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Untitled Document
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The
Pistons And How They Work
| Pistons:
Pistons must be designed to withstand the high pressure and temperature
generated during the combustion cycle. They must also be light weight
to minimize loading on the connecting rod and crankshaft. To meet
these requirements, pistons are usually made from cast aluminum.
A set of three rings prevents the leakage of combustion pressure
into the crankcase and oil leakage into the combustion chamber.
Shown are the Piston (1), Valve Relief (2), Piston Pin (3), Pin
Boss (4), Top Compression Ring (5), and Oil Ring (6).
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Piston
Pins:
Two types of piston pins (3) are used in modern engines. The full
floating pin is free to rotate in both the piston and the small end
of the connecting rod. It is secured in place by snap rings which
are fitted in grooves machined inside the piston's pin hole. The press-fit
pin is free to rotate in the piston but not the connecting rod. The
press-fit pin is very dependable and is the favored design.
Valve Reliefs:
Valve reliefs (2) are cast or machined into the piston's crown to
ensure adequate clearance exists between piston and valves. The piston's
upper surface is not always flat. Its shape is defined by the design
of the cylinder head and combustion chamber. Pistons can be contoured
with valleys and ridges.
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Piston
Rings:
Pistons have two compression rings (1 and 2) and one oil ring
(3). The compression rings prevent blow-by, the leakage of combustion
pressure into the crankcase. Ring tension and combustion pressure
seat the rings in place creating a seal. The oil ring is designed
to prevent crankcase oil from entering the combustion chamber.
Its design is different than the others. Openings allow oil, scraped
from the cylinder wall, back into the crankcase.
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Compression
Ring:
Compression ring tension (black arrow) closes the gap between the
piston's outside wall and the cylinder wall. Combustion pressure
(blue arrow) tightens the seal by applying a down force on the ring.
This vertical force is also present during the compression stroke.
A poor seal will result in blow-by, the leakage of pressure into
the crankcase, which results in reduced power. |
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Oil
Ring:
The primary purpose of the oil ring (3) is to prevent oil from leaking
into the combustion chamber. Excess oil is scraped from the cylinder
wall. Openings in the piston wall direct oil back into the engine's
crankcase. Blue smoke exhausting from the tail pipe is an indication
oil is leaking into the combustion chamber. Worn-out rings is one
possible cause.
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The
information on this page is provided from the "Road Machines"
learning CD. Buy it Today and continue
your learning !
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