less than atmospheric pressure (14.7 PSI at sea-level) • Gases, partially spent combustion gases & other engine gases are then routed to carburetor • Also serves as a oil mist collector • Prevents crankcase oil from escaping whenever breather opens • Required on most 4-stroke engines

23. Crankcase/Cylinder Block

24. Cylinder Blocks • Cast from materials strong enough to withstand the heat & stress inside the engine during operation • Must meet size & weight requirements of specific application

25. Cylinder Blocks • Common construction • Cast aluminum alloy • Lightweight & dissipates heat more rapidly than cast iron • Cast iron • Heavier & more expensive • More resistant to wear & less prone to heat distortion • Cast aluminum alloy with cast iron cylinder sleeves • Combine both for maximum strength

26. Cylinder Heads

27. Cylinder Head • Cast aluminum alloy or cast iron • Fastened to end of cylinder block farthest from crankshaft • Head gasket used between both to seal combustion chamber • Made from soft metals & graphite layered together • Allows for even heat distribution between block & head • Some 2-stroke engines have head & block cast together called a jug • Provides maximum structural integrity & eliminates potential for leaks in combustion chamber

28. Pistons • Slides back and forth in the cylinder bore • Piston pin • Skirt • Ring groove • Piston rings • Commonly made from cast iron • Compression Ring • Wiper Ring • Oil ring

29. Pistons • Generally made of cast aluminum alloy • Material will always be different than engine block to prevent piston merging into block • Excellent thermal conductivity • Ability of material to conduct & transfer heat • Aluminum expands when heated so proper clearance must be provided • Insufficient clearance can cause piston to seize in bore • Excessive clearance can cause loss of compression & increase piston noise

30. Connecting Rodsand Bearings • connecting rod transfers motion from piston to crankshaft • bearings reduce friction, maintain clearance

31. Crankshaft

32. Crankshaft • Converts liner motion of piston into rotary motion • Main rotating component of engine • Commonly made of iron • Vertical or Horizontal orientation

33. Crankshaft • Components: • Crankpin Journal • Precision ground surface provides pivot point to attach connecting rod to crankshaft • Bearing Journals • Precision ground surface which the crankshaft rotates • Mates with bearing surface in cylinder block

34. Crankshaft • Counterweights • Protruding mass used to partially balance the forces of a reciprocating piston

35. Crankshaft • Crank gear • Gear located on crankshaft used to drive other parts of an engine • Not required on all small engines • Power take-off (PTO) • Extension of crankshaft that allows engine to transmit power to an application

36. Flywheels and Valve Trains • a flywheel is a disk mounted at one end of a crankshaft • provides inertia for the engine • maintains crankshaft rotation between combustion intervals • a valve train controls the flow of gases into and out of the combustion chamber

37. Engine Operation • intake event • introduces air and fuel (or just air in a diesel engine) to the combustion chamber • piston moves from TCD to BDC • compression event • air and fuel (or just air in a diesel engine) in combustion chamber is compressed in the cylinder

38. Engine Operation • ignition (combustion) event • charge is ignited and rapidly oxidized to release energy • power event • expanding gases force the piston head away from the cylinder head

39. Engine Operation • exhaust event • spent gases are removed from the chamber and released to atmosphere • valve overlap is the point when both intake and exhaust valves are open

40. Engine Output • measured by torque and horsepower • units of measurement of horsepower • brake, friction, and indicated horsepower • dynamometer measures torque, load, speed, and horsepower

41. Engine Output • factors affecting engine output • displacement • volumetric efficiency • thermal efficiency • air density

42. Chapter 3 Review What is the function of a crankshaft? • A crankshaft is an engine component that converts the linear (reciprocating) motion of the piston into rotary motion. What are the five events of four-stroke cycle engines? • The five events of four-stroke cycle engines are intake, compression, ignition, power, and exhaust.