THIRD CLASS MARINE ENGINEERING

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THIRD CLASS MARINE ENGINEERING

• Engine Cycles
• 3 ME 4

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AIMS

• Describe the basic construction of a small
  marine diesel engine.
• Identify all major components and explain their
  function

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WHAT YOU SHOULD HAVE ALREADY COMPLETED!

• HEALTH AND SAFETY GENERAL
• 3 ME 1
• HEALTH AND SAFETY WORKSHOP
• 3 ME 2
• INTRODUCTION TO MARINE ENGINEERING
• 3 ME 3
• QUESTIONS ON PAGES 13, 19 AND 25

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General Arrangement of a Diesel Engine

• Diesel engines have an infinite number of jobs to
  carry out and are, therefore, of many various
  shapes and sizes. From simple single cylinder
  engines to multi-cylinder engines however, the
  basic components remain the same.

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The main components

• Engines may vary considerably but the majority
  incorporate the components listed on the next few
  slides.

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Cylinder block and crankcase.

• The unit forms the principal part of the engine.
  It may be cast as a single unit, or the cylinders
  may be detachable from the crankcase. A water
  cooling space is included in the cylinder block.
  Small air cooled engines have fins to provide a
  large surface area for the cooling air.

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Cylinder head.

• The head, usually detachable, is bolted to the
  cylinder block. In some small two-stroke engines,
  however the cylinder head is part of the cylinder
  and can not be detached. A water cooling space in
  the head communicates with the water cooling
  passages in the cylinder block. Air cooled types
  incorporate fins, as in the cylinder block.

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Sump

• This is a light casing fitted to the underside of
  the crankcase to contain the lubricating oil.
  This is normally OMD (oil mineral detergent), or
  a commercial equivalent.

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Piston.

• The piston is a sliding fit in the cylinder bore.
  It is fitted with piston rings to prevent gas
  leakage past the piston. The top of the piston is
  known as the crown. The lower part, which acts as
  the guide is called the piston skirt. Compression
  rings are fitted to maintain a seal between the
  piston and the cylinder wall and an oil control
  (scraper) ring removes excess oil from the
  cylinder wall.

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Connecting rod.

• The connecting rod is attached to the piston at
  one end by a gudgeon pin which is fitted to the
  piston. A bush in the end of the connecting rod
  provides a bearing surface. This is called the
  small end bearing. The bottom end of the
  connecting rod is attached to the crankshaft,
  with a halved bearing and bottom cap securely
  bolted to the connecting rod. This is called the
  big end bearing.

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Crankshaft.

• The crankshaft is carried in main bearings which
  are part of the crankcase casting, with halved
  bearing shells and bottom caps securely bolted to
  the crankcase.

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Flywheel.

• Most internal combustion piston engines have a
  heavy flywheel to carry the engine over the
  non-power strokes, particularly in the
  four-stroke engine where there is only one power
  stroke in four on each piston. The flywheel is
  attached to the crankshaft, and is outside the
  crankcase, but sometimes it is enclosed by the
  clutch case, or the reversing gear case. The
  flywheel usually incorporates a toothed ring into
  which the engine starter pinion engages to start
  the engine.

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Flywheel.
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Valves.

• The valves fitted to all four-stroke engines and
  some to two-stroke engines are mushroom-shaped.
  They have a long stem, over which a spring is
  installed to hold the valve on the seating. The
  valves are usually fitted in the cylinder head,
  these are called overhead valves.

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Camshaft.

• Each valve is lifted by a cam, which is pear
  shaped and machined in an angular position on the
  camshaft so that it will lift the valve for the
  correct period. The camshaft may lie above the
  cylinder head, when it is called an overhead
  camshaft, or alongside the crankshaft with push
  rods to lift the valves if they are of the
  overhead type.

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Rocker gear.

• Overhead valves are operated by rockers with one
  end of each rocker bearing on the cam or push
  rod, and the other end on the valve stem. Side
  valves are usually placed immediately above the
  cams so that the rockers are not required.

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Tappets.

• All valve gear is fitted with tappets, a form of
  set screw and locknut, which may be fitted to the
  rockers, the push rods, or the tappet spindle for
  side valve engines. The tappets are adjusted to
  give a slight clearance for expansion between the
  tappet and the valve stem. This clearance is
  always stated for inlet and exhaust valves of
  each engine.

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Timing gear.

• The camshaft must be accurately timed to the
  crankshaft so that the valves will open when
  required and for the correct period. The timing
  gear consists of gear wheels usually mounted at
  the front end of the crankcase and enclosed by a
  timing case. The gear wheels are marked to show
  the correct setting.

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Fuel pump and injectors.

• The fuel injection pump may be fitted on the side
  of the engine, or an individual fuel injection
  pump may be fitted alongside each cylinder. The
  pump is driven by the timing gear so that fuel is
  injected into each cylinder at the correct time.
  Each cylinder head is fitted with an injector,
  which consists of a non-return valve and a nozzle
  which produces a fine spray of fuel into the
  cylinder. The speed of the engine is varied by a
  governor which controls the quantity of fuel
  discharged by the fuel pump.

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Joints/Gaskets.

• These are fitted to prevent the leakage of oil,
  gas or water. The cylinder head gasket is
  normally made from copper and compressed fibre,
  whilst other joints are of treated paper or
  metal-to-metal with joining compound between the
  mating faces.

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OBJECTIVESYou should now be able

• Identify all major components and understand
  their purpose.
• Know the function of a gasket
• Answer the questions on page 31 of your task book.

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NEXT TIME

• You should have completed the questions on page
  31.
• Engine Cycles
• 3 ME 5