The internal
combustion engine is an amazing machine that has evolved for
more than 100 years. It continues to evolve as automakers
manage to squeeze out a little more efficiency, or a little
less pollution, with each passing year. The result is an
incredibly complicated, surprisingly reliable machine.
One could argue
that the ignition system is where it all comes together,
with a perfectly timed spark.
Spark Timing
The ignition
system on your car has to work in perfect concert with the
rest of the engine. The goal is to ignite the fuel at
exactly the right time so that the expanding gases can do
the maximum amount of work. If the ignition system fires at
the wrong time, power will fall and gas consumption and
emissions can increase.
The spark plug fires before the piston reaches top dead
center.
When the
fuel/air mixture in the cylinder burns, the temperature
rises and the fuel is converted to exhaust gas. This
transformation causes the pressure in the cylinder to
increase dramatically and forces the piston down.
Power
In order to get the most torque and power
from the engine, the goal is to maximize the pressure in the
cylinder during the power stroke. Maximizing pressure will
also produce the best engine efficiency, which translates
directly into better mileage. The timing of the spark is
critical to success.
There is a small delay from the time of
the spark to the time when the fuel/air mixture is all
burning and the pressure in the cylinder reaches its
maximum. If the spark occurs right when the piston reaches
the top of the compression stroke, the piston will have
already moved down part of the way into its power stroke
before the gases in the cylinder have reached their highest
pressures.
To make the best use of the fuel, the
spark should occur before the piston reaches the top of the
compression stroke, so by the time the piston starts down
into its power stroke the pressures are high enough to start
producing useful work.
Work = Force * Distance
In a cylinder:
Force = Pressure * Area of the piston
Distance = Stroke length
So when we're talking about a cylinder,
work = pressure * piston area * stroke length. And because
the length of the stroke and the area of the piston are
fixed, the only way to maximize work is by increasing
pressure.
Timing
The timing of the spark is important, and
the timing can either be advanced or retarded depending on
conditions.
The time that the fuel takes to burn is
roughly constant. But the speed of the pistons increases as
the engine speed increases. This means that the faster the
engine goes, the earlier the spark has to occur. This is
called spark advance: The faster the engine speed, the more
advance is required.
Other goals, like minimizing emissions,
take priority when maximum power is not required. For
instance, by retarding the spark timing
(moving the spark closer to the top of
the compression stroke), maximum cylinder pressures
and temperatures can be reduced. Lowering temperatures helps
reduce the formation of nitrogen oxides
(NOx),
which are a regulated pollutant. Retarding the timing may
also eliminate knocking; some cars that have knock sensors
that will do this automatically.
Next we'll go through the components that
make the spark. Let's start with the spark plug.
Spark Plug
The spark plug
is quite simple in theory: It forces electricity to arc
across a gap, just like a bolt of lightning. The electricity
must be at a very high voltage in order to travel across the
gap and create a good spark. Voltage at the spark plug can
be anywhere from 40,000 to 100,000 volts.
The spark plug is in the center of the four valves
in each cylinder.
The spark plug
must have an insulated passageway for this high voltage to
travel down to the electrode, where it can jump the gap and,
from there, be conducted into the engine block and grounded.
The plug also has to withstand the extreme heat and pressure
inside the cylinder, and must be designed so that deposits
from fuel additives do not build up on the plug.
Spark plugs use a ceramic insert to
isolate the high voltage at the electrode, ensuring that the
spark happens at the tip of the electrode and not anywhere
else on the plug; this insert does double-duty by helping to
burn off deposits. Ceramic is a fairly poor heat conductor,
so the material gets quite hot during operation. This heat
helps to burn off deposits from the electrode.
Some cars require a hot plug. This type
of plug is designed with a ceramic insert that has a smaller
contact area with the metal part of the plug. This reduces
the heat transfer from the ceramic, making it run hotter and
thus burn away more deposits. Cold plugs are designed with
more contact area, so they run cooler.
The difference between a "hot" and a "cold" spark plug is in
the shape of the ceramic tip.
The carmaker
will select the right temperature plug for each car. Some
cars with high-performance engines naturally generate more
heat, so they need colder plugs. If the spark plug gets too
hot, it could ignite the fuel before the spark fires; so it
is important to stick with the right type of plug for your
car.
The Coil
The coil is the
device that generates the high voltages required to create a
spark. It is a simple device -- essentially a high-voltage
transformer made up of two coils of wire. One coil of wire
is called the primary coil. Wrapped around it is the
secondary coil. The secondary coil normally has hundreds of
times more turns of wire than the primary coil.
Current flows from the battery through
the primary winding of the coil.
The primary coil's current can be
suddenly disrupted by the breaker points, or by a
solid-state device in an electronic ignition.
If you think the coil looks like an
electromagnet, you're right -- but it is also an inductor.
The key to the coil's operation is what happens when the
circuit is suddenly broken by the points. The magnetic field
of the primary coil collapses rapidly. The secondary coil is
engulfed by a powerful and changing magnetic field. This
field induces a current in the coils -- a very high-voltage
current (up to 100,000 volts)
because of the number of coils in the secondary winding. The
secondary coil feeds this voltage to the distributor via a
very well insulated, high-voltage wire.
The Distributor
The distributor
handles several jobs. Its first job is to distribute the
high voltage from the coil to the correct cylinder. This is
done by the cap and rotor. The coil is connected to the
rotor, which spins inside the cap. The rotor spins past a
series of contacts, one contact per cylinder. As the tip of
the rotor passes each contact, a high-voltage pulse comes
from the coil.
The pulse arcs
across the small gap between the rotor and the contact
(they don't actually touch) and
then continues down the spark-plug wire to the spark plug on
the appropriate cylinder. When you do a tune-up, one of the
things you replace on your engine is the cap and rotor --
these eventually wear out because of the arcing.
Also, the
spark-plug wires eventually wear out and lose some of their
electrical insulation. This can be the cause of some very
mysterious engine problems.
Older
distributors with breaker points have another section in the
bottom half of the distributor -- this section does the job
of breaking the current to the coil. The ground side of the
coil is connected to the breaker points.
A cam in the center of the distributor
pushes a lever connected to one of the points. Whenever the
cam pushes the lever, it opens the points. This causes the
coil to suddenly lose its ground, generating a high-voltage
pulse.
The points also control the timing of the
spark. They may have a vacuum advance or a centrifugal
advance. These mechanisms advance the timing in proportion
to engine load or engine speed.
Spark timing is so critical to an
engine's performance that most cars don't use points.
Instead, they use a sensor that tells the engine control
unit (ECU) the exact position
of the pistons. The engine computer then controls a
transistor that opens and closes the current to the coil.
We give our thanks to
HOW STUFF WORKS for allowing us to use their info and
graphics!
For more information on how engines
or anything else in this world works, visit
www.howstuffworks.com.
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