Multi-spark ignition systems are becoming more common even on base-line passenger vehicle applications, but how well do these systems really work? Do multi-spark ignition systems really improve power delivery or fuel economy, and do these systems really reduce emissions at low engine speeds? This article will discuss the benefits, as well as the disadvantages and limitations of multi-spark ignition systems as these relate to inductive ignition systems, starting with this question-
It should be noted that multi-spark ignition systems must not be confused with ignition systems that use spark plugs with multiple side electrodes, and which produce multiple sparks at the same time.
A true inductive multi-spark ignition system is one that causes the spark plug to fire multiple times in rapid succession, with the purpose of creating an overall sparking event that is more energetic, and of significantly longer duration than a single spark.
Note though that this article will only discuss multi-sparking on standard inductive ignition systems, and not CDI (Capacitor Discharge) based ignition systems. While CDI systems that have been enhanced with multi-spark abilities are used in a few isolated production vehicle applications, CDI systems are more suited to performance-oriented applications, although the particularly short spark duration of CDI-based systems makes multi-sparking on these systems possible only at very low engine speeds., which brings us to-
While multi-spark ignition systems have been in use in cold climates for several decades, these systems were generally capable of creating only two (or sometimes three) sparks in conditions where low ambient temperatures make the combustion process very inefficient.
In these conditions cylinder turbulence can extinguish the detonation flame front, which in turn, causes the coil voltage to “spike”, thus causing the coil to discharge, and release another spark. Note though this is mostly a normal characteristic of the ignition/combustion process under some conditions, and does not represent true multi-sparking.
However, in modern inductive ignition systems, most ignition coils store sufficient energy to create multiple sparks, which makes it possible to adapt existing ignition driver circuitry to utilise the excess energy without the need to add components to the ignition system.
The image below is a representation of a true multi-sparking event occurring in a Ford application where the idling speed is below 1500 RPM.
In this representation, the upper oscilloscope trace represents the voltage in an ignition coil’s primary winding, while the lower trace represents the voltage discharges from the secondary winding, and therefore the actual delivery of ignition sparks.
Note that the ignition driver sustains the first spark only for about 0.5 milliseconds, before the primary windings are shut off again to deliver the second spark, which is sustained for about 0.75 milliseconds. When the primary windings are shut off for the third time, the resulting spark is sustained for about 1 millisecond, after which time it extinguishes naturally as the magnetic fields in the coils dissipate.
In practice, the number, and duration of individual sparks vary between applications and operating conditions such as changes in engine speed and temperature, and in some applications such as some late-model BMW applications, the ignition system can deliver up to nine sparks during a single combustion event during cranking.
On most applications, the ignition system reverts to single-spark delivery at engine speeds of about 1500 RPM for reasons we will discuss later, but note that on some applications multi-sparking may remain active over a wide range of engine speed/loads and operating conditions, although the actual number of sparks delivered during an ignition event may be reduced. This brings us to this question-
Even the most sophisticated engines suffer from a phenomenon known as COV* (Coefficient of Variation) under some operating conditions, such as during idling, when the air/fuel mixture is deliberately leaned out to levels where complete combustion becomes problematic.
* COV is defined as cycle-to-cycle variations in cylinder pressures during the compression stroke. While COV can occur between different cylinders for a wide range of possible reasons, it can also occur between cycles in a particular cylinder, mostly as the result of deficiencies in the inlet ducting that produce varying results/effects with varying engine speeds and/or loads.
In practice though, inconsistencies in both the compression pressure and the uniformity of the air/fuel mixture, coupled with sub-optimal ignition energy can produce pockets, or areas within the combustion chamber where the air/fuel mixture may be richer or leaner than in other areas. As a result, the propagation of the flame front is uneven, and in extreme cases, the uneven expansion of the combustion mass can extinguish the flame front altogether.
Since the efficiency of the combustion process is largely a function of temperature at low engine speeds/temperatures, firing multiple sparks into the compressed air/fuel mixture in rapid succession ensures that sufficient energy is available to ignite the entire air/fuel mixture charge, even though the air/fuel mixture may not be fully homogenous.
Thus, the biggest practical advantage of this is that combustion is vastly improved under conditions where neither the movement of the piston, nor the swirl produced by advanced induction systems produces sufficient turbulence in the combustion chamber to ensure a homogenous air/fuel mixture during the compression stroke at low engine speeds/loads.
One other significant advantage of multi-spark ignition systems is that the technology allows lower idling speeds than is possible to achieve or sustain with single-spark systems. Flowing from this are improved fuel economy, reduced emissions, and measurable reductions in vibration, all of which are major selling points that all manufacturers leverage to the maximum in efforts to increase their market share.
Despite the above-mentioned advantages of multi-spark ignition systems, these systems are hampered by the very physics that make their operation possible. For instance, while the processing power of modern automotive microprocessors are certainly able to deliver any number of sparks during an ignition event, the relatively poor conductivity of the materials that are currently available for use in ignition systems is the single biggest factor that limit the number of sparks it is possible to create during an ignition event. Below are some details of the limitations of current multi-spark ignition systems-
Even though modern ignition coils can be charged in considerably under one millisecond for the purposes of delivering a single spark, charging that same coil several times to deliver multiple sparks within a single ignition event becomes problematic when the relationship between RPM, degrees (of rotation of the crankshaft), and time is considered.
