SPARK PLUGS.
By NGK
All
spark plugs are created equal ! Not true, some plugs can improve
performance, while others offer long life. Some make your lawnmower easier
to start, while others resist fouling. Some cost a few rands while other
cost a fortune. If you are just tuning your car to get rid of it then maybe
it isn’t cost effective to purchase the highest quality double platinum
plugs. But if you are planning on keeping the car, Landy, outboard motor etc
then the plug you choose can improve ease of starting, acceleration, fuel
economy, as well as how soon you have to change them. In short, there are
horsepower and mileage improvements available from different spark plugs.
The
spark plug is quite simple in theory: It forces electricity to arc
across a gap, just like a bolt of
lightning. The
electricity must be at a very high voltage in order to travel across the gap
and create a good spark. Voltage at the spark plug can be anywhere from
40,000 to 100,000 volts. The spark plug must have an insulated passageway
for this high voltage to travel down to the electrode, where it can jump the
gap and, from there, be conducted into the engine block and grounded. The
plug also has to withstand the extreme heat and pressure inside the
cylinder, and must be designed so that deposits from fuel additives do not
build up on the plug. Spark plugs use a ceramic insert to isolate the
high voltage at the electrode, ensuring that the spark happens at the tip of
the electrode and not anywhere else on the plug; this insert does
double-duty by helping to burn off deposits. Ceramic is a fairly poor heat
conductor, so the material gets quite hot during operation. This heat helps
to burn off deposits from the electrode.
Some
cars require a hot plug. This type of plug is designed with a ceramic
insert that has a smaller contact area with the metal part of the plug. This
reduces the heat transfer from the ceramic, making it run hotter and thus
burn away more deposits. Cold plugs are designed with more contact
area, so they run cooler. The carmaker will select the right temperature
plug for each car. Some cars with high-performance engines naturally
generate more heat, so they need colder plugs. If the spark plug gets too
hot, it could ignite the fuel before the spark fires; so it is important to
stick with the right type of plug for your car.
The
difference between a "hot" and a "cold" spark plug is in the shape of the
ceramic tip.
SPARK PLUG BASICS
: Spark plugs are the "window" into your engine (your only eyewitness to the
combustion chamber), and can be used as a valuable diagnostic tool. Like a
patient's thermometer, the spark plug displays symptoms and conditions of
the engine's performance. The experienced tuner can analyze these symptoms
to track down the root cause of many problems, or to determine air/fuel
ratios.
The spark plug has two primary functions:
1. To ignite
the air/fuel mixture
2. To
remove heat from the combustion chamber
Spark
plugs transmit electrical energy that turns fuel into working energy. A
sufficient amount of voltage must be supplied by the ignition system to
cause it to spark across the spark plug's gap. This is called "Electrical
Performance."
The
temperature of the spark plug's firing end must be kept low enough to
prevent pre-ignition, but high enough to prevent fouling. This is called
"Thermal Performance", and is determined by the heat range selected.
It is
important to remember that spark plugs do not create heat, they can
only remove heat. The spark plug works as a heat exchanger by
pulling unwanted thermal energy away from the combustion chamber, and
transferring the heat to the engine's cooling system. The heat range is
defined as a plug's ability to dissipate heat.
The rate
of heat transfer is determined by:
The
insulator nose length
Gas
volume around the insulator nose
The
materials/construction of the center electrode and porcelain insulator
A spark
plug's heat range has no relationship to the actual voltage transferred
though the spark plug. Rather, the heat range is a measure of the spark
plug's ability to remove heat from the combustion chamber. The heat range
measurement is determined by several factors; the length of the ceramic
center insulator nose and its' ability to absorb and transfer combustion
heat, the material composition of the insulator and center electrode
material.
Heat
rating and heat flow path of NGK Spark Plugs
The
insulator nose length is the distance from the firing tip of the insulator
to the point where insulator meets the metal shell. Since the insulator tip
is the hottest part of the spark plug, the tip temperature is a primary
factor in pre-ignition and fouling. Whether the spark plugs are fitted in a
lawnmower, boat, or a race car, the spark plug tip temperature must remain
between 500C-850°C. If the tip temperature is lower than 500°C, the
insulator area surrounding the center electrode will not be hot enough to
burn off carbon
and
combustion chamber deposits. These accumulated deposits can result in spark
plug fouling leading to misfire. If the tip temperature is higher than 850°C
the spark plug will overheat which may cause the ceramic around the center
electrode to blister and the electrodes to melt. This may lead to
pre-ignition/detonation and expensive engine damage. In identical spark plug
types, the difference from one heat range to the next is the ability to
remove approximately 70°C to 100°C from the combustion chamber. A projected
style spark plug firing tip temperature is increased by 10°C to 20°C.
