This guide was started for a motorcycle group seeking to solve
common ignition problems. But over time, this has expanded into
this stand-alone page because of the interest for a general explanation
of gas 4-stroke (not diesel or 2-stroke) engine ignition principals.
Everyone at one time or another has suffered through some ordeal
caused by a non-working ignition system. Whether its the lawn
mower, outboard, chainsaw, or the time you drove your Dad's car
through that big puddle..... you've been stuck somewhere or with
something that would not run. When troubleshooting non-running
engine problems I've always used the "GAS" method :
"gas, air, spark". Have those 3... something should
happen. So this is a discussion of how we get the Spark part.
Review of basic ignition designs
Basic Points/Distributor/ Coil.
A Frenchman named Etienne Lenoir
invented the electronic spark plug in 1860. Spark plugs haven't
really changed THAT much since then. But "firing" that
spark plug has been a better evolution. The Father of ignition
is Charles Franklin Kettering (man pictured left). In 1909, Kettering,
in association with Edward A. Deeds, organized the Dayton Engineering
Laboratories Company (Delco). As you've already started to guessed
from the name... they invented the first automobile generator.
That road to the generator invention brought them all the design
concepts that would be used to dream up starter motors, ignition
system components, etc.. So, it is in 1910 that Kettering
began work on new automobiles electrical systems. Also
notably, he invented the first "self-starter" in 1912.
Within two years, most cars were equipped with this new device.
Kettering went on to become head of General Motors research laboratories
and Vice President of the Corporation. "Boss Ket" would
eventually receive over 160 U.S. patents for his ideas.
It is in 1911 that he developed the first electrical ignition
system (or at least the design concepts that lead to that invention).
These early patents are hard to trace, but a little research
shows Kettering invented the "engine, starting, lighting,
and ignition system" (Patent
1171055 featured here in PDF). This early type of ignition
design is known as the Kettering system (points/condenser/coil)
or "induction" system. It became the standard in the
automotive industry replacing magnetos. It is rugged and reliable
but has drawbacks as you will see. A "lighting coil",
"lighting" system is a more tradition term for an auto/motorcycle
type self powered generator system. So you and I know Kettering
more as the man who invented the first practical engine-driven
generator (known as the "DELCO" generator). This was
Patent No 1150523.
A basic Kettering Ignition Design
A chain, belt, or gear from the engine drives a "DISTRIBUTOR".
Inside this distributor is a spring loaded contact switch ("POINTS")
riding on a revolving cam. The points would open and close to
fire a single coil which would produce the spark for the spark
plugs. Inside the distributor is also a "ROTOR"
which rotates to determine which plug wire gets the spark.
C.Points Dwell Adjustment
D. Point Riding On Cam
E. Rotating Cam Driven
Magnetism and Induced Current
In the mid 1800's Michael
Faraday (and others ... though "micro Farads " to this
day is the measurement of capacitance) developed the concept
that a current passing thru a coil wire creates a magnetic field.
More importantly, a magnet passing by a coil wire creates
a voltage current. The amount of current depends on the
magnetic size and speed (rate) the magnetic field passes (or
changes) by the coil. The math equation for this is e=-df/dt.
The change in magnetic field strength is df and the time
(rate of change) is dt. Technically, e is the "Induced
EMF". But we talk of this as the current produced in the
wire coil ... actually a PULSE.
Automotive COIL (also
called an "INDUCTOR")
An iron core is wrapped with 2
long "coils" of wire. The "PRIMARY" winding
on the outside and the longer "SECONDARY" winding on
the inside. The wire length ratio is typically 100:1 (the secondary
is 100 times longer than the primary).
The coil is fed 12v to the primary winding. This in turn creates
a large (enhanced by the iron rod) magnetic field which also
surrounds the Secondary windings. The coil is now storing a large
magnetic field (a Flux" field). When the +12v to the coil
primary winding is turned off the magnetic ("flux")
field inside the coil "collapses". This causes a "Back
EMF" (Electro Motive Force) current in the primary wire
of about 200-300volts. THIS IS IMPORTANT. Most think the
coil converts 12v to 30,000 volts. Not exactly. See,
this back EMF voltage of 300volts is now applied to both windings.
When the coil collapses this rapidly changing magnetic field
is also transferred to the "Secondary" windings as
current (remember the discussion above about magnetism... "a
changing magnetic field passing by a coil creates electric current").
The Secondary winding is 100 times longer so produces a voltage
about 100 times more than the Primary during collapse. Lets do
the math. The Primary ("Low Tension") wire is about
300v during the Back EMF spike. So the Secondary ("High
Tension") wire is 100 x 300=30,000 volts. This high voltage
is going somewhere, somehow to ground. The faster the power cutoff
is in the primary, the faster the collapse, and the faster (more
powerful) that spark is. So, when the points open (instantly
cutting off power to the coil) 30,000 volts goes to ground from
the secondary winding via the spark plug.
