A spark plug (sometimes,
in British English, a sparking plug, and, colloquially, a plug) is a device for delivering
electric current from an ignition system to the combustion chamber of a spark-ignition
engine to
ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A
spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain
a resistor, is connected by a heavily insulated wire to the output terminal of
an ignition coil or magneto. The spark plug's metal shell is screwed into the
engine's cylinder head and thus electrically grounded.
The central
electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and
usually one or more protuberances or structures attached to the inner end of
the threaded shell and designated the side, earth,
or ground electrode(s).
Spark plugs
may also be used for other purposes; in Saab Direct Ignition when they are not firing,
spark plugs are used to measure ionization in the cylinders – this ionic
current measurement is used to replace the ordinary cam phase sensor, knock
sensor and misfire measurement function. Spark plugs may also be used in
other applications such as furnaces wherein a combustible fuel/air mixture must be
ignited. In this case, they are sometimes referred to as flame igniters.
OPERATION
Components of a typical, four stroke cycle, DOHC
piston engine.
- · (E) Exhaust camshaft
- · (I) Intake camshaft
- · (S) Spark plug
- · (V) Valves
- · (P) Piston
- · (R) Connecting rod
- · (C) Crankshaft
The plug is connected to the high voltage generated by
an ignition coil or magneto. As current flows from the coil, a voltage develops between
the central and side electrodes. Initially no current can flow because the fuel
and air in the gap is an insulator, but as the voltage rises further it begins
to change the structure of the gases between the electrodes. Once the voltage
exceeds the dielectric strength of the gases, the gases
become ionized. The ionized gas becomes a
conductor and allows current to flow across the gap. Spark plugs usually
require voltage of 12,000–25,000 volts or more to "fire"
properly, although it can go up to 45,000 volts. They supply higher
current during the discharge process, resulting in a hotter and longer-duration
spark.
As the
current of electrons surges across the gap, it raises the temperature of the
spark channel to 60,000 K. The intense heat in the spark channel causes the ionized
gas to expand very quickly, like a small explosion. This is the
"click" heard when observing a spark, similar to lightning and thunder.
The heat and
pressure force the gases to react with each other, and at the end of the spark
event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this
fireball, or kernel, depends on the exact composition of the mixture between
the electrodes and the level of combustion chamber turbulence at the time of
the spark. A small kernel will make the engine run as though the ignition timing was retarded, and a large one
as though the timing was advanced.
SPARK PLUG CONSTRUCTION
A spark plug
is composed of a shell, insulator and the central conductor. It passes through the
wall of the combustion chamber and therefore must also seal
the combustion chamber against high pressures and temperatures without
deteriorating over long periods of time and extended use.
Spark plugs
are specified by size, either thread or nut (often referred to as Euro), sealing type (taper or crush
washer), and spark gap. Common thread (nut) sizes in Europe are 10 mm
(16 mm), 14 mm (21 mm; sometimes, 16 mm), and 18 mm
(24 mm, sometimes, 21 mm). In the United States, common thread (nut)
sizes are 10mm (16mm), 12mm (14mm, 16mm or 17.5mm), 14mm (16mm, 20.63mm) and
18mm (20.63mm).
PARTS OF THE PLUG
TERMINAL
The top of
the spark plug contains a terminal to connect to the ignition system. The exact terminal construction
varies depending on the use of the spark plug. Most passenger car spark plug
wires snap onto the terminal of the plug, but some wires have eyelet connectors
which are fastened onto the plug under a nut. Plugs which are used for these
applications often have the end of the terminal serve a double purpose as the
nut on a thin threaded shaft so that they can be used for either type of
connection.
INSULATOR
The main part
of the insulator is typically made from sintered alumina, a very hard ceramic material with high dielectric strength, printed with the manufacturer's
name and identifying marks, then glazed to improve resistance to surface spark tracking. Its
major functions are to provide mechanical support and electrical insulation for
the central electrode, while also providing an extended spark path for
flashover protection. This extended portion, particularly in engines with
deeply recessed plugs, helps extend the terminal above the cylinder head so as
to make it more readily accessible.
Dissected modern spark plug showing the one-piece showing
sintered alumina insulator. The lower portion is unglazed.
A further
feature of sintered alumina is its good heat conduction reducing the tendency for the insulator to
glow with heat and so light the mixture prematurely.
