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generates about 50 volts, but the
secondary wire is hair-thin and
produces over 15,000.
Inside the primary circuit is a set of
contact points that act as a circuit breaker
whose function is to make the flux
reversals more abrupt, since they are
not good enough to produce a clean
signal by themselves. In short, the
points are there to make and break
the primary circuit.
There is a cam which moves a rocker
arm to the open position as many
times per revolution as there are
lobes on the cam. There is a spring
to return the rocker arm to its
normal position. The separation of
the points needs to take place in
strict timing with the maximum
primary current, so the field
collapses at maximum intensity.
As the points separate, a spark is
generated across them as the primary
current ceases, due to a strong
follow-on current in the primary
(electricity has a momentum). A
condenser is used in parallel with the
points to minimise the spark, but the
points still need to be looked at for
pitting and damage because it can
never be eliminated entirely.
50 Canadian Private Pilot Studies
The high-tension current generated
is fed to the plugs by a distributor,
which is essentially a rotor spinning
inside a cap holding heavy cables
going to the plugs (part of the same
unit as a magneto, but separate in a
car). The one in the middle is the
King Lead. When fitted, they are not
matched to the plugs in order, as one
cylinder would receive the spark at
an entirely wrong time and strain the
crankshaft. Instead, the cables are
arranged out of order, on a fourcylinder
engine as 1342 or 1243 (you
will often see these numbers
moulded into the cylinder head as a
reminder, in case you were
wondering what they were for).
The switches in the cockpit ground
magnetos to Earth through the
primary circuit, because they cannot
be switched off (they work as long as
they spin), so they must always be treated
as live. Ground connections can fail.
The 4-Stroke (Otto) Cycle
Now that we are acquainted with
them, here, in excruciating detail, is a
description of the movements of all
the parts in an engine relative to
each other (it's not required for the
exam, but is included for fun) .
It all starts with the piston at top
dead centre, ready to move
downwards and suck in a fuel/air
mixture from the carburettor,
through the fuel valve, which has
just opened:
On top of the suction created by the
piston's downward movement,
atmospheric pressure helps to force
the fuel and air in (when it is less, the
supercharger or turbocharger helps).
The valve closes as the piston
reaches bottom dead centre, so the
chamber is filled.
With both valves closed, the piston
starts moving up again, compressing
and heating the mix, as well as
increasing the density, which helps the
flame ignite quicker because the
particles are closer:
Just before TDC, the spark plugs (in
the cylinder head, near the valves)
ignite the mixture with sparks from a
high-voltage electric current
provided by the magneto, which is
rotating in sympathy with the engine:
It is timed this way to give the fuel
time to catch fire, and produce the
optimum expansion at 10° after
TDC, which is when it is actually
required. The piston is forced
downwards again, in a smooth
Airframes, Engines & Systems 51
movement, making the crankshaft
rotate, and whatever is attached to it.
The volumetric efficiency is the measure
of mass charge to the theoretical
mass charge at ISA if the engine
were stationary. In other words, the
degree to which the cylinder is filled
with new mixture at full throttle, as
compared to an equivalent amount
of atmosphere. It is rarely more than
80%, due to various leakages and
losses, hence the need for
supercharging, above.
The momentum of the engine
(supplied by the flywheel) brings the
piston up again, to force the exhaust
gases out through the exhaust valve,
which has already opened, just as the
piston hit BDC:
It closes as the piston gets to TDC,
after the fuel valve has opened early,
to allow some atmospheric pressure
in that will help push the exhaust
out, as the piston by this time is
starting to slow down, ready to
reverse direction.
There – that wasn't so bad, was it?
Note that, although there were four
cycles, the crankshaft only went
　
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