Sparking Quality
Engine Testing and Instrumentation
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The Sparking Plug
Working environment
¾Reliable operation for at least 30 million cycles·
¾Normal operating temperature between 350°C and 900°C with a
possible temperature delta between these extremes in less than 4 seconds
¾Between 14 and 25 kV electrical stress
¾When running at 6,000 rev/min ( every 20mSec) near instantaneous
pressure change from negative to 1,000 psi (6.9 x106 Pa)
¾Corrosive environment including Sulphur, Bromine,Phoshorus etc.
¾Needs abrasion resistance from high velocity particles
Engine Testing and Instrumentation
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The working environment
• The heat generated by the combustion of petrol and air in a
modern engine can result in temperatures of more than
2,500°C inside the combustion chamber, while outside
the engine, at the terminal end of the plug, the air
temperature can be sub-zero in winter.
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The working environment
2,500 °C would melt all the metal around it if the flame heat
were continuous. The flame however, lasts for only one
stroke of the engine. At idle, 600 rev/min it lasts for only
0.05 seconds, and at 6,000 rev/min for 0.005 seconds. In
the four-stroke engine there are three strokes with out
flame (induction, compression and exhaust) for each power
stroke, during which the heat generated in the power stroke
is conducted to the cooling system via the engine walls.
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The working environment
• The new generation of 42 volt ignition systems have given
rise to high kV discharge plugs, new detergent additives
have been added to fuels, various methane and hydrogen
based gaseous fuels are being utilised which require higher
kV spark discharge and the spark plug is being design to
last for the life of the power unit.
• A truly daunting task for the spark plug designer and
development engineer.
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Design problems
• The spark plug serves the function of conducting high
tension current into the combustion chamber to provide
ignition of the fuel air mixture. Sealing is a major problem,
and this is undertaken by the composite metal/glass
conducting seal inside the insulator and by the metal gasket
between the insulator and the mild steel shell. These seals
pay an important role in the performance of the plug in
addition to sealing gas pressures. They provide the main
paths by which the heat may transfer into the bulk of the
cylinder head from the spark plug components.
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What stops it melting ?
• The heat generated at the firing end (tip) of the sparking
plug when the fuel air mixture is ignited is dissipated in the
engine by conduction via the ceramic insulator nose and
the central electrode and the mild steel shell.
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Heat conduction paths
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Heat transfer
• The metal components of the construction transfer heat
more rapidly than the ceramic insulator, the relevant
thermal conductivities differing by approximately a factor
of two. If the heat is not efficiently removed, with the
subsequent firings the ceramic tip (the ‘nose’) of the spark
plug soon attains a temperature at which it is able to preignite the fuel/air mixture before full compression is
achieved sand prior to the timed electrical discharge taking
place. This pre-ignition results n a drastic loss of power
and increased levels of gaseous emissions and if allowed to
continue, the increasing retention of heat can lead to
permanent damage of spark plug, piston, cylinder head and
catalytic converter formulation.
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Heat transfer
• It is this ability of the spark plug to remove heat from its
firing end determines its suitability for a particular type of
engine (heat range) It is evident that a standard quantitive
measurement of this ability is desirable to relate one plug
to another in the range of designs in existence and to
correlate the products of different manufactures. The
current method exclusively used in the industry is known
as the ‘Pre-ignition’ rating of spark plugs
Engine Testing and Instrumentation
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Sparking plug ratings
• The method used for ascertaining a spark plug rating is to
use a single cylinder 4-stroke engine whose essential
nature is constant speed running with changes in power
brought about by supercharging. Supercharging is when
air or the air/fuel mixture is presented to the cylinder at a
pressure that is higher than atmospheric pressure. Because
of this higher pressure, the air supplied to the cylinder has
a higher density and is able to absorb more fuel vapour.
This increases the power output.
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Running a sparkplug rating test
• For a given plug installed in a rating engine, by gradually
increasing the amount of supercharging and adjusting the
fuel mixture strength to give optimum temperature at each
setting, the plug experiences higher and higher
temperatures until it begins to run into re-ignition
(indicated by rapid rise in the measured plug temperature)
As Pre-ignition occurs, the fuel supply is instantly cut off
preventing uncontrolled temperature rise and possible
damage to the engine.
