The Bottom End
The ‘Bottom End’ is the rotating assembly, which we’ve already covered
briefly. This includes the crankshaft and some include the con-rod, which we
will here. The crankshaft sits inside the casing and runs in bearings which is
covered later. The pictures below show the relationship between the crankshaft,
con-rod and piston linked together with the crank pin and the wrist pin. These
‘pivots’ are highlighted below.
TDC
90° rotation
180° rotation or BDC
270° rotation
As you can see the piston
climbs and falls up and down the cylinder (the cylinder isn’t shown for clarity).
The piston is linked to the crankshaft by the con-rod. This transfers the
movement of the piston (linear – up and down) to the crankshaft which rotates. You
can see how con-rods direction changes as the crankshaft turns, this will
become important in a later explanation.
The rotational angle of the
crankshaft is known as the crank angle and
this is important in relation to when the spark plug fires.
There is a vital function of
the engine that isn’t one component but a function of the piston and the
casing. It is basically a pumping action that the piston exerts on the volume
of fuel/air mixture within the casing. The pictures below show how this works.
The piston is at TDC and the casing is already ‘charged’ (filled) with fresh
fuel/air mixture.
When the piston descends the
fuel/air mixture is squashed i.e compressed slightly. When the piston falls low
enough it uncovers the transfer ports and the slightly compressed mixture has a
void to fill within the cylinder. This causes the mixture to fill the cylinder.
The blue arrow above shows the flow
of mixture into the cylinder.