Heat range
The term spark plug heat range refers to the speed with which the plug can
transfer heat from the combustion chamber to the engine head. Whether the
plug is to be installed in a boat, lawnmower or racecar, it has been found
the optimum combustion chamber temperature for gasoline engines is between
500°C–850°C. When it is within that range it is cool enough to avoid
pre-ignition
and plug tip overheating (which can cause engine damage), while still hot
enough to burn off combustion deposits which cause
fouling.
The spark plug can help maintain the optimum combustion chamber temperature.
The primary method used to do this is by altering the internal length of the
core nose, in addition, the alloy compositions in the electrodes can be
changed. This means you may not be able to visually tell a difference
between heat ranges. When a spark plug is referred to as a “cold plug”, it
is one that transfers heat rapidly from the firing tip into the engine head,
which keeps the firing tip cooler. A “hot plug” has a much slower rate of
heat transfer, which keeps the firing tip hotter.
An unaltered engine will run within the optimum operating range straight
from the manufacturer, but if you make modifications such as a turbo,
supercharger, increase compression,
timing changes,
use of alternate racing fuels, or sustained use of
nitrous oxide,
these can alter the plug tip temperature and may necessitate a colder plug.
A rule of thumb is, one heat range colder per modification or one
heat range colder for every 75–100hp you increase. In identical spark plug
types, the difference from one full heat range to the next is the ability to
remove 70°C to 100°C from the combustion chamber.
The heat range numbers used by spark plug manufacturers are not universal,
by that we mean, a 10 heat range in Champion is not the same as a 10 heat
range in NGK nor the same in Autolite. Some manufacturers numbering systems
are opposite the other, for Manufacturers (Champion, Autolite,
Splitfire), the higher the number, the hotter the plug. For
Japanese manufacturers (NGK, Denso), the higher the number, the colder the
plug.
Do not make spark plug changes at the same time as another engine
modification such as injection, carburetion or
timing changes
as in the event of poor results, it can lead to misleading and inaccurate
conclusions (an exception would be when the alternate plugs came as part of
a single precalibrated upgrade kit). When making spark plug heat range
changes, it is better to err on the side of too cold a plug. The worst
thing that can happen from too cold a plug is a fouled spark plug, too
hot a spark plug can cause severe engine damage.
Tip Temperature and Firing End Appearance
The
firing end appearance also depends on the spark plug tip temperature. There
are three basic diagnostic criteria for spark plugs: good, fouled and
overheated. The borderline between the fouling and optimum operating regions
(500&def;C) is called the spark plug self-cleaning temperature. The
temperature at this point is where the accumulated carbon and combustion
deposits are burned off.
Bearing
in mind that the insulator nose length is a determining factor in the heat
range of a spark plug, the longer the insulator nose, the less heat is
absorbed, and the further the heat must travel into the cylinder head water
journals. This means the plug has a higher internal temperature, and is said
to be a hot plug. A hot spark plug maintains a higher internal operating
temperature to burn off oil and carbon deposits, and has no relationship to
spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose and absorbs more
combustion chamber heat. This heat travels a shorter distance, and allows
the plug to operate at a lower internal temperature. A colder heat range is
necessary when the engine is modified for performance, subjected to heavy
loads, or is run at high rpms for a significant period of time. The colder
type removes heat more quickly, and will reduce the chance of
pre-ignition/detonation and melting or damage to the firing end. (Engine
temperature can affect the spark plug's operating temperature, but not the
spark plugs heat range).
Below is a list of some of the possible external influences
on a spark plug's operating temperatures. The following symptoms or
conditions may have an effect on the actual temperature of the spark plug.
The spark plug cannot create these conditions, but it must be able to cope
with the levels of heat...if not, the performance will suffer and engine
damage can occur.