RIBS
By
lengthening the surface between the high voltage terminal and the grounded
metal case of the spark plug, the physical shape of the ribs functions to improve
the electrical insulation and prevent electrical energy from leaking along the
insulator surface from the terminal to the metal case. The disrupted and longer
path makes the electricity encounter more resistance along the surface of the
spark plug even in the presence of dirt and moisture. Some spark plugs are
manufactured without ribs; improvements in the dielectric strength of the
insulator make them less important.
INSULATOR TIP
Two spark plug in comparison views in multiple angle, one of
which is consumed regularly, while the other has the insulating ceramic broken
and the central electrode shortened, due to manufacturing defects and/or
temperature swing
On modern
(post 1930s) spark plugs, the tip of the insulator protruding into the
combustion chamber is the same sintered aluminium oxide (alumina) ceramic as the upper portion, merely unglazed. It is designed
to withstand 650 °C (1,200 °F) and 60 kV.
The
dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually
"cooler" plugs, while "hotter" plugs are made with a
lengthened path to the metal body, though this also depends on the thermally
conductive metal core.
Older spark
plugs, particularly in aircraft, used an insulator made of stacked layers
of mica, compressed by tension in the
centre electrode.
With the
development of leaded petrol in the 1930s, lead deposits on
the mica became a problem and reduced the interval between needing to clean the
spark plug. Sintered alumina was developed by Siemens in Germany to counteract this. Sintered alumina
is a superior material to mica or porcelain because it is a relatively good
thermal conductor for a ceramic, it maintains good mechanical strength and
(thermal) shock resistance at higher temperatures, and this ability to run hot
allows it to be run at "self cleaning" temperatures without rapid
degradation. It also allows a simple single piece construction at low cost but
high mechanical reliability.
SEALS
Because the
spark plug also seals the combustion chamber or the engine when installed,
seals are required to ensure there is no leakage from the combustion chamber.
The internal seals of modern plugs are made of compressed glass/metal powder,
but old style seals were typically made by the use of a multi-layer braze. The external seal is usually a crush washer, but some manufacturers use the cheaper method of a taper
interface and simple compression to attempt sealing.
METAL CASE/SHELL
The metal
case/shell (or the jacket,
as many people call it) of the spark plug withstands the torque of tightening
the plug, serves to remove heat from the insulator and pass it on to the
cylinder head, and acts as the ground for the sparks passing through the
central electrode to the side electrode. Spark plug threads are cold rolled to
prevent thermal cycle fatigue. It's important to install spark plugs with the
correct "reach," or thread length. Spark plugs can vary in reach from
0.095 to 2.649 cm (0.0375 to 1.043 in), such for automotive and small
engine applications. Also, a marine spark plug's shell is double-dipped,
zinc-chromate coated metal.
CENTRAL ELECTRODE
Central and lateral electrodes
The central
electrode is connected to the terminal through an internal wire and commonly a ceramic
series resistance to reduce emission of RF noise from the sparking. Non-resistor spark plugs,
commonly sold without an "R" in the plug type part number, lack this
element to reduce electro-magnetic interference with radios and other sensitive
equipment. The tip can be made of a combination of copper, nickel-iron, chromium, or noble metals. In the late 1970s, the development of engines reached a
stage where the heat range of conventional spark plugs with solid nickel alloy
centre electrodes was unable to cope with their demands. A plug that was cold
enough to cope with the demands of high speed driving would not be able to burn
off the carbon deposits caused by stop start urban conditions, and would foul
in these conditions, making the engine misfire. Similarly, a plug that was hot
enough to run smoothly in town could melt when called upon to cope with extended
high speed running on motorways. The answer to this problem, devised by the
spark plug manufacturers, was to use a different material and design for the
centre electrode that would be able to carry the heat of combustion away from
the tip more effectively than a solid nickel alloy could. Copper was the
material chosen for the task and a method for manufacturing the copper-cored
centre electrode was created by Floform.
The central
electrode is usually the one designed to eject the electrons (the cathode, i.e. negative polarity relative to the engine block)
because it is the hottest (normally) part of the plug; it is easier to emit
electrons from a hot surface, because of the same physical laws that increase
emissions of vapor from hot surfaces (see thermionic emission). In addition, electrons are
emitted where the electrical field strength is greatest; this is from wherever
the radius of curvature of the surface is smallest, from a sharp point or edge
rather than a flat surface (see corona discharge). Using the colder, blunter side
electrode as negative requires up to 45 percent higher voltage, so few
ignition systems aside from wasted spark are designed this way.
It would be
easiest to pull electrons from a pointed electrode but a pointed electrode
would erode after only a few seconds. Instead, the electrons emit from the
sharp edges of the end of the electrode; as these edges erode, the spark
becomes weaker and less reliable.