If you've understood some of this then you should be asking:
"how does the primary winding collapse to ground if the
points just opened its connection with ground?!??"
BINGO! To get the primary winding to collapse in the proper
fashion, we got to give it a way to get back to ground
during the collapse!
INTRODUCING THE "CONDENSER"
Ok, quick review: Due to magnetic
"flux" properties (research Teslar and the "left
hand rule" if you want to know more) the inductor (COIL)
encourages current flow towards the plug from the secondary winding.
But the collapsing magnetic field also produces the phenomenon
discussed above called "Back EMF". This 300+ voltage
spike in the primary winding would cause a mini-spark of it own
across the points. Another words, the primary winding would cause
a spark across the points just like the secondary will cause
a spark across the plugs. To facilitate the collapse of the primary
winding and to prevent point-gap spark a condenser is used.
The condenser is a large capacitor. Only the automotive
industry calls it a condenser (and no, I have no idea why). When
the points open this coil collapses. Remember, a coil output
is strongest when the collapse is fast and sharp. The condenser
slows this collapse by absorbing the initial shock (current)
of the primary winding. It helps shape the coil collapse to produce
the high power secondary collapse AND slows the collapse
of the coil just long enough for the points to get far enough
apart so the coil back EMF output won't arc across the points.
Without a condenser the backflow arcing and heat would destroy
the points (sometimes in a matter of seconds). However, the condenser
can't be too big either or the coil would collapse too slow and
not produce a strong spark. The charge the condenser absorbs
while the points are open is releases back to ground when the
points close again.
The capacitor also "harmonicly" tunes the coil,
raising the peak output voltage and increasing the secondary
voltage rise time. This increases the amount of energy transferred
to the spark plugs. If the coil secondary voltage rises too quickly,
excessive high frequency energy is produced. This energy is then
lost into the air-waves by electro-magnetic radiation from the
ignition wiring instead of going to the spark plugs where we
would like it to go.
Coil output is a function of coil windings "turns ratio"
and also voltage input. The more power you put in the more you
get out, right? And more power is better, right? Well.....
no. We'll talk more about higher coil outputs later, because
it becomes a bigger issue with CDI where you can REALLY pump
out some voltage. It only takes about 10-15,000 volts to
start the spark. Higher voltage is better because it can
jump a larger plug gap (which is good for igniting the charge)
and for overcoming ignition wear (worn/fouled plugs, wires, etc...).
A longer duration is also preferred because the EXACT millisecond
the fuel charge will ignite is shifting slightly. But big problems
occur with high coil outputs also. "Flashover"
refers to the discharge of the ignition voltage anywhere other
than at the spark plug gap. Too high a voltage and frequency
and the ignition is going to arc wires, leak out the side of
the coil, plugs, or convert to EMF.
The goal is to get a good strong spark with good duration
and one that jumps a good spark gap. Points are a mechanical
switch limited by how much current you can pump through them
without burning them up. So, in the ignitions points limit the
amount of power you can put into the coil. Points are limited
to about 250volts and 5 amps. Coils can handle up to about 7
amps and transistor switches about 10-20 amps. By the way, the
math for coil output is: e=L*( di/dt), which is.....
voltage = (coil inductance) times (the rate of change of primary
current as the stored coil current discharges).
MORE COMPLICATED COIL FOOD FOR THOUGHT
It gets more con"DENSER" than you thought. To really
get technical, a coil is really just an iron core transformer
(a step up transformer). This concept becomes more important
when we talk about CDI. But for now, we'll talk about a transformer
we've powered up with 12volt DC. That sounds reasonable
except that a transformer only works with "AC" current.
What? I'm so confused. Yeah, me too.
The points are opening and closing this electric flow thru
the coil (back-forth) at this really high rate. Hmmmmm, that
sounds a lot like "AC" current. See, the points have
actually created this psuedo sort-of AC current. The whole
system resonates at some frequency. That's why you hear
car ignitions on the radio. The circuit resonates at some
"tuned" frequency that any engine (at some RPM) can
produce in the am/fm frequency band.
In order to increase the coil voltage at startup some ignition
designs incorporate a "ballast" resistor. The
resistor is switched in and out of the supply voltage to the
coil. Once running, the resistor is switched in place and
the coil is actually getting less than 12volts. When the
engine is started, the resistor is removed and the coil gets
the full 12volts. This provides a much better spark at startup
to compensate for reduced battery voltage drawn by the starter.
When starting a cold engine, the plugs and the air are cold,
the cylinder pressure is up, and the fuel / air mixture is poorly
controlled. The oil is thick, the battery is cold and its voltage
drops as much as 60% because of the high current drained by the
Conventional ignition is affected by "Dwell time"
(or dwell angle). Dwell time refers to the time the points are
closed thus recharging the coil. Dwell angle refers to the crankshaft
angle of rotation made while the points are still closed. As
an example, if we talk about a 2 cylinder engine then the available
dwell angle would be 180 (360 degrees divided by
2=180 degrees). If dwell time or dwell angle (points closed)
is too short the coil may not have enough time to charge at high
RPM. So large dwell is better right? But, if dwell is too large
(points hardly open) then the points may not be open long enough
for the coil to collapse at high RPM. The ratio of closed points
to open is usually about 3:1.
