IGNITION AND COMBUSTION
IGNITION AND COMBUSTION
Possibly nowhere else in the performance engine world is there more
misinformation, myth and downright false claims than in the ignition component
aftermarket. Many manufacturers of these products prey on the naive, uninformed
consumer. We will try to help you understand what is really going on and what
works and what doesn't.
The Ignition/Combustion Process
Many people think that when the sparkplug fires, the fuel/air mixture explodes
instantaneously, driving the piston down. If this really happened, engines would
last only a few minutes before they literally grenaded.
Let's examine the process and dynamics involved from the moment that the intake
valve is fully open. With the piston moving down the bore, cylinder volume
increases, cylinder pressure decreases, allowing the higher pressure in the
intake tract to push the fuel/air mixture into the cylinder. As the piston
starts back up and the intake valve closes, cylinder volume decreases and
cylinder pressure increases.
When the crankshaft reaches about 30 degrees before top dead center, the spark
jumps the gap between the plug electrodes. The purpose of the spark is to raise
the temperature of a very small portion of the fuel/air mixture above its
ignition temperature. This is the point where true combustion begins. As the
exothermic reaction starts, the mixture directly adjacent to the spark plug is
also ignited and the process rapidly progresses out from the plug in a roughly
spherical shape.
At about 20 degrees BTDC, the rate of heat release causes the cylinder pressure
to rise above the compression line which is what the cylinder pressure would be
at a given piston position without ignition. Notice that it has taken 10 degrees
of crank rotation to generate this pressure level. This is known as the
ignition-delay period.
The rate of pressure rise is a function of the rate of energy release vs. the
rate of change of combustion space or cylinder volume. The rate of energy
release is directly related to the flame propagation rate and the area of
reacting surface. Flame speed is dependant on fuel/air ratio, charge density,
charge homogeny, fuel characteristics, charge turbulence and reaction with inert
gasses and the metal combustion chamber, cylinder walls and piston.
In technical terms, the pressure rise is referred to as flagregation. No two
combustion cycles progress at the same rate or at a uniform rate. Some start
slow and end slow. Some start slow and end fast. Some start fast and slow down.
Generally, only the ones that end too fast will lead to knocking as the rapid
pressure rise may happen too soon with the cylinder volume still decreasing or
not increasing fast enough. Usually, not all cylinders will knock at the same
time or on the same cycle because of this.
By the time the crank is at about 5 degrees ATDC, the cylinder pressure is about
double that of the compression line. From this point to roughly 15 degrees ATDC
the combustion process is very rapid due to the increasing area of inflamed
mixture and the high rate of energy release. The peak cylinder pressure (PCP)
occurs between 10 and 20 degrees ATDC on most engines and the combustion process
is complete by 20 to 25 degrees ATDC.. The peak temperature within the
combustion gasses will reach somewhere around 5000 degrees Fahrenheit and
pressures may be anywhere from 300 to 2500psi depending on the engine.
Obviously it is very important to have the crankpin at an advantageous angle
before maximum cylinder pressure is achieved in order that maximum force is
applied through the piston and rod to the crankshaft. If the mixture was ignited
too early, much of the force would simply try to compress the piston, rod and
crank without performing any useful work. In a worst case scenario, the cylinder
pressure would be rapidly rising before the piston reached TDC which would have
the cylinder volume decreasing at the same time. This will often result in knock
or detonation which is counterproductive to maximum power and engine life.
Detonation or knock is defined as a form of combustion which involves too rapid
a rate of energy release producing excessive temperatures and pressures,
adversely affecting the conversion of chemical energy into useful work.
If PCP was achieved too late, again, less work would be performed. Most of the
useful work is done in the first 100 degrees of crank rotation. Most combustion
must be done with the piston in close proximity to the chamber so that the
minimum amount of heat (energy) is lost into the water jackets and the maximum
amount of energy is delivered to the crankshaft.
Let's examine the different variables regarding flame speed separately and their
effects:
Fuel/air Ratio
Gasoline can be ignited in a non-stratified charge type engine between limits of
roughly 11 to 1 (rich) and 20 to 1 (lean) air/fuel ratios. Most gasolines will
burn fastest at ratios in the 17 to 1 range. The stoichiometric or chemically
correct ratio is around 14.7 to 1 which also results in the lowest average
emissions. Best power is obtained with a rich mixture of around 12 to 1.
