How Ignition Systems Work
Before you can
figure out how to make something work better you first need to understand how it works. An
ignition system builds up a charge then releases it at the right moment to ignite the air
fuel mixture in the cylinder. There are two ways to improve an ignition system, increase
the amount of energy that is thrown at the spark plug to increase the chances of lighting
the mixture, and better time the spark delivery to get the most from the burn. The timing
issue is covered on the Ignition Curve page. On this page I'll
do my best to explain how ignition systems work. But first...
What the ignition system does
The internal combustion
engine is the most popular motivator of vehicles big and small. Internal combustion
engines convert the energy of burning fuel into mechanical motion. An engine ingests fuel,
along with air so the fuel will burn, compresses it then burns it. The burning mixture
expands rapidly and pushes a piston, rotor, or turbine. Gasoline is the fuel of choice for
most road going vehicles because it packs a lot of power per gallon (you can go farther
with fewer and quicker fill-ups). The biggest problem with using gasoline is that it must
be vaporized and mixed with the proper ratio of air or it will not burn. The flammability
range is 1.4% to 7.6% (by mass not volume), so for every pound of air the motor pulls in
you must mix in 0.224 to 1.216 grams of vaporized gasoline. Any more or less and it will
not burn. The duty of the carburetor or fuel injection is to monitor the air input and
meter the appropriate amount of gasoline. The engine then compresses the volatile air fuel
mixture. The more its compresses the more power will be extracted from it when burned.
Compressing the mixture heats it, if you compress it too much it will ignite on its own.
That is how diesel engines work, however gasoline tends to explode rather than simply
ignite which will tear an engine apart. The goal is to compress it as much as you can
without it self igniting. Then apply more heat (from an external source) to a point in the
cylinder to initiate combustion. Early engines used a glow plug. Sometimes that was as
simple as a copper rod threaded into the head and heated with a torch. Eventually, they
figured out the same could be done by creating a spark inside the cylinder. The first
spark ignition systems made a constant spark so they functioned the same as a glow plug.
The revelation that changed gasoline engines forever was the timed spark ignition. Instead
of creating a constant spark that would light the mixture at any random time, a single
spark is delivered with precise timing to most efficiently burn the fuel.
There are several challenges with timed spark ignitions. The
spark only lasts about a millisecond. If the conditions are not just right for that one
millisecond then the fuel will not be ignited. The carburetor or fuel injection doesn't
always add the right amount of fuel. Even if the exact amount of gas was mixed with the
air it doesn't mean that every cubic centimeter of the cylinder has the perfect air fuel
ratio. There will always be pockets of rich and lean. On top of that, not all the gasoline
will be completely vaporized. Better fuel systems, intake manifolds, and heads are
constantly being developed so that the mixture inside the combustion chamber stays as
accurate and consistent as possible. But even with all the advancements and technology put
into modern engines the ideal conditions for combustion are not always met. A good
ignition system can not compensate for these bad conditions but it will increase your
chances of lighting a less than perfect mixture. One trick is to increase the size of the
spark gap. Making the spark travel through more air fuel mixture increases the odds that
it will hit a pocket that is combustible. Another trick is to make the spark last longer.
The air fuel mixture is swirling around in the chamber, the longer the spark is present
the better the odds are that a good pocket of fuel will run into it and burn. To get a
larger and/or longer spark you need to throw more energy at the spark plug. To do that you
need to know...
How ignition systems work
If you want to make a spark you need a spark
gap. The electricity has to jump from somewhere to somewhere. Since the spark gap is
subject to erosion and fouling it is made to be easily removed and replaced. There is a
threaded hole in the head that leads to the combustion chamber. The spark gap threads into
this hole and plugs it so the chamber is sealed. That's as good a reason as any to call it
a spark plug, I guess. A spark plug has three main parts, the center electrode, the
porcelain insulator, and the body. The center electrode is what carries the electricity
into the combustion chamber. The porcelain insulator keeps the electricity in the center
electrode from grounding out to the head before it has a chance to jump the gap. The body
of the spark plug is what threads into the head and it also has a ground electrode
connected to it which catches the spark from the center electrode and grounds it to the
head.
On many engines the spark plug is fed electricity through a spark
plug wire. Spark plug wires need to contain very high voltage so the insulation is
extremely thick. The conductor is often a resistive or inductive material to reduce
electro magnetic interference which can interfere with electronic components. On older
multi-cylinder engines the spark plug wires were connected to a distributor. A distributor
is a rotary "switch" that is used to feed the spark to the appropriate cylinder.