The "dwell" time (points closed) has to be long
enough for the coil to fully "charge". Typical dwell
times (charge time) for a Kettering designed ignition are 1.0
to 6.0 milliseconds. Obviously, dwell time limits the ability
of points to control a coil to deliver high power at high RPM.
At high RPM you simple run out of time (you simply don't have
6 milliseconds). In addition, points are inherently a sloppy
mechanical device to begin with. And worse, at high rpm they
start to "float" off the cam. You can't get the points
to "spring back" fast enough so instead of opening
and closing they would hover just off the cam. Points can also
have a phenomenon called "bounce", where they don't
ride evenly on the cam. The upshot is that you can't control
the coils fast enough at high RPMs. Some race teams got around
this by using dual point systems overlapping the dwell times
to get what they needed.
CHARGE TIME , VOLTAGE RISE TIME , SPARK DURATION
Again, an induction coil setup charges (is "fully
saturated") in typically 1-6 milliseconds. Race teams in
higher RPM applications use low resistance coils to speed up
the charge to about 3 milliseconds. (We'll see later it takes
CDI about 1 millisecond). The time it takes for the coil to collapse
and reach %90 of its peak potential voltage is referred to as
voltage "rise time". The voltage rise time in conventional
ignitions is about 100microseconds. The result is
spark durations from inductive coil systems between 1-2 milliseconds.
As we'll see, CDI is a different animal. Here the voltage
rise time is a short 6 microseconds, but the spark duration is
shortened considerably also. The quick charge time is an advantage
in high RPM settings but the short spark length is a disadvantage
for starting and other high rpm compression/ratio fuel/mix situations.
Because of this, the longer duration of inductive discharge systems
is sometimes preferred over CDI.
Distributor with dual points
SPARK "TIMING" and COMBUSTION PROGRESSION
Obviously, the timing of the spark is crucial to getting the
full horsepower capability from an engine. So, we need to talk
about the desired timing of the spark, why that changes,
and how that gets changed while an engine is running. Most of
us think when the spark plug fires, the gas/air mixture in the
piston instantly explodes and that "exploding" pressure
is what drives the piston down. Well.... sort of.
Except, if the gas really did ignite all at once we really
would have an "exploding" engine (pistons everywhere!).
We have to slow down the process and take a millisecond by millisecond
look at it.
In the perfect ignition process we would achieve efficient
combustion progression at exactly the right time to produce the
optimum pressure gradients in the cylinder in complete harmony
with the piston movement relative to the piston rod to crankshaft
rotation. Say W-H-A-T ??!!
The combustion process does not happen instantaneously-simultaneously
in the cylinder but rather (as preferred) in a progressive pattern
from the spark. The spark ignites gases near the electrode which
then continues to burn (propagates outward) away from the spark
plug usually a spherical pattern. You'll often hear this combustion
progression across the cylinder referred to as the "flame
front". The design of piston and cylinder heads, combined
with spark plug placement is largely to get the best flame front
possible. The time it takes the fuel/air mixture "charge"
to combust changes according to many variables to include: fuel/air
mixture ratio, density (temperature), octane, how well
the cylinder has filled given volumetric efficiency, the "charge"
turbulence inside the cylinder, compression ratio, the
physical shape of the combustion chamber and piston head, spark
plug placement, rear-end ratio and car weight (both translate
to engine "load"), etc...
The important point here is:
The combustion process does take a period of time and
the length of that time changes (so when you need to start it
An Example: Combustion Process
Timeline with Before and After Top Dead Center Piston Position
Timing at 3,000rpm
5Btdc - 15Atdc
Combustion pressure actually starts to overtake
normal cylinder pressure (without ignition).
This 10 degrees of lag is known as "Ignition-delay".
Normal cylinder pressure without ignition is called
the "compression line"
near double normal
is now very
The peak cylinder pressure occurs between 10 and 20 degrees
"after piston top dead center" (ATDC) on most engines
and the combustion process is complete by 20 to 25 degrees ATDC.
The rise in cylinder pressure (pressure gradient) is technically
called flagregation. Obviously we want the combustion process
to occur during the down stroke and that high pressure point
at the optimum point of leverage for the piston arm to crankshaft
angle. To get this, the spark and combustion process
must be started much earlier. Look at the graph above and
you'll see at low rpm the spark occurs near 18 degrees before
piston TDC (BTDC). From 5 degrees BTDC to roughly 15 degrees
ATDC the combustion process is very rapid due to the progressing
flame front and the high rate of energy release.
As RPM increases it becomes obvious that we must start the
spark earlier or the combustion process will complete later in
the "power stroke". Another words, we're not getting
the best bang for our buck. Consider an example timeline.
At 1,200rpm the crank is rotating 1 per every .05 second.