Charge Density
Charge density is affected by the pressure and temperature of the charge. The
higher the charge density, the more rapidly it will burn.
Charge Homogeny
This refers to how uniformly the air and fuel molecules are distributed in the
charge. This is a very important factor with regards to successful ignition. If
there is a big lump of fuel molecules with no air present between the spark plug
electrodes or vice versa, at the time that the spark jumps, there will be no
ignition. If the charge between the electrodes is leaner than 20 to 1 or richer
than 11 to 1, there is little chance for ignition. Ideally, the molecules
throughout the whole charge should be evenly spaced and distributed. This allows
for a smooth rate of burn. If the charge is randomly mixed, there will be local
variations in flame front propagation rates which will not produce maximum power
as these may advance or delay when PCP is achieved. This phenomenon is known as
ignition probability.
Charge Turbulence
Because the charge is in constant motion from the valve and port flow
characteristics along with inertial effects and piston motion, the mixing of
fuel and air molecules is dynamic. From one split second to the next, the actual
mixture and molecular distribution changes. This can mean in some instances,
that if a very short duration spark was initiated at one instant, the mixture
might not ignite, whereas only a half millisecond later, conditions might be
perfect for ignition. For this reason, very short duration sparks are
undesirable. A long duration spark or multispark ignition system will ensure the
highest ignition probability.
Fuel Characteristics
Low compression engines usually run well on low octane fuel because they have
relatively low charge densities and the burn rate within these confines is
usually predictable. A low compression engine switched to 118 octane race fuel
will always lose power unless the ignition advance is increased to compensate
for the slower burn rates. Even then, a low CR engine may lose power with the
timing optimized for high octane fuel.
A high compression or turbocharged engine operates with much higher charge
densities and consequently faster burn rates. The high octane fuel permits these
rapid burn rates because it has far less tendency to autoignite and detonate
under these conditions. As a result, high compression and turbo engines cannot
realize their full hp potential without high octane fuel.
Inert Effects
Inert effects constitute 2 areas. Residual exhaust gasses left over from the
last exhaust stroke tend to dilute the fresh charge and slow down burn rates.
Camshaft timing, port flow and exhaust backpressure will affect charge dilution.
Nitrogen is the major constituent of air and is essentially inert in the
combustion process. Its presence substantially lowers the burn rate but since
there is little that we can do about it, it is generally ignored. Nitrous oxide
can be injected along with extra fuel to increase charge density as it contains
a much higher concentration of oxygen than does air. Oxygen is the essential
element in the combustion process.
The second inert effect concerns the relatively cold, metal engine parts in
direct contact with the combustion gasses. Combustion will not easily take place
in areas where the gas temperatures are well below the ignition temperature.
This property is often used to advantage on engines to reduce the tendency to
knock.
On many engines, a squish or quench area is used to negate combustion in certain
areas to avoid knock. By having a matched area where the piston and combustion
chamber come in close proximity at TDC, the gasses are kept cool enough so that
they will not ignite until the piston has moved down the bore and cylinder
volumes are increasing. This keeps the rate of pressure rise below the knock
limit. Some people are dismayed when they install a thicker head gasket to lower
the CR and have knocking worse than before. This is because they have negated
the designed-in quench effect. A large squish area also tends to promote
increased chamber turbulence which is important for mixing and power at high
rpm.
Combustion Chamber Shape and Spark Plug Location
Combustion chambers and spark plug location and the number of plugs will have a
marked effect on the time required to complete the combustion process. A large
open chamber like a hemi which has a high surface to volume ratio, will combust
more slowly than a wedge or modern pentroof chamber simply because it has more
cold, metal molecules in contact with the combustion gasses which tends to slow
reaction rates. For this reason, these chambers will require that the spark be
initiated sooner to achieve PCP at the correct time.
The slowest combusting chamber would be an open chamber with a large bore size
and the spark plug at one edge of the chamber. The flame front has a long
distance to cover to complete combustion. By placing the plug in the center of
the chamber, you halve the distance that the front needs to travel and will be
able to reduce the spark advance needed to achieve maximum power. Another
solution would be to add another spark plug to create two flame fronts which
would also require much less time to combust. This is the solution in most
aircraft engines where big bores and poor fuel distribution and homogeny require
solutions to increase ignition probability.
