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Items relevant to "6-Digit Retro Nixie Clock Mk.2, Pt.1":
Items relevant to "What’s In A Spark? – Measuring The Energy":
Items relevant to "Spark Energy Meter For Ignition Checks, Pt.1":
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How to measure
SPARK ENERGY
in an ignition system
By
Dr Hugo Holden
K
Modern car ignition systems are reputed to deliver
very “hot” sparks but how do you measure their
energy? And which system is better: CDI or transistorassisted ignition? And what about multi-spark CDI
systems? This article discusses how the energy of
sparks can be measured, as a prelude to a Spark
Energy Meter presented elsewhere in this issue.
nowing the energy intensity of an ignition spark –
and that they are equal across all cylinders – is an
essential part of engine service. But how can you tell?
In general, a spark is best defined as plasma, with the
physical properties of a gas and the electrical properties of a
metal conductor. Plasma is an ionised gas stream where the
atoms’ electrons have been mobilised by the applied electric field and are free enough to carry an electrical current.
A spark’s ability to ignite a gas mixture is related to its
peak temperature and this is proportional to the spark’s
peak power. Since a spark has a fairly stable voltage drop,
the peak power is also related to peak spark current.
A spark initially starts in the gas ionisation phase where
a fine streamer of ionised gas forms between the spark
plug’s electrodes. This creates an increasingly hot electrically conductive pathway and helps to excite adjacent
gas molecules and mobilise their electrons until a spark is
fully established.
A fixed amount of energy delivered to the spark over
a shorter time frame results in more heating or a hotter
spark than if that same amount of energy is delivered over
a longer period.
This is not dissimilar to delivering electrical energy to a
resistor, although unlike a resistor the spark plasma existing between two electrodes tends to adopt a fairly stable
voltage, largely independent of the current. (It is actually
a negative resistance.)
One way to assess a spark’s gas ignition ability is to
divide the spark’s burn time energy in Joules by the time
over which this energy is delivered, in seconds. This is the
44 Silicon Chip
spark’s pulse power or SPP which has units of Watts and
this can be used as a parameter which indicates a spark’s
ability to ignite gases.
In practice though, measuring the individual spark’s
energy alone is a very useful measurement regardless of
the spark’s duration.
Spark sustaining
In a typical automotive set-up, with the engine running,
the spark plug voltage drop during the spark burn time in
the combustion chamber is around 1000V but this varies,
depending on the spark gap, mixture, etc.
In air though, a spark plug typically has a voltage drop
of around 500V to 600V. Once established it has similar
electrical properties to a zener diode. Hence the industry
standard electrical equivalent or “dummy spark plug” is
a 1000V zener diode.
In systems with a mechanical distributor, the voltage
drop of the distributor’s air gap spark is around 500V so
the ignition coil experiences a total constant voltage drop
of about 1500V during the spark time. This is a low value
compared to the ignition coil’s open-circuit output voltage;
often as high as 30kV to 40kV for some coils.
Spark ionisation energy versus spark burn
time energy
In general there are two aspects or phases to the spark’s
energy. The initial early phase is the establishment of the
spark or initial ionisation the gases between the spark plug’s
electrodes. The voltage has to climb high enough to ionise
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a fine streamer of gas between the spark plug’s electrodes
and initiate the spark.
The capacitance of the ignition coil secondary winding
and the HT wiring set-up and the spark plug body (the total
being around 70pF) must be charged to a high voltage very
briefly prior to spark ionisation. This could be 10kV or more.
When the spark strikes, usually after less than a microsecond, these system capacitances are rapidly discharged
down to a low voltage of around 1000V with very high peak
currents in the order of 50 to 100A (unless there is added
series resistance to reduce this peak current).
The capacitance is suddenly shunted into the low impedance when the spark strikes. The electric field energy of
½CV2 is generally in the order of 3.5mJ (millijoules) with
a 70pF capacitance charged to 10kV prior to the spark’s
ionisation.
This energy is not the energy of the “spark burn time”
which is the longer phase in which the spark is seen to
exist by an observer.