Let's say the combustion process takes about 60degrees of that
revolution so... about .008 second. At 6,000 the timeframe
is 1/10 of that at .0008 sec. Typically, timing starts at a preset
level (about 10-20 degrees) and then is advanced to be earlier
as RPM increases. At about 3-4,000rpm the variables
that change timing tend to even out and so no further advance
is needed. In the example above timing advance caps at
about 3,500 rpm to 38 BTDC. The graph above is known as
the timing curve and while typical, can quite obviously be hugely
different for other motors.
DETONATION ("engine knock", "ping",
While not advancing the timing would result in power loss,
advancing the time to far ahead is even worse. If spark
occurs too early then the combustion (and resulting PCP) can
occur before TDC and the piston is still moving upward (not downward).
This is BAD! Detonation in any form is destroying the
piston head. I mean.... you can hear it banging itself to death!
Pre-ignition is like detonation except it is less related
to incorrect timing. Something else is causing the "charge"
to ignite too early. Often, a lower octane gas could cause this
because of its increased tendency to "auto ignite".
Could be a hot spot in the piston head or hot carbon buildup
on the spark plug end. Maybe the wrong "heat"
rated spark plugs is being used for the type of engine use (plug
running too hot). In any event, adjusting the timing can
solve this phenomenon. Unfortunately, older motors were
not adjusted for this while the motor was running, so the type
of gas you used was locked in at the tune-up. Now, there's
more lee-way as modern auto engines compensate for engine knock
as part of the timing adjustments.
Early Electronic Ignition Distributor with
dual points, centrifugal advance, and vacuum advance
In early ignitions the timing was advanced mechanically with
RPM. Normally this done was with small weights and counter
springs inside the distributor. As RPM increased, the weights
were pull outwards by centrifugal force and shifted the points
around the cam, changing the time they would open/close. The
photo above shows the mechanical advance with spring. Can
you notice also that the photo above is an early "electronic"
ignition? So, ignition advance is necessary despite the "type"
of ignition being used.
We advance the timing trying get the perfectly optimum time
for combustion during the power stroke. But as I mentioned earlier,
there are many variables that effect how long the complete combustion
process takes. So, advancing the timing based solely on RPM probably
doesn't cover all the possible scenarios. But how do you tie
those other variables into a mechanical contraption to adjust
timing? You see the problem. To adjust the timing ever slightly
more, autos in the 70's - mid 80's were fitted with an additional
device called "Vacuum Advance".
A large factor in the rate of combustion is the density of
the fuel/air mix (charge) in the cylinder. A low density
mixture burns slower than a high density mixture. So, the spark
needs to occur earlier when the fuel/mix charge is less dense
which happens at closed (idle) or part open throttle (cruise
= flat road) operation. At wide-open throttle (WOT) operation
the fuel/air mix is dense and combustion is rapid, so no additional
advance is needed.
This additional timing adjustment was made with a "vacuum
advance". This is a clever device that ties manifold pressure
to a mechanical advance mechanism. At "part-throttle"
operation (like cruising on a flat road) the manifold vacuum
is high and engine load is low. At full throttle (WOT)
the manifold vacuum is low and engine load is high. Look
at the above photo. The black device is the vacuum advance.
The open port would normally be connected by hose to the intake
manifold vacuum lines. A vacuum driven diaphragm on the distributor
advances the spark even more when the manifold vacuum is higher.
SO.... the spark is advanced more during closed or part throttle
TIMING ADVANCE SETTINGS/ADJUSTMENTS
Advance at Wide Open throttle = Initial Advance setting + Advance
from centrifugal advance only
Advance at Close or Part-Throttle = Initial Advance setting +
Centrifugal advance + Vacuum advance
In earlier conventional ignitions you could adjust the initial
timing by physically moving where the points were screwed onto
the distributor. This would need to be periodically changed as
the points wore down. They would also need to be changed
as the engine got older and the cylinder compression was less
due to ring/piston wear.
As for the timing advance: the centrifugal and vacuum advances
were set by their physical mechanical design in the factory.
No adjustments were normally possible. To change timing
you could swap out the device with one from another car or another
after market device.
SPARK PLUGS TELL THE TALE
The spark plug itself speaks volumes about how "optimum"
the combustion process is. Visually, if the burn is good
and combustion heating of the plug is correct (you have the correct
plug heat rating) then ..... the plug looks like this.
The insulator around the tip of the electrode will appear slightly
off-white (light beige). There won't be any heat disfiguring
of the electrode and no carbon buildup or soot.
WAIT A MINUTE .......
How important can timing advance be? My lawn mower doesn't
have it, or my small magneto outboard, weed-wacker, etc.... Well,
your right. And, I don't know exactly. So here's my guess.
I won't talk about 2-cycle 'cause that's a different animal.
But other small motors... you simply aren't demanding that much
out of them (throttle acceleration, load under changing rpm)
to notice any major performance loss from no advance. So the
motor is set at a worse case timing. Not optimum, but still runs
consitantly. At some point the motor and motor load/performance/hp
makes timing advance worth it.