The distributor is fed a spark through a coil lead, which is the same thing as a spark
plug wire. The other end of the coil lead is hooked to the coil. The coil is what produces
the high voltage necessary to generate a spark. In an effort to simplify systems and
increase service life, the coil lead was eliminated and the coil was mounted inside the
distributor. The next step was to eliminate the distributor and instead hook several coils
straight to the spark plug wires, often referred to as a Distributorless Ignition System
or DIS. The next inevitable step was to eliminate the spark plug wires and mount the coils
right to the end of the spark plugs, called Coil On Plug or COP.
The ignition coil is what produces the several thousand volts
needed to make a spark across the spark plug gap. Surprisingly the basic design of
ignition coils hasn't changed at all in the last century. What we call a coil is actually
two separate coils wrapped around a common metal core. In any other electrical system this
would be called a transformer. The
coil you put electricity into is called the primary and the coil that sends the
electricity out is the secondary. Transformers are used to step up or step down voltage.
The voltage is changed according to the ratio of the turns. Say you have a transformer
with 50 turns of wire on the primary and 100 turns on the secondary, a 1:2 ratio. If you
put 10 volts into the primary you will get 20 volts out of the secondary. The trick to
transformers is that to see anything from the secondary the current of the primary must be
changing. If you put a constant DC current through the primary you will have no current
flow through the secondary. Transformers are usually used in AC systems since the current
is constantly changing. If you put 120 volts AC through a transformer with a turns ratio
of 10:1 then you will get 12 volts AC out of the secondary. An ignition coil isn't fed
with a constant AC signal. All we need is one spark so all we are going to put into the
primary is one voltage spike. An ignition coil typically has a 1:100 turns ratio, so if
10,000 volts is required to generate a spark then we need to feed the primary a 100 volt
spike.
Notice I said if 10,000 volts is required. The required
voltage changes constantly. Firing a spark plug outside the engine requires far less
voltage than it does when firing an engine at wide open throttle. Pressure plays a major
role, the higher the pressure in the chamber the more voltage is required. The size of the
spark gap is the other big factor, the further the spark has to jump the more voltage is
required. Firing a spark through an air fuel mixture requires more voltage than it would
in pure air, especially with exotic fuels. There are many other small factors that
determine how much voltage will be required to make a spark. You may see "high
performance" coils advertises as 50,000 volt coils but all that is meaningless since
under most conditions you will only need 5,000 - 10,000 volts. Now if you are running wide
gap plugs in an alcohol burning, supercharged engine then you will likely need to step up
to a system with a higher voltage potential.
So all that's left is to explain where we get the large voltage
spike for the primary. The ignition coil may not have changed in the last 100 years but
the primary circuit sure has. There are two different types of ignition systems, inductive
and CDI. Each system has its strengths and weaknesses. The ignition coil, spark plugs and
such are the same between them. The difference is how they generate the primary voltage
spike that drives the coil.
Inductive Ignition
Inductive
ignition systems have been around almost as long as the internal combustion engine. It is
a simple and rugged design which is why it is still being used on new cars today. With an
inductive ignition all that's needed to make enough juice for a spark is the ignition
coil, a switch and a low voltage power source. Wait a minute, how do you get a big
voltage spike from a 12 volt battery with only a switch? The truth is, an inductive
ignition doesn't actually throw a voltage spike at the primary. Instead, it makes the
primary generate its own voltage spike. Oh, ok... wait... what? A coil of wire, also
called an inductor, exhibits strange properties when you run electricity through it. You
probably remember from elementary school what happens when you wrap a bunch of wire around
a metal object then hook it to a battery. That's right, you get a magnet. The more current
you run through a coil the greater the magnetic field. What you probably don't
remember (unless you happened to be touching both ends of the wire when you disconnected
the battery) is that the magnetic field is actually stored energy, and when the current
flow through the coil stops the stored energy causes a voltage spike. Inductors resist current changes. Think of them like the electrical equivalent of a flywheel. When current is low and you try increasing it, the inductor will add resistance to try and keep the current low. When current is high and you try reducing it, the inductor will increase voltage to try and keep the current high. Since the current flow in the primary of our ignition coil goes from several amps to zero amps almost instantly, the voltage will rise very fast until it can get the current flowing again. The goal is to prevent any more current flow in the primary so the coil will start the current flowing again in the secondary, giving us a spark.