The ionisation phase is probably important in overcoming fouled plugs and initiating combustion in some cases.
A Spark Energy Meter does not measure the ionisation
energy but it measures the spark burn time energy which
is the substantially larger of the two values.
The diagram of Fig.1 shows the capacitances and the
discharge current pathways at the moment the spark strikes
for a real spark plug in the initial or ionisation phase. The
ignition coil’s self capacitance, the wiring capacitance and
the spark plug’s capacitance all contribute to these high
initial peak currents.
Clearly if a resistor spark plug (about 5k) is used,
these high initial and brief peak currents are significantly
reduced to a value of 10kV/5k or about 2A. This is why
resistor spark plugs suppress radio interference. The same
applies to resistive ignition cable which reduces RFI from
the ignition system. Inductive ignition cable also reduces
the peak currents.
As shown in Fig.1 though, all cable has some components of resistance Rw, inductance Lw and distributed
capacitance. There was a method tried many years ago to
increase these initial spark ionisation currents by adding
capacitance at the spark plug. While this probably had some
small benefits the idea never took off. Probably because it
is the spark burn time energy that is largely responsible
for initiating combustion, not the initial spark ionisation
energy.
When using a zener diode as a dummy spark plug a series
5k resistor is also helpful in providing a ballast for the
zener to reduce initial peak currents from the capacitances
of the ignition coil secondary and wiring. A convenient
feed through or coupling device into a spark energy meter
Lw
IGNITION
COIL
50pF
CDI versus MDI spark current characteristics
The basis of a Magnetic Discharge Ignition or MDI system
(Kettering) whether it is electronically assisted or not, is
the storage of energy in the magnetic field of the ignition
coil, then the release of this energy to generate the spark.
In MDI systems the spark always extinguishes before all
of the stored magnetic field energy has been dissipated.
The residual magnetic field energy that remains after the
spark burn time is dissipated later as decaying oscillations
visible on the primary or secondary of the ignition coil in
an oscilloscope recording.
The same applies to CDI. In most cases after the spark
burn time, there is still some residual energy in the discharge capacitor or in the ignition coil’s field (which has
acquired that energy from the capacitor in a series of oscillations during the spark burn time).
Energy transfer efficiency
Measurements with a spark energy meter for an MDI system show that the spark burn time energy is typically about
60% of the total magnetic stored energy prior to the spark.
The value for CDI is much lower. About 16% of the energy
stored in the discharge capacitor’s electric field becomes
spark energy for a CDI system using a standard oil filled
Kettering style coil. However there are other mitigating
L2
Rp
HT
WIRING
10pF
Rs
L1
PRIMARY
SECONDARY
Fig.2(a): the transformer’s actual leakage inductance and
resistance (winding capacitance not shown).
‘Rs
Rp
L1
R(total)
BATTERY
Rw
‘L2
SHORT CIRCUIT
OR CONSTANT
VOLTAGE LOAD
(EG, = SPARK)
Lip
CB CAPACITOR
CONTACT BREAKER
SPARK
PLUG
10pF
SPARK
GAP
Fig.1: the capacitances and the discharge current
pathways at the moment the spark strikes for a real spark
plug immediately after the initial or ionisation phase.
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is therefore a typical resistor style spark plug.
For a spark energy meter, the spark burn time energy is
calculated from the product of the spark plug’s (or zener
diode’s) voltage drop and electrical charge in Coulombs
which has passed by that voltage drop over the duration of
the spark. This is because work (in Joules) is equal to the
product of the charge (in Coulombs) and the voltage field
(in Volts) which the charge has traversed.
Since the spark current may have a variety of amplitude
versus time profiles, the current needs to be integrated over
the course of the spark time to yield the transferred charge.
Fig.2(b): leakage inductance and resistance transposed to
primary circuit.
L2
‘L1
‘Rp
Rs
CONSTANT
APPLIED
VOLTAGE
(CONTACT
BREAKER
CLOSED)
+
Lis
R(total)
SECONDARY
CAPACITANCE
–
Fig.2(c): leakage inductance and resistance transposed to
secondary circuit.