Common Problems with a conventional
ignition system are:
- Points wear and erode (poor current flow and sloppy timing)
- Points limit power input to coil (limiting coil output)
- Point dwell limits and "point float" or "bounce"
limit high power at high RPM
- Mechanical Advance and vacuum advance wears
- Advance cannot be mechanically adjusted for all the variables,
- Points get wet and stop working altogether
- Timing belt (chain) wears and/or breaks
Basic Electronic Ignition Transistors and Pickups
The first improvement of electronic ignition was to replace
the mechanical points with a "solid state" semiconductor
switch called a transistor (pictured above) This is called a
"fast switching transistor" to be exact. The advantage
of an ignition transistor is that it can conduct up to near 400-500
volts (more power than needed), is extremely accurate fast (in
nanoseconds vs. milliseconds), and can last a long time in the
heat / vibration of an engine. The trick of course if how to
trigger the transistor switch. The common types of sensor systems
that have evolved are: magnetic, "Hall" effect, optical,
and (for trivia purposes only) "ECKO". In order of
"Hall Effect" Pickups Most Commonly
Used in Modern Autos
This is the most widely used type of ignition sensor. The
Hall effect (named after the American physicist Edwin Herbert
Hall, 1855-1938) involves the generation of an "electric
potential perpendicular to both an electric current flowing along
a conducting material and an external magnetic field applied
at right angles to the current upon application of the magnetic
field". SAY W-H-A-T ?!??. Practically speaking, a
current is passed though a silicon wafer. When a exposed to a
magnetic field this disrupts the current flow and distributes
more "potential" on one side of the wafer. This can
be measured, conditioned, and amplified to trigger the ignition
module. Hall Effect sensors are extremely accurate, they produce
a "square" wave signal perfect for solid-state applications,
and are very durable against heat / vibration. The rotor magnet
does not need to be as strong (you may not feel its pull with
a heavy screwdriver).
Most Hall effect rotors involve a stationary Hall Switch
and stationary magnet. What rotates is an "Interrupter Blade".
When the blade passes between the sensor and the magnet it blocks
the magnetic pull on the Hall Switch. When a "shutter blade"
is open, the magnetic field projects onto the Hall Sensor switching
it on. The easy way to identify a Hall system is the fact that
it must be externally powered. So, there's going to be that extra
Magnetic Pickups (Most common
in everything other than Auto)
This is very popular and still used for many applications
today because its a rugged durable design. In addition the sensor
is not powered (like in Hall Effect) so it can be used in self
powered magneto ignition applications. A coil sensor is used
to detect the "flux" a magnet produces when it passes
close by the sensor. This magnetic rotor is called a "Reluctor"
(I betch ya didn't know that one). The problem with magnetic
is that at higher RPM the sensor has trouble seeing "teeth"
close together on the magnetic rotor. This is a bigger problem
with many cylinder engines and/or high RPM applications. Also,
you may remember magnets tend to loose their strength with vibration
and heat. Also, reluctors are stronger magnets (you can definitely
feel the pull if you get a screwdriver close) which tends to
magnify things around it.
An infrared sensor triggers
when a rotor blade blocks the light path. Although accurate,
the sensor is sensitive to dirt and dust. This is not used much
but is very common in aftermarket ignition kits because its easy
to adapt to almost any application.
"Eddy Current Killed Oscillator" systems were used
by Lucas (yeah, the British "Dark Lord"). It involves
a 2 coil sensor that has current flowing. The sensor detects
the current disruption cause by a magnet passing by. It is similar
to "Hall Effect" in that it is extremely accurate and
durable. But for whatever reason is only used mainly in manufacturing
Modern Electronic Ignition Modules
For the most part, there are 2 types of ignition systems in
use today (or variations of them): Induction ignition (TCI=Transistor
Controlled Ignition) or CDI ("Capacitive Discharge Ignition").
Both systems use a sensor (discussed above) to trigger a transistor
switch (which has replaced the points). CDI is becoming the standard
and you'll see why.
Induction Ignition (Kettering design):
This is called an induction system because the coil is used
as a power storage (an "inductor") device for the spark.
Remember, the coil is powered up, stores near 30,000 volts, and
unleashes it when the coil collapses (power supply cutoff). A
feature of induction ignition is the slightly longer spark duration
while the coil collapses. This is an advantage when starting
and for igniting lean/high compression mixtures at high RPM.
These type of systems require coils meant for "induction"
ignitions (they have a higher resistance typically than CDI coils).
Induction ignitions are simpler in design (cheaper) and used
often on less sophisticated motors.
CDI ("Capacitive Discharge Ignition)
Commercial development of CDI happened around the mid 60's.
Up till then it was regarded as worthless and even dangerous.
Well.. the dangerous part is somewhat right as you will see.