Modern 4 valve engines with shallow pentroof chambers and a central plug
location are fast, efficient combustors, requiring minimal advance for maximum
power.
Inductive Discharge Coils
Generating the spark on most production automotive systems is accomplished by
the coil. Coils have 2 sets of windings, a primary and a secondary. The typical
coil will have around 250 turns of wire on the primary and about 25,000 on the
secondary for a ratio of 100 to 1. The secondary section often uses an iron core
to increase its inductance. Coil resistance on the primary will be from .5 to
2.5 ohms usually and on the secondary, between 5000 and 12,000 ohms.
The inductance and resistance of the coil will determine how quickly a coil can
be charged and discharged.
A transistor is used to switch the current flow off and on in the primary coil.
When the transistor is switched on, current rapidly builds from 0 to a maximum
value determined by the coil inductance and resistance. This current flow
induces a magnetic field within the primary. When the current is turned off,
this magnetic field collapses which cuts the windings of the secondary coil and
induces a high voltage surge.
The output voltage is determined by the rate of field collapse and the windings
ratio between primary and secondary. Because the path to ground for the current
involves the spark gap, the initial resistance is extremely high. This allows
the voltage to build to a very high value until it gets high enough to jump the
plug gap. The potential difference must be high enough to first ionize the gas
between the electrodes. The ionized gas creates a conductive path for the
current to flow. At this point, the arc jumps and current flow is established.
It is important to note that if only 10,000 volts are required to jump a plug
gap under a given condition, that will be the maximum delivered. It is also
important to note that the spark duration is determined by coil inductance and
total resistance of the circuit. Most inductive discharge systems have a spark
duration of between 1 and 2 milliseconds.
As cylinder pressure increases, the voltage required to jump the plug gap
increases. This is especially true in turbocharged engines under boost. The
second problem on high performance engines with high rev limits, is that there
is less time to charge the coil with increasing rpm. As such, a high rpm, high
output, turbo engine puts greater demands on the ignition system than does a
5000 rpm naturally aspirated engine. Additionally, with a single coil, the more
cylinders that you are firing, the less rpm you can run before the spark voltage
becomes insufficient to jump the plug gap. A V8 engine would only run to about
half the rpm that a 4 would before encountering misfire.
Coil Charge Time and Saturation
The amount of time it takes to charge the coil or bring the current to maximum
in the primary windings is called charge time. Input voltage and coil resistance
are the main parameters relating to charge time. When the current has reached
its maximum value in the primary, it is said to be fully saturated.
If current is applied longer than the time needed to fully saturate the primary,
energy is wasted and there is nothing more to be gained. If the current is cut
off before saturation is achieved, the maximum spark energy available will be
reduced.
Typical coils require charge times of between 2.1 and 6 milliseconds. Obviously,
a coil requiring 6 milliseconds to saturate would be unsuitable on a high
revving engine as there is not 6 milliseconds available to charge it between
discharges at high rpm. For this reason, most performance and racing coils have
low primary resistances between .5 and .7 ohms and are fully saturated in less
than 3 milliseconds. This permits full coil output at very high rpms.
Most 4 cylinder engines below 200hp/L specific output will run fine below 9000
rpm with a good inductive discharge coil setup.
Capacitive Discharge Ignition
On very high output engines, especially V8 and V12 engines, a single inductive
discharge coil is inadequate to supply spark at high rpm and high cylinder
pressures. This is where the CD ignition or CDI is used to reduce charge times.
The MSD line is very popular worldwide, especially on American V8 engines fitted
with distributors.
In normal inductive discharge coils, only 12-14 volts is available from the
battery to charge the primary. The CDI charges capacitors to store a high
voltage kick to fire to the primary side, putting between 30 and 500 volts onto
the primary windings which reduces the charge time substantially. A coil that
would take 3 milliseconds to become fully saturated with 12 volts is now fully
saturated in less than 1 with a CDI. The same engine now will be able to turn
twice the rpm and experience a major increase in cylinder pressure before
encountering misfire.