February 2015 45
factors because the peak spark currents are higher in CDI
than MDI and with a good transformer ignition coil for CDI
the energy transfer efficiency can reach 25%.
The energy losses in MDI primarily relate to the resistances of the ignition coil windings and also the spark
ionisation energy is not factored into a spark burn time
energy measurement and there is some residual magnetic
field energy left behind at the end of the spark burn time
in the coil’s magnetic field.
There are also other losses related to the magnetic and
dielectric properties of the ignition coil.
The spark as an electrical load and viewed from an alternating current perspective acts much like a short circuit
on the ignition coil secondary because the spark voltage
drop is low compared with what would be the ignition
coil’s open circuit secondary voltage (as already noted).
Fig.2(a) shows a model transformer. There is leakage
reactance, winding resistances and distributed winding
capacitances. Fig.2(b) shows the heavy loading on the secondary winding by the spark during the spark burn time
and this has some interesting effects, shown in Fig.2(c).
The primary circuit can be regarded as containing the
total leakage inductance Lip. This represents a series inductance due to the fact that the primary & secondary turns
are not perfectly magnetically coupled. There is also the
primary winding resistance Rp and a resistance reflected
into the primary winding 'Rs, which is the secondary winding resistance transformed into the primary by the square
of the turns ratio.
Therefore as the magnetic field of the core collapses,
Lip resonates with the points capacitor (CB Capacitor) and
R(total) damps the oscillations so decaying oscillations
are seen in the spark current. These oscillations are typically around 8kHz and are seen in the scope screen photo,
Scope1. The top trace is the primary voltage on a standard
Kettering ignition coil. Even with no contact breaker capacitor fitted oscillations still occur at a higher frequency
because of the self capacitance of the primary winding.
Although the negative-going spark current (second trace)
is oscillatory in the early phase of the spark, the oscillations
damp out prior to the end of the spark burn time and are
never large enough to make the spark current swing to a
positive value in the MDI system.
When the spark current extinguishes at F not all the
stored magnetic energy of the core was dissipated, so then
the coil primary inductance resonates with the contact
breaker capacitor (contact breaker is still open) at around
2kHz. The coil’s secondary with its self-inductance and
distributed capacitance also resonate at a similar frequency.
This is seen in the recording of primary voltage between B
& C. This 2kHz oscillation is abruptly terminated when the
contact breaker closes at C, however it has almost decayed
away by then anyway.
With 12V applied to the ignition coil primary (the points
close or the switching transistor or Mosfet conducts) this
effectively shorts out the primary from the alternating
current perspective and again the current builds and the
magnetic field climbs in the ignition coil’s core. A constant
12V is applied across the primary at as shown in Fig.2(c)
and in Scope1 at D.
Note that after the points close, the peak secondary
voltage is the 12V supply times the coil’s turns ratio. So a
Scope 2: a typical MDI spark current profile in more detail
with the negative going current and the oscillations in the
early phase of the spark current.
Scope 3: the timing of the coil voltage and spark current
and SCR current for a typical CDI. Note the spark current
is bidirectional.
B C
A
D
F
E
Scope 1: this photo shows the relationship between the
primary voltage on a standard Kettering ignition coil (top
trace) and spark current when the points open and close.
46 Silicon Chip
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+12V
L = Lip
R
‘Rs
‘Xs
C
1.5F
Xp
SECONDARY
Rp
PRIMARY
–360V
Rs
BIDIRECTIONAL
CONSTANT
VOLTAGE LOAD
1500V
INVERTER
(ROYER
OSCILLATOR)
350Vp-p
SQUARE
WAVE
BRIDGE RECTIFIER
K
K
A
A
K
K
A
A
HT
CAPACITOR,
TYP. 1.5F
A
SCR
G
Fig.3: CDI system capacitor discharging into coil primary.
positive voltage appears on the secondary terminals that
can be as high as 1200V with a 1:100 ratio coil. While this
is not enough voltage to initiate a spark with a real spark
plug it can result in a small current transient when a 1000V
bidirectional zener diode is being used as a “dummy spark
plug” measuring an ignition coil’s output directly and not
via the spark gap in a distributor.