Air/Fuel Mixtures
seriously affect engine performance and spark plug operating temperatures.
-
Rich air/fuel mixtures
cause tip temperature to drop, causing fouling and poor drive ability
-
Lean air/fuel mixtures
cause plug tip and cylinder temperature to increase, resulting in
pre-ignition, detonation, and possibly serious spark plug and engine
damage
-
It is important to read
spark plugs many times during the tuning process to achieve the optimum
air/ fuel mixture
Higher Compression
Ratios/Forced Induction
will elevate spark plug tip and in-cylinder temperatures
-
Compression can be
increased by performing any one of the following modifications:
a) reducing combustion chamber volume (i.e.: domed pistons, smaller
chamber heads, milling heads, etc.)
b) adding forced induction (Nitrous, Turbo charging or Supercharging)
c) camshaft change
-
As compression increases, a
colder heat range plug, higher fuel octane, and careful attention to
ignition timing and air/fuel ratios are necessary. Failure to select a
colder spark plug can lead to spark plug/engine damage
Advancing Ignition Timing
Engine Speed and Load
-
Increases in firing-end
temperature are proportional to engine speed and load. When traveling at a
consistent high rate of speed, or carrying/pushing very heavy loads, a
colder heat range spark plug should be installed
Ambient Air Temperature
-
As air temperature falls,
air density/air volume becomes greater, resulting in leaner air/fuel
mixtures.
-
This creates higher
cylinder pressures/temperatures and causes an increase in the spark plug's
tip temperature. So, fuel delivery should be increased.
-
As temperature increases,
air density decreases, as does intake volume, and fuel delivery should be
decreased
Humidity
-
As humidity increases, air
intake volume decreases
-
Result is lower combustion
pressures and temperatures, causing a decrease in the spark plug's
temperature and a reduction in available power.
-
Air/fuel mixture should be
leaner, depending upon ambient temperature.
Barometric Pressure/Altitude
-
Also affects the spark
plug's tip temperature. The higher the altitude, the lower cylinder
pressure becomes. As the cylinder temperature de-creases, so does the plug
tip temperature
-
Many mechanics attempt to
"chase" tuning by changing spark plug heat ranges
-
The real answer is to
adjust jetting or air/fuel mixtures in an effort to put more air back into
the engine
Types of Abnormal Combustion
Pre-ignition
Caused by hot spots in the combustion chamber...can be caused (or amplified)
by over advanced timing, too hot a spark plug, low octane fuel, lean
air/fuel mixture, too high compression, or insufficient engine cooling. A
change to a higher-octane fuel, a colder plug, richer fuel mixture, or lower
compression may be in order . You may also need to retard ignition timing,
and check vehicle's cooling system. Pre-ignition usually leads to
detonation; pre-ignition and detonation are two separate events
Detonation
The
spark plug's worst enemy! (Besides fouling). Can break insulators or break
off ground electrodes. Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process
(in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is
being forced upward by mechanical action of the connecting rod, the
pre-ignited explosion will try to force the piston downward. If the piston
can't go up (because of the force of the premature explosion) and it can't
go down (because of the upward motion of the connecting rod), the piston
will rattle from side to side. The resulting shock wave causes an audible
pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from
excessive heat
The spark plug is damaged by both the elevated temperatures and the
accompanying shock wave, or concussion
Misfires
A spark
plug is said to have misfired when enough voltage has not been delivered to
light off all fuel present in the combustion chamber at the proper moment of
the power stroke (a few degrees before top dead center)
A spark plug can deliver a weak spark (or no spark at all) for a variety of
reasons...defective coil, too much compression with incorrect
plug gap, dry fouled or wet fouled spark plugs, insufficient ignition
timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel
is not lit, no energy is be-ing created)
Severe
misfires will cause poor fuel economy, poor drive ability, and can lead to
engine damage
Fouling
-
Will
occur when spark plug tip temperature is insufficient to burn off carbon,
fuel, oil or other deposits
-
Will
cause spark to leach to metal shell...no spark across plug gap will cause
a misfire
-
Wet-fouled spark plugs must be changed...spark plugs will not fire
-
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to
operating temperature
-
Before changing fouled spark plugs, be sure to eliminate root cause of
fouling
Last Modified :
09/02/2006 06:58