At one time
it was common to remove the spark plugs, clean deposits off the ends either
manually or with specialized sandblasting equipment and file the end of the electrode to restore
the sharp edges, but this practice has become less frequent for three reasons:
1. cleaning with tools such as a wire
brush leaves traces of metal on the insulator which can provide a weak
conduction path and thus weaken the spark (increasing emissions)
2. plugs are so cheap relative to labor
cost, economics dictate replacement, particularly with modern long-life plugs.
3. iridium and platinum plugs that have
longer lifetimes than copper have become more common
The development of noble metal high temperature electrodes
(using metals such as yttrium, iridium, tungsten, or palladium, as well as the relatively high value platinum, silver or gold) allows the use of a smaller center
wire, which has sharper edges but will not melt or corrode away. These
materials are used because of their high melting points and durability, not
because of their electrical conductivity (which is irrelevant in series with
the plug resistor or wires). The smaller electrode also absorbs less heat from
the spark and initial flame energy. At one point, Firestone marketed plugs with polonium in the tip, under the (questionable theory)
that the radioactivity would ionize the air in the gap, easing spark formation.
SIDE (GROUND, EARTH)
ELECTRODE
The side
electrode (also known as the "ground strap") is made from high
nickel steel and is welded or hot forged to
the side of the metal shell. The side electrode also runs very hot, especially
on projected nose plugs. Some designs have provided a copper core to this
electrode, so as to increase heat conduction. Multiple side electrodes may also
be used, so that they don't overlap the central electrode.
SPARK PLUG GAP
Gap gauge: A disk with sloping edge; the edge is thicker going
counter-clockwise, and a spark plug will be hooked along the edge to check the
gap.
Spark plugs
are typically designed to have a spark gap which can be adjusted by the
technician installing the spark plug, by bending the ground electrode slightly.
The same plug may be specified for several different engines, requiring a
different gap for each. Spark plugs in automobiles generally have a gap between
0.6 and 1.8 mm (0.024 and 0.071 in). The gap may require adjustment
from the out-of-the-box gap.
A spark plug gap gauge is
a disc with a sloping edge, or with round wires of precise diameters, and is
used to measure the gap. Use of a feeler gauge with flat blades instead of round wires, as is used
on distributor points or valve lash, will give erroneous results, due to the shape of
spark plug electrodes. The simplest gauges are a collection of keys of
various thicknesses which match the desired gaps and the gap is adjusted until
the key fits snugly. With current engine technology, universally incorporating
solid state ignition systems and computerized fuel injection, the gaps used are larger on
average than in the era of carburetors and breaker point distributors, to the
extent that spark plug gauges from that era cannot always measure the required
gaps of current cars. Vehicles using compressed natural gas generally require
narrower gaps than vehicles using gasoline.
The gap
adjustment can be crucial to proper engine operation. A narrow gap may give too
small and weak a spark to effectively ignite the fuel-air mixture, but the plug
will almost always fire on each cycle. A gap that is too wide might prevent a
spark from firing at all or may misfire at high speeds, but will usually have a
spark that is strong for a clean burn. A spark which intermittently fails to
ignite the fuel-air mixture may not be noticeable directly, but will show up as
a reduction in the engine's power and fuel efficiency.
VARIATIONS ON THE BASIC
DESIGN
Over the years variations on the basic spark plug design
have attempted to provide either better ignition, longer life, or both. Such
variations include the use of two, three, or four equally spaced ground
electrodes surrounding the central electrode. Other variations include using a
recessed central electrode surrounded by the spark plug thread, which
effectively becomes the ground electrode (see "surface-discharge spark
plug", below). Also there is the use of a V-shaped notch in the tip of the
ground electrode. Multiple ground electrodes generally provide longer life, as
when the spark gap widens due to electric discharge wear, the spark moves to
another closer ground electrode. The disadvantage of multiple ground electrodes
is that a shielding effect can occur in the engine combustion chamber
inhibiting the flame face as the fuel air mixture burns. This can result in a
less efficient burn and increased fuel consumption.
SURFACE-DISCHARGE SPARK
PLUG
A piston
engine has a part of the combustion chamber that is always out of reach of the
piston; and this zone is where the conventional spark plug is located. A Wankel engine has a permanently varying
combustion area; and the spark plug is inevitably swept by the tip seals.
