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Spark plugs are one
of the most misunderstood components of an engine. Numerous questions have
surfaced over the years, leaving many people confused.
This guide was designed to assist the technician, hobbyist, or race mechanic
in understanding, using, and troubleshooting spark plugs. The information
contained in this guide applies to all types of internal combustion engines:
two stroke engines, rotary engines, high performance/racing engines and
street vehicles.
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.
Spark Plug Basics:
The spark plug has two primary functions:
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.
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
driveability
- 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, mill ing heads, etc.)
b) adding forced induction (Nitrous, Turbocharging or Supercharging)
c) camshaft change
- As compression
increases, a colder heat range plug, higher fuel octane, and careful attention
to igni-tion timing and air/fuel ratios are necessary. Failure to select
a colder spark plug can lead to spark plug/engine damage
Advancing Ignition
Timing
- Advancing
ignition timing by 10° causes tip temperature to increase by approx.
70°-100° C
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 tem-perature 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
- Defined as:
ignition of the air/fuel mixture before the pre-set ignition timing mark
- 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 an 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 mo-tion 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 driveability, 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
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