If your really bored here's a 1965 Danish sketch of an early CDI design and the bike it was tested on (a 90cc Kawasaki
motorcycle). Automotive CDI was pioneered mainly by Bosch in
Europe. In 1979 they introduced the "Bosch Motronic".
Today we see a variety of names to include: Ford's TFI (Thick
Film Integrated), GM's HEI (High Energy Ignition), DIS (Distributorless
Ignition System), ECU (electronic control unit), and many others.
Again.... the differences Kettering (TCI) vs CDI
CDI ignition is most widely used today on automotive and marine
engines. A CDI module has "capacitor" storage of its
own and sends a short high voltage (about 250+ volts) pulse through
the coil. The coil now acts more like a transformer (instead
of a storage inductor) and multiplies this voltage even higher.
Modern CDI coils step up the voltage about 100:1. So, a typical
250v CDI module output is stepped up to over 25,000v output from
the coil. The CDI output voltage of course can be higher. So
you'll see CDI systems claiming coil output capability over 40,000-60,000
volts!!? As you will see this is not exactly what happens at
the plug but for math purposes it works out. The huge advantage
of CDI is the higher coil output and "hotter" spark.
The spark duration is much shorter (about 10-12 microseconds)
and accurate. This is better at high RPM but can be a problem
for both starting and/or lean mixture/high compression situations.
CDI systems can and do use "low" resistance coils.
With the Kettering Induction ignition design, the coils are
powered all the time at 12 volts and are commanded to collapse
to spark by the ignition module. Here, the ignition module disconnects
the primary winding coil ground. The coil secondary winding collapses
to spark at about 30,000 volts. In the CDI design, the coils
are not powered. They receive a short high (250 volt) pulse from
the ignition module and then amplify that (100:1) to a much larger
voltage spike (about 40,000 volts) . Since the potential output
of a CDI coil can be over 40,000 volts you have stickers all
over your engine bay reminding you that: This
can KILL you!!
Advantages / Disadvantages of Electronic
The advantages of solid state are numerous but the big one
is : "no moving" parts. This should translate to control
and reliability impossible to achieve in any mechanical system.
The term "engine tune-up" is nearly meaningless with
respect to modern ignition systems. Outside of replacing plugs
and inspecting wiring there is not much else to do. More than
a few mechanic shops exploit the public misunderstanding of modern
engines. Having said that, the disadvantage of electronic
ignition is simply reliability.
A desktop computer circuit board should last a LONG
time in theory, and yet you know quite clearly it does not sometimes.
Ignitions have suffered the same evolution of making electronics
that can stand the test of time. Early ignition systems were
particularly prone to "component" breakdown. Anyone
who has owned an older British sports cars will understand the
term "the dark Lord of Lucas" (Lucas Electronics were
notorious for their failure).
Solid state components are particularly sensitive to
heat, thermal stress, vibration, moisture, and power surges (basically,
everything an engine is about). So, great strides have been made
to beef up and improve CDI reliability. These include things
What's Better? Induction
- Improved cooling and heat sinks
- Epoxy resin or Epoxy-rubber encasing components so they can't
- Using separate ignition modules for each plug (so a single
failure won't kill the whole engine). This concept was first
exploited in outboards where each plug has a separate "power
pack". Power pack failures were a big problem in the outboard
industry for awhile and not funny when you are 60 miles from
land in a small sport fisherman with 1 engine.
- Heavier duty components that can withstand the heat, vibration,
and "duty cycles".
Clearly, CDI is being used for most all modern auto / motorcycle
/ marine applications. It is also the choice for most high
rpm race engines. This is simply due to the ability to fine tune
all aspects of the combustion process electronically. Where
simplicity and reliability is a factor, induction systems have
an advantage. That is why you see them most often in aircraft
engines. High revving RPM control is not the emphasis but
rather reliability. The longer spark duration of induction systems
gives a better chance that combustion WILL take place!
Anyone who has ever flown at night over mountainous terrain and
has heard "auto-rough" knows what I'm talking about
MISC IGNITION INFO
Dwell time refers to the time the distributor points are closed.
The dwell angle was the amount of rotation of the crankshaft
that corresponded to the points being closed. This affects the
charge time of the coil. Dwell was important then because at
higher RPM the dwell time (points are closed to charge the coil)
was not enough to fully charge the induction coil. That meant
less voltage spark at higher RPM (...BAD). There was also the
problem of how fast a point could open and close without "floating"
(a problem you have with valves also). There was a real balance
between dwell time at high RPM, how much voltage you needed for
high RPM spark, how much voltage you could actually push thru
a point without burning it up, and then what would happen at
low rpm (long dwell times) when all that voltage was just heating
up the coils.
Electronic Timing Curves
In newer CDI systems this term is near meaningless for several
reasons. Solid state transistors control the discharge pulses
electronically with near instantaneous timing in the nanoseconds.
So the dwell times can be finely controlled to achieve the best
coil output. Transistors can handle a LARGE amount of voltage/current
(compared to points). And, newer generation coils are extremely
fast with charge "saturation" times around 1milliseond.