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Running a sparkplug rating test (cont)
• When stable operation is obtained 34 millibar of
supercharge boost below the pre-ignition point for three
minutes, the torque is measured, allowing an IMEP value
to be calculated according to equation 4 above. At any
fixed set of engine conditions there is a definite boost IMEP relationship which is a straight-line function.
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Engine boost against output for optimum
plug temperature at 30° spark advance
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Hot and Cold Plugs
• Various IMEP values are required because of the very
different demands on engine performance. A racing car for
example, needs to run at high temperatures for maximum
efficiency and power output over a relatively long period,
The best spark plug for this environment would be one
which could dissipate heat rapidly to the engine mass. For
a ‘HOT’ engine therefore, a plug that remains as cold as is
necessary to prevent Pre-ignition – a COLD- plug is
required.
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Hot and Cold Plugs
• Its IMEP value would be relatively high. However, if the
same plug was used on a family saloon car used on short
urban journeys, and which as a result never fully warms
up, combustion deposits would soon build up leading to
misfiring. In this situation what is required is a plug with a
relatively low IMEP value which does not dissipate the
heat so readily and whose operating temperature will be
sufficiently high to burn off the combustion deposits. Thus
a COLDER engine calls for a HOT plug. Other terms that
are used are HARD = COLD or SOFT = HOT
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Hot and cold sparking plugs
• The much shorter heat path of the cold plug can, however,
cause problems in that there is much less area of the
ceramic insulator exposed to the cylinder. Under certain
engine conditions such as cold start with full enrichment,
carbon combustion deposits build up on the nose of the
plug, offering a leakage path for the current to earth and
leading rapidly to a situation wherein the plug ceases to
function, known as cold fouling
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Hot and cold sparking plugs
• Substitution by a normal plug of lower IMEP rating, thus
providing a longer nose to overcome this problem, may be
unsatisfactory due to the reduction in maximum safe
operating temperature. The plug designer and the engine
development engineer have however, one or two options
available to satisfy the provision of a greater surface area
of the insulator nose whilst maintaining the IMEP rating.
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Design advances
•
Recent years have seen the adoption of centre electrodes containing a
core of copper. These electrodes are more thermally conductive than
the normal nickel-alloy types and enable a longer electrode and
ceramic nose to be employed. This solution elegantly achieves the
aims discussed above. I.e. larger surface area for the same heat rating.
An alternative means of increasing heat removal rate is to eliminate the
‘air gap’ between the ceramic nose and the centre electrode by filling
the space with a refractory cement material. This is normally achieved
by the application of a vacuum to the tip of the plug and allowing the
re-establishment of atmospheric pressure to force the cement into the
evacuated space.
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Heat conduction
•
•
Air can be a very effective thermal insulator and its replacement by the
solid cement enables the heat to transfer from the ceramic considerably
more efficiently by conduction into the centre electrode along its entire
length and hence be more readily dissipated.
Since most of the heat conduction takes place by way of the electrodes,
the use of materials more efficient in this respect must obviously be
considered. However, the employment of, say nickel in place of
nickel-alloy increases heat dissipation, and hence IMEP, but at the
expense of electrode durability. This is due to more rapid chemical,
electrical and mechanical erosion of the softer material, despite its
higher temperature tolerance.
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Improvement in heat transfer rates by
vacuum-cementing
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Heat conduction
•
One problem that the designer has to be aware of in producing a very
cold plug is a possible change in the site from which pre-ignition may
occur. Normally, initiation of pre-ignition will take place from the
overheated ceramic nose which is unable to dissipate the heat rapidly
enough, provided there are no incandescent sharp burrs or a glowing
protrusion of combustion deposits present to pre-empt such
occurrence. As the IMEP value is increased, the removal of heat from
the nose becomes more efficient, lessens the chance of the ceramic
overheating and the emphasis shifts to the side electrode which then
becomes the prime site for pre-ignition. The plug is then said to ‘rate’
off the side wire instead of the nose. Attempts to control this
phenomenon centre on improving heat removal rate of the side
electrode either by a change of material, as discussed above, or by
cutting back the side wire to provide a shorter path.
