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of something like a horseshoe
magnet, called an armature, the bulk
of which is surrounded by two sets
of coils, a primary and a secondary
winding. As the rotor spins, the flux
along the armature reverses as the
lobes pass the ends (in the above
case, you will get four flux reversals
and four sparks per revolution, so it
runs at ¼ of the engine speed).
The primary coil oscillates in sympathy
with the flux reversals, and a current
is induced in the secondary coil
wrapped around it (because the
primary field is expanding and
collapsing – the essential point is
movement). The ratio between the
coils is significant, which is how
such a high voltage is generated. The
primary has thicker wire and
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.
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 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 all starts with the piston at top
dead centre, ready to move down
and suck in a fuel/air mixture from
the carburettor, through the fuel
valve, which has just opened:
208 Canadian Professional Pilot Studies
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, somewhere 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
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
　
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