This false spark current can be called a “Dwell Artefact”
and can be seen in spark current recordings with 1000V
bidirectional zener dummy spark plugs directly connected
to an ignition coil output. Also a zener dummy spark plug
has to be bidirectional or it would conduct like a normal
diode in reverse when the contact breaker closed and effectively short out the coil secondary at that time when the
current was building up in the primary.
One might also expect that after the points close, there
should be some oscillations visible on the secondary winding caused by the leakage reactance now appearing in the
secondary circuit and oscillating with the coil’s secondary
self capacitance as shown in Fig.2(c). These are easy to
record with an oscilloscope loosely coupled to the insulation of the high voltage cable and they have a frequency
around 7.5kHz with a typical ignition coil.
Scope2 shows a typical MDI spark current profile in more
detail with the negative going current and the oscillations
in the early phase of the spark current. The small positive
going spike or “Dwell Artefact” is seen at the start of the
dwell time (points closed) because this scope photo was
taken using a 1000V bidirectional dummy zener spark plug.
Ignoring the spark current oscillations that peak at -60mA,
Scope 4: the spark current profile from a Delta 10B CDI
unit which uses an SCR. The spark current in CDI is
bidirectional.
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TRIGGER
K
STANDARD
IGNITION
COIL
Fig.4: functional diagram of a capacitor discharge ignition.
Fig.4: CAPACITOR DISCHARGE IGNITION – FUNCTIONAL DIAGRAM
the waveform is roughly triangular with a starting point
roughly around -30mA and decaying to zero over about
two milliseconds.
A similar situation applies with the spark loading the
ignition coil in a CDI system, in that the ignition coil’s
leakage reactance resonates with the discharge capacitor
value during the spark time. However in the CDI case the
ignition coil is acting as a pulse transformer rather than
an energy storage device and the stored energy was in the
electric field of the discharge capacitor rather than in the
magnetic field of the coil.
Transformer style ignition coils are much more efficient
for use with a CDI units than using the conventional oilfilled Kettering style coil. In MDI the energy storage and
energy release occur at separate times, so the coil properties such as the leakage reactance between the primary and
secondary are less important than for CDI, where ideally the
ignition coil behaves as an ideal transformer. Fig.3 shows
the electrical arrangement when a CDI is transferring the
stored energy from the discharge capacitor into the spark.
The general format for a CDI unit is shown in Fig.4 but
there are many variations using SCRs or Mosfets (as in the
latest SILICON CHIP design in the December 2014 issue).
The capacitor’s initial voltage is typically in the order of
360V to 400V and its charge is dumped into the primary
winding of the ignition coil by the SCR which is triggered
by the contact breaker or electronic sensor in the distributor.
In CDI the spark current oscillations during the spark time
Scope5: when a primary winding clamp diode is added
to the circuit, the positive-going component of the spark
current flips around to become a negative-going spark.
February 2015 47
Scope photos in this
feature are based
on the venerable
Mark10B Capacitor
Discharge Ignition
from Delta
Products.
As they say, “an
oldie but a goodie!”
are the result of the discharge capacitor, typically about 1F
to 2F in value, resonating with the leakage inductance Lip
of the ignition coil.
The timing of the coil voltage and spark current and SCR
current for a typical CDI are shown in the scope photo of
Scope3.
The measured spark current is the ignition coil’s secondary current. The discharge capacitor has lost its energy (has
zero volts) at about the time the spark current first peaks
to its negative value of -140mA. The discharge capacitor
then charges in reverse to +200V. The energy required to
do this has not come from the DC:DC converter directly in
the CDI unit but has come from magnetic energy imparted
to the core of the ignition coil by the discharging capacitor.
The capacitor again discharges this time from +200V
(with the currents in the reverse direction) to generate the
positive peak of spark current to +80mA.The circuit which
allows the positive going spark current does not involve the
SCR at that time which is switched off and a little reverse
biased. The reverse primary current (and positive polarity
spark current) flows in a circuit completed by the bridge
rectifier diodes on the output of the DC:DC converter which
become forward biased.