A slight drawback to CDIs are their shorter spark discharge times although it is
better to have a shorter spark rather than no spark. One other concern when
using a CDI and a distributor especially ones having closely spaced wire
terminals is the possibility of crossfiring. This may happen when the coil
voltage is so high that the spark will jump to adjacent terminals which can be
very destructive. Most high output CDI systems will also run a larger diameter
cap to reduce this possibility. Ignition rotor life may also be somewhat
reduced.
Some CDIs also include a multispark function where more than 1 spark is
generated after the first spark. This improves ignition probability but it is
usually discontinued above 3000 rpm because there just isn't enough time
available to make this useful. If the first spark didn't ignite the mixture at
8000 rpm, the 3rd spark a few milliseconds later would light off the mixture
very late, leading to PCP occuring late with little useful power being
delivered. Igniting late is probably better than not at all though.
One company who makes CDIs claims that their system cures all misfires among
other dubious benefits. This is a physical impossibility as we have seen above
that many factors could contribute to a misfire which are totally outside the
realm of the ignition system. An over rich or over lean condition or broken
parts cannot be fixed by ANY CDI system.
Many CDIs also claim increased fuel economy which is unlikely as well. Besides
the high rpm coil saturation advantages, perhaps the only other one would be
greater resistance to plug fouling. However on modern, well tuned engines in
proper repair, plug fouling is really a thing of the past anyway.
Direct Ignition
Commonly known as DIS. Most DIS units are of the inductive discharge type. They
use a double ended, isolated coil which fires one cylinder on the compression
stroke and one on the exhaust stroke simultaneously. This is referred to as a
waste spark strategy.
The advantages of DIS are the elimination of the distributor and the associated
rotor to terminal air gap and moving parts, plus the addition of twice the
number coils so that one can be charged while the other is discharging. This
feature allows DIS to produce a very powerful spark up to around 10,000 rpm.
The disadvantage of the waste spark strategy is that the coils are firing at
twice the frequency needed which reduces the charge time window at extreme rpm.
DIS systems will usually fire the plugs on any engine up to 10,000 rpm and
300hp/L specific output.
Coil on Plug
The latest, greatest ignition, is the coil on plug setup (COP). This method uses
a small inductive discharge coil clipped directly to each spark plug. It
eliminates plug wires entirely and does not usually use the waste spark strategy
so it has twice the amount of time available to saturate. This basically doubles
the RPM capability of the system over other ignition systems. This is the system
used in FI and Indy Car engines which generate outputs of over 300hp/L and
16,000 rpm.
Many new top line production engines are starting to use COP.
Ignition Wires
The purpose of the ignition wires is to conduct the maximum coil output energy
to the spark plugs with a minimum amount of radiated electromagnetic
interference (EMI) and radio frequency interference (RFI). On most street
applications using digital computers for engine management control, excessive
EMI and even RFI can interfere with ECUs and cause running problems.
There are 3 basic types of conductors used in automotive applications: Carbon
string, solid and spiral wound. Most production engines come equipped with
carbon string or spiral wound. The solid core types are used exclusively for
racing, mainly with carbureted engines because they offer no EMI or RFI
suppression. They generally have a low resistance stainless steel conductor.
These types are rapidly losing favor, even in racing circles.
The carbon string type is the most common and work just fine in most stock type
applications. The conductor is usually a carbon impregnated fiberglass
multistrand. Suppression qualities are fine with resistances in the 5K to 10K
ohms per foot. They are cheap and reliable for 2 to 5 years usually, then they
may start to break down and should be replaced. High voltage racing ignitions
will likely hasten their demise.
The spiral wound type is probably the best type for any application. The better
brands offer excellent suppression, relatively low resistance and don't really
wear out. Construction quality and choice of material vary widely between
brands.
NGK makes low priced wire sets which work well in performance and street
applications however the terminal ends tend to be a bit fragile.
Magnecor makes excellent quality spiral types with high suppression qualities.
These are reasonably priced for the quality you are getting and proven worldwide
over many years under extreme conditions.
Some amount of resistance is required along with proper construction to achieve
high suppression levels. Resistance is also important to avoid damaging some
types of coils and amplifiers due to flyback and coil harmonics. Beware of wires
claiming to have very low resistance. These CANNOT have good suppression
qualities.
