14-02-2013, 09:38 AM
Internal Combustion Engines
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Basic principals of mechanical construction
The function of the major components of Internal Combustion Engines and their construction
materials will now be reviewed.
The engine cylinders are contained in the engine block. The block has traditionally been
made of gray cast iron because of its good wear resistance and low cost. Passages for the
cooling water are cast into the block. Heavy-duty and truck engines often use removable
cylinder sleeves pressed into the block that can be replaced when worn. These are called wet
liners or dry liners depending on whether the sleeve is in direct contact with the cooling water.
Aluminium is being used increasingly in smaller SI engine blocks to reduce engine weight. Iron
cylinder liners may be inserted at the casting stage, or later on in the machining and assembly
process. The crankcase is often integral with the cylinder block.
The crankshaft has traditionally been a steel forging; nodular cast iron crankshafts are
also accepted normal practice in automotive engines. The crankshaft is supported in main
bearings. The maximum number of main bearings is one more than the number of cylinders;
there may be less. The crank has eccentric portions (crank throws). The connecting rod bigend
bearings attach to the crank pin on each throw.
Both main and connecting rod bearings use steel backed precision inserts with bronze, babbit,
or aluminium as the bearing materials. The crankcase is sealed at the bottom with a pressed
steel or cast aluminium oil pan which acts as an oil reservoir for the lubricating system.
Pistons are made of aluminium in small engines or cast iron in larger slower-speed
engines. The piston both seals the cylinder and transmits the combustion-generated gas
pressure to the crank pin via the connecting rod. The connecting rod, usually a steel or alloy
forging (though sometimes aluminium in small engines), is fastened to the piston by means of a
steel piston pin through the rod upper end.
The piston pin is usually hollow to reduce its weight.
The oscillating motion of the connecting rod exerts an oscillating force on the cylinder walls
via the piston skirt (the region below the piston rings). The piston skirt is usually shaped to
provide appropriate thrust surfaces. The piston is fitted with rings which ride in grooves cut in
the piston head to seal against gas leakage and control oil flow.
The upper rings are compression rings which are forced outward against the cylinder wall and
downward onto the groove face. The lower rings scrape the surplus oil from the cylinder wall
and return it to the crankcase. These crankcase must be ventilated to remove gases which blow
by the piston rings, to prevent pressure build-up.
Wankel rotary engine
In order to reduce engine components and produce more compact engine, and to reduce losses
wich caused by alternating movements in traditional engine, an egine with rotary piston (or
pistons) were invented which is called Wankel engine.
But now it is not used because its sealing and leakage problems.
There are two rotating parts, the triangular-shaped rotor and the output shaft with its integral
eccentric. The rotor revolves directly on the eccentric. The rotor has an integral timing gear
which meshes with the fixed timing gear on one side housing to maintain the correct phase
relationship between the rotor and eccentric shaft rotations.
Thus the rotor rotates and orbits around the shaft axis. Breathing is through ports in the center
housing. The combustion chamber lies between the center housing and rotor surface and is
sealed by seals at the apex of the rotor and around the perimeters of the rotor sides. Wankel
rotary engine operates with four stroke cycle.
As the rotor makes one complete rotation, during which the eccentric shaft rotates through
three revolutions, each chamber produces one power stroke.
Three power pulses occur, therefore, for each rotor revolution; thus for each eccentric (output)
shaft revolution there is one power pulse.