Their coil pulse "voltage rise time" to the plug is
VERY fast at around 6 microseconds. So charge / discharge times
are not a huge factor (unless racing). Newer racing ignitions
(like MSD) are NOT producing bigger sparks with long durations
but in fact getting more efficient burn by producing very controlled
multiple short duration sparks to the plug.
Timing curves can be manipulated in great detail to maximize
engine horsepower. Replaceable "high performance" chips
for many sport cars are routinely offered by aftermarket companies
as a byproduct of racing technology. While it seems logical that
auto manufacturers already put the "best timing" they
had in an engine design, you could argue that they also may detune
an engine slightly to address reliability and longevity of an
engine. It has always been a balance between performance (Horsepower)
and "how long" an engine will last. You can often squeeze
a few more HP out of an engine by improving the timing curves.
Low vs High resistance coils
Induction ignition uses higher resistance coils compared to
CDI is ystems that can use lower resistance coils. So....Do
Not Use a "racing" -or- low resistance type
coil in an "induction" ignition (or TCI) system unless
it is specifically designed for that. The low resistance coil
will flow more current thru the TCI and produce the legendary
"Hot Toaster" effect. Though it will work for awhile,
you will eventually burn the TCI module out.
Obviously, it seems harsh on the components to be powering
the ignition system when when the engine is NOT running but the
key is on. So, both types of ignition designs employ some auto
shutdown of the ignition modules. This is usually tied to the
pickup sensors. If no RPM is observed then the ignition is shut
off (as well as is the high pressure fuel pumps for the fuel
injection system). I mention this because in older Induction
(TCI) designs when the ignition module shuts off, it collapses
the coil. You would occasionally get a single backfire a second
or two after trying to start a engine that didn't run.
The Big Myth:
Your engine needs a tune up!
If you've understood any of this then the question that should
come to mind is: What do they adjust when you take your car in
for a "tune-up". The obvious answer: NOT MUCH !! If you own an older electonic
ignition (80-late 90's) a shop can:
- Replace the spark Plugs
- Check the plug wires
- Check the wear and replace the distributor rotor (if there
- Check the wear and replace the timing belt (if there is one)
There are no points to replace, no dwell or timing to change.
In the "New Millennium" high-end car & marine engines
have neither a timing belt, distributor, plug wires, etc... You
guessed it., there ARE NO MOVING PARTS and NO
ADJUSTMENTS. It either WORKS -or- IT DOESN'T.
All they can do is change the spark plugs. AND YOU SHOULD.
Now... in fairness, the truth is: a dealer (or very good
shop) WILL plug your high end machine into a $300,000
diagnostic computer which will tell whether all those sensors
(you don't know about) are working correctly to produce max horsepower,
best gas mileage, no knocking, and clean "California"
exhaust so LA people can breath what little is left of their
air and your catalytic converter doesn't have a Chernobyl meltdown.
And... these things could be important.
But generally, if your car is running good (no engine light)....
just change your spark plugs occasionally (if you can find them)
The common question is: how much power do you need in an ignition
system? The more the better right? Well, not exactly. Lately
it seems the talk is always centered around high voltage (50,000+)
low resistance racing coils, aftermarket ignitions (MSD, Accel),
etc... What is really important?
Coil aspects: "Rise time" refers to the time
needed for the coil voltage to reach 90% of its peak. Fast rise
times are desired as they help prevent and breakdown plug fouling
(or "plug tracking"). Plug fouling occurs when the
spark is dissipated and runs to "ground" across deposits
on the plug's surface instead of across the plug gap.
These deposits can be carbon buildup, corrosion, lead salts,
water, etc. Rise times for ignition systems are typically 80-120
microseconds for induction systems and 6 microsecond for CDI.
It takes about 10-14,000 volts to initiate the spark
across the plug gap. After the initial arc the voltage required
to sustain the arc is much less and drops off significantly.
So while you may have a manufacturer claimed 60,000 volt racing
coil you can't actually get that across the plug. Since the advantage
of CDI is the higher coil output, how does that get used. Well,
normally it doesn't. The extra power possible in the coil is
"Reserve Voltage". As the plugs wear, fouling, plug
wires and connections get worse then the required firing voltage
may go up 1-5,000 volts. So the "hotter" CDI coil output
can help overcome these obstacles and the ignition system will
last longer. So, its not that its working better.... but rather
lasting longer that makes a hot coil good. The ideal coil output
needed for normal applications is about 30,000 volts.
High RPM / High Compression / Racing applications: Newer
techniques are being used to increase spark output. Additionally,
CDI typically has a very short spark duration near 10-12 microseconds.
