The Piston
The Piston, or piston assembly has three main functions in the engine,
acts as a plug to seal the cylinder, opens and closes the ports within the
cylinder and transfers the increase in pressure to the rod and eventually the
crankshaft.
The design of the piston is quite complex but all we need to be concern
with right now is what does what and what it looks like. First the assembly,
the piston body has a profile that ensures that it is strong, but lightweight.
The main body is hollow inside, imagine it like a coke bottle lid.
The piston interior has to ‘bosses’ (inserts) that hold the PISTON PIN (also known as the gudgeon
pin or wrist pin) from now on I’m going to refer to it as the WRIST PIN, as it makes more sense to
me.
The wrist pin goes through the piston and through the con-rod. This is how
force is transfered through the piston and down the con-rod to the crankshaft.
The wrist pin is hollow to save weight.
The piston is slightly smaller than the cylinder that it fits into, This
helps seal the cylinder when the fuel/air mixture is compressed. If the piston
was a tight fit it couldn’t move up and down the cylinder. So the piston has
to be smaller that the cylinder, however this is a problem, as the compressed
mixture just rushes though the gap.
To stop this from happening a seal is needed. This comes in the form of PISTON RINGS. The picture below has
been exaggerated below to show you the gap between the piston and the cylinder
wall. The blue highlighted areas shows the recessess where the rings are held.
To seal this gap the rings are made slightly larger in diameter than the
cylinder so they ‘spring’ outwards to hold the rings in place. The rings are
quite small and delicate. They have a split so the ring can expand.
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