Inductive ignitions are self adjusting. If a 100 volt spike is
needed to initiate a spark the primary will only rise to 100 volts. The spark will then
burn until all the energy in the coil is used up. The lower the voltage the longer the
spark will last. Let's say you open up the spark plug gap and now 150 volts is required to
fire the coil. When the current is interrupted on the primary, the magnetic field will
cause the voltage to rise rapidly. The voltage will rise until it reaches 150 volts at
which point the spark plug will fire. After the spark is started the voltage will actually
drop since it takes less voltage to maintain a spark than it does to start one. The spark
will continue to burn as long as there is energy left in the coil. Only now the spark
duration is shorter since the spark fired at a higher voltage than before. Increasing the
spark plug gaps without increasing the amount of energy stored in the coil can actually
hurt performance. If most of the energy is used to initiate the spark then there will be
little left to maintain it. It's even possible there won't be enough energy to make a
spark in the first place. To get the benefits of wider spark gaps you need to put more
energy into the coil. When you put more energy into the coil you will increase spark
duration. When you have longer spark duration you can afford to lose some by opening up
the spark gaps.
Early inductive ignitions actually used a mechanical switch,
commonly referred to as points, to physically break the primary circuit. When the points
close it closes the primary circuit and current flows through the primary, when the points
open the circuit is open and there is no current flow. In the seventies points were phased
out in favor of electronic ignitions. Electronic ignitions use a transistor to switch the
current on and off instead of a mechanical switch. There is no magic to electronic
ignitions, they are just a switch the same as points. It's the switch opening
(stopping current flow) that triggers the voltage spike that fires the coil.
When the switch closes current flows through the primary, but it
takes a while for the current to build up. That is the biggest issue with inductive
ignitions. The inductance of the coil limits change in current. When the switch is open
there is zero amps flowing through the primary. When the switch closes the current starts
at zero and ramps up until it reaches its limit. It takes time for the current to build
up. You can calculate how long it takes for a coil to charge with this formula, T = ( L *
I ) / V where T is time in seconds, L is inductance in henries, I is amps, and V is volts.
Say you have a 7 millihenry coil and want to charge it to 6 amps with 14 volts. T = ( .007
* 6 ) / 14 It will take 3 milliseconds (0.003 sec.) for the coil to reach six amps. That
is not much time at all, so what's the problem? Believe it or not you don't always have
that much time to charge the coil. For example, a V8 with a single coil running at 5000
rpm fires every 3 milliseconds. If you take away one millisecond for the spark that only
leaves two milliseconds to charge the coil. In two milliseconds the coil will only reach
four amps before it has to fire again. The energy stored in the coil is determined by the
inductance and the current. The formula is, J = 0.5 * L * I * I where J is the energy in
joules, L is henries and I is amps. Notice amps is squared which means small changes in
amps will greatly effect the amount of energy stored. Our 7 millihenry coil at 6 amps
holds 126 millijoules (0.126 joules), the same coil at 4 amps only has 56 millijoules of
energy available to fire the spark. So more amps mean more energy but it also means more
heat. The current needs to be limited or the coil will go up in smoke.
One strategy for controlling current is to add resistance to the
primary circuit. Ohm's law says that current equals volts divided by resistance, I = V /
R. If you run 12 volts through a coil with a primary resistance of 3 ohms then the peak
current will be 4 amps. Running a coil with a high primary resistance is the simplest way
to limit current but far from the best. When voltage drops, like when the starter motor is
engaged, the peak current drops and as a result you're spark output drops. A band aid fix
is to use a lower resistance coil and add a ballast resistor. On many old cars you will
see a 1.5 ohm coil and a 1.5 ohm ballast resistor, total series resistance is still 3 ohms
so the peak current is the same. The trick is that the ballast resistor is bypassed when
the starter motor is engaged so the circuit consists of only the 1.5 ohm coil. Now if the
voltage drops to 9 volts when the starter is engaged your peak current will be 6 amps.
That means you will have a more powerful spark when starting than you do when the engine
is running which is good since it's harder to initiate combustion in a cold engine. The
coil can handle the increased current because it is only done for a short period of time.