Beware of any wires claiming to increase hp. Ignition wires CANNOT increase hp.
As long as the wires that you have are allowing the spark to jump the gap
properly, installing a set of $400 wires is strictly a waste of money.
Lately, some truly "magic"wires have come onto the market claiming to not only
increase power but also to shorten the spark duration from milliseconds to
nanoseconds. As we have seen above, spark duration is determined primarily by
coil inductance and coil resistance so these wires CANNOT shorten the spark
duration by the amount claimed. The wire resistance has a minimal effect on
discharge time because of the high voltage involved. We have also seen above
that a very short duration spark is in fact detrimental to ignition because of
lower probability.
These same wires claim to increase flame front propagation rates and the ability
to ignite over- rich mixtures for more power. We have again seen that once
ignited, the mixture undergoes the flagregation process and that the progression
rate of the flame front is totally independent of the spark. We have also
learned above that most gasolines will not ignite nor burn at air fuel ratios
richer than 11 to 1, period, and that maximum power is actually achieved at
around 12 to 1 AFR so the second claim also has no basis in fact.
These wires use a braided metal shield over the main conductor which is grounded
to the chassis. This arrangement offers poor suppression because it does not
cover the entire conductor. Any energy leaking out of the main conductor by
induction is actually wasted to ground and will not make it to the spark plug.
These wires also have very low resistance which as we have seen above, can have
a detrimental effect on coils and ignition amplifiers due to severe flyback
effects which are normally damped by circuit resistance.
Other claims for these wires include current flows of up to 1000 amps. The
current flow in the ignition circuit is determined by the coil construction and
drive circuits, not by the ignition wires. Most ignition systems are current
limited to between 5 and 15 amps. The most powerful race systems rarely exceed
30 amps. To flow current at 1000 amps, you would require #0 welding cable for
the ignition system!
Spark Plugs
The last part in the ignition system is the spark plug itself. The average plug
consists of steel shell which threads into the cylinder head, a ceramic
insulator, an iron or copper core leading to a nickel or platinum center
electrode and a ground electrode of similar material. The spark jumps between
the center and ground electrode. Certain special application plugs may have
multiple ground electrodes.
Different heat ranges are available depending on application. For constant high
power applications, a colder than stock plug is usually selected to keep
internal temperatures within limits.
Again, many "magic" plugs come onto the market from time to time expounding the
virtues of their incredible new design, usually offering more hp of course.
Split electrode plugs are a waste of money because the spark will only jump to
one of the electrodes at a time in any case.
You will find that most reputable engine builders in the higher forms of racing
use pretty standard NGK, Bosch or Champion plugs with pretty standard electrode
setups. A properly selected, standard plug will easily last 25,000 miles of hard
use in most engines. A platinum tipped plug will easily last twice as long on
most engines. There just isn't any rocket science here. Modern spark plugs
coupled to modern ignition systems in a modern engine are extremely cheap and
reliable. In most cases, on street performance and even race engines, a $2, off
the shelf, NGK plug will work just fine.
Wild Claims
If you see an ad for any ignition system component claiming substantial power
increases over stock, BEWARE. Most of these claims are total bullshit with no
basis in fact. Even if the seller advertises a money back guarantee, you will
still be responsible for shipping the product back to them and probably a
restocking fee. These companies rely on hype and unsubstantiated claims to sell
their products to a predominantly gullible buying public. These people count on
the fact that you will probably not time your vehicle's acceleration before and
after installation. The seat of the pants "feel" of "increased" acceleration
after installing the latest $400 trick gadget is usually enough to sell most
people.
If your engine runs clean to redline with the modifications that you have done,
it is very questionable that you will make it any faster by modifying the
ignition system. If you encounter a high rpm miss at full throttle, there is a
good chance that something needs replacing or upgrading.
If you must buy something, stick to reputable manufacturers making reasonable
product claims. Steer clear of any company using hype and hard sell tactics and
ones claiming vastly increased power or fuel economy. This just does not happen
in the real world.
If you see a company making wild claims in their ads, do everyone a favor in the
industry as well as the buying public, report them to the Federal Trade
Commission.
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