Clearly, if a spark plug were to protrude into the Wankel's combustion chamber
it would foul the rotating tip; and if the plug were recessed to avoid this,
the sunken spark might lead to poor combustion. So a new type of "surface
discharge" plug was developed for the Wankel. Such a plug presents an
almost flat face to the combustion chamber. A stubby centre electrode projects
only very slightly; and the entire earthed body of the plug acts as the side
electrode. The advantage is that the plug sits just beneath the tip-seal that
sweeps over it, keeping the spark accessible to the fuel/air mixture. The
"plug gap" remains constant throughout its life; and the spark path
will continually vary (instead of darting from the centre to the side electrode
as in a conventional plug). Whereas a conventional side electrode will
(admittedly, rarely) come adrift in use and potentially cause engine damage,
this is impossible with a surface discharge plug, as there is nothing to break
off. Surface-discharge spark plugs have been produced by inter alia, Denso,
NGK, Champion and Bosch.
SEALING TO THE CYLINDER
HEAD
Old
spark plug removed from a car, new one ready to install.
Most spark plugs seal to the
cylinder head with a single-use hollow or folded metal washer which is crushed
slightly between the flat surface of the head and that of the plug, just above
the threads. Some spark plugs have a tapered seat that uses no washer. The
torque for installing these plugs is supposed to be lower than a washer-sealed
plug. Spark plugs with tapered seats should never be installed in vehicles
with heads requiring washers, and vice versa. Otherwise, a poor seal or
incorrect reach would result because of the threads not properly seating in the
heads.
TIP PROTRUSION
Different spark plug sizes. The left and right plug are identical in
threading, electrodes, tip protrusion, and heat range. The centre plug is a
compact variant, with smaller hex and porcelain portions outside the head, to
be used where space is limited. The rightmost plug has a longer threaded
portion, to be used in a thicker Cylinder head.
The length of
the threaded portion of the plug should be closely matched to the thickness of
the head. If a plug extends too far into the combustion chamber, it may be
struck by the piston, damaging the engine internally. Less dramatically, if the
threads of the plug extend into the combustion chamber, the sharp edges of the
threads act as point sources of heat which may cause pre-ignition; in addition, deposits which form between the exposed
threads may make it difficult to remove the plugs, even damaging the threads on
aluminium heads in the process of removal. The protrusion of the tip into the
chamber also affects plug performance, however; the more centrally located the
spark gap is, generally the better the ignition of the air-fuel mixture will
be, although experts believe the process is more complex and dependent on
combustion chamber shape. On the other hand, if an engine is "burning
oil", the excess oil leaking into the combustion chamber tends to foul the
plug tip and inhibit the spark; in such cases, a plug with less protrusion than
the engine would normally call for often collects less fouling and performs better, for a longer period. In fact,
special "anti-fouling" adapters are sold which fit between the plug
and the head to reduce the protrusion of the plug for just this reason, on
older engines with severe oil burning problems; this will cause the ignition of
the fuel-air mixture to be less effective, but in such cases, this is of lesser
significance.
HEAT RANGE
Construction of hot and cold spark plug a longer
insulator tip makes the plug hotter
The operating
temperature of
a spark plug is the actual physical temperature at the tip of the spark plug within the running
engine, normally between 500 and 800 °C (932 and 1,472 °F). This is
important because it determines the efficiency of plug self-cleaning and is
determined by a number of factors, but primarily the actual temperature within
the combustion chamber. There is no direct relationship between the actual
operating temperature of the spark plug and spark voltage. However, the level
of torque currently being produced by the engine will strongly
influence spark plug operating temperature because the maximal temperature and
pressure occur when the engine is operating near peak torque output (torque and
rotational speed directly determine the power output). The temperature of the insulator responds to
the thermal conditions it is exposed to in the combustion chamber, but not vice
versa. If the tip of the spark plug is too hot, it can cause pre-ignition or
sometimes detonation/knocking, and damage may occur. If it is too
cold, electrically conductive deposits may form on the insulator, causing a
loss of spark energy or the actual shorting-out of the spark current.
A spark plug
is said to be "hot" if it is a better heat insulator, keeping more
heat in the tip of the spark plug. A spark plug is said to be "cold"
if it can conduct more heat out of the spark plug tip and lower the tip's
temperature. Whether a spark plug is "hot" or "cold" is
known as the heat range of the spark plug. The heat range of a spark plug is
typically specified as a number, with some manufacturers using ascending
numbers for hotter plugs, and others doing the opposite using ascending numbers
for colder plugs.