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Long life – 160,000 km
• Changes in Spark plugs have been caused by demands for
longer life and also improved ignitability in today's lean
burning modern engines. It is critical that each cylinder
fires to prevent unburnt fuel from reaching the catalytic
converter and causing damage. Variations in burn rates on
a cycle to cycle basis are critical when new emission
regulations are reviewed, a few parts in one million of
trace emission gases can be difference between a
regulatory pass or fail.
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Noble metals
• With engine manufacturers striving to lean off engines to
improve fuel consumption the possibility of misfire
becomes a problem, in addition manufactures are striving
to produce an engine where the sparking plugs will never
need changing. A very high voltage with very large gaps
appears to be the answer.
It has been proven that a very thin electrode will improve
ignitability, however a thin electrode made from nickel
would quickly erode and fail, therefore spark plug
manufacturers have necessarily turned to precious metals
to withstand the harsh environment. This has lead to the
introduction of precious metals to withstand the harsh
environment.
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Noble metals
• The introduction of precious metals like platinum and
iridium gave spark plugs the added benefit of long life. It is
important to note that spark plugs have historically been
regarded as consumable with a limited service life and as
such requiring to be changed at regular intervals. NGK
Spark Plugs designed and Patented V-Groove Spark Plugs
for improving ignitability in nickel alloy electrodes. This
style of spark plug incorporates a 90-degree V Groove in
the centre electrode and ensures that sparking occurs at the
periphery of the electrode, thus enhancing
ignitability.
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42 Volt
• The introduction of 42 volt ignition systems means that
even higher spark plug voltages can be applied leading to
larger plug gaps (up to 3mm) which have very long lives
indeed.
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R.F.I.
• All modern engines require the use of resistor type spark
plugs. A resistor type spark plug is one that incorporates a
5 K ohm resistor to suppress ignition noise generated
during sparking. (Radio frequency interference). Radio
frequency interference is commonly exhibited by the
crackle sound coming from the car radio and there are now
international standards covering R.F.I., which is
considered a type of pollution.
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R.F.I.
• As R.F.I. can also cause premature failure to other
electronic components in a modern vehicle, for example
the ECU it is important that resistor spark plugs are used to
prevent this possibility. As stated earlier, the use of
precious metals in spark plugs has increased their service
life. The use of Iridium spark plugs is only just starting
with Japanese car manufacturers who are finding them
ideal for very low emission engines.
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Electrode temperatures
• We have discussed heat range selection; it is vital to ensure
optimum performance of spark plugs. A spark plug's
optimum operating temperature is between 450 degrees C
and 870 degrees C. Spark plug tip temperatures outside
this range can occur when an incorrect heat rating is
selected. Viz to re-cap:
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When the heat rating is too high
• The spark plug temperature remains too low and causes
deposits to build up on the firing end; the deposits offer an
electrical leakage path that gives rise to loss of sparks.
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When the heat rating is too low
• The spark plug temperature rises too high and induces
abnormal combustion (pre-ignition): this leads to melting
of the spark plug electrodes as well as piston seizure and
erosion. Many plug manufacturers have pioneered the use
of a copper cored electrode NGK being the first in 1958,
which enables a spark plug to heat up quickly and also
dissipate heat quickly giving an ultra wide heat range. It is
essential to use a spark plug that fits a specific engine and
its conditions of use
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Platinum Tipped Electrodes
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Use your eyes and nose !
•
•
•
•
It is important to observe spark plugs that have removed from engines
and learn from the visual evidence. Oxidization, metal deposits on the
ceramic, copper migration from gaskets all tell a story.
When this data is reviewed along side rate of cylinder pressure rise
against crankshaft angle (combustion analysis) and a measurement of
the gaseous and particulate emissions then the development engineer
has a very good tool.
To date, the sparking plug has been used only in gasoline, or gas based
fuels.
New emissions regulations, may force diesel engines to control the
point of burn initiation by utilising sparking plugs.
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