As discussed you can't push more than about 20,000 volt across
the plug without other strange phenomenon happening. If you were
to try you would see arcing down the side of the plug, across
carbon buildups at the electrode end, out any weak points in
the wire insulation, connections, etc... So how do you get a
better spark? Newer ignitions (like MSD-5 for example) are outputting
a finely controlled multiple spark pattern into the plug. Instead
of one big spark a shower of short duration sparks are flooded
across the combustion stroke. This makes for a much more efficient
burn. Using this technique newer CDI can achieve longer spark
duration times (near 250). This is particularly better for starting,
lean mixtures (which are hard to ignite), and high compression
Ignition Memory: Most modern auto ignitions keep a
profile of engine performance with respect to recent usage.
So the type of gas you use, environment you drive in, and the
way you drive with effect how the engine anticipates its timing
curves and ignition / fuel / emission settings. Another
words, if you drive mostly in Florida and then move to the high
altitude Rockies.... you've car is going to run rough for awhile
till it catches up. Most ignition memories can be "zeroed"
out by removing power from the car for a "noticeable"
length of time (30 minutes?)
Ignitions Today and in the Future
The GOOD NEWS is the ignitions systems are getting
very reliable, accurate, and sophisticated. The BAD NEWS
is that you can't work on it. I looked under the hood of a new
Ford F-150several years back .... and .... "where's the
spark plugs?" For that matter, where are the spark plug
....wires..... and ..... coil !??! Yeah, do I feel dumb.
New generation auto ignitions are designed for more accuracy,
better efficiency and reliability. This includes" crank
angle" sensors to improve timing and fuel injection accuracy
(example, mounting the Hall sensor and magnets on the flywheel).
Newer coils will be wound around an "E" shaped pole
(not "center wound"). They will look like a square
module and not the round cylinder you've seen all these years.
"DIRECT IGNITION": (first seen on
Saab's and GM's ). In this setup each spark-plug has its
own coil -or- a single coil will supply opposing cylinders. The
advantage here is no more rotor. DIS systems are usually
inductive ignitions and employ the "wasted spark" strategy.
This refers to a coil that supplies the spark on every revolution
so the cylinder will get one during compression and also during
the exhaust stroke. The wasted spark design cuts the coil charge
time in half so is not often used on extreme high rpm race engines.
"COIL ON PLUG": This design is becoming more
prevalent. You'll see separate coils mounted directly atop
each spark plug. This improves spark power (no plug wires), accuracy,
reduces RFI (radio frequency interference) problems, and most
importantly eliminates the distributor (the last moving mechanical
device to wear in ignition systems). The advantage here is total
control, no moving parts, and extreme high rpm capability.
This is the design used on most Indy and F1 engines which are
generating nearly 15,000-16,000 rpms.
Timing and spark duration times can adjusted across the RPM
range. Additionally, timing advance will be calculated for a
variety of inputs including increasing RPM vs. decreasing RPM
(braking, coasting, or accelerating), throttle position, temperature,
altitude, etc. Even "engine knock" (detonation) will
be eliminated by timing adjustments (using "listening"
sensors like piezoelectric crystals bolted to the engine )
Making Yours a Better Ignition
(For normal operations)
Simple , and in order of importance: Plugs, Plug-wires,
The biggest enemy against ignition is RFI and insulation leakage.
Basically, you want the best insulated (thicker, 12mm?) and well
made plug wires you can get. Remember this simple gradeschool
phrase: "... electricity takes the path of least resistance".
In the case of your engine this will be arc tracking across the
plug and/or out the plug wires due to insulation breakdown. Upgrade
your CDI module (example: performance timing curves, high output,
etc...) if you want to spend a little more for higher performance.
Although a good coil is always a plus, we've discussed how meaningless
SOME manufacturers high output claims can be in practicality.
In short, spend your money on a good well insulated system.
As long as the coil and CDI can meet the rpm demand's of the
engine you are on top of the game.
Ignition Plug Wires
There are 3 basic types of conductors used in automotive applications:
Carbon string, solid and spiral wound. Spiral wound is becoming
the more popular so there's probably a good reason. I recommend
getting on some really good "wire" sites and scoping
out the differences. MAGNECOR or NGK would be a good place
This is a bar conversation if ever there was one.....right
up there with synthetic vs normal oil. But I believe most standard
plugs these days are more than adequate. NGK, Bosche, Champion.....
are all good. Platinum plugs are the way to go because
they simply last longer. A good plug should last about
a years worth of normal use (20-30,000 miles). If using the engine
in more extreme load/rpm ranges you should consider a plug with
a different "heat" rating. As an example, an engine
operated in constant high rpm/power/load situations might benefit
from a colder (than "stock") plug rating. Since
the engine is running hotter, a "cold" plug would run
cooler thereby preventing pre-ignition (helping the plug and
engine last longer).
Ignition Discussion Links
From dave andrews motorcycle repair page
dan motorcycle garage
from simple digital systems fm-4
runtronic spark ignition
Some Ignition Links and searches
Try to use search word that would only be used in discussion
of ignition systems. Examples:
capacitive discharge ignition
votage rise time ignition
centrifugal advance ignition
vacuum advance ignition
hall effect ignition
Just throwing some names out here IN NO ORDER.
Intelligent Ign System (Iss)