The heat
range of a spark plug is affected by the construction of the spark plug: the
types of materials used, the length of insulator and the surface area of the plug exposed within the combustion chamber. For
normal use, the selection of a spark plug heat range is a balance between keeping
the tip hot enough at idle to prevent fouling and cold enough at maximal power
to prevent pre-ignition or engine knocking. By examining "hotter"
and "cooler" spark plugs of the same manufacturer side by side, the
principle involved can be very clearly seen; the cooler plugs have a more
substantial ceramic insulator filling the gap between the center electrode and
the shell, effectively allowing more heat to be carried off by the shell, while
the hotter plugs have less ceramic material, so that the tip is more isolated
from the body of the plug and retains heat better.
Heat from the combustion chamber escapes through the
exhaust gases, the side walls of the cylinder and the spark plug itself. The
heat range of a spark plug has only a minute effect on combustion chamber and
overall engine temperature. A cold plug will not materially cool down an
engine's running temperature. (A too hot plug may, however, indirectly lead to
a runaway pre-ignition condition that can increase engine temperature.) Rather, the main effect
of a "hot" or "cold" plug is to affect the temperature of
the tip of the spark plug.
It was common
before the modern era of computerized fuel injection to specify at least a
couple of different heat ranges for plugs for an automobile engine; a hotter
plug for cars that were mostly driven slowly around the city, and a colder plug
for sustained high-speed highway use. This practice has, however, largely
become obsolete now that cars' fuel/air mixtures and cylinder temperatures are
maintained within a narrow range, for purposes of limiting emissions. Racing
engines, however, still benefit from picking a proper plug heat range. Very old
racing engines will sometimes have two sets of plugs, one just for starting and
another to be installed for driving once the engine is warmed up.
Spark plug
manufacturers use different numbers to denote heat range of their spark plugs.
READING SPARK PLUGS
The spark
plug's firing end will be affected by the internal environment of the
combustion chamber. As the spark plug can be removed for inspection, the
effects of combustion on the plug can be examined. An examination, or
"reading" of the characteristic markings on the firing end of the
spark plug can indicate conditions within the running engine. The spark plug
tip will bear the marks as evidence of what is happening inside the engine.
Usually there is no other way to know what is going on inside an engine running
at peak power. Engine and spark plug manufacturers will publish information
about the characteristic markings in spark plug reading charts. Such charts are
useful for general use but are of almost no use in reading racing engine spark
plugs, which is an entirely different matter.
A light
brownish discoloration of the tip of the block indicates proper operation;
other conditions may indicate malfunction. For example, a sandblasted look to
the tip of the spark plug means persistent, light detonation is occurring, often unheard. The damage that is
occurring to the tip of the spark plug is also occurring on the inside of the
cylinder. Heavy detonation can cause outright breakage of the spark plug
insulator and internal engine parts before appearing as sandblasted erosion but
is easily heard. As another example, if the plug is too cold, there will be
deposits on the nose of the plug. Conversely if the plug is too hot, the
porcelain will be porous looking, almost like sugar. The material which seals
the central electrode to the insulator will boil out. Sometimes the end of the
plug will appear glazed, as the deposits have melted.
An idling
engine will have a different impact on the spark plugs than one running at
full throttle. Spark plug readings are only valid
for the most recent engine operating conditions and running the engine under
different conditions may erase or obscure characteristic marks previously left
on the spark plugs. Thus, the most valuable information is gathered by running
the engine at high speed and full load, immediately cutting the ignition off
and stopping without idling or low speed operation and removing the plugs for
reading.
Spark plug
reading viewers, which are simply combined flashlight/magnifiers, are available
to improve the reading of the spark plugs.
Two spark plug viewers
INDEXING SPARK PLUGS
"Indexing"
of plugs upon installation involves installing the spark plug so that the open
area of its gap, not shrouded by the ground electrode, faces the center of the
combustion chamber rather than one of its walls. The theory holds that this
will maximize the exposure of the fuel-air mixture to the spark, also ensuring
that every combustion chamber is even in layout and therefore resulting in
better ignition. Indexing is accomplished by marking the location of the gap on
the outside of the plug, installing it, and noting the direction in which the
mark faces. Then the plug is removed and washers are added to change the orientation
of the tightened plug. This must be done individually for each plug, as the
orientation of the gap with respect to the threads of the shell is random. Some plugs are made with a non-random orientation of the
gap and are usually marked as such by a suffix to the model number; typically
these are specified by manufacturers of very small engines where the spark plug
tip and electrodes form a significantly large part of the shape of the combustion
chamber. The Honda Insight has indexed spark plugs from
factory, with four different part numbers available corresponding to the
different degrees of indexing to achieve most efficient combustion and maximal
fuel efficiency.
WORKING