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Monday, October 22, 2012

MAIN COMPONENTS OF AN ENGINE

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Engines may vary considerably but the majority incorporate the components listed
below.
1. 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.
2. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
Marine Engineering – Third Class – Study Guide and Training Manual
- 29 -
Engine components cont…
8. 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.
9. 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.
10. 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.
OVERHEAD VALVE GEAR
11. 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.
12. 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.
13. 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

1 comment:

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