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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 siliconchip.com.au 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. siliconchip.com.au 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 siliconchip.com.au +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. siliconchip.com.au 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 1F to 2F 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 200s 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 100s, the charge in Coulombs transferred is the average current x time which is roughly 0.64 x 0.14 x 100s = 8.96C, 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 100s 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/200s = 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 siliconchip.com.au