If you bypassed the ballast resistor permanently you would likely burn up the coil. Early
electronic ignitions still used a ballast resistor but they soon figured out the same task
could be accomplished with the switching transistor. This makes the circuit simpler since
there is no longer a ballast resistor or bypass circuit. But the big advantage is that it
can automatically adjust resistance to keep the peak current consistent. If you have a 1.5
ohm coil it will add 1.5 ohms to keep peak current at 4 amps. If voltage drops to 10 volts
it will only add 1 ohm so peak current will remain 4 amps. If you swap to a coil with a
0.5 ohm primary it will add 2.5 ohms. An automatic current limited works well but puts a
greater load on the ignition module. A big heat sink is required to dissipate the heat
from the switching transistor since it is still using resistance to limit current. With
computer controlled dwell came the "ramp-and-fire" dwell strategy. Since the
current ramps up gradually and at a known rate, the computer starts charging the coil late
enough that there won't be enough time for it to go over current. If it takes three
milliseconds for the coil to reach 7 amps then the computer will start charging the coil
exactly three milliseconds before it has to fire. With this strategy you can run higher
peak current without over heating the coil or module.
That segues us nicely into the next critical element of
inductive ignitions, coil charge time also known as dwell. Most guys when they hear the
word dwell immediately think of old breaker point systems where you would set the dwell by
adjusting point gap. Dwell readings were in degrees. Thirty degrees dwell meant that the
distributor rotated 30 degrees between the time the points closed and when they opened
again. When the points are closed the coil is charging. A V8 fires every 45 degrees of
distributor rotation so the coil is charging for 30 degrees and is given 15 degrees to
discharge. When the engine is running the coil is being charged 2/3 of the time regardless
of engine speed. This is not a good dwell strategy. To see why, you have to look at dwell
as time and not some arbitrary distance. At 1000 rpm it takes 15 milliseconds for the
distributor to turn 45 degrees, so the coil will be charging for 10 milliseconds and given
5 milliseconds to discharge. It only takes 2 milliseconds for the coil to reach 4 amps
where it stays for 8 milliseconds before the points open and the coil fires. The coil has
reached its full power potential after 2 milliseconds, the additional 8 milliseconds does
nothing but heat up the coil and waste power. The dwell strategy on early electronic
ignitions wasn't much better than points (some were considerably worse) but as they
improved the dwell times got closer and closer to the actual charge time of the coil. This
means less power was being wasted. Coils were running cooler, so the peak current could be
increased. More amps into the coil mean more energy out of the coil. As stated before,
with more energy you will have longer spark duration and can run larger spark gaps. When
you improve both of those you increase your chances of lighting the fuel mixture which
ultimately means more power and better fuel economy.
To sum it all up, inductive ignitions are simple and make a nice
long duration spark. To do so they need as much current run through the primary as
possible, which if not controlled can burn up the coil. It takes time to build up primary
current and as rpm increases the time available to build up current decreases. When there
is insufficient time to reach a high current, the energy available to fire the spark plugs
drops significantly. This leads to misfiring at higher rpms.
CDI (Capacitive Discharge Ignition)
CDI systems came out about
the time inductive ignitions were becoming electronic. Conventional electronic ignitions
simply replaced the points with a transistor but CDI completely reinvented the way the
spark is generated. Instead of slowly charging the coil then relying on it to generate
its own voltage spike, capacitive discharge ignitions charge a capacitor with high
voltage which is discharged through the coil to make a spark. The capacitor can charge and
discharge much faster than a coil so a CDI can operate at a much higher speed than an
inductive ignition. The basic components of a CDI are a high voltage power supply, a
capacitor, a switch, and a coil. The construction of a CDI system is a bit more
complicated but the principle is quite simple. The capacitor is connected to the high
voltage supply and charged. When it's time to fire, the capacitor is connected to the
coil. The high voltage applied to the primary causes current to rise very rapidly and
that's where the secondary get the power to make the spark. So the coil is basically just
used to step up the voltage that the CDI module produces. That, in a nutshell, is the
difference between a conventional electronic ignition module and a CDI module. A CDI
module puts out power where a conventional ignition module simply switches the power to
the coil on and off like a switch. Some guys refer to any electronic ignition as a CDI but
it's only a CDI if spark is achieved by discharging a high voltage capacitor through the
coil.
It all starts with the high voltage power supply. There are two
common approaches to generating the necessary high voltage. One is to have a voltage
converter built into the module that converts the 12 volts from the vehicles electrical
system into a high voltage. The converter consists of an oscillator to convert the 12 volt
DC to 12 volt AC, a transformer to step up the AC voltage to several hundred volts, and a
rectifier to make the AC back into DC. This arrangement is what makes CDI modules more
complex and costly than a conventional electronic ignition. You will see voltage
converters used mainly on cars and street bikes where there is a battery and charging
system to power it. The alternative is to use the alternator to produce a high voltage AC
signal. Then the CDI module only has to rectify it before it can be used to charge the
capacitor. This design is generally used on dirt bikes and lawn equipment where there is
no battery or charging system. It's also used on many quads and enduros, even those that
have a battery and charging system. In a setup like that the alternator has two outputs, a
high voltage output to power the ignition and a 12 volt output to run the lights and
charge the battery.