Therefore although CDI is called “capacitive discharge
ignition” it is a combination of energy exchange in a resonant circuit between the electric field of the capacitor and
the magnetic field of the coil. Even if one just considers the
initial negative-going peak of spark current, half of that was
formed by magnetic energy of the ignition coil returning
to the electric field of the capacitor.
CDI might have been better called “Capacitive Oscillatory
Ignition” or COI. So really it is not true CDI as it requires
the magnetic component and voltage step up function from
the ignition coil to operate. This is the case when standard
ignition coils are used and the capacitor is initially charged
to only around 400V prior to discharge.
True CDI does exist in aviation exciter systems when a
capacitor charged to a very high voltage, discharges after
a separate spark ionisation process, directly into the spark
plug. Typically this produces a high initial peak current and
an exponential decay. In this instance there is no energy
exchange with magnetic field energy.
Scope4 shows the spark current profile from a Delta
10B CDI unit which uses an SCR. Note that unlike an MDI
system which has a unidirectional negative-going spark
current, the spark current in CDI is bidirectional.
Some brands of CDI are modified with an additional energy recovery or clamp diode on the ignition coil primary
to only generate a negative-going spark current, for example
the MSD 6A unit.
The CDI spark burn time has a much shorter duration
than MDI at about 200s versus 1ms or more for the MDI
system. However the peak currents are much higher at
around -140mA for the first negative peak.
Some CDIs can produce another half cycle of oscillation
of spark current if the SCR gate is held on for a longer period
than a full cycle of current. Yet others can put a sequence
of sparks thought to improve the probability of combustion.
When the primary winding clamp diode is added to
the circuit as in the MSD 6A CDI unit, the positive-going
component of the spark current flips around to become a
negative-going spark current (See Scope5) but this has little
effect on the total spark energy.
Estimating spark energy from scope recordings
Typical spark energies in MDI ignition systems are in the
order of 20 to 60mJ per spark and have durations of around
0.5 to 2ms; 1ms being common.
Assuming the ignition coil is wired correctly, the polarity
of the spark current is negative-going and has a roughly
right-angle triangle profile. Ignoring the initial oscillations
of spark current, the peak currents are typically about
-30mA, decaying nearly linearly to zero over the spark
burn time.
The exact energy depends on the dwell time and how
much energy is stored in the coil prior to the spark. So for
this example a -30mA peak spark current, has an average
current of about 15mA over a 2ms interval. The charge
transferred across a 1000V load (the spark) is about 30µQ
(millicoulombs) resulting in about 30mJ (millijoules) per
spark.
CDI system spark energies are typically lower than MDI;
usually less than half, however the peak spark currents are
higher than MDI and the spark duration is usually much
shorter. Also CDI spark currents are roughly sinusoidal
in shape. So in Scope4 for the Delta 10B unit above, the
negative peak spark current is nearly sinusoidal.
It peaks at -140mA and has a duration of about 100s,
the charge in Coulombs transferred is the average current
x time which is roughly 0.64 x 0.14 x 100s = 8.96C, and
multiplying that by the spark voltage (1000V) yields 8.96mJ.
Likewise for the positive-going spark current, the energy
is 0.08A x 0.64 x 100s x 1000 = 5.12mJ, the total energy
being 5.12 + 8.96 = 14mJ.
The Spark Energy Meter described elsewhere in this issue (with a proper current-time integrator) reported 15mJ
for that particular example.
Although CDI overall spark energies are lower than MDI,
they are delivered over a shorter time frame than MDI
sparks and they have higher peak currents and peak power.
Therefore they have a higher temperature than MDI sparks.
For example the CDI spark cited above has an SPP value
of 15mJ/200s = 75W and the MDI spark cited above has
an SPP of 30mJ/2ms = 15W.
While it is easy to estimate spark energy from an oscilloscope recording of the spark current profile and the
knowledge of the spark sustaining voltage it is much more
convenient to use the Spark Energy Meter which can measure the energy immediately.
SC
Now see the build-it-yourself Spark Energy Meter, commencing on page 57
48 Silicon Chip
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