For instance, on an average 2.0L petrol engine that is running at 1000 RPM, the crankshaft can rotate through 6 degrees in only about one millisecond, which translates into an angular movement of 1 degree of rotation every 0.17 milliseconds. Considering that it can take up to one millisecond to induce a sufficiently strong magnetic field in a standard, OEM ignition coil to produce a single spark, it becomes obvious that multi-sparking can only be used at engine speeds that are low enough to allow for multiple charging cycles to occur in an ignition coil within a single ignition event.
While the primary current in a conventional ignition system starts from a zero value when a coil is charged up, this is not the case when the ignition system is in multi-spark mode. In practice, when a spark is extinguished by switching on the primary circuit when the ignition system is in multi-spark mode, the coil is usually not fully discharged. This means that in a poorly designed system, or in a system where abnormal resistances are present, there may not be enough dwell* time for the coil to build up a sufficiently strong magnetic field, which in turn, will produce low- energy second or even third sparks.
* “Dwell time” is the time required for thecoil to reach its nominal, or rated magnetic field strength.
The actual strength of the spark is directly related to the strength of the magnetic field the ignition coil was able to accumulate. In practice, and because the strength and duration of a spark can (and does), change in response to changes in temperature and other factors (such as the condition of the spark plugs, among others) the residual energy level(s) in the coil changes after the first spark had been delivered, as well.
Therefore, since most ECU and/or ignition drivers cannot know how much energy the ignition coil retains after the first spark, it can happen that the coils become magnetically saturated when the primary circuit is switched off for the second (or third)time, which can lead to severe overheating and subsequent failure of the coils.
As a practical matter, magnetic saturation refers to a condition where there is always a magnetic field present in the coils’ secondary windings, which has the negative effect of preventing the coils’ secondary windings from shedding heat between their “ON” and “OFF” states. In individual, COP (Coil over Plug) ignition coils, this issue is aggravated by the facts that dwell time is pre-programmed, which precludes the ECU from compensating for factors that affect the duration of ignition sparks, and that COP coils are almost always operating at, or close to their maximum allowable temperatures.
However, in order to reduce the incidence of COP coil failures caused by excessive temperatures, engine designers have introduced measures to reduce the energy levels in ignition coils progressively for each successive charge period. While this has gone some way towards reducing both the temperatures and duty cycles of COP ignition coils, much of the benefit of having multiple sparks in the first place is negated by the lower-energy sparks that result from shorter dwell times, which in turn, results in poor combustion, increased fuel economy, and undesirable increases in emissions.
In addition to regulating energy levels during ignition coil dwell times to control coil temperatures, some OEM ECU’s now have the ability to control the duration of dwell times based on look-up tables that can predict coil temperatures fairly accurately. In essence, this means that such ECU’s can effectively control coil temperatures to some extent by adapting dwell times according to the predicted temperature of individual ignition coils.
However, because COP coils are often poorly designed and constructed, or used on multi-spark systems they were not designed for, the high incidence of COP coil failures remains a significant problem that affects engine operation in vehicles in all markets.
When ignition coils are new, they generally cope reasonably well with multi-sparking loads, but as we know, especially COP coils deteriorate rather rapidly over time. Moreover, COP coils don’t always deteriorate at a predictable rate, nor do individual COP coils deteriorate at the same rate.
What this means in practice is that over time, it takes progressively longer for COP coils to charge up, which means that even at low engine speeds and loads, one or more coils on an engine might begin to miss one or more sparks in the spark delivery sequence. The effects of this depend on the degree of deterioration of a coil: in some cases, there may be no discernible symptoms, while in other cases a distinct deterioration in the idle quality may be present, which may or may not vary with changes in the engine temperature.
In the absence of trouble codes that identify possible causes of a poor engine idle, the best course of action to take is to obtain waveforms from each COP coil with an oscilloscope to verify, or eliminate coil deterioration as the cause of the poor idle.
Bear in mind that once an ignition system reverts to single-spark operation at engine speeds of above about 1500 RPM, a marginally defective ignition coil may have enough time to charge up to full strength, and the misfire (if present) will disappear. If an oscilloscope is not available, or if waveforms are inconclusive, use a laser or infrared-based thermometer to obtain at least three temperature readings of each coil at different engine temperatures.
The actual temperatures of the coils are less important than significant temperature differences that might be present, since magnetic saturation can cause an ignition coil’s temperature to spike, while a coil that is colder than adjacent coils is indicative of a coil that is not reaching its full charge potential. However, it is important always to refer to relevant technical information for details on any ignition coil’s allowable maximum and minimum temperature before condemning coils out of hand.
In addition, bear in mind that while (as a rule of thumb) most engines require spark durations of at least 0.5 to 0.6 milliseconds to ensure efficient combustion at idling speeds, some engines that use very lean mixtures during idling may require spark durations of at least 1.5 milliseconds, or sometimes longer, to ensure efficient combustion at low engine speeds.
Note that since spark duration (especially during multi-sparking operation) can be affected by factors such as worn/incorrect/unsuitable spark plugs, defects in an ignition coils’ primary circuit/windings, and even software issues such as defective ignition drivers, it is important to ensure that none of these issues are present before an ignition coil is condemned.