The high voltage supply is used to charge the capacitor. The
characteristics of a capacitor charging are the opposite of a coil. When you put power to a
coil the current starts at zero then ramps up at a linear rate as high as it's allowed,
and you need to keep the current flowing through it to keep it charged. The amount of time
the power is applied to the coil is critical. Too little time and the coil will not be
charged. Too much time means that the current used to maintain the charge is being wasted
heating up the coil. CDI eliminates these concerns. When you put power to a capacitor the
current starts extremely high then drops exponentially as the capacitor charges. When the
capacitor is charged the current flow is nearly zero amps. You can leave the capacitor
hooked to the power and almost no energy is wasted. You can even disconnect the power and
the capacitor will retain its charge. As you can see, with CDI there is no need for a
current limiter or finely tuned dwell time. Generally, CDIs are more efficient since very
little energy is wasted charging the capacitor.
The energy stored in a capacitor is determined by the size of the
capacitor and the stored voltage. J = .5 * C * V * V Inductive ignitions take a long time
to charge because of the large coil required to hold a charge. A CDI can hold the same
charge with a very small capacitor since it is being charged with several hundred volts.
For example, an MSD 6 ignition charges a 1 microfarad (.000001 farad) capacitor to 500
volts. Do the math and you see that it stores 125 millijoules (0.125 joules). That's the
same as the inductive ignition example in the previous segment. However, the inductive
ignition took 3 milliseconds to charge where the CDI charges in only a few microseconds.
OK, so the cap is charged. The only step left is to dump the
charge into the coil. For that we need a switch. The schematic above is an overly
simplified representation of a CDI circuit. An actual CDI circuit doesn't use a physical
switch. Instead, an SCR is most commonly used as the switch. An SCR is basically a
transistor that when turned on, stays on until current stops flowing through it. That
eliminates the need to control the switch time. All you have to do is trigger the SCR, it
will stay on as long as the capacitor is discharging and shuts off automatically when the
cap is empty.
There are a few variations to CDI circuits but the one shown is a
very common scheme. It's a bit hard to see how it works when you first look at it. I
animated the drawing to make it a little clearer. The process starts when the switch is
open. The high voltage supply charges the capacitor. Notice how the current flows through
the diode, bypassing the coil. [A diode is like an electric check valve, current can only
flow through it one way.] The diode is not mandatory, many circuits don't have this diode,
I only added it to make it easier to see when the cap is charging and when it's
discharging through the coil. When the capacitor is fully charged the current flow stops.
The fully charged capacitor sits waiting to be dumped. When the switch closes it connects
the positive side of the capacitor to ground. This in effect bypasses the power supply and
hooks the capacitor straight to the coil. Notice the positive side of the capacitor is
hooked to the negative side of the coil. This is why, on many CDIs, you see a negative
voltage when probing the positive coil lead. Once the capacitor is drained the switch
turns itself off and the capacitor charges again.
An inductive ignition
charges a coil then allows the primary voltage to rise high enough to initiate a spark.
CDIs don't wait for the coil to do the work. The high voltage surge from the capacitor
causes the voltage on the primary to rise much faster and higher than it would on its
own. This creates a very intense spark. [CDI is great for two strokes since the hotter
spark can better fire an oily spark plug] Unfortunately to get the big hot spark you have
to give up duration. A CDI spark may last only 50 microseconds (0.00005 seconds) where
the spark from an inductive ignition typically lasts about 1 millisecond (.001 seconds).
The short spark can hurt both driveability and gas mileage. A common approach to covering
up this deficiency is to fire the CDI multiple times at lower rpm. While a few short
sparks are better than one short spark, it's still not as effective as one long duration
spark.
Which is better
Each system has it's
strengths and weaknesses. The question isn't 'Which is better' but rather 'Which is better
for my application'. Inductive ignitions are rugged and simple. An inductive ignition with
good current control and well timed dwell works excellent for most applications. However,
inductive ignitions can't keep up on a high revving, multi-cylinder engine running a
single coil. Modern motorcycle engines spin fast enough that even with a distributorless
setup there is not enough time to fully charge the coils. That's why capacitive discharge
ignitions are seen mostly on race cars and motorcycles.
If you have any questions or comments
please e-mail me at mrriggs@gofastforless.com.
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