Combustion 101

Ok so we know how the spark happens, but how does
the spark happen at the right time?  (just before the
piston reaches the top of the compression stroke)  The
timing of the spark is controlled by a distributor on older
cars (pre-1990s) or a computer on modern cars, both
work on similar principles.  

The powerful electric spark is created by an ignition coil,
essentially a voltage amplifier.  The coil is a transformer
that takes 12 volt current from the car battery and
increases it to 10,000+ volts, similar to the way your cell
phone charger takes 115 volts from the wall outlet and
turns it into about 3 or 4 volts for your cell phone-just
the other way around.  

The electricity from the ignition coil now travels to the
distributor which does what its name suggests, it
distributes electricity to each of the cylinder spark plugs.  
A distributor is a shaft that spins at the same speed as
the camshaft (its usually connected by a gear) and at
one end has an electrode that looks and spins like the
hand of a clock.  

On the outside of the distributor or its case (which
doesn't spin) are electrodes, one for each of the
cylinders spaced evenly around in a circle-like the
numbers on a clock.  When the hand of the clock passes
near any of the clock numbers as it is spinning- an
electric spark jumps between the hand and the number.  
This spark then travels down a wire to the spark plug
just in time for its big boom.  This happens once for
each cylinder every time the distributor and camshaft
spin.  Because the camshaft, crankshaft and distributor
spin together, the timing of the spark for each cylinder
can be set precisely to the timing of the combustion

On modern cars the mechanical distributor has been
replaced with a computer, but that computer still relies
on measuring the spin of the crankshaft and camshaft to
know when to send the spark.  This is accomplished by a
wheel with a certain number of teeth cut into it (like a
bicycle gear) which is attached to the front or back of
the crankshaft.  A sensor counts the teeth as they go
spin by to know what position the crankshaft is in- and
when to fire each spark plug.  When its time to fire a
spark, the computer sends an electrical signal to each
cylinder’s dedicated ignition coil, to send electricity to its
spark plug.  This results in greater accuracy and
adjustability of the spark due to strict computer control.  
Also the is less power loss by sending the spark a
shorter distance, usually the dedicated coil is near to the
spark plug.  

In a distributor system, the spark has to travel from the
coil to the distributor through a long wire to the spark
plug- losing some of its power along the way.  Also If
the long spark plug wires come in contact with
something metal-like the engine itself the spark can
jump from the wire to the block and miss the spark plug
altogether.  This is referred to as a misfire, and makes
the engine run roughly.  As a consequence, the spark
plug wires must be carefully routed around the engine
bay to make sure they aren’t touching anything metallic.  
Plastic clips or brackets are often used to guide the
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